Head-tracked rendering of audio

Abstract

An apparatus and method for tracking user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space. Responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the audio sources are then modified so as to counter the change in user orientation from the reference orientation to the new orientation. This is followed by tracking a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation from the reference orientation when the user travels along the portions. This is followed by updating the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions, wherein the one or more audio sources are thereafter rendered at the respective spatial positions with respect to the updated reference orientation.

Description

Technical Field Example embodiments relate to head-tracked rendering of audio data, for example immersive audio data. Background Immersive audio refers to audio which, when output to a user device such as a pair of earphones, enables a user to perceive audio sources as coming from respective directions with respect to the user's position. For example, one audio source may be perceived as coming from a position in front of the user whereas one or more other audio sources may be perceived as coming from positions to the left and / or right -hand side of the user. The user may perceive such audio sources as coming from positions external to the user's position, in contrast to, for example, stereoscopic audio in which audio is effectively perceived within the user's head and where an audio source may be panned between the ears or, in some cases, played back to one ear only. Summary The scope of protection sought for various embodiments of the invention is set out by the independent claims. The embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention. According to a first aspect, there is described an apparatus comprising: means for tracking user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space and wherein, responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation; means for tracking a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation from the reference orientation when the user travels along the one or more portions; and means for updating the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions, wherein the one or more audio sources are thereafter rendered at the respective spatial positions with respect to the updated reference orientation. In some example embodiments, the tracked spatial position of the user may indicate that the user is travelling along a first portion of the travel route, and the reference orientation may be updated by changing the reference orientation using the respective change in user orientation for said first portion of the travel route. In some example embodiments, the spatial position of the user and the travel route may be provided by a navigation system of a vehicle in which the user is travelling. In some example embodiments, the travel route may be received from a remote source. In some example embodiments, the spatial position of the user may be determined based at least on an estimate of a position of a vehicle in which the user is travelling, the estimate being received from the remote source. In some example embodiments, the travel route may be associated with a public transportation route. In some example embodiments, the apparatus may further comprise: means for comparing, when the tracked spatial position of the user indicates that the user is travelling along a first portion of the travel route, a change in the tracked user orientation with the respective change in user orientation for said first portion, wherein the reference orientation is updated based on identifying a correspondence between the change in the tracked user orientation and the respective change in user orientation for said first portion. In some example embodiments, the reference orientation may be updated if the tracked change in user orientation and the respective change in user orientation for said first portion are within a predetermined angular range of one another. In some example embodiments, the apparatus may further comprise: means for determining, based on identifying no correspondence between the tracked change in user orientation and the respective change in user orientation for said first portion, a time delay, wherein the time delay is determined based on the time taken for the respective change in user orientation to catch-up with the tracked change in user orientation, wherein the comparing subsequently comprises comparing the tracked change in user orientation with one or more respective changes in user orientation for one or more further portions of the travel route determined based on the time delay. In some example embodiments, the apparatus may further comprise: means for identifying, based on the tracked spatial position of the user and the direction the user is travelling, a plurality of possible travel routes which are ahead of the tracked spatial position; and means for selecting, prior to updating the reference orientation, one of the possible travel routes as the travel route of the user. In some example embodiments, the selected travel route is that which most closely corresponds to the direction of travel of the user. In some example embodiments, the apparatus may further comprise: means for assigning respective probability scores to the plurality of possible travel routes, the respective probability scores indicating a likelihood that the user will travel along said possible travel routes, wherein the selected travel route is that with the highest probability score. In some example embodiments, the plurality of possible travel routes may comprise at least first and second possible travel routes which will cause different respective changes in user orientation from the reference orientation if the user were to travel along said first and second possible travel routes, and the probability scores for the first and second possible travel routes may be determined based on speed of user movement as the tracked spatial position of the user approaches an intersection between the first and second possible travel routes. In some example embodiments, the apparatus may further comprise: means for comparing the tracked change in user orientation with the respective changes in user orientation for the possible travel routes; and means for selecting the possible travel route that corresponds, or most closely corresponds, to the tracked change in user orientation. In some example embodiments, the reference orientation may be a local reference orientation associated with the audio output device and the apparatus is configured to update the local reference orientation. In some example embodiments, the updated reference orientation may be provided as an external reference orientation input to a decoder for receiving and decoding the data representing the spatial audio scene. The decoder may, for example, comprise a 3GPP Immersive Voice and Audio Services (IVAS) decoder. In some example embodiments, the apparatus may comprise the audio output device or a user device having a paired relationship with the audio output device. In some example embodiments, the apparatus may comprise a server or service in communication with the audio output device. According to a second aspect, there is described a method comprising: tracking user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space and wherein, responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation; tracking a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation from the reference orientation when the user travels along the one or more portions; and updating the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions, wherein the one or more audio sources are thereafter rendered at the respective spatial positions with respect to the updated reference orientation. In some example embodiments, the tracked spatial position of the user may indicate that the user is travelling along a first portion of the travel route, and the reference orientation may be updated by changing the reference orientation using the respective change in user orientation for said first portion of the travel route. In some example embodiments, the spatial position of the user and the travel route may be provided by a navigation system of a vehicle in which the user is travelling. In some example embodiments, the travel route may be received from a remote source. In some example embodiments, the spatial position of the user may be determined based at least on an estimate of a position of a vehicle in which the user is travelling, the estimate being received from the remote source. In some example embodiments, the travel route may be associated with a public transportation route. In some example embodiments, the method may further comprise: comparing, when the tracked spatial position of the user indicates that the user is travelling along a first portion of the travel route, a change in the tracked user orientation with the respective change in user orientation for said first portion, wherein the reference orientation is updated based on identifying a correspondence between the change in the tracked user orientation and the respective change in user orientation for said first portion. In some example embodiments, the reference orientation may be updated if the tracked change in user orientation and the respective change in user orientation for said first portion are within a predetermined angular range of one another. In some example embodiments, the method may further comprise determining, based on identifying no correspondence between the tracked change in user orientation and the respective change in user orientation for said first portion, a time delay, wherein the time delay is determined based on the time taken for the respective change in user orientation to catch-up with the tracked change in user orientation, wherein the comparing subsequently comprises comparing the tracked change in user orientation with one or more respective changes in user orientation for one or more further portions of the travel route determined based on the time delay. In some example embodiments, the method may further comprise: identifying, based on the tracked spatial position of the user and the direction the user is travelling, a plurality of possible travel routes which are ahead of the tracked spatial position; and selecting, prior to updating the reference orientation, one of the possible travel routes as the travel route of the user. In some example embodiments, the selected travel route is that which most closely corresponds to the direction of travel of the user. In some example embodiments, the method may further comprise assigning respective probability scores to the plurality of possible travel routes, the respective probability scores indicating a likelihood that the user will travel along said possible travel routes, wherein the selected travel route is that with the highest probability score. In some example embodiments, the plurality of possible travel routes may comprise at least first and second possible travel routes which will cause different respective changes in user orientation from the reference orientation if the user were to travel along said first and second possible travel routes, and the probability scores for the first and second possible travel routes may be determined based on speed of user movement as the tracked spatial position of the user approaches an intersection between the first and second possible travel routes. In some example embodiments, the method may further comprise comparing the tracked change in user orientation with the respective changes in user orientation for the possible travel routes; and means for selecting the possible travel route that corresponds, or most closely corresponds, to the tracked change in user orientation. In some example embodiments, the reference orientation may be a local reference orientation associated with the audio output device and the apparatus is configured to update the local reference orientation. In some example embodiments, the updated reference orientation may be provided as an external reference orientation input to a decoder for receiving and decoding the data representing the spatial audio scene. The decoder may, for example, comprise a 3GPP Immersive Voice and Audio Services (IVAS) decoder. In some example embodiments, the method may be performed at the audio output device or a user device having a paired relationship with the audio output device. In some example embodiments, the method may be performed at a server or service in communication with the audio output device. According to a third aspect, there is provided a computer program product comprising a set of instructions which, when executed on an apparatus, is configured to cause the apparatus to carry out a method comprising: tracking user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space and wherein, responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation; tracking a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation from the reference orientation when the user travels along the one or more portions; and updating the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions, wherein the one or more audio sources are thereafter rendered at the respective spatial positions with respect to the updated reference orientation. The third aspect may also comprise any feature described in relation to the second aspect. According to a fourth aspect, there is provided a non-transitory computer readable medium comprising program instructions stored thereon for performing a method, comprising: tracking user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space and wherein, responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation; tracking a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation from the reference orientation when the user travels along the one or more portions; and updating the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions, wherein the one or more audio sources are thereafter rendered at the respective spatial positions with respect to the updated reference orientation. The fourth aspect may also comprise any feature described in relation to the second aspect. According to a fifth aspect, there is provided an apparatus comprising: at least one processor; and at least one memory including computer program code which, when executed by the at least one processor, causes the apparatus to: track user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space and wherein, responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation; track a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation from the reference orientation when the user travels along the one or more portions; and update the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions, wherein the one or more audio sources are thereafter rendered at the respective spatial positions with respect to the updated reference orientation. The fifth aspect may also comprise any feature described in relation to the second aspect. Brief Description of the Drawings Example embodiments will now be described with reference to the accompanying drawings, in which: FIG. 1 illustrates a system useful for understanding example embodiments; FIG. 2 illustrates a user when consuming spatial audio; FIG. 3 illustrates the FIG. 2 user when consuming the spatial audio without head-tracked rendering; FIG. 4 illustrates the FIG. 2 user when consuming the spatial audio with head-tracked rendering; FIG. 5 is a flow diagram illustrating operations in accordance with example embodiments; FIG. 6 illustrates a system for operating in accordance with example embodiments; FIG. 7 illustrates a user interface of a navigation system or module which may be used with the FIG. 6 system; FIG. 8 illustrates the FIG. 7 user interface at a subsequent time; FIG. 9 illustrates a further system for operating in accordance with example embodiments; FIG. 10 illustrates a user interface of a navigation system or module which may be used with the FIG. 9 system; FIG. 11 illustrates the FIG. 10 user interface at a subsequent time; FIG. 12 illustrates a further system for operating in accordance with example embodiments; FIG. 13 illustrates a further system including a spatial encoder in accordance with example embodiments; FIG. 14 illustrates an apparatus which may be configured to operate in accordance with example embodiments; and FIG. 15 illustrates a non-transitory computer-readable medium for storing computer-readable instructions for causing the FIG. 14 apparatus to operate in accordance with example embodiments. Detailed Description In the description and drawings, like reference numerals refer to like elements throughout. Example embodiments relate to an apparatus, method and computer program relating to head-tracked rendering of audio data, for example immersive audio data. Immersive audio data may comprise spatial audio data which encodes one or more audio sources in a spatial audio scene. When the spatial audio data is decoded and rendered for output to an audio output device, the user may perceive the one or more audio sources as coming from respective directions with respect to the user's position. Example formats for spatial audio data may include, but are not limited to, multichannel mixes, Ambisonics, parametric spatial audio (e.g., metadata-assisted spatial audio (MASA)), object-based audio, or any combination thereof. The spatial audio data may be encoded and decoded using a codec which may comprise, but is not limited to, the 3GPP Immersive Video and Audio Services (IVAS) format. In cases where, for example, the audio output device comprises a head-worn device comprising a pair of loudspeakers (examples being a pair of earphones, earbuds, headphones, or an extended reality (XR) headset) the spatial audio data may be rendered using binaural rendering. In binaural rendering, various algorithms may be used by a binaural rendering module of the audio output device, or an associated media player, based on a head-related impulse response (HRIR) or frequency domain equivalent to provide spatial reproduction such that the user perceives the audio sources as if they were positioned within the spatial audio scene. It may also be possible to perform head-tracking as part of the rendering process. This may involve tracking the orientation of the user's head and compensating the rendering such that audio sources are perceived as staying still even if the user's head rotates. This may be performed by providing a reference orientation, wherein one or more audio sources are perceived from respective spatial positions with respect to the reference orientation. In response to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation. In this way, the user's perception is that the one or more audio sources remain static in the spatial audio scene, which mimics how the user perceives sounds in the real world and provides a strong cue for spatial audio perception. It is commonly known in spatial audio research that reliable head-tracking significantly improves the quality of binaural rendering and allows good perceptual quality even in cases where rendering otherwise might be lacking in accuracy. FIG. 1 is a block diagram of a system 100 which may be useful for understanding example embodiments. The system 100 may comprise a server 110, a media player 120, a network 130 and an audio output device comprising, in this example, set of earphones 140 worn by a user 150. The server 110 may be connected to the media player 120 by means of the network 130 for sending data, e.g., spatial audio data, to the media player 120. The server 110 may for example comprise an internet protocol (IP) telecommunications server which transmits spatial audio data comprising part of a voice call to the media player 120. The spatial audio data may represent one or more other users which are party to the voice call such that their respective audio data will be perceived from respective directions when rendered by the media player 120 and output to the set of earphones 140. Alternatively, the spatial audio data may represent a music track or an audio track of a movie. Transmission may be by means of any suitable streaming data protocol. Alternatively, or additionally, the server 110 may provide one or more files representing spatial audio data to the media player 120 for storage and processing thereat. At the media player 120, the spatial audio data may be processed, rendered and output to the set of earphones 140. In example embodiments, the set of earphones 140 may comprise head tracking sensors for providing head-tracking data to the media player 120, indicating, using any suitable method, the user's head orientation or change in user's head orientation, in order to determine how the spatial audio data is to be rendered and output by the earphones 140. The user's head orientation may be determined in real-time or near real-time using one or more known head-tracking methods, such as by use of one or more inertial sensors (e.g., gyroscopes and / or accelerometers) within the earphones 140. Alternative or additional examples may include using one or more cameras which may identify facial features in real-time. In some example embodiments, the media player 120 may comprise one of a mobile telephone, a tablet computer, a games console, a laptop computer, a personal computer, a vehicle navigation computer, or wearable device. In some example embodiments, the media player 120 may comprise part of the set of earphones 140. The network may be any suitable data communications network including, for example, one or more of a radio access network (RAN) whereby communication is via one or more base stations, a WiFi network whereby communications is via one or more access points, or a short-range network such as one using the Bluetooth or Zig bee protocol. FIGs. 2, 3 and 4 are representational drawings of the user 150 when wearing the set of earphones 140 which may also be useful for understanding example embodiments. Referring to FIG. 2, the user 150 is shown listening to a rendered spatial audio field comprising first to fourth audio sources (collectively indicated by reference numeral 220) corresponding to distinct respective sounds labelled "1", "2", "3" and "4." Referring to FIG. 3, the respective perceived spatial positions of the first to fourth audio sources 220 are indicated with respect to the user's head in the case that head-tracked rendering is not used. It will be seen that clockwise rotation of the user's head results in no modification of the spatial audio scene and the respective perceived spatial positions of the first to fourth audio sources 220 follow the user's movement. Referring to FIG. 4, the respective perceived spatial positions of the first to fourth audio sources 220 are indicated with respect to the user's head in the case that head-tracked rendering is used, as in the case of example embodiments. It will be seen that clockwise rotation of the user's head results in modification of the spatial audio scene and the respective perceived spatial positions of the first to fourth audio sources 220 do not follow the user's movement and instead remain static. The general approach in applying head-tracking to rendering means that all directions of the rendered audio sound scene are compensated for head rotations. This is relatively simple in the sense that head rotation may be represented by a rotation matrix R from a reference orientation to a current head orientation. Any direction in the spatial audio scene may be then rotated with the inverse R1 of the head rotation matrix to provide head rotation compensated directions. These compensated directions also remain static compared to the reference orientation unless the directions themselves move. This state is often also called "world-locked" whereas the state of moving with the head orientation (i.e., without headtracking compensation, as shown in FIG. 3) is often called "head-locked". The reference orientation can be represented as a rotation matrix R. In this case, the reference orientation may be considered to define a local reference orientation which is used as the reference for any further head-orientation. The local reference orientation may be a rotation from a global zero orientation. In practice, the global zero orientation may not necessarily have any further meaning, but it can be considered as e.g., a natural zero state of the sensors that detect rotations. The local reference orientation is often set during software or hardware startup, or it could be set via calibration. Some head-tracking application programming interfaces (APIs) may also enable automatic adjustment of the reference orientation with some form of averaging. The application of the reference orientation can be done in various ways and over different points of the rendering chain. As described, a head orientation may provide a further rotation from the reference orientation. As any orientation can be represented by rotation matrix, there may be a chain of rotation matrices R applied when both head rotation and reference orientation are done from a global zero orientation. Note that rotations can be described in a multitude of formats other than by rotation matrices, such as by using quaternions, Euler angles, Tait-Bryan angles and direction cosine matrices, and conversions between the formats can be made where needed. For head-tracked audio output devices, there are usually two possible options, or APIs, available. One option is for the head-tracked audio output device to return one orientation with no possibility to add a reference orientation. In this case, the reference orientation may be set by the user (or software) and may be provided by selecting a specific head orientation as the reference orientation and then a rotation matrix may be formed from it, which can be applied to the directions after they have been first compensated with the combined orientation from the tracker. The second option is that the head-tracker is provided with a reference orientation, e.g., an external reference orientation, or it is set with a request, and the provided tracked rotation is always with respect to the reference orientation and no further adjustments are required. As these orientations are simply rotations, regardless of the stage of processing, it follows that the "reference orientation" can also be applied at any later stage of processing if it is more practical. For example, the 3GPP Immersive Voice and Audio Services (IVAS) codec provides for input of an external orientation in addition to a user's current head orientation. The external orientation may provide, or be used to provide, the reference orientation that can be used to produce a desired combined orientation internally. Example embodiments relate to updating the reference orientation, however provided, in the case that the user 150 is travelling along a travel route which may comprise one or more portions which will cause an expected change in user orientation. In this regard, the user 150 may be walking, running or travelling in a vehicle whilst consuming a spatial audio scene using a head-tracked audio output device. For example, the user 150 may be listening to a spatial voice call or an audio track of a movie whilst driving a car or travelling on a bus or train. The user 150 may make one or more intentional rotational inputs, e.g., when the user rotates their head from the current reference orientation. A binaural rendering module of a user device, which user device may comprise the media player 120, may responsively change the respective spatial positions of one or more audio sources, as described with reference to FIG. 4, such as to counter the change in user orientation from the reference orientation to the new orientation. However, other rotational inputs may result from the vehicle changing orientation as it travels along the travel route, e.g., when a turn is performed. This form of rotational input may be considered unintentional as the user 150 does not make an intentional head rotation. However, the binaural rendering module may still responsively change the respective spatial positions of the one or more audio sources such as to counter the unintentional rotational input, which will affect how the rendered spatial audio scene will be perceived in an unwanted way. FIG. 5 is a flow diagram showing operations 500 according to one or more example embodiments. The operations 500 may be performed in hardware, software, firmware or a combination thereof. For example, the operations 500 may be performed individually, or collectively, by a means, wherein the means may comprise at least one processor and at least one memory storing instructions that, when executed by the at least one processor, cause the performance of the operations. The operations 500 may, for example, be performed by user device associated with the user 150. For example, the operations 500 may, for example, be performed by a user device which may be the media player 120 of FIG. 1 or by a binaural rendering module which may comprise a part of the media player. Alternatively, the operations 500 may be performed by a network entity, such as by the server 110 wherein the server updates an external reference orientation which is transmitted with the encoded spatial audio data to the media player 120. A first operation 501 may comprise tracking user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space and wherein, responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation. A second operation 502 may comprise tracking a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation when the user travels along the one or more portions. A third operation 503 may comprise updating the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions. In some example embodiments, tracking user orientation may comprise tracking only the orientation of the user's head. In some example embodiments, where for example the tracked spatial position of the user indicates that the user is travelling along a first portion of the travel route, the reference orientation may be updated so as to comprise the respective change in user orientation for said first portion. FIG. 6 illustrates an example system 600 which may comprise one or more components of the FIG. 1 system 100. For example, the user 150 may be wearing the set of earphones 140 configured to provide head-tracking data to a binaural rendering module 620. The binaural rendering module may comprise part of a user device equivalent to the media player 120 described in relation to FIG. 1. The head-tracking data may comprise at least an indication of a current head orientation of the user 150. A telecommunications (telco) module 610 may be configured to transmit and receive spatial audio data via the network 130 with one or more remote users as part of an immersive audio call, given here by way of example. The spatial audio data may encode one or more audio sources such that they will be perceived from respective spatial positions with respect to a current reference orientation (however provided) in the real-world space in which the user 150 is located. For example, the current reference orientation may correspond to the current frontal direction of the user 150. The received spatial audio data may be decoded by the binaural rendering module 620 using a suitable decoder which may comprise part of a codec, such as the 3GPP IVAS codec. The binaural rendering module 620 may render the decoded spatial audio data for output to the head-tracking earphones 140 such that the one or more audio sources will be perceived by the user 150 from the respective spatial positions with respect to the current reference orientation. The binaural rendering module 620 may also update the current reference orientation based on a tracked spatial position of the user 150 in the real-world space. A travel route, for example in the form of active navigation data 635, may be received by the binaural rendering module 620. The travel route (e.g., active navigation data 635) may be provided by a navigation module or system 630. For example, the navigation module or system 630 may comprise a navigation module or system of a vehicle in which the user 150 is travelling. For example, the user 150 may initiate, via the navigation module or system 630, selection of a travel route from a location A to a location B. The navigation module or system 630 may responsively generate an active travel route indicating, e.g., the shortest and / or fastest route, from location A to location B and provide the travel route (e.g., active navigation data 635) to the binaural rendering module 620. As shown in FIG. 7, the navigation module or system 630 may provide a user interface 700. The user interface 700 may display map data, i.e., a two or three -dimensional representation of a geographical area, indicating a current tracked spatial position 702 of the vehicle and part of the active travel route 704. The map data may also show other roads 706, 708 which are not part of the active travel route 704. A current reference orientation 705 is assumed and is shown for ease of reference in FIG. 7. The current tracked spatial position 702 of the vehicle may be determined by means of an on-board satellite navigation receiver which receives positioning signals, e.g., global navigation satellite system (GNSS) signals, via an antenna provided on the vehicle. The active travel route 704 may comprise a first portion 710 which will cause a respective change, i.e., an expected or future change, in user orientation when the user 150, or in this case their vehicle, travels along the first portion. The binaural rendering module 620 may determine or estimate the respective change in user orientation for the first portion 710 based on the active travel route (e.g., the active navigation data 635). Based on the activ travel route (e.g., the active navigation data 635), the respective change in user orientation is determined to be approximately +30 degrees for the first portion 710. In accordance with the third operation 503, when it is determined based on the tracked spatial position 702 of the vehicle that the user 150 is travelling along the first portion 710, the reference orientation may be updated. In some example embodiments, the reference orientation may be updated such as to comprise the respective change in user orientation, i.e., +30 degrees. FIG. 8 shows the user interface 700 after it is determined that the current tracked spatial position 702 of the vehicle is travelling along the first portion 710. It is seen that the map data is rotated by approximately -30 degrees and the current reference orientation 705' used by the binaural rendering module 620 is updated by approximately +30 degrees. The active travel route 704 may further comprise a second portion 720 which will cause a respective change, i.e., an expected or future change, in user orientation which, in this case, is different from that of the first portion 710, e.g., approximately - 40 degrees with respect to the first portion. In this way, the perceived spatial positions of the one or more audio sources will not change even though the head-tracking data from the head-tracking earphones 140 indicates a +30 degree change in orientation. The unwanted effects described above are not therefore experienced. In some example embodiments, the third operation 503 may be performed in response to setting the active travel route 704 by the navigation module or system 630. That is, the binaural rendering module 620 may be aware that the navigation module or system 640 has generated the active travel route 704 which the user 150 intends to follow. In some example embodiments, the telecommunications module 610 may comprise part of the navigation module 630. It may sometimes be the case that the tracked spatial position 702 of the vehicle lags in time behind the actual position of the vehicle or may be incorrect. In some example embodiments, therefore, a check may be performed before updating the reference orientation in the third operation 503. For example, when the tracked spatial position 702 of the vehicle indicates that the user 150 is travelling along said first portion 710 (or any subsequent portion, e.g., the second portion 720, where there is a respective change in user orientation) a tracked change in user orientation based on the head-tracking data from the headtracking earphones 140 may be compared with the respective change in user orientation, i.e., +30 degrees for the first portion 710. If there is a correspondence between the tracked change in user orientation and the respective change in user orientation for, in this example, the first portion 710, the check is passed and the reference orientation may be updated. A correspondence may occur if the tracked change in user orientation and the respective change in user orientation are the same or are otherwise within a predetermined angular range of one another, e.g., within 10 degrees of one another. In some example embodiments, where there is no correspondence between the tracked change in user orientation and the respective change in user orientation for a given portion, a time delay may be computed. The time delay may represent the lag or time taken for the respective change in user orientation to catch-up with the tracked change in user orientation, e.g., 3 seconds. This may involve finding a highest correlation between the tracked change in user orientation and one or more respective changes in user orientation for one or more portions of the travel route when there will be a clear expected change in user orientation; this process may be performed one or more times to keep the time delay relatively accurate. The time delay may thereafter be used for subsequent comparisons. For example, the next time there is a tracked change in user orientation, the comparing may comprise comparing the tracked change in user orientation with one or more respective changes in user orientation for one or more further portions of the travel route determined based on the time delay and possibly also the current direction of travel. Effectively, the process may check how the user orientation is expected to change, e.g., 3 seconds in the future. The expected change may be compared with to the tracked change in user orientation. If there is a correspondence, then the reference orientation may be updated at this time rather than at a later time (due to the time delay) in order to more accurately maintain the head-tracked rendering. FIG. 9 illustrates an example system 900 similar to the FIG. 6 system 600. In this example, the navigation module 630 provides no active travel route but rather map data which comprises a plurality of possible travel routes which are ahead of the tracked spatial position 702. The system 900 may further comprise a selection module 910 for selecting, prior to updating the reference orientation in the third operation 503, one of the possible travel routes as the travel route of the user 150. The operations 500 described above with reference to FIGs. 5 to 8 may thereafter be performed for the selected travel route. In some example embodiments, the selection module 910 may comprise part of the navigation module 630, part of the binaural rendering module 620 (and therefore the media player 120) or a stand-alone module in signal communication with the navigation module and the binaural rendering modules. In some example embodiments, the selection module 910 may be configured to select one of the possible travel routes based on one or more heuristic rules. In some example embodiments, where only one of the possible travel routes corresponds to the direction of travel of the user, that travel route may be the selected travel route. A possible travel route may correspond to the direction of travel if it is within + / - 90 degrees of the direction of travel of the user, or an alternative lower value. To illustrate this, FIG. 10 shows a further user interface 1000 that may be displayed by the navigation module or system 630, although it will be appreciated that display of the user interface is not required. The user interface 700 may display map data indicating the current tracked spatial position 702 of the vehicle in which the user travels and first and second possible routes 1004, 1006, both of which are ahead of the current tracked spatial position. It will be seen that the first possible route 1004 differs by approximately +15 degrees from the route of travel whereas the second possible route 1006 differs by approximately +120 degrees from the route of travel, and hence the first possible route 1004 may be selected as the travel route of the user 150. In some example embodiments, the selection module 910 may be configured to assign respective probability scores to the plurality of possible travel routes. The respective probability scores may indicate a likelihood that the user will travel along the possible travel routes. The possible travel route having the highest probability score may be selected as the travel route of the user 150. In some example embodiments, the respective probability scores may be based on speed of user movement (speed-of-travel) as the tracked spatial position of the user 150, for example as the tracked spatial position of the user 150 approaches an intersection between the plurality of possible travel routes. In some example embodiments, the respective probability scores may be based on changes in the speed-of-travel, which may be determined based on measured changes in the tracked spatial position 702 of the user 150 over measured time intervals. For example, and with reference to FIG. 11, it will be seen that the expected change in user orientation will be different depending on whether the user 150 decides to travel along the first possible travel route 1004 or the second possible travel route 1006. When the tracked spatial position 702 of the user 150 approaches or passes an intersection 1008 between the first and second possible travel routes 1004, 1006, the speed-of-travel will reduce by a greater amount if the user 150 intends to travel along the second possible travel route 1004. Therefore, the first possible travel route 1004 may be assigned a relatively higher probability score if the speed-of-travel of the user 150 is maintained, increased or is reduced by a relatively small amount at or just before the intersection 1008. Conversely, the second possible travel route 1006 may be assigned a relatively higher probability score if the speed-of travel is reduced by a relatively high amount. Where the tracked spatial position 702 of the user 150 is static, then the same probability score may be assigned to the first and second possible travel routes 1006. In some example embodiments, the selection module 910 may compare the tracked change in user orientation with expected changes in user orientation for the possible travel routes and may select the possible travel route that corresponds, or most closely corresponds, to the tracked change in user orientation. This selection method may be used as a stand-alone selection method or where two or more possible travel routes are identified with, for example, identical or near-identical probabilities based on the above-described method. In example embodiments described above with reference to FIGs. 5 to 11, the spatial position of the user and the travel route may be provided by a navigation system or navigation module 630 of a vehicle in which the user 150 is travelling. In one or more other example embodiments, the travel route may instead be received from a remote source. For example, a user device, which may be equivalent to the media player 120 of FIG. 1, may transmit a request message to the remote source and the remote source may responsively transmit the travel route to the user device. Otherwise, the above-described operations may be the same. For example, the user 150 may be travelling on a public transport vehicle, e.g., a bus or train, which has an identifier of a public transportation route, e.g., a bus or train number which corresponds to a route from location A to location B. FIG. 12 illustrates an example system 1200 which is similar to the FIG. 6 system 600. Instead of using a navigation module 630 to provide the active route, the user device may provide an open application programming interface (API) 1230 for communicating with a public transportation server 1240. In this example, the user device may also comprise a movie streaming module 810 for receiving via the network 130 audio associated with a movie that the user 150 may be watching. In other examples, the user device may include another type of module, for example the telecommunications module 610 as described in relation to FIG. 6. The user 150 may via the user device transmit a request message including the public transportation route identifier to the public transportation server 1240. The public transportation server 1240 may responsively transmit to the user device navigation data, e.g., map data, for the route associated with the public transportation route identifier. The spatial position 702 of the user may be tracked in this example based on an estimate of where the public transport vehicle is currently located with respect to the received navigation data; tracked spatial position data may be readily available from the public transportation server 1240 although the abovementioned lag or time delay between the tracked spatial position 702 of the user and the actual location of the user may be more pronounced. Hence, the abovementioned methods of checking and, if required, determining and compensating for the determined time delay may be required and hence utilized. In some example embodiments, the encoding and decoding of the spatial audio data in any of the above examples may be by means of the IVAS codec or any codec that may utilize external orientation data. FIG. 13 shows a system 1300 comprising a IVAS encoder 1302 (as may be used by a remote telecommunications or video service) and an IVAS decoder 1304. The IVAS decoder 1304 may comprise a decoding portion 1306 for performing decoding of spatial audio data representing the spatial audio scene and an IVAS internal renderer portion 1308 for performing known head-tracking processing. The IVAS internal renderer portion 1308 may comprise an orientation data combiner module 1310 which includes an external orientation input 1312. In some example embodiments, rather than the user device, e.g., the media player 120, maintaining a local version of the reference orientation, the updated reference orientation determined according to any above example may be provided to the external orientation input 1312 which operates to cancel the expected change in orientation due to the travel route. In some example embodiments, the above-described operations may be performed by a user device, e.g., the media player 120 or any user device comprising the media player, which user device may have a paired relationship with the earphones 140, e.g., by means of a short-range link such as Bluetooth, Zigbee or WiFi. Alternatively, the above-described operations may be performed remotely from the user device and / or earphones 140, for example by a server such as a telecommunications or navigation server. The updated reference orientation may be formatted according to, e.g., the IVAS Real Time Protocol (RTP) payload format specification and transmitted using processing information (PI) frames. The PI frames may be packetized along with IVAS frames, and the user device may be configured to unpack and process the PI Frames and the updated reference orientation provided to the external orientation input 1312 of the IVAS decoder 1304. Example Apparatus FIG. 14 illustrates an example apparatus capable of supporting at least some embodiments. Illustrated is a device 1400, which may the media player 120 or server 110 configured to perform the FIG. 5 operations. Comprised in device 1400 is a processor 1410, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multi-core processor comprises more than one processing core. The processor 1410 may comprise, in general, a control device. The processor 1410 may comprise more than one processor. The processor 1410 may be a control device. A processing core may comprise, for example, a Cortex-A8 processing core manufactured by ARM Holdings ora Steamroller processing core produced by Advanced Micro Devices Corporation. The processor 1410 may comprise at least one Qualcomm Snapdragon and / or Intel Atom processor. The processor 1410 may comprise at least one Application-Specific Integrated Circuit, ASIC. The processor 1410 may comprise at least one Field-Programmable Gate Array, FPGA. The processor 1410 may be means for performing method steps in device 1400. The processor 1410 may be configured, at least in part by computer instructions, to perform actions. A processor may comprise circuitry, or be constituted as circuitry or circuitries, the circuitry or circuitries being configured to perform phases of methods in accordance with embodiments described herein. As used in this application, the term "circuitry" may refer to one or more or all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and / or digital circuitry, and (b) combinations of hardware circuits and software, such as, as applicable: (i) a combination of analog and / or digital hardware circuit(s) with software / firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as the network node 120, or a device configured to control the functioning thereof, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation. This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device. The device 1400 may comprise a memory 1420. The memory 1420 may comprise random access memory and / or permanent memory. The memory 1420 may comprise at least one RAM chip. The memory 1420 may comprise solid-state, magnetic, optical and / or holographic memory, for example. The memory 1420 may be at least in part accessible to processor 1410. The memory 1420 may be at least in part comprised in processor 1410. The memory 1420 may be means for storing information. The memory 1420 may comprise computer instructions that processor 1410 is configured to execute. When computer instructions configured to cause the processor 1410 to perform certain actions are stored in the memory 1420, and the device 1400 overall is configured to run under the direction of the processor 1410 using computer instructions from the memory 1420, the processor 1410 and / or its at least one processing core may be considered to be configured to perform said certain actions. The memory 1420 may be at least in part comprised in the processor 1410. The memory 1420 may be at least in part external to the device 1400 but accessible to the device 1400. The device 1400 may comprise a transmitter 1430. The device 1400 may comprise a receiver 1440. The transmitter 1430 and the receiver 1440 may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. The transmitter 1430 may comprise more than one transmitter. The receiver 1440 may comprise more than one receiver. The transmitter 1430 and / or the receiver 1440 may be configured to operate in accordance with Global System for Mobile Communication, GSM, Wideband Code Division Multiple Access, WCDMA, 5G / NR, 5G-Advanced, i.e., NR Rel-18, 19 and beyond, Long Term Evolution, LTE, IS-95, Wireless Local Area Network, WLAN, Ethernet and / or Worldwide Interoperability for Microwave Access, WiMAX, standards, for example. The device 1400 may comprise a Near-Field Communication, NFC, transceiver 1450. The NFC transceiver 1450 may support at least one NFC technology, such as NFC, Bluetooth, Wibree or similar technologies. The device 1400 may comprise a User Interface, UI, 1460. The UI 1460 may comprise at least one of a display, a keyboard, a touchscreen, a vibrator arranged to signal to a user by causing device 1400 to vibrate, a speaker and a microphone. A user may be able to operate the device 1400 via the UI 1460, for example to accept incoming telephone calls, to originate telephone calls or video calls, to browse the Internet, to manage digital files stored in memory 1420 or on a cloud accessible via the transmitter 1430 and the receiver 1440, or via NFC transceiver 1450, and / or to play games. The device 1400 may comprise or be arranged to accept a user identity module 1470. The user identity module 1470 may comprise, for example, a Subscriber Identity Module, SIM, card installable in device 1400. The user identity module 1470 may comprise information identifying a subscription of a user of device 1400. The user identity module 1470 may comprise cryptographic information usable to verify the identity of a user of device 1400 and / or to facilitate encryption of communicated information and billing of the user of the device 1400 for communication effected via device 1400. The processor 1410 may be furnished with a transmitter arranged to output information from processor 1410, via electrical leads internal to the device 1400, to other devices comprised in the device 1400. Such a transmitter may comprise a serial bus transmitter arranged to, for example, output information via at least one electrical lead to the memory 1420 for storage therein. Alternatively to a serial bus, the transmitter may comprise a parallel bus transmitter. Likewise, the processor 1410 may comprise a receiver arranged to receive information in The processor 1410, via electrical leads internal to the device 1400, from other devices comprised in the device 1400. Such a receiver may comprise a serial bus receiver arranged to, for example, receive information via at least one electrical lead from the receiver 1440 for processing in the processor 1410. Alternatively to a serial bus, the receiver may comprise a parallel bus receiver. The device 1400 may comprise further devices not illustrated in Figure 14. For example, where the device 1400 comprises a smartphone, it may comprise at least one digital camera. Some devices 1400 may comprise a back-facing camera and a front-facing camera, wherein the back-facing camera may be intended for digital photography and the front-facing camera for video telephony. The device 1400 may comprise a fingerprint sensor arranged to authenticate, at least in part, a user of the device 1400. In some embodiments, the device 1400 lacks at least one device described above. For example, some devices 1400 may lack a NFC transceiver 1450 and / or user identity module 1470. The processor 1410, memory 1420, transmitter 1430, receiver 1440, NFC transceiver 1450, UI 1460 and / or user identity module 1470 may be interconnected by electrical leads internal to the device 800 in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to the device 1400, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the embodiment various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the present invention. Figure 15 shows a non-transitory media 1500 according to some embodiments. The non-transitory media 1500 is a computer readable storage medium. It may be e.g. a CD, a DVD, a USB stick, a blue ray disk, etc. The non-transitory media 1500 stores computer program instructions, causing an apparatus to perform the method of any preceding process for example as disclosed in relation to the flow diagrams in this specification and related features thereof. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the preceding description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. While the forgoing examples are illustrative of the principles of the embodiments in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from 5 the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below. The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of also un-recited 10 features. The features recited in dependant claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, a singular form, throughout this document does not exclude a plurality.

Claims

1. An apparatus comprising:means for tracking user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space and wherein, responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation;means for tracking a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation from the reference orientation when the user travels along the one or more portions; andmeans for updating the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions, wherein the one or more audio sources are thereafter rendered at the respective spatial positions with respect to the updated reference orientation.

2. The apparatus of claim 1, whereinthe tracked spatial position of the user indicates that the user is travelling along a first portion of the travel route, andthe reference orientation is updated by changing the reference orientation using the respective change in user orientation for said first portion of the travel route.

3. The apparatus of claim 1 or claim 2, whereinthe spatial position of the user and the travel route are provided by a navigation system of a vehicle in which the user is travelling.

4. The apparatus of claim 1 or claim 2, whereinthe travel route is received from a remote source.

5. The apparatus of claim 4, whereinthe spatial position of the user is determined based at least on an estimate of a position of a vehicle in which the user is travelling, the estimate being received from the remote source.

6. The apparatus of claim 4 or claim 5, wherein the travel route is associated with a public transportation route.

7. The apparatus of any preceding claim, further comprising:means for comparing, when the tracked spatial position of the user indicates that the user is travelling along a first portion of the travel route, a change in the tracked user orientation with the respective change in user orientation for said first portion,wherein the reference orientation is updated based on identifying a correspondence between the change in the tracked user orientation and the respective change in user orientation for said first portion.

8. The apparatus of claim 7, wherein the reference orientation is updated if the tracked change in user orientation and the respective change in user orientation for said first portion are within a predetermined angular range of one another.

9. The apparatus of claim 7 or claim 8, further comprising:means for determining, based on identifying no correspondence between the tracked change in user orientation and the respective change in user orientation for said first portion, a time delay, wherein the time delay is determined based on the time taken for the respective change in user orientation to catch-up with the tracked change in user orientation,wherein the comparing subsequently comprises comparing the tracked change in user orientation with one or more respective changes in user orientation for one or more further portions of the travel route determined based on the time delay.10.The apparatus of any preceding claim, further comprising:means for identifying, based on the tracked spatial position of the user and the direction the user is travelling, a plurality of possible travel routes which are ahead of the tracked spatial position; andmeans for selecting, prior to updating the reference orientation, one of the possible travel routes as the travel route of the user.11.The apparatus of claim 10, wherein the selected travel route is that which most closely corresponds to the direction of travel of the user.12.The apparatus of claim 10 or claim 11, further comprising:means for assigning respective probability scores to the plurality of possible travel routes, the respective probability scores indicating a likelihood that the user will travel along said possible travel routes,wherein the selected travel route is that with the highest probability score.

13. The apparatus of claim 12, whereinthe plurality of possible travel routes comprises at least first and second possible travel routes which will cause different respective changes in user orientation from the reference orientation if the user were to travel along said first and second possible travel routes; andthe probability scores for the first and second possible travel routes are determined based on speed of user movement as the tracked spatial position of the user approaches an intersection between the first and second possible travel routes.14.The apparatus of any of claims 10 to 13, further comprising:means for comparing the tracked change in user orientation with the respective changes in user orientation for the possible travel routes; andmeans for selecting the possible travel route that corresponds, or most closely corresponds, to the tracked change in user orientation.

15. The apparatus of any preceding claim, wherein the reference orientation is a local reference orientation associated with the audio output device and the apparatus is configured to update the local reference orientation.16.The apparatus of any of claims 1 to 14, wherein the updated reference orientation is provided as an external reference orientation input to a decoder for receiving and decoding the data representing the spatial audio scene.

17. The apparatus of claim 16, wherein the decoder comprises a 3GPP Immersive Voice and Audio Services (IVAS) decoder.

18. The apparatus of any preceding claim, wherein the apparatus comprises the audio output device or a user device having a paired relationship with the audio output device.19.The apparatus of any of claims 1 to 17, wherein the apparatus comprises a server or service in communication with the audio output device.

20. A method, comprising:tracking user orientation in a real-world space, wherein the user consumes, via an audio output device, data representing a spatial audio scene comprising one or more audio sources which are rendered at respective spatial positions with respect to a reference orientation in the real-world space and wherein, responsive to a tracked change in user orientation from the reference orientation to a new orientation, the respective spatial positions of the one or more audio sources are modified so as to counter the change in user orientation from the reference orientation to the new orientation;tracking a spatial position of the user in the real-world space with respect to a travel route, wherein the travel route comprises one or more portions which will cause one or more respective changes in user orientation from the reference orientation when the user travels along the one or more portions; andupdating the reference orientation based on the tracked spatial position of the user indicating that the user is travelling along one of said one or more portions, wherein the one or more audio sources are thereafter rendered at the respective spatial positions with respect to the updated reference orientation.

21. The method of claim 20, whereinthe tracked spatial position of the user indicates that the user is travelling along a first portion of the travel route, andthe reference orientation is updated by changing the reference orientation using the respective change in user orientation for said first portion of the travel route.

22. The method of claim 20 or claim 21, whereinthe spatial position of the user and the travel route are provided by a navigation system of a vehicle in which the user is travelling.

23. The method of claim 21 or claim 22, wherein the travel route is received from a remote source.24.The method of any of claims 20 to 23, further comprising: comparing, when the tracked spatial position of the user indicates that the user is travelling along a first portion of the travel route, a change in the tracked user orientation with the respective change in user orientation for said first portion,wherein the reference orientation is updated based on identifying a correspondence between the change in the tracked user orientation and the respective change in user orientation for said first portion.5 25.The method of claim 24, further comprising:determining, based on identifying no correspondence between the tracked change in user orientation and the respective change in user orientation for said first portion, a time delay, wherein the time delay is determined based on the time taken for the respective change in user orientation to catch-up with the tracked10 change in user orientation,wherein the comparing subsequently comprises comparing the tracked change in user orientation with one or more respective changes in user orientation for one or more further portions of the travel route determined based on the time delay.33

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