Digital audio signal processing method and system for improving low-frequency reproduction

Abstract

Method and system for processing digital audio signals to improve low-frequency performance. This method is implemented in an audio system comprising a transducer (8) including a magnetic element adapted to generate vibrations according to the frequency of the audio signal.The process is implemented by one or more processing chains (101 to 103) comprising: high-pass filtering of the digital audio signal having a frequency greater than or equal to 10 Hz and less than two-thirds of the transducer's natural cutoff frequency; a first low-pass filter with a predetermined first amplification gain; an estimation of the transducer's excursion value for the signal obtained after application of the first low-pass filter; if the estimated excursion value of the transducer exceeds the maximum excursion value, a calculation of a second amplification gain lower than the first gain, and a second low-pass filter with a gain equal to the second calculated gain. Figure 1 for the abbreviation.

Description

Title of the invention: Method and system for processing digital audio signals to improve the rendering of low frequencies

[0001] The present invention relates to a digital audio signal processing method for improving the rendering of low frequencies, and an associated digital audio signal processing system.

[0002] The invention lies in the field of audio systems, and more particularly audio systems for vehicles, in particular for passenger transport vehicles.

[0003] In this type of audio system, it is known to use various types of transducers to promote the rendering of audio signals, for example for listening to music. Such transducers comprise loudspeakers, which produce a sound wave in response to an electrical signal applied as input, integrated into the passenger compartment of a vehicle, for example a motor vehicle, or integrated into the seat headrest to improve the sound rendering quality for each user. In addition, it has also been proposed to integrate vibrators, which are also transducers, into seats, for example motor vehicle seats, to improve the user experience by accompanying the sound rendering with corresponding vibrations.

[0004] Such a transducer comprises a magnetic element adapted to generate vibrations as a function of the frequency of the audio signal supplied as input. For example, for a loudspeaker, the magnetic element is composed of a coil, through which an electric current is passed, immersed in a magnetic field created by pole pieces and a magnet, the loudspeaker further comprising a membrane linked to the coil, the vibrations of which create the reproduced sound wave. The displacement of the membrane has a displacement amplitude relative to its equilibrium position, also called excursion. For operation under good conditions, a maximum excursion is predefined. The restitution of the low frequencies of the audio signal by such a transducer is limited by a cut-off frequency, specific to each transducer.

[0005] With the aim of improving the sound reproduction quality for the user, it has been proposed to artificially increase the level of the audio signal in the low frequencies, by appropriate filtering of the audio signal, analog or digital.

[0006] However, it has been found that an amplification of the signal level in the low frequencies has the effect of considerably increasing the excursion of the transducer, which induces the risk of reaching the maximum excursion value of the transducer, and consequently damage, or rupture of the membrane if the sound volume is increased by the user. This risk is all the greater if the transducer, for example the loudspeaker, is small, which is the case for transducers integrated into a seat headrest or in a mobile phone.

[0007] Thus, it is useful to develop methods for improving sound reproduction while protecting against excessive excursions of the transducer membrane.

[0008] In particular, patent EP 2 571 286 B1 proposes a technique for dynamically reinforcing low frequencies in a sound reproduction installation, by applying amplification filtering of the low shelf filter type, with amplification gain calculated as a function of an estimated excursion value of the loudspeaker, in order to avoid excessive excursion of the loudspeaker membrane. Thus, the method proposed in this patent makes it possible to calculate a suitable amplification gain, and this amplification gain is applied to all the low frequencies of the audio signal. However, this method increases the level including very low frequencies, which contribute to the excursion of the transducer membrane, while being barely audible to a user.

[0009] The object of the invention is to further improve the reproduction of low frequencies of the audio signal while protecting the transducer from possible degradation.

[0010] To this end, the invention proposes, according to a first aspect, a method for processing a digital audio signal to improve the rendering of low frequencies implemented in an audio system comprising a transducer comprising a magnetic element adapted to generate vibrations as a function of the frequency of the audio signal and having an associated maximum excursion value, the transducer further having a specific cut-off frequency. This method comprises:

[0011] - A) high-pass filtering of the digital audio signal having an associated filtering frequency, the filtering frequency being greater than or equal to 10 Hz and less than two-thirds of the natural cut-off frequency of the transducer, the high-pass filtering producing a filtered digital audio signal, - B) an application, on said filtered digital audio signal, of a first low-shelf filter, said first low-shelf filter carrying out an amplification of the frequencies, included in a frequency range lower than a predetermined limit frequency, said first low-shelf filter having a first predetermined amplification gain, - C) an estimate of a transducer excursion value for the signal obtained after application of the first low-shelf filter, - D) if the estimated transducer excursion value exceeds the maximum excursion value, a calculation of a second amplification gain lower than the first amplification gain, - E) an application, on said filtered digital audio signal, of a second low-shelf filter carrying out an amplification of the frequencies included in said frequency range lower than the predetermined limit frequency, and of gain equal to the second calculated gain, the application of the second low-shelf filter making it possible to obtain a filtered and amplified digital audio signal, for which the transducer has an excursion value lower than the maximum excursion value.

[0012] Advantageously, the proposed digital audio signal processing method makes it possible to preserve the transducer by avoiding reaching the maximum excursion value and to improve the sound rendering of the frequencies of interest to the user.

[0013] The digital audio signal processing method for improving the rendering of low frequencies according to the invention may also have one or more of the characteristics below, taken independently or in any technically conceivable combination.

[0014] The filtering frequency is less than half of the natural cutoff frequency of the transducer.

[0015] The method comprises a phase of determining said filtering frequency of the high-pass filter to be applied, comprising the application of steps A) to E) a number P of times, P being greater than or equal to two, each application being carried out with a high-pass filtering having a distinct test filtering frequency, chosen between 10 Hz and less than two-thirds of the natural cut-off frequency of the transducer, each application making it possible to obtain a second amplification gain associated with a test filtering frequency, then a selection of a high-pass filtering to be applied to the digital audio signal of filtering frequency chosen from among the test filtering frequencies.

[0016] Each application of steps A) to E) makes it possible to obtain a filtered and amplified digital test audio signal, and the selection implements a calculation of the power of each filtered and amplified digital test audio signal, and the selection of the test filtering frequency making it possible to obtain the filtered and amplified digital test audio signal of maximum power.

[0017] The test filter frequencies comprise P frequency values, comprising a first test filter frequency equal to 10 Hz, a last test filter frequency equal to the natural cutoff frequency of the transducer divided by two.

[0018] In one embodiment, P is strictly greater than two, and the test filtering frequencies comprise P frequencies regularly distributed between 10 Hz and the natural cutoff frequency of the transducer divided by two.

[0019] The test filter frequencies comprise P frequency values, incremented by a 10 Hz increment from a chosen floor frequency value.

[0020] According to another aspect, the invention relates to an audio system comprising a transducer comprising a magnetic element adapted to generate vibrations as a function of the frequency of the audio signal and having an associated maximum excursion value, the transducer further having a specific cut-off frequency, the audio system further comprising a calculation unit configured to implement a digital audio signal processing method to improve the rendering of low frequencies as briefly described above.

[0021] The advantages of the audio system are analogous to the advantages of the audio processing method recalled above.

[0022] According to advantageous embodiments: the transducer is a loudspeaker and / or the audio system is integrated into a seat headrest, and / or the transducer is a vibrator, said audio system being integrated into a seat.

[0023] Other characteristics and advantages of the invention will emerge from the description given below, for information purposes only and in no way limiting, with reference to the appended figures, among which:

[0024] [Fig-1] [Fig.l] schematically represents a signal processing system digital audio to improve the rendering of low frequencies according to one embodiment;

[0025] [Fig.2] [Fig.2] is a block diagram of a digital audio signal processing method for improving the rendering of low frequencies according to a first embodiment;

[0026] [Fig.3] [Fig.3] is a block diagram of a digital audio signal processing method for improving the rendering of low frequencies according to a second embodiment.

[0027] [Fig.l] schematically represents a system 2 for processing digital audio signals to improve the rendering of low frequencies according to one embodiment.

[0028] An audio signal S_in is provided at the input of a block 4, which applies a gain dependent on the amplifier 6, which will be used at the end of the processing chain, before providing the audio signal to the transducer 8. The audio signal obtained at the output of block 4 is a digital audio signal to which sound rendering improvement processing is applied.

[0029] In one embodiment, the transducer 8 is a loudspeaker, which is for example integrated into a seat headrest audio system, more particularly a transport vehicle seat.

[0030] In another embodiment, the transducer 8 is a loudspeaker, which is for example integrated into a mobile telephone audio system.

[0031] In another embodiment, the transducer 8 is a vibrator integrated into a seat, for example in a seat, more particularly a vehicle seat. transportation.

[0032] The processing system 2 comprises one or more processing chains 10; at 10P, the example of [Fig.l] comprising P=3 processing chains.

[0033] In the embodiment where P is an integer strictly greater than 1, the system implements, substantially in parallel or successively, processing chains 10i to 10P, any one of these processing chains being referenced 10;.

[0034] Each processing chain 10; implements a test phase to dynamically determine optimized processing parameters of the digital audio signal.

[0035] Each processing chain comprises a module 12; for applying high-pass filtering of the digital audio signal, having an associated filtering frequency Fi.

[0036] Such a high-pass filter produces, in a known manner, a cut-off at the frequency Fi, so as to retain only the frequencies greater than or equal to the filtering frequency Fi. A filtered digital audio signal Shi is thus obtained.

[0037] The processing chain 10; further comprises a module 14; for applying a first “lowshelf” type filter, designated by the abbreviation LS, also called a low shelf filter.

[0038] A low-shelf filter or low-shelf filter is, in a known manner, defined by a limit frequency or cut-off frequency F;, and an amplification gain G;. The low-shelf filter provides an amplification of gain G; for frequencies included in a frequency range, delimited by the limit frequency F; above which no amplification is applied. For example, such a filter provides an amplification of G;=+2dB in a chosen frequency range, for example notably lower than 100 Hz, and no amplification clearly above 100 Hz. The limit frequency is in the middle of a transition period whose steepness (or slope) is configurable.

[0039] For example, each LS filter 14; has a limit frequency of 130Hz and a first amplification gain of 5dB.

[0040] The processing chain 10; also comprises a module 16; for estimating a current excursion value of the transducer for the signal obtained after application of the first test low plateau filter 14;.

[0041] In addition, the module 16; also performs the calculation of a correction, or attenuation, xi to be applied to the first amplification gain to obtain a second amplification gain G2=G;-xi which makes it possible to obtain a current excursion value which does not exceed the maximum excursion value.

[0042] Any method of estimating the current value of the transducer excursion is applicable, for example the method described in patent EP 2 571 286 Bl.

[0043] An LS 18i low-shelf filter is then applied to the filtered digital audio signal, with the second amplification gain calculated, resulting in a signal digital filtered by high-pass filtering and amplified by the LS 18 filter;.

[0044] The system 2 comprises, in the case where the number P of processing chains is greater than or equal to two, a module 20 for selecting a high-pass filtering, of filtering frequency Fj, to be applied to the digital audio signal as a function of the filtered and amplified digital signals obtained respectively by each processing chain 10i.

[0045] In one embodiment, the selected high-pass filter 12j is the one that provides the maximum power filtered and amplified digital signal. The corresponding filtered and amplified digital signal is provided at the input of the amplifier 6, then provided to the transducer 8 for sound reproduction.

[0046] The modules 12 to 18 of each processing chain and the module 20 are preferably digital processing modules, implemented within a calculation unit which is for example a processor, a microcontroller or a digital signal processing chip of the DSP type.

[0047] In the embodiment where P=1, a single processing chain 10 is implemented, and the filtering frequency is chosen between 10 Hz and two-thirds of the natural cut-off frequency of the transducer, preferably between 10 Hz and half of the cut-off frequency of the transducer. In this case, the selection module 20 is not implemented.

[0048] [Fig.2] is a block diagram of the main steps of a digital audio signal processing method for improving the rendering of low frequencies in a first embodiment.

[0049] The method comprises the following steps, implemented on a digital audio signal.

[0050] The method comprises a step 30 of obtaining parameters, for example from an electronic memory or via a human-machine interface allowing a user to provide parameters.

[0051] In particular, these parameters include the natural cut-off frequency Fo of the transducer and the maximum excursion value of the transducer.

[0052] These parameters further include cutoff frequency parameters Fb and an amplification gain Gi for each first LS filter (“lowshelf” filter).

[0053] The method comprises a step 32 of obtaining the digital audio signal from an analog audio signal, by analog-digital conversion, executed continuously, and the application of a gain by the gain block 4.

[0054] Step 32 is followed by a step 34 of high-pass filtering of the digital audio signal, the high-pass filter having a chosen filtering frequency Fhp.

[0055] For example, the filtering frequency Fhpest is between 10Hz and 2 / 3 Fo, preferably between 10Hz and F0 / 2.

[0056] The filtering step 34 provides a filtered audio signal Sh.

[0057] The method then comprises a step 36 of applying a first low-shelf filter (“lowshelf” filter). This filter is characterized by parameters which are the limit frequency Fi and the first amplification gain Gb

[0058] For example, Fi=130 Hz and Gi=5dB.

[0059] The method also comprises a step 38 of estimating a transducer excursion value for the signal obtained after applying the first low-level shelf filter to the filtered digital signal Sh.

[0060] The estimated transducer excursion value is compared in comparison step 40 to the maximum transducer excursion value, and if it exceeds this value, step 40 is followed by a step 42 of calculating a second amplification gain, which is a function of the first amplification gain and of the exceeding of the maximum transducer excursion value.

[0061] If the estimated transducer excursion value is less than the maximum transducer excursion value, the method continues at step 32.

[0062] In one embodiment, step 42 implements the calculation of an attenuation of XdB of amplification gain to be applied to avoid exceeding the maximum excursion value of the transducer: G2=GrX in dB.

[0063] For example, if for a first amplification gain Gi, the estimated excursion x(Gi) exceeds the excursion limit x_lim, the attenuation value X is calculated by X=20*log(x(Gi) / x_lim).

[0064] Step 42 is followed by a step 44 of applying a second low-shelf amplification filter, having a gain equal to the second amplification gain, and the same limit frequency as the first low-shelf filter applied in step 36.

[0065] After step 44, a digital audio signal is obtained, filtered and amplified by the LS filtering, which is then supplied to the transducer for reproduction.

[0066] Step 44 is followed by step 32, the low frequency rendering improvement processing being applied in a loop to an audio signal received as input.

[0067] [Fig.3] is a block diagram of the main steps of a digital audio signal processing method for improving the rendering of low frequencies in a second embodiment.

[0068] This second embodiment comprises steps of obtaining parameters and acquiring the digital audio signal to be processed, similar to steps 30 and 32.

[0069] The method further comprises a step 50 of determining P values ​​of filtering frequencies of high-pass filters to be tested, called test filtering frequencies and noted Fi to FP.

[0070] For example, given the natural cutoff frequency of the transducer Foet a integer P chosen, for example P=3 or P=4, at step 50 the step is calculated

[0071] d=(F0 / 2-10) / P

[0072] and the test filter frequencies Fi are calculated as follows: Fi=10Hz, and then Fi+1=(Fi+d) Hz.

[0073] In other words, in this embodiment, the test filtering frequencies are regularly distributed between the floor frequency value equal to 10 Hz and the natural cut-off frequency of the transducer divided by two.

[0074] According to a variant, a floor frequency value other than 10Hz is chosen, for example 15Hz, and the calculation described above is applied with the floor frequency fp, to obtain test filtering frequencies regularly distributed between the floor frequency and F0 / 2.

[0075] According to another variant, filtering frequencies incremented by 1 Ohz by 1 Ohz, from a floor frequency value, for example equal to 10 Hz, are chosen, for example Fi=10 Hz, F2=20 Hz, F3=30 Hz, F4=40 Hz, for Fo=12 Ohz.

[0076] According to another variant, for P=4, filtering frequencies of Fi=10Hz, F2=30Hz, F3=50Hz, F4=70 Hz are chosen, for Fo=12OHz.

[0077] Of course, other progression variants, in the form of an arithmetic or geometric series, or otherwise, can be implemented to determine P test filtering frequencies, each test filtering frequency being greater than or equal to the floor frequency value, preferably equal to 10 Hz, and less than two-thirds of the natural cutoff frequency of the transducer.

[0078] Step 50 is followed by the application of several test treatments 52_1 to 52_P.

[0079] Each of these treatments implements steps 34 to 44 described with reference to the [Fig.2], with, for each processing 52_i, the implementation of a high-pass filtering of corresponding frequency F;, determined in step 50.

[0080] In other words, each test processing 52_i implements a test filtering frequency F; and makes it possible to calculate a second plateau filtering gain, corresponding to the first amplification gain attenuated by a value x; calculated to limit the value of the transducer excursion to an excursion value lower than the maximum excursion value.

[0081] The method then comprises a selection 54 of a high-pass filtering to be applied to the digital audio signal of filtering frequency chosen from the test filtering frequencies Fi to FP.

[0082] The selection 54 comprises, in one embodiment, a calculation 56 of the power Pi of each filtered and amplified digital audio test signal obtained by the processing 52_i, and the determination 58 of the processing 52j for which the power Pj is maximum.

[0083] At the end of the selection step 54, the filtering frequency pair Fj and gain attenuation xj to be applied to obtain the second low-shelf filter gain LS to be applied.

[0084] The processing 52 j, implementing a high-pass filtering of filtering frequency F j, and an application of a filter LSj of second gain Gl-Xj is implemented.

[0085] Advantageously, the method makes it possible to amplify the low or medium frequencies in a self-adaptive manner, while avoiding any degradation of the transducer by limiting the excursion value so as not to exceed the maximum excursion value.

[0086] Advantageously, the proposed method makes it possible to improve the restitution of the audio signal in the low frequencies and in the medium frequencies, without introducing non-linear distortion.

Claims

Claims

1. Method for processing a digital audio signal to improve the rendering of low frequencies, implemented in an audio system comprising a transducer (8) comprising a magnetic element adapted to generate vibrations as a function of the frequency of the audio signal and having an associated maximum excursion value, the transducer further having a specific cut-off frequency, the method being characterized in that it comprises: - A) high-pass filtering (34) of the digital audio signal having an associated filtering frequency, the filtering frequency being greater than or equal to 10 Hz and less than two-thirds of the natural cut-off frequency of the transducer, the high-pass filtering producing a filtered digital audio signal, - B) an application (36), on said filtered digital audio signal, of a first low-shelf filter, said first low-shelf filter carrying out an amplification of the frequencies, included in a frequency range lower than a predetermined limit frequency, said first low-shelf filter having a first predetermined amplification gain, - C) an estimate (38) of a transducer excursion value for the signal obtained after application of the first low-shelf filter, - D) if the estimated excursion value of the transducer exceeds the maximum excursion value, a calculation (42) of a second amplification gain lower than the first gain, - E) an application (44), on said filtered digital audio signal, of a second low-shelf filter carrying out an amplification of the frequencies included in said frequency range lower than the predetermined limit frequency, and of gain equal to the second calculated gain, the application of the second low-shelf filter making it possible to obtain a filtered and amplified digital audio signal, for which the transducer has an excursion value lower than the maximum excursion value.

2. The method of claim 1, wherein said filtering frequency is less than half the transducer's natural cutoff frequency.

3. Method according to one of claims 1 or 2, comprising a phase of determining said filtering frequency of the high-pass filter to be applied, comprising the application of steps A) to E) a number P of times, P being greater than or equal to two, each application (52_1,..., 52_P) being carried out with a high-pass filtering having a distinct test filtering frequency, chosen between 10 Hz and less than two-thirds of the natural cut-off frequency of the transducer, each application making it possible to obtain a second amplification gain associated with a test filtering frequency, then a selection (54) of a high-pass filtering to be applied to the digital audio signal of filtering frequency chosen from among the test filtering frequencies.

4. The method of claim 3, wherein each application of steps A) to E) makes it possible to obtain a filtered and amplified test digital audio signal, and the selection (54) implements a calculation (56) of the power of each filtered and amplified test digital audio signal, and the determination (58) of the test filtering frequency making it possible to obtain the maximum power filtered and amplified test digital audio signal.

5. A method according to claims 3 or 4, wherein the test filter frequencies comprise P frequency values, comprising a first test filter frequency equal to 10 Hz, a last test filter frequency equal to the natural cut-off frequency of the transducer divided by two.

6. Method according to claim 5, in which P is strictly greater than two, and the test filtering frequencies comprise P frequencies regularly distributed between 10 Hz and the natural cut-off frequency of the transducer divided by two.

7. A method according to claims 3 or 4, wherein the test filter frequencies comprise P frequency values, incremented by an increment of 10 Hz from a chosen floor frequency value.

8. Audio system comprising a transducer (8) comprising a magnetic element adapted to generate vibrations as a function of the frequency of the audio signal and having an associated maximum excursion value, the transducer (8) further having a specific cut-off frequency, the audio system further comprising a calculation unit configured to implement a digital audio signal processing method

9.

10.

11. to improve the rendering of low frequencies in accordance with claims 1 to 7. The audio system of claim 8, wherein said transducer is a loudspeaker. Audio system according to claim 9, integrated into a seat headrest. The audio system of claim 8, wherein said transducer is a vibrator, said audio system being integrated into a seat.

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