Audio apparatus and method of operation therefor

Abstract

An audio apparatus comprises a first renderer (205) applying a directional rendering to a mono downmix with the rendering being based on a channel transfer function retrieved from a store (211) storing transfer functions for different directions. The rendering is for a direction determined in response to spatial upmix parameters provided for the downmix by a direction determining circuit (209). A second intermediate stereo signal may be rendered from a decorrelated version of the mono downmix by a second renderer (217) using a predetermined rendering, such as for example from a specific direction / position that is not signal dependent. A combiner (219) generates an output stereo signal by combining the intermediate stereo signals. An adapter (221) adapts the level of the first intermediate stereo signal relative to the level of the second intermediate stereo signal in the output stereo signal dependent on a first scaling value determined from the spatial parameters.

Description

FIELD OF THE INVENTION

[0001] The invention relates to an audio apparatus and method of operation therefor, and specifically, but not exclusively, to rendering of Parametric Stereo encoded signals.BACKGROUND OF THE INVENTION

[0002] Spatial audio applications have become numerous and widespread and increasingly form part of many audiovisual experiences. New and improved spatial experiences and applications are continuously being developed which result in increased demands on the audio processing and rendering.

[0003] A lot of research and development effort has focused on providing efficient and high quality audio encoding and audio decoding for spatial audio. A frequently used spatial audio representation is multichannel audio representations, including stereo representation, and efficient encoding of such multichannel audio based on downmixing multichannel audio signals to downmix channels with fewer channels have been developed. One of the main advances in low bit-rate audio coding has been the use of parametric multichannel coding where a downmix signal is generated together with parametric data that can be used to upmix the downmix signal to recreate the multichannel audio signal.

[0004] In particular, instead of traditional mid-side or intensity coding, in parametric multichannel audio coding, a multichannel input signal is downmixed to a lower number of channels (e.g. two to one) and multichannel image (stereo) parameters are extracted. Then the downmix signal is encoded using a more traditional audio coder (e.g. a mono audio encoder). The bitstream of the downmix is multiplexed with the encoded multichannel image parameter bitstream. This bitstream is then transmitted to the decoder, where the process is inverted. First the downmix audio signal is decoded, after which the multichannel audio signal is reconstructed guided by the encoded multichannel image / upmix parameters.

[0005] An example of stereo coding is described in E. Schuijers, W. Oomen, B. den Brinker, J. Breebaart, "Advances in Parametric Coding for High-Quality Audio", 114th AES Convention, Amsterdam, The Netherlands, 2003, Preprint 5852. In the described approach, the downmixed mono signal is parametrized by exploiting the natural separation of the signal into three components (objects): transients, sinusoids, and noise. In E. Schuijers, J. Breebaart, H. Pumhagen, J. Engdegård, "Low Complexity Parametric Stereo Coding", 116th AES, Berlin, Germany, 2004, Preprint 6073 more details are provided describing how parametric stereo was realized with a low (decoder) complexity when combining it with Spectral Band Replication (SBR).

[0006] Parametric Stereo (PS) is a technology which is widely used to efficiently code a stereo signal as a mono downmix and a set of spatial parameters allowing an accurate reconstruction of the stereo image. PS has been used to substantially improve the compression efficiency for AAC and USAC at lower bit-rates, say 32kbps and below.

[0007] In addition to accurately reproducing a stereo signal, it has also been of interest to create high quality binaural rendering of (encoded) stereo signals to emulate a virtual loudspeaker playback. Binaural rendering of content authored for multi-channel playback can be achieved by the sum of convolutions of the input channel signals with left and right Head Related Impulse Responses (HRIRs), where each HRIR pair corresponds to a measured / simulated impulse response from a loudspeaker location to the ears. This can be expressed compactly in the z-domain as: Y L , R z = ∑ ∀ c X C z ⋅ H L , R φ c z where X C (z) represents the z-transform of the time domain input signal x c [n] with channel c, Y L,R (z) represents the z-transform of the left and right time domain output signals l[n] and r[n] respectively and H L , R φ c z is the z-transform of the HRIR of the left and right channels h l [n] and h r [n] for the angle (and distance) corresponding to loudspeaker position φ c . Approaches for rendering binaural stereo are disclosed in WO2010 / 122455A1 and WO2007 / 031896 A1.

[0008] However, whereas current approaches for audio rendering may provide acceptable performance in many applications and scenarios, they tend not to be ideal and may exhibit suboptimal behavior in some scenarios. In particular, it may result in suboptimal perceived quality and / or a reduced user experience with e.g. perceived suboptimal spatial perception / audio scene in some cases. Complexity and / or resource usage may also be higher than desired and may in some case make the approach undesired for implementations, such as applications based on small and cheap portable devices.

[0009] Hence, an improved approach would be advantageous. In particular an approach allowing increased flexibility, improved adaptability, improved performance, increased audio quality, improved perceived quality, an improved rendering of an audio scene, improved spatial representation, reduced complexity and / or resource usage, reduced computational load, facilitated implementation, improved user experience, and / or an improved spatial audio experience would be advantageous.SUMMARY OF THE INVENTION

[0010] Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

[0011] According to an aspect of the invention there is provided an audio apparatus comprising: a receiver arranged to receive a data signal comprising encoded data for a mono downmix audio signal being a downmix of a stereo signal and a set of spatial upmix parameters for upmixing the mono downmix audio signal to the stereo signal, the set of spatial upmix parameters being indicative of relative signal properties of channels of the stereo signal; a store comprising directional transfer functions for different directions, a directional transfer function for a given direction representing a mapping of a mono audio signal to stereo channels such that the mono audio signal is positioned in the given direction in a stereo image of the stereo channels; a decoder arranged to generate the mono downmix audio signal by decoding the encoded data; a decorrelator arranged to apply a decorrelation to the mono downmix audio signal to generate a first decorrelated mono downmix audio signal; a direction determining circuit arranged to determine a first direction from the spatial parameters; a first renderer arranged to perform a first rendering of the mono downmix audio signal to generate a first intermediate stereo signal, the first rendering being a directional rendering using a first channel transfer function retrieved from the store for the first direction; a second renderer arranged to perform a second rendering being a rendering of the first decorrelated mono downmix audio signal to generate a second intermediate stereo signal, the second rendering being a predetermined rendering employing a predetermined mapping of the decorrelated mono downmix audio signal to channel signals of the second intermediate stereo signal; a combiner arranged to combine at least the first intermediate stereo signal and the second intermediate stereo signal to generate an output stereo signal; and an adapter arranged to adapt a ratio between a level of the first intermediate stereo signal in the output stereo signal and a level of the second intermediate stereo signal in the output stereo signal dependent on a first scaling value determined from the spatial parameters.

[0012] The approach may provide an improved audio experience in many embodiments. For many signals and scenarios, the approach may provide improved rendering of a stereo audio signal allowing improved generation / reconstruction of a stereo audio signal with an improved perceived audio quality. The approach may provide improved representation of an audio scene by a stereo signal.

[0013] The approach may provide an efficient implementation and may in many embodiments allow reduced complexity and / or resource usage. The approach may in many scenarios allow a reduced computational burden while providing a perceived high quality rendering of a stereo signal, and in particular based on an efficient representation of a stereo signal using a downmix and spatial upmix parameters, such as specifically a PS encoded stereo signal.

[0014] The adapter may be arranged to adapt the ratio by adapting the level of the first intermediate stereo signal in the output stereo signal, by adapting the level of the second intermediate stereo signal in the output stereo signal; or by adapting both the level of the first intermediate stereo signal and the level of the second intermediate stereo signal in the output stereo signal.

[0015] The adapter may be arranged to adapt a level of an intermediate stereo signal in the output stereo signal by adapting a gain of any suitable process or function for the rendering path for that intermediate stereo signal and / or e.g. by adapting a weight for the intermediate stereo signal in the combination. The adapter may be arranged to adapt a level of an intermediate stereo signal in the output stereo signal by adapting a gain for a signal of the rendering path prior to or after the rendering. The adapter may for example adapt a level / gain for a rendered intermediate stereo signal generated by a renderer or may e.g. adapt a level / gain for a signal prior to the rendering.

[0016] The processing may be in time frequency segments or tiles. Each time frequency segment / tile may represent a frequency interval in a time interval. In many embodiments, the mono downmix audio signal may be divided into time segments / intervals and a frequency representation of the signal in the time segment / interval may be provided by signal values representing different frequency segments of the signal in the time segment / interval. Some or all of the processing may be performed in the frequency domain / frequency subbands.

[0017] The spatial upmix parameters may comprise sets of upmix parameters, each set of upmix parameters comprising at least one of: a level difference parameter indicative of a level difference between channels of the multichannel audio signal; a correlation parameter indicative of a coherence between channels of the multichannel audio signal; a timing difference parameter indicative of a timing difference between channels of the multichannel audio signal, and a phase difference parameter indicative of a phase difference between channels of the multichannel audio signal.

[0018] The first direction may be a desired / target rendering direction. A direction may be an angle and / or orientation from a listening position / in a stereo image.

[0019] The directional rendering may be a binaural rendering generating the first intermediate stereo signal as a binaural stereo signal comprising a point source positioned in the first direction, the binaural rendering comprising selecting binaural impulse response values as values for a binaural impulse response for a sound source in the first direction. The directional transfer functions may be parameterized transfer functions, and specifically may be represented in the frequency domain as weights for each of a plurality of subbands. A weight may be provided for each stereo channel. The weights may typically be complex.

[0020] The binaural impulse response values may be parametric values and may be frequency tile values. The binaural impulse response values may be values representing any suitable binaural impulse response in any suitable way, including HRIR, HRTF, BRIR values etc.

[0021] In many embodiments, the encoded data and the spatial upmix parameters are part of a Parametric Stereo encoding of the stereo signal.

[0022] The adapter may be arranged to adapt a level of the first intermediate stereo signal in the output stereo signal relative to a level of the second intermediate stereo signal in the output stereo signal in dependence on a first scaling value determined from the spatial parameters.

[0023] According to an optional feature of the invention, the first scaling value is a scaling value for the second intermediate stereo signal and the adapter is arranged to adapt a gain for the second intermediate stereo signal in dependence on the first scaling value.

[0024] This may provide an advantageous approach for many scenarios, including e.g. providing an advantageous trade-off between complexity, computational resources, data rate and / or the perceived audio quality of the generated output stereo signal. The first gain may be applied at any stage in the rendering process / path including prior to or after rendering. The first scaling value may specifically be used to set a gain for the decorrelated mono downmix audio signal and / or for the second intermediate stereo signal.

[0025] In some embodiments, the adapter is arranged to determine the scaling value for the second intermediate stereo signal substantially as g n = IID + 1 − IID − 1 2 + 4ICC 2 IID 2 IID + 1 or g n = IID ⋅ 1 − IID − 2 ⋅ ICC 2 + 4 ⋅ ICC 2 ⋅ IDD + IID − 1 2 1 − IDD 2 + IID + 1 ⋅ 4 ⋅ ICC 2 ⋅ IID + IID − 1 2 where IID is an interchannel intensity difference, expressed as the ratio of the signal power of the original left and right signals and ICC is an inter-channel cross-correlation, expressed as a normalized cross-correlation coefficient.

[0026] According to an optional feature of the invention, the adapter is arranged to determine a second scaling value from the spatial parameters, the second scaling value is a scaling value for the first intermediate stereo signal and the adapter is arranged to adapt a gain for the first intermediate stereo signal in dependence on the second scaling value.

[0027] This may provide an advantageous approach for many scenarios, including e.g. providing an advantageous trade-off between complexity, computational resources, data rate and / or the perceived audio quality of the generated output stereo signal.

[0028] In some embodiments, the second scaling value is a scaling value for the first intermediate stereo signal and the adapter is arranged to adapt a gain for the first intermediate stereo signal in dependence on the second scaling value.

[0029] The gain may be applied at any stage in the rendering process / path including prior to or after rendering. The second scaling value may specifically be used to set a gain for the mono downmix audio signal and / or for the first intermediate stereo signal.

[0030] In many embodiments, the adapter may be arranged to determine the scaling value for the first intermediate stereo signal substantially as g x = IID − 1 2 + 4 ICC 2 IID IID + 1 or g x = IID − 1 2 + 4 ⋅ ICC 2 ⋅ IDD + 1 − IID ⋅ 4 ⋅ ICC 2 ⋅ IID + IID − 1 2 1 − IID 2 + IID + 1 ⋅ 4 ⋅ ICC 2 ⋅ IID + IID − 1 2 where IID is an interchannel intensity difference and ICC is an inter-channel cross-correlation as described above.

[0031] According to an optional feature of the invention, the first rendering is a binaural rendering and the directional transfer functions are binaural transfer functions.

[0032] This may provide a particularly advantageous operation and user experience.

[0033] According to an optional feature of the invention, the second rendering is arranged to generate the second intermediate stereo signal using a set of directional transfer functions retrieved from the store for a set of predetermined directions.

[0034] This may provide an advantageous approach for many scenarios, including e.g. providing an advantageous trade-off between complexity, computational resources, data rate and / or the perceived audio quality of the generated output stereo signal.

[0035] According to an optional feature of the invention, the set of predetermined directions consists of one predetermined direction.

[0036] This may provide a particularly advantageous trade-off between complexity and user experience / perceived audio quality in many scenarios and embodiments.

[0037] According to an optional feature of the invention, the set of predetermined directions comprises a plurality of predetermined directions.

[0038] This may provide a particularly advantageous trade-off between complexity and user experience / perceived audio quality in many scenarios and embodiments.

[0039] According to an optional feature of the invention, the audio apparatus further comprises: a decorrelator arranged to decorrelate the mono downmix audio signal to generate a second decorrelated mono downmix audio signal, a third renderer arranged to perform a third rendering being of the second decorrelated mono downmix audio signal to generate a third intermediate stereo signal, the third rendering being a predetermined rendering employing a predetermined mapping of the second decorrelated mono downmix audio signal to channel signals of the third intermediate stereo signal; and the combiner is arranged to combine at least the first intermediate stereo signal, the second intermediate stereo signal, and the third intermediate stereo signal to generate the output stereo signal.

[0040] This may provide an advantageous approach for many scenarios, including e.g. providing an advantageous trade-off between complexity, computational resources, data rate and / or the perceived audio quality of the generated output stereo signal.

[0041] According to an optional feature of the invention, the second renderer is arranged to map the first decorrelated mono downmix audio signal to a first channel of the output stereo signal and the third renderer is arranged to map the second decorrelated mono downmix audio signal to a second channel of the output stereo signal.

[0042] This may provide an advantageous approach for many scenarios, including e.g. providing an advantageous trade-off between complexity, computational resources, data rate and / or the perceived audio quality of the generated output stereo signal.

[0043] According to an optional feature of the invention, the second rendering is arranged to generate the second intermediate stereo signal using a first set of directional transfer functions retrieved from the store for a first set of predetermined directions, and the third rendering is arranged to generate the third intermediate stereo signal using a second set of directional transfer functions retrieved from the store for a second set of predetermined directions, the first set of set of predetermined directions being different from the second set of predetermined directions.

[0044] This may provide a particularly advantageous trade-off between complexity and user experience / perceived audio quality in many scenarios and embodiments.

[0045] According to an optional feature of the invention, the direction determining circuit is arranged to determine a point source direction in a stereo image of the stereo signal from the spatial upmix parameters, and to determine the first direction by applying a mapping function to the point source direction.

[0046] This may provide an advantageous approach for many scenarios, including e.g. providing an advantageous trade-off between complexity, computational resources, data rate and / or the perceived audio quality of the generated output stereo signal.

[0047] The mapping may be non-uniform. The mapping may be from one range to a different range (e.g. from [0,90°] to [-30°,30°]). The mapping may be non-linear.

[0048] In some embodiments, the direction determining circuit may be arranged to determine the point source direction as: γ = arctan 1 − IID + IID − 1 2 + 4 ⋅ ICC 2 ⋅ IID 2 ⋅ ICC ⋅ IID where IID is an interchannel intensity difference and ICC is an inter-channel cross-correlation.

[0049] According to an optional feature of the invention, the spatial upmix parameters include an interchannel intensity difference parameter and an interchannel correlation parameter.

[0050] This may be particularly advantageous approach in many scenarios and embodiments.

[0051] According to an optional feature of the invention, the spatial upmix parameters and the directional transfer functions are provided for frequency subbands and the first renderer is arranged to generate subband values for subbands of the first intermediate signals from subband values of the mono downmix audio signal based on spatial upmix parameters and directional transfer functions for the subbands.

[0052] This may provide an advantageous approach for many scenarios, including e.g. providing an advantageous trade-off between complexity, computational resources, data rate and / or the perceived audio quality of the generated output stereo signal.

[0053] In many embodiments, the second renderer is arranged to generate subband values for subbands of the second intermediate signals from subband values of the decorrelated mono downmix audio signal, and specifically based on spatial upmix parameters and directional transfer functions for the subbands if these are used.

[0054] In many embodiments, the spatial upmix parameters and the directional transfer functions are provided for frequency subbands and the adapter is arranged to determine first scaling values for subbands of the mono downmix audio signal and / or the decorrelated mono downmix audio signal from spatial parameters of the subbands; and the adapter is arranged to adapt a level of subbands of the first intermediate stereo signal relative to a level subbands of the second intermediate stereo signal in the output stereo signal dependent on the first scaling value for the subbands.

[0055] The processing may be performed in subbands. The processing may be performed in time segments. The processing in each subband may for some (any) or all steps be performed separately / independently in each subband (with respect to the processing in other subbands). The processing in each time segment may for some (any) or all steps be performed separately / independently in each time segment (with respect to the processing in other time segments).

[0056] The processing may be time interval / segment based with all processing being performed for each time segment. Equivalently, the signal(s) for each segment may be considered a signal (and in particular signals of different time segments, may be considered different signals).

[0057] According to another aspect of the invention, there is provided a method of generating an output audio stereo signal, the method comprising: receiving a data signal comprising encoded data for a mono downmix audio signal being a downmix of a stereo signal and a set of spatial upmix parameters for upmixing the mono downmix audio signal to the stereo signal, the set of spatial upmix parameters being indicative of relative signal properties of channels of the stereo signal; storing directional transfer functions for different directions, a directional transfer function for a given direction representing a mapping of a mono audio signal to stereo channels such that the mono audio signal is positioned in the given direction in a stereo image of the stereo channels; generating the mono downmix audio signal by decoding the encoded data; applying a decorrelation to the mono downmix audio signal to generate a first decorrelated mono downmix audio signal; determining a first direction from the spatial parameters; performing a first rendering of the mono downmix audio signal to generate a first intermediate stereo signal, the first rendering being a directional rendering using a first channel transfer function retrieved from the store for the first direction; performing a second rendering being a rendering of the first decorrelated mono downmix audio signal to generate a second intermediate stereo signal, the second rendering being a predetermined rendering employing a predetermined mapping of the decorrelated mono downmix audio signal to channel signals of the second intermediate stereo signal; combining at least the first intermediate stereo signal and the second intermediate stereo signal to generate an output stereo signal; and adapting a level of the first intermediate stereo signal relative to a level of the second intermediate stereo signal in the output stereo signal dependent on a first scaling value determined from the spatial parameters.

[0058] These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which FIG. 1 illustrates some elements of an example of an audio distribution system; FIG. 2 illustrates some elements of an example of an audio apparatus in accordance with some embodiments of the invention; FIG. 3 illustrates some elements of an example of an audio apparatus in accordance with some embodiments of the invention; and FIG. 4 illustrates some elements of a possible arrangement of a processor for implementing elements of an audio apparatus in accordance with some embodiments of the invention. DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

[0060] FIG. 1 illustrates an example of an audio system wherein a stereo signal may be distributed / communicated for remote rendering. In the system, an audio source device 101 generates a data signal including a representation of a stereo audio signal. The stereo audio signal may be one captured at the audio source device 101 or may be received from another source or indeed may be an artificially generated stereo signal (e.g. it may be a virtual audio stereo signal).

[0061] The audio source device 101 may generate the data signal to include encoded data that represents a mono downmix audio signal for the stereo signal. For example, the mono downmix audio signal may be generated as a weighted combination, and specifically as a weighted summation of the channel signals of the stereo signal. In many cases, the weights may be fixed and may specifically be the same for the two channel signals.

[0062] The generated mono downmix audio signal is encoded using a suitable mono audio encoding algorithm / standard to generate encoded data representing the mono downmix audio signal.

[0063] In addition to the mono downmix audio signal, the audio source device 101 generates spatial upmix parameters for upmixing the mono downmix audio signal to recreate the original stereo signal.

[0064] The spatial upmix parameters are generated to be indicative of / reflect relative properties of the channel signals of the stereo audio signal. In particular, the spatial upmix parameters may be indicated to include parameters that are indicative of at least one of relative intensities / levels of the stereo channels, relative (frequency domain) phases of the stereo channels, a relative time difference between the channels, and / or a correlation between the channels. Specifically, the audio source device 101 may generate spatial upmix parameters including one or more of an inter-channel intensity difference, inter-channel level difference, inter-channel time difference, inter-channel phase difference, and / or inter-channel correlation.

[0065] The data signal may specifically comprise a Parametric Stereo (PS) encoding of the stereo signal.

[0066] A classical PS downmix is calculated as: m = c l + r where the parameter c is chosen such that the power of the stereo signal is preserved in the downmix, the power being defined using the 2-norm: m 2 = l 2 + r 2 and thus e.g.: c = l 2 + r 2 l + r 2

[0067] The PS parameters are specifically an Inter-channel Intensity Difference IID, an Inter-channel Correlation ICC, and in some cases an Inter-channel Phase Difference IPD parameter. These may specifically be defined as: IID = l 2 r 2 ICC = < l , r > l 2 r 2 IPD = arg < l , r > where the complex-valued inner product is defined as: < x ,y > = ∑ ∀ i x i ⋅ y i ∗

[0068] The spatial upmix parameters are typically generated for specific time frequency tiles, and thus specifically each parameter value is generated / provided for a given frequency subband and for a given time segment.

[0069] The audio source device 101 may accordingly encode a stereo signal as encoded data representing a mono downmix audio signal of the stereo signal and associated spatial upmix parameters that are indicative of relative properties of the channels (channel signals) of the stereo signal. Specifically, the audio source device 101 may be arranged to generate a data signal comprising a conventional PS encoded stereo signal.

[0070] The system of FIG. 1 further comprises an audio apparatus which henceforth will be referred to as the audio render apparatus 103. The audio render apparatus 103 is arranged to receive the data signal generated by the audio source device 101. In the example, the audio render apparatus 103 and audio source device 101 are both coupled to a network 105 through which the data signal can be communicated and specifically through which it can be communicated from the audio source device 101 to the audio render apparatus 103. The network 105 may specifically be, or include, the Internet.

[0071] The render apparatus 103 comprises a receiver 201 which is arranged to receive the data signal from the audio source device 101. It will be appreciated that the data signal may be communicated and received via any suitable communication means and channel.

[0072] In the example the receiver 201 comprises functionality for establishing a communication connection with the audio source device 101, and specifically in the example the receiver 201 comprises a network interface for coupling to the network 105 and for establishing a communication with the audio source device 101 through the network.

[0073] Thus, the receiver 101 receives a data signal which comprises encoded data for a mono downmix audio signal set of spatial upmix parameters for upmixing the mono downmix audio signal to the stereo signal. The set of spatial upmix parameters comprises one or more parameters indicative of relative signal properties of channels of the stereo signal, and may specifically be indicative of a level / intensity difference between the channels of the stereo signal, a cross-correlation between the channels of the stereo signal, and / or a phase difference or time difference between the channels of the stereo signals. In many cases, the data signal may comprise ICC, IID and / or IPD parameters. The data signal may specifically comprise a PS (Parametric Stereo) encoded stereo signal. The audio source device 101 may be arranged to generate a data signal which comprises a stereo signal encoded in accordance with the Parametric Stereo (PS) specifications / standard. The receiver 201 may accordingly receive a representation of a stereo signal encoded by a mono downmix audio signal and spatial upmix parameters, and specifically a PS encoded stereo signal.

[0074] The audio render apparatus 103 is arranged to process the data signal to render a stereo signal. However, the audio render apparatus 103 uses a specific approach where parallel paths process the received mono downmix audio signal in different ways to generate different stereo signal components which are then combined to generate the output stereo signal.

[0075] The approach is based on consideration / signal model that the stereo signal can be represented as: l = cos γ e jϕ l x + n l r = sin γ e jϕ r x + n r

[0076] This signal model considers the stereo signal to correspond to a combination of a directional signal and of diffuse background audio. The directional signal component x is phase shifted using two parameters φ l and φ r , and is further panned / positioned in the stereo image of the original stereo channels l and r. The panning is to an angle represented by the panning angle γ. Furthermore, a diffuse signal component is represented by diffuse residual signal components n l and n r of the respective channels. It is noted that the signal model description does not necessarily refer to a time-domain signal, but rather can alternatively or additionally refer to individual (potentially relatively small) frequency subbands. For example, the described signal model may individually apply to each of the frequency subbands for which separate spatial upmix parameters are provided.

[0077] A rendering approach could be to seek to decompose the received mono downmix audio signal into respectively the directional signal x and the diffuse signal(s) n (n l , n r ) and then to generate the output stereo signal in accordance with the equations above. However, decomposition of the mono downmix audio signal into such individual components is extremely challenging and tends to require substantial complexity and resource usage yet tends to yield suboptimal decomposition.

[0078] The audio render apparatus 103 is arranged to render the output stereo signal based on a consideration of such a signal model but without requiring specific decomposition of the mono downmix audio signal. It uses different rendering approaches to generate different intermediate stereo signals which may be considered estimates of the different signal components of the signal model with these intermediate stereo signals being combined to generate an output stereo signal. The intermediate stereo signals may be considered estimates or approximations of respectively the directional signal x and diffuse signals n of the signal model but are generated using low resource demanding approaches. The approach provides an advantageous rendering in many scenarios, embodiments, and applications and in particular may often provide an advantageous audio and spatial perception while allowing low complexity and resource demanding implementation and operation.

[0079] FIG. 2 shows examples of elements of the audio render apparatus 103.

[0080] The audio render apparatus 103 comprises a receiver 201 which is arranged to receive the data signal from the audio source device 101. Thus, the receiver 201 receives a data signal comprising encoded data for a mono downmix audio signal of the original stereo signal. In addition, the data signal includes spatial upmix parameters for upmixing the mono downmix audio signal to the stereo signal where the spatial upmix parameters are indicative of relative signal properties of channels of the stereo signal. The spatial upmix parameters may as mentioned specifically be inter-channel time, phase, level, intensity differences and / or inter-channel correlation measures.

[0081] The receiver 201 is coupled to a decoder 203 which is arranged to receive the encoded data representing the mono downmix audio signal and to decode this data to generate the mono downmix audio signal. It will be appreciated that any suitable method for encoding and decoding the mono downmix audio signal may be used and in particular that any suitable standardized encoding format and algorithm may be used.

[0082] The receiver 201 may be arranged to receive a time domain audio signal and / or a frequency domain audio signal version / representation of the mono downmix audio signal. In some cases, the received data signal may include the mono downmix audio signal in only one representation, i.e. the data signal may include only one of the frequency domain audio signal and the time domain audio signal. In such cases, the received data signal may be transformed to the other domain as appropriate. Thus, in some cases, a received data signal may include a time domain audio signal being the time domain representation of the mono downmix audio signal, and a time to frequency domain transformer may from this generate the frequency domain audio signal for the mono downmix audio signal. In some cases, a received data signal may include a frequency domain audio signal being the frequency domain representation of the mono downmix audio signal and a frequency to time domain transformer may from this generate the time domain audio signal for the mono downmix audio signal if necessary.

[0083] In particular, in some embodiments, the receiver may comprise a filter bank which is arranged to generate a frequency subband representation of a received time domain mono downmix audio signal. The receiver 201 may comprise a filter bank that is applied to the mono downmix audio signal such that it is divided into frequency subbands.

[0084] The filter bank may be Quadrature Mirror Filter (QMF) bank or may e.g. be implemented by a Fast Fourier Transform (FFT), but it will be appreciated that many other filter banks and approaches for dividing an audio signal into a plurality of subband signals are known and may be used. The filterbank may specifically be a complex-valued pseudo QMF bank, resulting in e.g. 32 or 64 complex-valued sub-band signals.

[0085] The processing is furthermore typically performed in time segments or time slots. In most embodiments, the audio signal is divided into time intervals / segments with a conversion to the frequency / subband domain by applying e.g. an FFT or QMF filtering to the samples of each signal. For example, each channel of the downmix audio signal may be divided into time segments of e.g. 2048, 1024, or 512 samples. These signals may then be processed to generate samples for e.g. 64, 32 or 16 subbands. Thus, a set of samples may be determined for each subband of the mono downmix audio signal.

[0086] It should be noted that the number of time domain samples is not directly coupled to the number of subbands. Typically, for a so-called critically sampled filterbank of N bands, every N input samples will lead to N sub-band samples (one for every sub-band). An oversampled filterbank will produce more output samples. E.g. for every N input samples, it would generate k*N output samples, i.e., k consecutive samples for every band.

[0087] In some embodiments, the subbands are generated to have the same bandwidth but in other embodiments subbands are generated to have different bandwidths, e.g. reflecting the sensitivity of human hearing to different frequencies.

[0088] For example, the receiver 201 may employ a hybrid filterbank with logarithmic filter band center-frequency spacings that follow that of human perception similar to equivalent rectangular bandwidths (ERBs). In order to compensate for the delay of the filtering by the small filter bank, a delay may be introduced for higher frequency subbands.

[0089] As a specific example, a time-domain signal x[n] may be fed through a downsampled complex-exponential modulated QMF bank with K bands. Each frame of 64 time domain samples x[n] results in one slot of QMF samples X[k,l] with k = (0, ..., K - 1) at slot l. The lower slots may then be filtered by additional complex-modulated filterbanks splitting the lower bands further. The higher slots are delayed ensuring that the filtered mono downmix audio signals of the lower bands are in sync with the higher bands as the filtering introduces a delay. This finally results in a structure where for every 64 time-domain samples x[n], one slot m of hybrid QMF samples Y[k, l] is produced with k = (0, ..., L - 1) at slot l, e.g. with a total number of hybrid bands M = 77.

[0090] Thus, in many embodiments, the signals and the processing may be performed in subbands and for individual segments. Such blocks of a frequency interval / subband in a given time interval / segment will also be referred to as time frequency segments / tiles.

[0091] The mono downmix audio signal is in the example fed to a first renderer 205 via a first gain block 207. The first renderer 205 is arranged to render the (potentially gain modified / scaled) mono downmix audio signal to generate a first intermediate stereo signal. The rendering by the first renderer 205 (also referred to as a first rendering) is a directional rendering which renders the first intermediate stereo signal with a given direction / position in the stereo image of the first intermediate stereo signal. The first rendering may specifically render the mono downmix audio signal as a point source with a given direction / position in the stereo image.

[0092] The first renderer 205 is coupled to a direction determining circuit 209 which is arranged to determine a direction γ' which is fed to the first renderer 205 resulting in this rendering the mono downmix audio signal from this position / direction. Thus, the first rendering is specifically such that the mono downmix audio signal in the first intermediate stereo signal is perceived as a point audio source positioned in the direction corresponding to the direction γ' determined by the direction determining circuit 209. The direction γ' will also be referred to as the rendering direction or rendering angle.

[0093] The direction determining circuit 209 is arranged to determine the direction from the received spatial upmix parameters. The spatial upmix parameters provide information on the relationship between the channels of the stereo signal that is downmixed and as such provide information of the position / orientation of the audio, and specifically of a dominant signal component in the stereo image of the stereo signal. For example, for a PS encoded signal, the spatial upmix parameters provide information of the position of the dominant signal component in the stereo signal, and specifically it provides information of an orientation angle for the dominant signal.

[0094] The direction determining circuit 209 may specifically determine the rendering direction γ' from the spatial upmix parameters. The rendering direction will typically be determined on a frequency tile basis, and specifically in frequency subbands and time segments matching those for which the spatial upmix parameters are provided.

[0095] The first renderer 205 may accordingly proceed to render the mono downmix audio signal such that is perceived from the given direction and it specifically achieves this directional rendering by applying a channel transfer function to the mono downmix audio signal with the channel transfer function generating the intermediate stereo signal from the mono downmix audio signal. The channel transfer function may specifically include a sub-transfer function for each channel, i.e. it may include one (sub)transfer function for generating a left channel signal and one (sub)transfer function for generating the right channel signal.

[0096] In many cases, the transfer function may be provided as a set of complex weights for the different subbands of a frequency representation of the mono downmix audio signal. The audio apparatus may perform many or all of the operations in the frequency domain and thus the transfer function may also be expressed and applied in the frequency domain. For example, for each frequency subband of the representation of the mono downmix audio signal, the transfer function may provide a complex weight for each of the output channels and a frequency representation of the first intermediate stereo signal may be generated by applying / multiplying the subband samples of the mono downmix audio signal by these weights to generate the subband samples of the first intermediate stereo signal.

[0097] The first transfer function is determined to correspond to the desired direction, i.e. it reflects the mapping from the mono downmix audio signal to the channels of the first intermediate stereo signal such that it is perceived as / corresponds to an audio source at a position in the stereo image corresponding the rendering direction / angle.

[0098] For example, in some cases, the transfer function for a given direction may correspond to a panning of the mono downmix audio signal to the given direction in the stereo image.

[0099] In many embodiments, the first rendering may be a binaural rendering and the first intermediate stereo signal may be a binaural stereo signal providing an enhanced spatial experience / perception when heard through headphones. Thus, the first renderer 205 may specifically be a binaural audio renderer which generates binaural audio signals for the left and right ear of a user. Binaural audio signals are generated to provide a desired spatial experience and are typically reproduced by headphones or earphones that specifically may be part of a headset worn by a user (the headset typically also comprises left and right eye displays).

[0100] Thus, in many embodiments, the audio rendering by the first renderer 205 is a binaural render process using suitable binaural transfer functions to provide the desired spatial effect for a user wearing a headphone. For example, the first renderer 205 may be arranged to generate an audio component to be perceived to arrive from a specific position using binaural processing.

[0101] Binaural processing is known to be used to provide a spatial experience by virtual positioning of sound sources using individual signals for the listener's ears. With an appropriate binaural rendering processing, the signals required at the eardrums in order for the listener to perceive sound from any desired direction can be calculated, and the signals can be rendered such that they provide the desired effect. These signals are then recreated at the eardrum using either headphones or a crosstalk cancelation method (suitable for rendering over closely spaced speakers). Binaural rendering can be considered to be an approach for generating signals for the ears of a listener resulting in tricking the human auditory system into perceiving that a sound is coming from the desired positions.

[0102] The binaural rendering is based on binaural transfer functions which vary from person to person due to the acoustic properties of the head, ears and reflective surfaces, such as the shoulders. Binaural transfer functions may therefore be personalized for an optimal binaural experience. For example, binaural filters can be used to create a binaural recording simulating multiple sources at various locations. This can be realized by convolving each sound source with the pair of e.g., Head Related Impulse Responses (HRIRs) that correspond to the position of the sound source.

[0103] A well-known method to determine binaural transfer functions is binaural recording. It is a method of recording sound that uses a dedicated microphone arrangement and is intended for replay using headphones. The recording is made by either placing microphones in the ear canal of a subject or using a dummy head with built-in microphones, a bust that includes pinnae (outer ears). The use of such dummy head including pinnae provides a very similar spatial impression as if the person listening to the recordings was physically present during the recording.

[0104] By measuring e.g., the responses from a sound source at a specific location in 2D or 3D space to microphones placed in or near the human ears, the appropriate binaural filters can be determined. Based on such measurements, binaural filters reflecting the acoustic transfer functions to the user's ears can be generated. The binaural filters can be used to create a binaural recording simulating multiple sources at various locations. This can be realized e.g., by convolving each sound source with the pair of measured impulse responses for a desired position of the sound source. In order to create the illusion that a sound source is moving around the listener, a large number of binaural filters is typically required with a certain spatial resolution, e.g., 10 degrees.

[0105] The head related binaural transfer functions may be represented e.g., as Head Related Impulse Responses (HRIR), or equivalently as Head Related Transfer Functions (HRTFs) or, Binaural Room Impulse Responses (BRIRs). The (e.g., estimated or assumed) transfer function from a given position to the listener's ears (or eardrums) may for example be represented in the frequency domain in which case it is typically referred to as an HRTF or BRTF, or in the time domain in which case it is typically referred to as a HRIR or BRIR. In some scenarios, the head related binaural transfer functions are determined to include aspects or properties of the acoustic environment and specifically of the environment in which the measurements are made, whereas in other examples only the user characteristics are considered. Examples of the first type of functions are the BRIRs and BRTFs.

[0106] The audio render apparatus 103 comprises a store 211 which stores directional transfer functions for different directions. The directional transfer function for a given direction represents the mapping of a mono audio signal to stereo channels such that the mono audio signal is positioned in the given direction in a stereo image of the stereo channels. Thus, applying the directional transfer function for a given direction to the mono downmix audio signal may generate a stereo signal representing the mono downmix audio signal as an audio source positioned in the given direction. The mapping may in some cases be a time domain mapping (such as a gain, filter or other transfer function) or may in many cases be a frequency domain mapping, such as a set of parameter values / scale values (typically complex values) for different subbands. In the latter case, a frequency domain intermediate stereo signal may be generated by for each subband multiplying the subband sample of the mono downmix audio signal with respectively a complex value for that subband for a first channel of the intermediate stereo signal and with a complex value for that subband for a second channel of the intermediate stereo signal.

[0107] For example, in examples where a panning is performed in the horizontal 2D plane, the store 211 may comprise panning parameters for different directions. For example, panning parameters for azimuth angles in a 0-360° interval may be provided for each 1° angle increment. The first renderer 205 may be coupled to the store 201 and be arranged to extract the directional transfer function for the rendering direction and then proceed to perform the rendering using the extracted directional transfer function. The rendering of the (potentially gain compensated) mono downmix audio signal may accordingly be rendered such that it is positioned / perceived in the stereo image to arrive from the rendering position.

[0108] It will be appreciated that the store 211 may not have directional transfer function stored for the desired rendering direction. In such cases, the first renderer 205 may be arranged to retrieve the nearest directional transfer function from the store 211 and use this for rendering. In such cases, the rendering direction may be considered to correspond to the direction for the retrieved directional transfer function, i.e. the rendered direction may be a quantized value γ' of the desired rendering direction determined by the direction determining circuit 209.

[0109] In other embodiments, the first renderer may be arranged to estimate a desired directional transfer function for a desired rendering direction by interpolating between two directional transfer functions from the store 305 corresponding to the two rendering angles nearest to the desired rendering direction determined by the direction determining circuit 303.

[0110] In most embodiments, the first renderer 205 is as mentioned arranged to perform a binaural rendering and the directional transfer functions stored in the store 211 are binaural transfer functions. Thus, the store may store data describing binaural transfer functions for different directions. The binaural transfer functions may for example be HRTFs, BRIRs, or HRIRs. The store 211 may specifically store frequency subband complex values for each channel for each frequency subband for a range of different frequencies. The first renderer 205 may thus perform the binaural rendering by multiplying the subband samples of the mono downmix audio signal with the corresponding subband coefficients / complex values of the selected binaural transfer function to generate subband sample values of the intermediate binaural stereo signal.

[0111] It will be appreciated that in many embodiments, the directional transfer functions may be stored as a plurality of functions linked with different directions. For example, the store 211 may be a look-up table which can receive the rendering direction as an index and provide a set of values of the directional transfer function for that direction. The directional transfer function may for example be represented by individual subband values / coefficients, or may e.g. in other embodiments by represented by e.g. parameter values defining the directional transfer function operation (e.g. coefficients for the transfer function), a mathematical description / function from which suitable values of the transfer function can be generated etc.

[0112] Thus, the audio render apparatus 103 comprises a processing path which generates an intermediate stereo signal comprising the mono downmix audio signal represented as an audio source at a specific position in the spatial image of the first intermediate stereo signal. The mono downmix audio signal may typically be represented as a point audio source at the given direction. The rendering is adaptive with the direction being given by the spatial parameters and thus is dynamically adapted to reflect the characteristics of the stereo signal.

[0113] In addition, the audio render apparatus 103 comprises a second processing path which generates a second intermediate stereo signal. The second processing path includes an optional second gain block 213 Which may adapt the level of the second intermediate stereo signal as will be described in more detail later. The (optionally gain compensated) mono downmix audio signal is fed to a decorrelator 215 which is arranged to apply a decorrelation to the mono downmix audio signal to generate a first decorrelated mono downmix audio signal. It will be appreciated that a large number of different algorithms and functions for decorrelating an audio signal is known to the skilled person, and that any suitable approach or algorithm may be used without detracting from the invention.

[0114] The first decorrelated mono downmix audio signal is fed to a second renderer 217 which is arranged to perform a second rendering being a rendering of the first decorrelated mono downmix audio signal to generate a second intermediate stereo signal. However, in contrast to the first rendering process, the second rendering process is a predetermined rendering which is not dependent on the spatial parameters, and which typically is not depending on properties of the stereo signal. The second rendering may typically be a diffuse rendering seeking to generate the second intermediate stereo signal to provide a perception of a more diffuse and spatially less definite audio source. The second rendering is specifically a predetermined rendering employing a predetermined mapping of the decorrelated mono downmix audio signal to channel signals of the second intermediate stereo signal.

[0115] As a specific example, the second rendering may typically generate the second intermediate stereo signal by simply mapping the first decorrelated mono downmix audio signal to two phase inverse signals, i.e. the second intermediate stereo signal may be generated with the first decorrelated mono downmix audio signal being mapped to both channels but with a 180° phase offset between them (the first decorrelated mono downmix audio signal may specifically be inverted for one of the channels). For example, in some embodiments, the first decorrelated mono downmix audio signal may be mapped to the right and left signals of the second intermediate stereo signal but with the mapping being 180° out of phase for the two channels of the second intermediate stereo signal.

[0116] The first renderer 205 and second renderer 217 are coupled to a combiner 219 which is arranged to combine at least the first intermediate stereo signal and the second intermediate stereo signal to generate an output stereo signal. In many embodiments, the combiner 219 may be arranged to combine / sum the samples / values of the individual channels of the first and second intermediate stereo signals to generate the samples / values of the output stereo signal. In many cases, the combination may be performed by combining / summing subband values of the intermediate stereo signals. In other embodiments, the combination may be performed in the time domain by combining / summing time domain values of the intermediate stereo signals.

[0117] In many embodiments, the combination of the intermediate stereo signals may be by a (possibly weighted) combination / summation of corresponding channel signals for the first intermediate stereo signal and the second intermediate stereo signal.

[0118] The audio render apparatus 103 accordingly generates an output stereo signal which is the combination of a directional rendering putting an audio source at a desired position as determined from the received spatial upmix parameters, and of a predetermined rendering providing a more diffuse and decorrelated perception of the corresponding audio source. The approach provides two parallel rendering processes / paths for the mono downmix audio signal with the rendered results being combined to generate the output stereo signal.

[0119] In addition to the described flexible and adaptable generation of the output signal to provide an output stereo signal that includes both a directionally rendered component and a more diffuse / predeterminedly rendered component, the audio render apparatus 103 is arranged to flexibly and adaptively adapt / control the relative level between these components.

[0120] Specifically, in the example of FIG. 2, the audio render apparatus 103 comprises an adapter 221 which is arranged to adapt the level of the first intermediate stereo signal relative to the level of the second intermediate stereo signal in the output stereo signal, where the adaptation and the relative levels are dependent on a first scaling value which is determined from the spatial parameters.

[0121] The spatial parameters are indicative of the relative properties of the channel signals for the stereo signal and, as has been realized by the inventors, this may also provide information on the relative levels for respectively a directional component and for a non-directional component. The adapter 221 may specifically determine a scaling factor that scales the effective gain of one of the rendering paths relative to the other path thereby adapting the relative levels of the first intermediate stereo signal component and the second intermediate stereo signal component in the output stereo signal. The relative gain adaptation may be by an adaptation of the gain for the first intermediate stereo signal / of the first rendering path (including an adaptation of the weight / gain for the first intermediate stereo signal in the combination), may be by an adaptation of the gain for the second intermediate stereo signal / of the second rendering path (including an adaptation of the weight / gain for the second intermediate stereo signal in the combination), or of the gain of both of these.

[0122] In the example of FIG. 2, the relative levels may be adapted by adapting the gains of the first gain block 207 and of the second gain block 213, and thus the gain of both the directionally rendered component and the non-directionally and predetermined rendered component are adapted to provide an output stereo signal with suitable levels for the rendered mono downmix audio signal components.

[0123] The audio apparatus may accordingly be arranged to generate an output stereo signal, and often an output binaural stereo signal from the received mono downmix audio signal and spatial upmix parameters. The audio apparatus specifically implements two different rendering paths with one being a directional (binaural) rendering of a directional (e.g. a dominant) signal component while the other is a predetermined rendering / mapping of a decorrelated audio signal generated from the mono downmix audio signal. The rendering of the output stereo signal is not a conventional adaptive upmixing of the received and decorrelated mono signals, and is specifically not a conventional 2x2 matrix upmixing of the mono signal and a decorrelated signal, but rather is a direct generation of a stereo signal by parallel processing of respectively the mono downmix audio signal and one or more decorrelated versions of this, with the former rendering being directional dependent on the spatial upmix parameters and the latter rendering being a predetermined rendering.

[0124] The processing seeks to render direct / dominant / directional components using a direct rendering with a direction that is given by the spatial parameters. The rendering employs a directionally dependent transfer function to the left and right stereo output signal for that purpose. The approach further seeks to render a residual / remaining signal component as a more diffuse signal, and specifically it uses a predetermined rendering where a decorrelated signal is mapped directly to the channels of the output binaural signal using a transfer function. The mapping is predetermined and may specifically be such that it allows a more diffuse and non-directional perception of this signal component. The rendering process thus uses fundamentally different approaches to provide different signal components in the output binaural signal, but does so without specifically decomposing the mono downmix audio signal into a dominant and diffuse / residual signal component with these subsequently being individually rendered. Rather a direct rendering of respectively the mono downmix audio signal and a decorrelated version thereof is performed to generate the binaural output signal.

[0125] The approach is further arranged to adapt the relative levels (and in many cases the absolute levels) of the two different signal components in the binaural output signal based on the spatial upmix parameters, and indeed typically only on the spatial upmix parameters. In particular, the spatial upmix parameters that indicate differences between the channels of the original stereo signal are directly used to determine the levels in the binaural output signal for respectively the directionally rendered mono downmix audio signal component and for the predetermined (and often non-directionally) rendered decorrelated mono downmix audio signal component.

[0126] The approach allows low complexity and computationally efficient rendering of a stereo signal encoded as a downmix and spatial upmix parameters, such as a PS encoded signal. It may further allow high performance rendering with a perceived improved audio quality. In many cases, a substantially improved spatial perception and user experience may be achieved, and indeed can be provided using headphones. The approach may in many cases provide a user perception of an audio scene where individual / dominant audio sources are well defined at specific directions / positions in a stereo image whereas other sources (e.g. ambient or background audio sources) are perceived more diffuse.

[0127] The approach may typically allow a very efficient operation and rendering with reduced complexity. A particular advantage of the approach is that it does not require a decomposition of the received mono downmix audio signal into different components with different specific properties.

[0128] In the specific example, the adapter 221 is arranged to determine and set gains for each of the rendering paths, and thus for both the directional rendering of the mono downmix audio signal (and the first intermediate stereo signal) and for the predetermined rendering of the first decorrelated mono downmix audio signal (and the second intermediate stereo signal). This may be highly advantageous in many embodiments as it may allow appropriate setting of the levels of the corresponding signal components in the output stereo signal, especially with respect to other component that may possibly be present in the output stereo signal.

[0129] However, in other embodiments, the gain of only one of the rendering paths may be adapted. For example, the directional rendering of the mono downmix audio signal may be with a fixed gain and / or with automatic gain that sets the first intermediate stereo signal to a given (predetermined / fixed) level, with the adapter 221 then adapting the gain for the predetermined rendering path to result in the desired relative gain between the first intermediate stereo signal and the mono downmix audio signal.

[0130] In the present case, the adapter 221 may seek to determine the gains to reflect the signal model as previously described, i.e. where the original stereo signal is considered to represent a main component x and residual / more diffuse component n: l = cos γ e jϕ l x + n l r = sin γ e jϕ r x + n r

[0131] The audio render apparatus 103 may seek to generate a directly rendered component that is an estimate of the directional signal component x generated by scaling the mono downmix audio signal and rendering it directionally. The estimated direct signal component may be represented by: x ′ = g x ⋅ m = x 2 m 2 m where the resulting approximation x' is then rendered as the directional component.

[0132] Similarly, the audio render apparatus 103 may seek to approximate the (typically left and right) diffuse / ambient / residual / noise signal(s) n by scaling a decorrelated version of the mono downmix, i.e. by a scaling of the first decorrelated mono downmix audio signal. The estimate diffuse residual signal may be represented by: n ′ = g n ⋅ H m = n 2 m 2 H m where H{. } is a decorrelation process (filter) and ∥n∥ 2< is the power of the (left, right) diffuse residual signal. The approximated diffuse residual component(s) are rendered as diffuse components.

[0133] In many embodiments, the adapter 221 is arranged to generate the gains in line with the above equations. In particular, the gain for the directional rendering may be set in dependence on a relative power level of a directional component of the stereo signal relative to a power level of the mono downmix audio signal. The spatial upmix parameters provide information on the relative properties of the channel signals of the stereo signal and specifically may provide information on both the interchannel levels / intensity differences as well as on the interchannel correlation. Accordingly, the spatial upmix parameters can be considered to provide information on the directional signal component x and the diffuse / residual component n. Accordingly, the gains g x and g n may be estimated / calculated from the received spatial parameters.

[0134] The gains may in many embodiments be determined to ensure power preservation with the combined power level of the intermediate signals corresponding to the power level of the mono downmix audio signal. For the signal model of the stereo signal, the gains for an actual point source with no other audio being present may be 1 for the directional rendering path and 0 for the predetermined rendering path. For a completely diffuse signal, the gains would be the opposite, i.e. 1 for the predetermined rendering path and 0 for the directional rendering path. In practice, when two diffuse signals are considered, one for the left channel and one for the right channel, the gains would typically be 1 / 2 .

[0135] The gains may in particular in many embodiments advantageously be determined in line with one or more of the following: g x = IID − 1 2 + 4ICC 2 IID IID + 1 g x = IID − 1 2 + 4 ⋅ ICC 2 ⋅ IID + 1 − IID ⋅ 4 ⋅ ICC 2 ⋅ IID + IID − 1 2 1 − IID 2 + IID + 1 ⋅ 4 ⋅ ICC 2 ⋅ IID + IID − 1 2 g n = IID + 1 − IID − 1 2 + 4ICC 2 IID 2 IID + 1 g n = IID ⋅ 1 − IID − 2 ⋅ ICC 2 + 4 ⋅ ICC 2 ⋅ IID + IID − 1 2 1 − IID 2 + IID + 1 ⋅ 4 ⋅ ICC 2 ⋅ IID + IID − 1 2 where IID is an interchannel intensity difference and ICC is an inter-channel cross-correlation, and specifically IID = l 2 r 2 ICC = < l , r > l 2 r 2 and < x , y > = ∑ ∀ i x i ⋅ y i ∗

[0136] Thus, in many embodiments, one or more of the gains may be determined based on received interchannel intensity differences, and interchannel correlations (being part of the spatial upmix parameters).

[0137] In many embodiments, the gains g x and g n may by the adapter 221 be set by changing the gains of the different rendering paths (e.g. by setting the gains of the gain blocks 207, 213). In other cases where only one rendering path is adapted, the variable gains may for example be determined considering both of the calculated gains. For example, if the gain of the direct rendering path is fixed (e.g. at unity), the gain 213 of the predetermined rendering path may be set to a value corresponding to g n / g x .

[0138] Different approaches for determining the rendering direction from the spatial upmix parameters may be used in different embodiments. In particular, the signal model as indicated above is based on directional component x being at a direction γ in the stereo image of the stereo signal, henceforth also referred to as the orientation direction. In many embodiments, the direction determining circuit 209 may determine the orientation direction γ and then determine the (desired) rendering direction γ' from the orientation direction γ. Indeed, in some embodiments or scenarios, the rendering direction γ' may simply be set equal to the orientation direction γ.

[0139] The determination of the orientation direction γ may be based on the signal model indicated above. The spatial upmix parameters provide information on the relative properties of the channel signals of the stereo signal and specifically they may provide information on both the interchannel levels / intensity differences as well as on the interchannel correlation. Accordingly, the spatial upmix parameters can be considered to provide information on the directional signal component x and on the position of this in the stereo image of the stereo signal, i.e. the spatial upmix parameters provide information on the orientation direction γ allowing this to be determined from the provided parameter values.

[0140] The direction determining circuit 209 may determine the orientation direction γ as a direction to a directional signal component in a stereo image of the stereo signal from the spatial upmix parameters, and to map this to a direction in a stereo image of the output stereo signal. The directional signal component may be a dominant signal component. The direction determining circuit 209 may be arranged to determine the orientation direction γ as a direction of a dominant sound source in the stereo signal where the direction of the dominant sound source is represented by the spatial upmix parameters.

[0141] The directional signal component may specifically be a signal component (estimated / determined) to originate from a point source. Specifically, the direction determining circuit 209 may be arranged to determine the orientation direction γ as a direction for which a single point source audio source will result in spatial upmix parameter values matching the spatial upmix parameters of the data signal.

[0142] In some embodiments, the direction determining circuit may be arranged to determine the first direction in line with: γ = arctan 1 − IID + IID − 1 2 + 4 ⋅ ICC 2 ⋅ IID 2 ⋅ ICC ⋅ IID where IID is an interchannel intensity difference and ICC is an inter-channel cross-correlation, and specifically with these given by the equations provided above in connection with the equations for determining gains. The determination of the orientation direction γ above results from an assumption that the signals x, n l and n r of the signal model are mutually decorrelated, and that the power of the left and right diffuse signals n l and n r are equal.

[0143] The direction determining circuit 209 may, as previously mentioned, in some embodiments be used directly as the rendering direction γ', i.e. γ = γ'. However, in many embodiments, a mapping may be included which for at least some values of the orientation direction γ may result in a different rendering direction γ'.

[0144] Thus, in many embodiments, the direction determining circuit 209 may be arranged to apply a mapping function to the orientation direction γ to determine the rendering direction γ'.

[0145] For example, the mapping may map the position in the stereo image of the original stereo signal as represented by the orientation direction γ to a desired position in the stereo image of the output stereo signal as represented by the rendering direction γ'. In many cases, where the output stereo signal is a binaural signal, the mapping may include a consideration / determination of a distance to the audio sources. For example, a range of the orientation direction γ in the interval of [0,180°] may be mapped to a location between two virtual stereo speakers in the audio scene created by the binaural rendering. Such speakers may for example be positioned at angles of -30° and +30° relative to a center direction for the binaural signal. Thus, in such situations, the direction determining circuit 209 may include a mapping between an orientation direction γ in the range of [0,180°] to a rendering direction γ' in the range of [-30°,+30°].

[0146] Thus, in some embodiments, the directional component (the mono downmix audio signal) may be rendered to a virtual angle in the range of a virtual loudspeaker angle range generated by a binaural rendering. The rendered directional component may be combined with a diffuse rendering of the residual signal(s).

[0147] In many embodiments, the direction determining circuit 209 may be arranged to map an orientation direction γ representing an angle in one interval / range to a rendering direction γ' representing an angle in a different interval / range.

[0148] In the previous examples, the rendering has been based on one intermediate stereo signal representing the diffuse signal component. However, in many embodiments, there may be two (or possibly more) parallel paths for the rendering of the residual / non-directional signal components.

[0149] An example of such an approach is illustrated in FIG. 3. In addition to the audio render apparatus of FIG. 2, this apparatus includes a second path for rendering the diffuse / non-directional component.

[0150] In particular, the audio render apparatus of FIG. 3 comprises an additional decorrelator 301 followed by a third renderer 303. The additional decorrelator 301 is arranged to generate a second decorrelated mono downmix audio signal which is then rendered by the third renderer 303 generating a third intermediate stereo signal. The third intermediate stereo signal is then combined with the first intermediate stereo signal and the second intermediate stereo signal.

[0151] The second decorrelated mono downmix audio signal is generated to have low correlation with both the mono downmix audio signal and the first decorrelated mono downmix audio signal, and typically will be uncorrelated with both of these. Accordingly, the output stereo signal includes two components for the residual signal. The two signals may be rendered differently such that an improved diffuseness is perceived. For example, the second decorrelated mono downmix audio signal may be rendered from one position (e.g. fully in a first channel of the output stereo signal or from a first virtual speaker position for a binaural signal) with the first decorrelated mono downmix audio signal being rendered from a second position (e.g. fully in a second channel of the output stereo signal or from a second virtual speaker position for a binaural signal).

[0152] The approach may typically provide an improved performance and perception relative to what can be achieved with a single decorrelated signal representing the non-directional component. In particular, the approach may more accurately represent the signal model indicated previously with different and uncorrelated diffuse signal components n l and n r .

[0153] It will be appreciated that the operation and implementation of the additional decorrelator 301 and the third renderer 303 may use the same functionality as employed for the decorrelator 215 and the second renderer 217.

[0154] The predetermined rendering of the second and / or third renderer 217, 303 may as previously mentioned simply be achieved by rendering the corresponding decorrelated signal in one channel of the corresponding intermediate stereo signal, and with no signal being included in the other channel. For example, the second decorrelated mono downmix audio signal may be rendered in the left channel of the second intermediate stereo signal and the first decorrelated mono downmix audio signal may be rendered in the right channel of the third intermediate stereo signal.

[0155] In some embodiments where binaural processing is used, each of the decorrelated signals may be rendered from a specific position, such as each decorrelated signal being rendered from a different virtual position, such as for example from different virtual positions.

[0156] In some embodiments, the rendering for a decorrelated signal, such as the rendering of the first decorrelated mono downmix audio signal, may be to position the signal at a specific position.

[0157] In many embodiments, the rendering of a decorrelated mono downmix audio signal may be performed by the renderer 217, 303 retrieving a set of directional transfer functions from the store 201 and rendering the decorrelated mono downmix audio signal using the retrieved transfer function(s).

[0158] In many embodiments, the renderer 217, 303 may be arranged to extract a directional transfer function for a single predetermined direction and render the decorrelated mono downmix audio signal using this directional transfer function. Accordingly, the decorrelated mono downmix audio signal may be rendered from one predetermined direction / position, such as a direction / position corresponding to a virtual speaker position.

[0159] An example of subband parametric rendering may e.g. result in left and right signals: l = g x ⋅ m ⋅ G l f γ ⋅ e jϕ l f γ + g n ⋅ H 1 m ⋅ G l β l ⋅ e jϕ l β l + g n ⋅ H 2 m ⋅ G l β r ⋅ e jϕ l β r r = g x ⋅ m ⋅ G r f γ ⋅ e jϕ r f γ + g n ⋅ H 1 m ⋅ G r β l ⋅ e jϕ r β l + g n ⋅ H 2 m ⋅ G r β r ⋅ e jϕ r β r where G l , G r , ϕ l , ϕ r form the parametric HRIRs, f(γ) is a mapping function converting the estimated angles (orientation direction γ) to HRIR direction angles, β l and β r are two pre-determined angles and H 1 {.} and H 2 {.} are two mutually independent decorrelators (215, 301).

[0160] In some embodiments, one or both renderer(s) 217, 303 may retrieve directional transfer functions for a plurality of predetermined directions and it may use multiple directional transfer functions in performing the predetermined rendering. For example, different directional transfer functions may be used for different frequency subbands. This may provide a more diffuse perception with the audio being generated such that it is perceived from different directions for different subbands thereby resulting in a perception of a more distributed and spread audio source.

[0161] Such approaches may be used both in embodiments in which a single decorrelated mono downmix audio signal is generated and rendered, or indeed in cases where multiple decorrelated mono downmix audio signals are generated and rendered. In the latter case, the sets of predetermined directions for the different decorrelated mono downmix audio signals are different in order to enhance the perceived diffuseness of the non-directional signal component.

[0162] The second renderer 217 may generate the second intermediate stereo signal using a first set of directional transfer functions retrieved from the store 211 for a first set of predetermined directions, and the third renderer 303 may generate the third intermediate stereo signal using a second set of directional transfer functions retrieved from the store 211 for a second set of predetermined directions where the first set of set of predetermined directions is different from the second set of predetermined directions.

[0163] In particular, the directional transfer functions may be binaural transfer functions and the second renderer 217 may be arranged to perform binaural rendering using binaural impulse response values for a first set of predetermined directions and the third renderer 303 may be arranged to perform binaural rendering using binaural impulse response values for a second set of predetermined directions where the first set of predetermined directions are different from the second set of predetermined directions.

[0164] In many cases, the use of multiple directional transfer functions may be achieved by using directional transfer functions for different directions in different frequency subbands.

[0165] Thus, instead of rendering the diffuse / non-directional signals using fixed angles, e.g. mimicking a virtual stereo speaker setup, the diffuse signals may also be rendered using composite, e.g. pre-calculated HRIRs for many sources / directions, e.g. spread over a (part of a) circle, or (part of) a sphere. l = g x ⋅ m ⋅ G l f γ ⋅ e jϕ l f γ + g n ⋅ H 1 m ⋅ G l , comp ⋅ e jϕ l , comp + g n ⋅ H 2 m ⋅ G l , comp ⋅ e jϕ l , comp r = g x ⋅ m ⋅ G r f γ ⋅ e jϕ r f γ + g n ⋅ H 1 m ⋅ G r , comp ⋅ e jϕ r , comp + g n ⋅ H 2 m ⋅ G r , comp ⋅ e jϕ r , comp where e.g.: G l , comp = g norm ⋅ ∑ β ∈ B l G l β ⋅ e jϕ l β ϕ l , comp = ∠ ∑ β ∈ B l G l β ⋅ e jϕ l β G r , comp = g norm ⋅ ∑ β ∈ B r G r β ⋅ e jϕ r β ϕ r , comp = ∠ ∑ β ∈ B r G r β ⋅ e jϕ r β with B l being a set of angles at which the left diffuse signal is to be rendered, B r a set of angles at which the right diffuse signal is to be rendered, and g norm a normalisation factor.

[0166] In some embodiments, the diffuse signal component may be directly rendered onto left and right channels without any HRIR processing.

[0167] In some embodiments where only a single decorrelation path is used, the left and right diffuse signals may be approximated by an out-of-phase approximation, such as n l ′ = g n ⋅ H m n r ′ = − g n ⋅ H m

[0168] As described, the operation may typically be performed in a frequency domain representation and most or all of the described processing may be performed on a subband basis. In particular, the spatial upmix parameters may be provided for frequency subbands and the first renderer 205 may be arranged to generate subband values for subbands of the first intermediate signals from subband values of the mono downmix audio signal based on spatial upmix parameters and directional transfer functions for the subbands. The processing may be performed in time intervals / segments with a time interval / segment of the output stereo signal being generated for the corresponding time interval / segment of the mono downmix audio signal.

[0169] Similarly, in many embodiments, the second renderer is arranged to generate subband values for subbands of the second intermediate signals from subband values of the decorrelated mono downmix audio signal, and specifically based on spatial upmix parameters and directional transfer functions for the subbands (if these are used).

[0170] In many embodiments, the spatial upmix parameters and the directional transfer functions are provided for frequency subbands and the adapter 221 is arranged to determine first scaling values for subbands of the mono downmix audio signal and / or the decorrelated mono downmix audio signal from spatial parameters of the subbands; and the adapter is arranged to adapt a level of subbands of the first intermediate stereo signal relative to a level of subbands of the second intermediate stereo signal in the output stereo signal dependent on the first scaling value for the subbands.

[0171] The processing may be performed in subbands and may be performed in time segments. The processing in each subband may for some (any) or all steps be performed separately / independently in each subband (with respect to the processing in other subbands). The processing in each time segment may for some (any) or all steps be performed separately / independently in each time segment (with respect to the processing in other time segments).

[0172] The processing may be time interval / segment based with all processing being performed for each time segment. Equivalently, the signal(s) for each segment may be considered a signal (and in particular signals of different time segments, may be considered different signals).

[0173] The audio apparatus(s) may specifically be implemented in one or more suitably programmed processors. An example of a suitable processor is provided in the following.

[0174] FIG. 4 is a block diagram illustrating an example processor 400 according to embodiments of the disclosure. Processor 400 may be used to implement one or more processors implementing an apparatus as previously described or elements thereof (including in particular one more artificial neural network). Processor 400 may be any suitable processor type including, but not limited to, a microprocessor, a microcontroller, a Digital Signal Processor (DSP), a Field ProGrammable Array (FPGA) where the FPGA has been programmed to form a processor, a Graphical Processing Unit (GPU), an Application Specific Integrated Circuit (ASIC) where the ASIC has been designed to form a processor, or a combination thereof.

[0175] The processor 400 may include one or more cores 402. The core 402 may include one or more Arithmetic Logic Units (ALU) 404. In some embodiments, the core 402 may include a Floating Point Logic Unit (FPLU) 406 and / or a Digital Signal Processing Unit (DSPU) 408 in addition to or instead of the ALU 404.

[0176] The processor 400 may include one or more registers 412 communicatively coupled to the core 402. The registers 412 may be implemented using dedicated logic gate circuits (e.g., flip-flops) and / or any memory technology. In some embodiments the registers 412 may be implemented using static memory. The register may provide data, instructions and addresses to the core 402.

[0177] In some embodiments, processor 400 may include one or more levels of cache memory 410 communicatively coupled to the core 402. The cache memory 410 may provide computer-readable instructions to the core 402 for execution. The cache memory 410 may provide data for processing by the core 402. In some embodiments, the computer-readable instructions may have been provided to the cache memory 410 by a local memory, for example, local memory attached to the external bus 416. The cache memory 410 may be implemented with any suitable cache memory type, for example, Metal-Oxide Semiconductor (MOS) memory such as Static Random Access Memory (SRAM), Dynamic Random Access Memory (DRAM), and / or any other suitable memory technology.

[0178] The processor 400 may include a controller 414, which may control input to the processor 400 from other processors and / or components included in a system and / or outputs from the processor 400 to other processors and / or components included in the system. Controller 414 may control the data paths in the ALU 404, FPLU 406 and / or DSPU 408. Controller 414 may be implemented as one or more state machines, data paths and / or dedicated control logic. The gates of controller 414 may be implemented as standalone gates, FPGA, ASIC or any other suitable technology.

[0179] The registers 412 and the cache 410 may communicate with controller 414 and core 402 via internal connections 420A, 420B, 420C and 420D. Internal connections may be implemented as a bus, multiplexer, crossbar switch, and / or any other suitable connection technology.

[0180] Inputs and outputs for the processor 400 may be provided via a bus 416, which may include one or more conductive lines. The bus 416 may be communicatively coupled to one or more components of processor 400, for example the controller 414, cache 410, and / or register 412. The bus 416 may be coupled to one or more components of the system.

[0181] The bus 416 may be coupled to one or more external memories. The external memories may include Read Only Memory (ROM) 432. ROM 432 may be a masked ROM, Electronically Programmable Read Only Memory (EPROM) or any other suitable technology. The external memory may include Random Access Memory (RAM) 433. RAM 433 may be a static RAM, battery backed up static RAM, Dynamic RAM (DRAM) or any other suitable technology. The external memory may include Electrically Erasable Programmable Read Only Memory (EEPROM) 435. The external memory may include Flash memory 434. The External memory may include a magnetic storage device such as disc 436. In some embodiments, the external memories may be included in a system.

[0182] The invention can be implemented in any suitable form including hardware, software, firmware, or any combination of these. The invention may optionally be implemented at least partly as computer software running on one or more data processors and / or digital signal processors. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the invention may be implemented in a single unit or may be physically and functionally distributed between different units, circuits and processors.

[0183] Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

[0184] Furthermore, although individually listed, a plurality of means, elements, circuits or method steps may be implemented by e.g. a single circuit, unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and / or advantageous. Also, the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims do not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc. do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example shall not be construed as limiting the scope of the claims in any way.

Claims

1. An audio apparatus comprising: a receiver (201) arranged to receive a data signal comprising encoded data for a mono downmix audio signal being a downmix of a stereo signal and a set of spatial upmix parameters for upmixing the mono downmix audio signal to the stereo signal, the set of spatial upmix parameters being indicative of relative signal properties of channels of the stereo signal; a store (211) comprising directional transfer functions for different directions, a directional transfer function for a given direction representing a mapping of a mono audio signal to stereo channels such that the mono audio signal is positioned in the given direction in a stereo image of the stereo channels; a decoder (203) arranged to generate the mono downmix audio signal by decoding the encoded data; a decorrelator (215) arranged to apply a decorrelation to the mono downmix audio signal to generate a first decorrelated mono downmix audio signal; a direction determining circuit (209) arranged to determine a first direction from the spatial parameters; a first renderer (205) arranged to perform a first rendering of the mono downmix audio signal to generate a first intermediate stereo signal, the first rendering being a directional rendering using a first channel transfer function retrieved from the store for the first direction; a second renderer (217) arranged to perform a second rendering being a rendering of the first decorrelated mono downmix audio signal to generate a second intermediate stereo signal, the second rendering being a predetermined rendering employing a predetermined mapping of the decorrelated mono downmix audio signal to channel signals of the second intermediate stereo signal; a combiner (219) arranged to combine at least the first intermediate stereo signal and the second intermediate stereo signal to generate an output stereo signal; and an adapter (221) arranged to adapt a ratio between a level of the first intermediate stereo signal in the output stereo signal and a level of the second intermediate stereo signal in the output stereo signal dependent on a first scaling value determined from the spatial parameters.

2. The audio apparatus of claim 1 wherein the first scaling value is a scaling value for the second intermediate stereo signal and the adapter (221) is arranged to adapt a gain for the second intermediate stereo signal in dependence on the first scaling value.

3. The audio apparatus of claim 2 or wherein the adapter (221) is arranged to determine a second scaling value from the spatial parameters, the second scaling value is a scaling value for the first intermediate stereo signal and the adapter (221) is arranged to adapt a gain for the first intermediate stereo signal in dependence on the second scaling value.

4. The audio apparatus of any previous claim wherein the first rendering is a binaural rendering and the directional transfer functions are binaural transfer functions.

5. The audio apparatus of any previous claim wherein the second rendering is arranged to generate the second intermediate stereo signal using a set of directional transfer functions retrieved from the store for a set of predetermined directions.

6. The audio apparatus of claim 4 wherein the set of predetermined directions consists of one predetermined direction.

7. The audio apparatus of claim 4 wherein the set of predetermined directions comprises a plurality of predetermined directions.

8. The audio apparatus of any previous claim further comprising: an additional decorrelator (301) arranged to decorrelate the mono downmix audio signal to generate a second decorrelated mono downmix audio signal; a third renderer (303) arranged to perform a third rendering being of the second decorrelated mono downmix audio signal to generate a third intermediate stereo signal, the third rendering being a predetermined rendering employing a predetermined mapping of the second decorrelated mono downmix audio signal to channel signals of the third intermediate stereo signal; and the combiner (219) is arranged to combine at least the first intermediate stereo signal, the second intermediate stereo signal, and the third intermediate stereo signal to generate the output stereo signal.

9. The audio apparatus of claim 8 wherein the second renderer (217) is arranged to map the first decorrelated mono downmix audio signal to a first channel of the output stereo signal and the third renderer is arranged to map the second decorrelated mono downmix audio signal to a second channel of the output stereo signal.

10. The audio apparatus of claim 7 and 8 wherein the second rendering is arranged to generate the second intermediate stereo signal using a first set of directional transfer functions retrieved from the store for a first set of predetermined directions, and the third rendering is arranged to generate the third intermediate stereo signal using a second set of directional transfer functions retrieved from the store for a second set of predetermined directions, the first set of set of predetermined directions being different from the second set of predetermined directions.

11. The audio apparatus of any previous claim wherein the direction determining circuit (209) is arranged to determine a point source direction in a stereo image of the stereo signal from the spatial upmix parameters, and to determine the first direction by applying a mapping function to the point source direction.

12. The audio apparatus of any previous claim wherein the spatial upmix parameters include an interchannel intensity difference parameter and an interchannel correlation parameter.

13. The audio apparatus of any previous claim wherein the spatial upmix parameters and the directional transfer functions are provided for frequency subbands and the first renderer is arranged to generate subband values for subbands of the first intermediate signals from subband values of the mono downmix audio signal based on spatial upmix parameters and directional transfer functions for the subbands.

14. A method of generating an output audio stereo signal, the method comprising: receiving a data signal comprising encoded data for a mono downmix audio signal being a downmix of a stereo signal and a set of spatial upmix parameters for upmixing the mono downmix audio signal to the stereo signal, the set of spatial upmix parameters being indicative of relative signal properties of channels of the stereo signal; storing directional transfer functions for different directions, a directional transfer function for a given direction representing a mapping of a mono audio signal to stereo channels such that the mono audio signal is positioned in the given direction in a stereo image of the stereo channels; generating the mono downmix audio signal by decoding the encoded data; applying a decorrelation to the mono downmix audio signal to generate a first decorrelated mono downmix audio signal; determining a first direction from the spatial parameters; performing a first rendering of the mono downmix audio signal to generate a first intermediate stereo signal, the first rendering being a directional rendering using a first channel transfer function retrieved from the store for the first direction; performing a second rendering being a rendering of the first decorrelated mono downmix audio signal to generate a second intermediate stereo signal, the second rendering being a predetermined rendering employing a predetermined mapping of the decorrelated mono downmix audio signal to channel signals of the second intermediate stereo signal; combining at least the first intermediate stereo signal and the second intermediate stereo signal to generate an output stereo signal; and adapting a level of the first intermediate stereo signal relative to a level of the second intermediate stereo signal in the output stereo signal dependent on a first scaling value determined from the spatial parameters.

15. A computer program product comprising computer program code means adapted to perform all the steps of claim 14 when said program is run on a computer.

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