EP1123638B1 - System and method for evaluating the quality of multi-channel audiosignals - Google Patents

Abstract

A system for evaluating the quality of an audio test signal derived from an audio reference signal by coding and decoding, said audio test signal and said audio reference signal each comprising a plurality of channels, comprises a unit for converting the audio reference signal into a first audio reference sum signal at a first reference point and into a second audio reference sum signal at a second reference point and for converting the audio test signal into a first audio test sum signal at the first reference point and into a second audio test sum signal at the second reference point, the audio reference sum signals and the audio test sum signals at the first and second reference points being a superposition of the respective channels, which can be emitted by a plurality of loudspeakers, weighted with a respective transfer function between the respective loudspeaker and the reference point in question, and a unit for evaluating the quality of the audio test sum signals while taking into consideration the audio reference sum signals so as to provide an indication of the quality of the audio test signal. The system according to the present invention permits real rooms and an arbitrary number of channels of the audio test signal to be taken into account so as to execute a listening-adapted evaluation of the quality of a specific coding/decoding method.

Description

Since the standardization of hearing-adapted digital coding methods the same are used to an increasing extent. Examples of this are the digital compact cassette, the mini disc, digital terrestrial broadcasting and digital Video disc. When coding using a hearing-adapted coding method however, artifacts or artifacts may occur that did not occur in analog audio signal processing are.

For the evaluation or assessment of a specific encoder hearing tests have been carried out with test persons in the past Service. Although the hearing tests on average are relatively reliable Delivering results is still subjective Component. There are also hearing tests with a certain number of test subjects relatively complex and therefore relative expensive. Therefore, measurement methods have been used for hearing-appropriate assessment developed by audio signals.

Such a measuring method is described, for example, in DE-C-196 47 399. The procedure described therein for hearing-appropriate Quality assessment models all non-linear Hearing effects on both a reference signal as well on a test signal. The hearing-appropriate quality assessment by means of a comparison in the cochlea area ("Cochlear Domain"). The excitement in the ear by the test signal or by the reference signal compared. For this purpose, both the audio reference signal as well as the audio test signal through a filter bank broken down into their spectral compositions. By a large number of filters, which differ in frequency overlap, will be adequate both temporally and frequency resolution ensured. Thus a Mono audio test signal by coding and subsequent Decoding derived from an audio reference signal in its quality can be assessed.

The measuring method described in DE-C-196 47 399 allows furthermore the quality assessment of stereo signals, i. H. two-channel signals. For this, a non-linear Preprocessing that highlights transients in a frequency-selective manner and stationary signals decreased, with the left and right Channel of the audio test signal or the audio reference signal carried out. In particular, various detections the probability of error with the left channel of the audio reference signal and with the left channel of the audio test signal as input signals, with the right channel of the audio reference signal and with the right channel of the audio test signal as input signals, with the left channel of the preprocessed Audio reference signal and with the left channel of the preprocessed audio test signal as input signals and with the right channel of the preprocessed audio reference signal and with the right channel of the preprocessed audio test signal performed as input signals to measure for maintain the quality of the stereophonic audio test signal.

A disadvantage of the known method for the hearing-adapted Quality assessment of audio signals is the fact that the stereo capability only on headphone playback is limited. In other words, the audio test signal that enters a listener's ear with the audio reference signal, that enters a listener's ear. This means effects caused by a room be such. B. reflections on walls, ceiling and floor, Multiple reflections, attenuations, etc. not taken into account become. Known quality assessment methods can also be used no directional characteristic of the human Consider ear, d. H. it doesn't matter if a Signal comes from behind, in front or from the side. Known measuring methods only work for headphone playback, at the the sound signal from the headphone speaker, the is usually arranged directly on the ear, exits and in the ear or in the process of quality assessment.

Another disadvantage of the known method is that until now the hearing-adapted quality assessment of more and more emerging multi-channel signals, such as. B. 5-channel signals, which is known under the keyword "Dolby Surround" are completely impossible.

WO-A-97 25 834 discloses processing a multi-channel Signals and EP-A-0 165 733 a method and an apparatus for measuring and correcting acoustic characteristics in a sound spring.

The object of the present invention is a improved concept for quality assessment of audio signals to create that takes space effects into account.

This task is accomplished by a quality assessment device according to claim 1 and by a method for Quality assessment according to claim 14 solved.

The present invention is based on the finding that that the human listener who is ultimately at issue despite the presence of signals with any number Channels are only ever available to two ears. The directional hearing is due to the different impulse responses for different directions of incidence of sound signals into the human ear. The different Impulse responses for different directions of incidence in technology as header-related transfer functions or "Head Related Transfer Functions". In real Case not only the direct sound paths between Ear and loudspeaker also emit reflections the walls, the ceiling and the floor. This can act as a room impulse response be summarized. The HRTFs and the room impulse response together lead to a sound change that can also be evaluated according to the invention by measuring systems, which do not explicitly model binaural effects, such as B. different masking thresholds for binaural Signals compared to monoaural signals, perception of phase shifts, precedent effects, etc.

When evaluating audio signals using hearing tests usually standardized listening rooms, for example standardized according to ITU-R BS.1116. It is the size, speaker layout and reverberation time largely set. According to the invention, the extended Quality assessment of audio signals both the Head-related transfer functions (HRTFs) as well as room impulse responses be taken into account. It is also for the irrelevant auditory quality assessment according to the invention, whether a signal is a stereo signal, that of two Speakers for the left or right channel broadcast or whether the signal is a multi-channel signal that for example, has five channels and five speakers is broadcast, the z. B. in such a way Positioned that the loudspeakers on the left rear, left front, right rear, right front or front are.

For this purpose, the device for quality assessment according to of the present invention means for converting the audio reference signal into a first audio reference sum signal at a first reference point and into a second audio reference sum signal at a second reference point and a Means for converting the audio test signal into a first one Audio test sum signal at the first reference point and in second audio test sum signal at the second reference point, wherein the audio reference sum signals and the audio test sum signals one at the first and second reference points Superimposition of the respective channels by the plurality of Speakers are output, weighted with a respective Transfer function between the respective loudspeaker and the corresponding reference point. The audio reference sum signals and the audio test sum signals will eventually fed into a quality assessment facility, an indicator of the quality of the audio test signal to obtain. The quality assessment facility can be any known device, such as is disclosed in DE 196 47 399 C1, or how they in the international standard ITU-R BS 1387 (PEAQ) has been established.

An advantage of the method according to the invention is the fact that if the audio signal is a stereo signal, the Influences of the listening room on everyone's signal propagation Speakers to each reference point, d. H. every ear, considered can be.

Another advantage is the fact that the method applicable for audio signals with any number of channels is because the channels have appropriate transfer functions, which is the propagation of a signal from one Model loudspeakers to one ear on two sum signals be converted so that any method for quality assessment that is suitable for two channels, can be used.

The individual transfer functions can usually by measurement using built-in microphones with an artificial head or with probe microphones with a human listeners. Particularly advantageous is the inventive method, however, if the Head-related transfer functions of any person are already known and for example on the Internet can be downloaded from an appropriate server. In this case the impulse response of a listening room, that can be measured or simulated with a certain present HRTF can be folded to a transfer function to obtain. This is particularly advantageous there where the listening room does not yet exist, d. H. where the sound properties of a room are simulated before the room is built at all, for example at the planning of concert halls or recording studios the sound properties to simulate and even before the construction of the listening room to optimize it.

Fig. 1

ein schematisches Blockdiagramm einer erfindungsgemäßen Vorrichtung;

Fig. 2

ein schematisches Diagramm zur Ermittlung der Kopf-bezogenen Übertragungsfunktionen (HRTFs); und

Fig. 3

ein schematisches Blockdiagramm zur Darstellung der Situation in einem realen Abhörraum.

Preferred embodiments of the present invention are explained in detail below with reference to the accompanying drawings. Show it:

Fig. 1

a schematic block diagram of a device according to the invention;

Fig. 2

a schematic diagram for determining the header-related transfer functions (HRTFs); and

Fig. 3

a schematic block diagram to illustrate the situation in a real listening room.

1 shows a schematic block diagram of a device for quality assessment of an audio test signal that by encoding and decoding an audio reference signal is derived. The audio test signal and the audio reference signal each have a plurality of channels, wherein each channel through a speaker of a plurality of Speakers 11 to 15 that are in different positions positioned in an at least fictional room, audible can be made, and wherein two reference points 17, 18 to Simulation of hearing regarding the positions of the majority are defined by speakers 11 to 15. The device for quality assessment comprises means 19 for converting the audio reference signal into a first audio reference sum signal at the first reference point 17 and in a second audio reference sum signal at the second reference point 18 and for converting the audio test signal into a first one Audio test sum signal at the first reference point 17 and in a second audio test sum signal at the second reference point 18, the audio reference sum signals and the audio test sum signals at the first and second reference points 17, 18 an overlay of the respective channels by the A plurality of speakers 11 to 15 are deliverable, weighted with a respective transfer function ÜF11 to ÜF52 between the respective loudspeaker 11 to 15 and the corresponding one Reference points 17, 18 are. The device for Quality assessment also includes a device 20 for Quality assessment of the audio test sum signals taking into account the audio reference sum signals to a display the quality of the audio test signal at an output 21 to deliver.

In the following, the device 19 for converting received. It includes the majority of transfer functions ÜF11 to ÜF52, which are either the HRTFs, though an anechoic room, d. H. a room in which no reflections occur, is considered, or the whole Transfer function of the room from one of the loudspeakers 1 to 5 to a reference point 1, 2. As shown in Fig. 1 is, the output signals of the speakers weighted the corresponding transfer functions. The Output signals when weighting the input signals with the corresponding transfer functions, are summed by means of a first summer 22 by first To get audio sum signals. The same applies to the second reference point 18 a second totalizer 23 is provided, to the output signals of the transfer functions from the respective speakers 11 to 15 to the second reference point 18 sum to add the second audio sum signals deliver. Of course, both the audio test signal as well as the audio reference signal of the processing by means of subjected to the conversion device 19 such that for the audio reference signal and the audio test signal have the same ratios prevail, such that the device 20 for quality assessment for 2-channel signals only the quality the encoding / decoding measures and no other effects Disrupt measurement result.

1 shows the situation for a 5-channel audio signal is shown, the device according to the invention is also for stereo signals with only two channels or for signals applicable with three, four or more than five channels. In in this case, only corresponding transfer functions need to be carried out added or omitted. Furthermore, be on it noted that the positioning of the speakers in Fig. 1 is only schematic. Correct positioning the loudspeaker with respect to the reference points is in the 2 and 3 for the example of 5-channel signals.

Regarding the notation of the individual transfer functions it should be noted that the first digit always refers to the Loudspeaker, while the second digit refers to the Reference point, d. H. Reference point No. 1 (17) or reference point No. 2 (18).

Fig. 2 shows a possible arrangement of the five speakers 11 to 15 with respect to a receiver 24, the head of which is shown in FIG. 2 is shown schematically in plan view. alternative the head 24 could be an artificial head. Definitely includes the head 24 the first reference point 17 and the second reference point 18, d. H. the ears 17, 18 in the case of a human Handset or the built-in microphones 17, 18 at an artificial head 18. In Fig. 2 transmission paths in anechoic chamber from each of the speakers 11 to 15 to each Reference points 17, 18 entered. The header-related transfer functions (HRTFs) are shaded for example the head or shoulders of the listener and determined by different transmission times. So arrow 31a, for example, represents the transmission path from the first loudspeaker 11 to the first reference point 17 The arrow 31b, dashed in the area of the head 24 is drawn, the HRTF provides the first speaker 11 to the second reference point 18 Arrow 32a the transfer function from the second speaker 12 to the first reference point, i. H. ÜF21 in Fig. 1, Accordingly, arrow 32b represents the transfer function from the second speaker 12 to the second reference point 18, d. H. ÜF22 in Fig. 1, by summing the Partial signals with the corresponding transfer function weighted speaker output signals at the reference points 17, 18 then the first or second audio test sum signals result or audio reference sum signals, which in a any device 22 for quality assessment for 2-channel signals can be fed to a measure of the Quality of the audio test signal used in that shown in FIG Case is a 5-channel signal.

As already mentioned, the scenario in Fig. 2 the acquisition of the header-related transfer functions in the anechoic chamber. This means that if the HRTFs obtained by measurement, the space procured in this way must be that there are no sound reflectors within the Space, d. H. that the room is completely sound absorbing must be lined.

Fig. 3 shows a schematic representation of transmission paths in a listening room 30, in which the loudspeakers 11, 12, 13, 14, 15 are arranged as in Fig. 2. additionally direct sound is an indirect path for everyone Speakers shown to the left ear 17. It was on it noted that the scenario in Fig. 3 is only partial reflects reality, because here reflections occur on all walls, floors and ceilings and further multiple reflections also exist. Specifically, the first speaker 11 further sound from the as it through a line 31c is shown on the front wall of the Room 30 is reflected and from there to the first reference point 17 arrives. The transfer function from the first Speaker 11 to the left ear 17, i. H. ÜF11 in Fig. 1, therefore models not only the direct sound propagation 31a from the speaker to the ear but also the sound propagation by means of reflection 31c from the first loudspeaker 11 to the first ear 17. Similarly, there is also an indirect one Away from the second speaker 12 through one Arrow 32c is indicated to the first ear 17. This means that the transfer function ÜF21 in Fig. 1 by the second speaker 12 to the first reference point 17 only the direct sound propagation 32a but also the sound propagation modeled by reflection to the first ear 17.

The following is based on the determination of the individual transfer functions ÜF11 to ÜF52 (Fig. 1) received. To there are different options.

The first possibility is to use one as in FIG. 3 shown, positioning of the speakers 11 to 15 to to choose the reference points 17 and 18. Then the first loudspeaker 11 excited by means of an excitation signal, whereupon arriving at the first reference point 17 Sound signal is measured, which is a superimposition of the Signals 31a, 31c when looking at Figure 3. Moreover the sound signal is measured at the second reference point 18, which is a superimposition of the signal 31b and one in Fig. 3 signal, not shown, could be that of the first Speaker 11 is output and on any wall like this it is reflected that it arrives at the second reference point 18.

The transfer function from the first speaker to the first reference point 17 (ÜF11 in Fig. 1 can from the excitation signal and that measured at the first reference point 17 Sound signal can be calculated. If the speaker 11 with stimulated with an ideal impulse, it results at the reference points directly the respective impulse response that the Transmission of the sound signal in the time domain describes. However, this is only due to practical limitations a theoretical method. In practice, the speaker 11, however, excited with a pseudo noise signal. This The procedure for the other speakers 12 to 15 repeated in such a way that all other transfer functions ÜF21 to ÜF52 from the measured sound signal on the respective reference point and the excitation signal at the respective Have the speaker determined.

Find how it was carried out in such measurements a real room with non-absorbent walls, etc. instead, the entire transfer function, the from the room impulse response and the head-related transfer functions (HRTFs) for the individual speaker positions exist, determined. Are such measurements in an anechoic room, d. H. a completely sound absorbing Space, the HRTFs can be determined directly, which are then the transfer functions ÜF11 to ÜF52 are.

Such sound measurements are independent of the fact whether measurement using two built-in microphones and one Artificial head or by means of two probe microphones and one Test person can be carried out, if only because of the very expensive probe microphones complex and expensive.

However, head-related transfer functions (HRTFs) are for certain people or for an "average person" known, so they can be used to with the Impulse response of a room, which can also be simulated to be folded. In this case, no measurements needed to transfer functions ÜF11 to ÜF52 determine. A major advantage of this procedure is that it can also be used to simulate rooms, that are not yet built to before the actual construction a recording studio, for example, the same for optimal Sound propagation for certain speaker configurations to design. In this case it can no longer be spoken of that the space in which the quality of a coded and re-decoded audio test signal should actually exist. Instead, the room is only present in the simulation and therefore a fictitious one Room.

Regardless of whether the room actually exists or just based on a simulation as a fictitious room , it is usually assumed that test persons in such a listening room, for example a standardized one Interception room can be at the optimal listening location sit or stand. However, many test subjects move during your head forward, back, left or right, which is also called translation. Furthermore people usually move slightly the optimal listening position, d. H. people turn her head to the left and right, also called DF movements or rotation. Thus, one will any center speaker, d. H. the speaker 13, no longer exactly in the middle. This occurs because the directional perception is often uncertain at the very front is. In particular, there is often confusion at the front and back. This is also called "front-back" in technology Confusion ". When referring to Figs. 2 and 3 it can be seen that with every movement of the Head of the first reference point 17 and the second reference point 18 change with respect to the fixed speaker positions.

To do justice to this situation, for several Positions of the reference points 17, 18 through the device for quality assessment, which is shown in Fig. 1, procedures for quality assessment carried out, whereupon different quality displays for the different Positions. Needless to say for any different positions of the reference points 17, 18 different transfer functions determined and used in carrying out the method according to the invention become. The result is then several quality advertisements for different positions of the reference points 17, 18, d. H. for different head positions.

For the evaluation of the different quality displays there are different options. On the one hand, a Average is taken to be a general statement about it to be able to make a certain coding / decoding process is perhaps optimal if the position of the Head is not changed at all, or that this at certain translations or DF movements or rotations of the head is no longer as cheap as another coding method.

On the other hand, the "worst case" of the individual measurements be found out to make a statement about it can determine whether a particular coding / decoding method at a specific position of the head with respect to the five speakers is suboptimal in the case of 5-channel audio signals. advantageously, are such quality assessments on the one hand for several positions of the reference points 17, 18 close the optimal reference listening position. on the other hand such measurements can also be made for other places are not at the reference listening position for example certain other seats in one Be able to judge recording studio to see if here Coding / decoding errors are more audible or not.

From the previous description it has become clear that the device according to the invention and the invention Process existing devices and processes for Quality assessment with a substantial amount of flexibility provided such that not only a quality assessment of audio signals with more than two channels but that a quality assessment for different Scenarios of positioning the reference points 17, 18 are played through with respect to the loudspeakers 11 to 15 can, and that the device according to the invention and the inventive Process even when designing sound studies or other listening rooms, such as. B. cinemas can be used can check the quality of certain coding / decoding methods to be able to judge in a certain room. Furthermore, the inventive method and the inventive Device for designing listening rooms to be the best for a particular room Coding methods from a variety of possible coding methods select.

The transfer functions ÜF11 - ÜF52 can be different Types can be implemented in terms of circuitry. An implementation via an FIR filter for each is preferred Impulse response. It should be noted that for large Spaces that FIR filters can take up a considerable length, which for example at a sampling frequency of 48 kHz can be over 100,000 samples long. Here offers itself, the first milliseconds of that length, in the rather discrete reflections occur to represent more accurately than the Time range rather at the end of filters, where more diffuse reflections occur.

Claims (21)

1. A system for evaluating the quality of an audio test signal derived from an audio reference signal by coding and decoding, said audio test signal and said audio reference signal each comprising a plurality of channels, each channel being adapted to be made audible by one loudspeaker (11 -15) of a plurality of loudspeakers which are positioned at different positions in an at least fictitious room (30), and two listening reference points (17, 18) being defined with respect to the positions of the plurality of loudspeakers. said system comprising:

   a unit (19) for converting the audio reference signal into a first audio reference sum signal at the first reference point (17) and into a second audio reference sum signal at the second reference point (18) and for converting the audio test signal into a first audio test sum signal at the first reference point (17) and into a second audio test sum signal at the second reference point (18), the audio reference sum signals and the audio test sum signals at the first and second reference points (17, 18) being a superposition of the respective channels, which can be emitted by said plurality of loudspeakers (11 - 15), weighted with a respective transfer function (ÜF11 - ÜF52) between the respective loudspeaker and the reference point in question; and

   a unit (20) for evaluating the quality of the audio test sum signals while taking into consideration the audio reference sum signals so as to provide an indication of the quality of the audio test signal.
2. A system according to claim 1, wherein the transfer functions (ÜF11 - ÜF52) between the respective loudspeakers (11 - 15) and the respective reference points (17, 18) are individual head-related transfer functions (HRTFs) so as to take into account the different impulse responses for different sound incidence directions into the human ear (17, 18).
3. A system according to claim 2, wherein the transfer functions (ÜF11 - ÜF52) between the respective loudspeakers (11-15) and the respective reference points (17, 18) are mean head-related transfer functions (HRTFs) obtained by averaging a large number of individuals.
4. A system according to one of the preceding claims, wherein the transfer function (ÜF11 - ÜF52) between the respective loudspeaker (11 -15} and the respective reference point (17, 18) is a transfer function which corresponds to the convolution of the head-related transfer function with a room impulse response in such a way that the sound reflections of the room in which the plurality of loudspeakers (11-15) and the two reference points (17, 18) are positioned are taken into account.
5. A system according to one of the preceding claims, wherein the transfer functions (ÜF11 - ÜF52) between the respective loudspeakers (11 -15) and the respective reference points (17, 18) are averaged transfer functions which are the result of averaging individual transfer functions between fixed loudspeaker positions and varying positions of the reference points (17, 18).
6. A system according to one of the claims 1 to 4, wherein said conversion unit (19) is arranged for providing transfer functions for various positions of said first and second reference points (17, 18) with respect to fixed loudspeaker positions, and wherein the quality-evaluating unit (20) is arranged for providing the indication of the quality of the audio test signal for various transfer functions and for providing the positions of the reference points (17, 18) for the indication of the poorest quality.
7. A system according to one of the preceding claims, wherein the room (30) is a standardized reference listening room and wherein the two reference points (17, 18) simulate the auditory organs of a test person at a reference listening position.
8. A system according to one of the claims 1 to 6, wherein the room (30) is a sound studio and wherein the two reference points simulate the auditory organs of a test person at an arbitrary seated/standing position in said room.
9. A system according to one of the claims 5 to 8, wherein the different positions of the first and second reference points (17, 18) deviate only slightly from a reference position so as to simulate a bearing movement of a test person.
10. A system according to one of the claims 5 to 8, wherein the different positions of the first and second reference points deviate markedly from the reference position so as to simulate a rotation of the head of a test listener.
11. A system according to one of the preceding claims, wherein the audio test signal comprises five channels, said five channels being a left rear, a right rear, a left front, a right front and a middle front channel.
12. A system according to one of the claims 1 to 10, wherein the audio test signal is a stereo signal.
13. A system according to one of the preceding claims, wherein the conversion unit (19 ) comprises:

    an FIR filter for each loudspeaker/reference-point combination, the filter coefficients of each FIR filter being determined by the transfer function of the transmission path from the respective loudspeaker to the respective reference point;

    a first adder (22) for the first reference point (17) for adding the output signals of the FIR filters (ÜF11 - ÜF51), which represent transmission paths to the first reference point (17), so as to provide the first audio test sum signal and the first audio reference sum signal, respectively; and

    a second adder (23) for the second reference point (18) for adding the output signals of the FIR filters (ÜF12 - ÜF52), which represent a transmission path to the second reference point (18), so as to provide the second audio test sum signal and the second audio reference sum signal, respectively.
14. A method for evaluating the quality of an audio test signal derived from an audio reference signal by coding and decoding, said audio test signal and said audio reference signal each comprising a plurality of channels, each channel being adapted to be made audible by one loudspeaker (11 - 15) of a plurality of loudspeakers which are positioned at different positions in an at least fictitious room (30), and two reference points (17, 18) being defined with respect to the positions of the plurality of loudspeakers, said method comprising the following steps:

    converting (19) the audio reference signal into a first audio reference sum signal at the first reference point (17) and into a second audio reference sum signal at the second reference point (18);

    converting the audio test signal into a first audio test sum signal at the first reference point (17) and into a second audio test sum signal at the second reference point (18);

    weighting the respective channels, which can be emitted by said plurality of loudspeakers (11 - 15), with a respective transfer function (ÜF11 - ÜF52) between the respective loudspeaker and the reference point in question;

    superimposing the weighted channels at said first and at said second reference point (17, 18) so as to obtain the audio reference sum signals and the audio test sum signals; and

    conducting the audio test sum signals and the audio reference sum signals to a unit (20) for evaluating the quality of the audio test sum signals while taking into consideration the audio reference sum signals so as to obtain an indication of the quality of the audio test signal.
15. A method according to claim 14,wherein the following step precedes the step of converting (19):

    obtaining the individual transfer functions (ÜF11 - ÜF52) between each loudspeaker (11 - 15) and each reference point (17, 18).
16. A method according to claim 15, wherein the step of obtaining comprises the following sub-steps:

    exciting a loudspeaker (11-15) with an excitation signal;

    measuring the signal at each reference point (17, 18);

    determining the transfer function between the excited loudspeaker and the first reference point (17);

    determining the transfer function between the excited loudspeaker and the second reference point (18); and

    repeating the steps of exciting, measuring and determining until all the loudspeakers (11-15) have been excited so as to obtain the individual transfer functions.
17. A method according to claim 16, wherein the first and second reference points (17, 18) are the ears of a human listener.
18. A method according to claim 16, wherein the first and second reference points are built-in microphones of an artificial head.
19. A method according to one of the claims 16 to 18, wherein the excitation signal is pseudo-noise signal.
20. A method according to claim 15, wherein the step of obtaining comprises the following sub-steps:

    accessing a head-related transfer function (HRTF) for a determined positioning of a loudspeaker (11 - 15) relative to the first reference point (17);

    determining the room impulse response for the position of the loudspeaker in the room;

    convoluting the head-related transfer function (HRTF) with said room impulse response so as to obtain the transfer function from said loudspeaker to the first reference point (17);

    repeating the steps of accessing, determining and convoluting so as to obtain the transfer function (ÜF11 - ÜF52) from said loudspeaker to the second reference point; and

    executing the steps of accessing, determining, folding and repeating for each additional loudspeaker so as to obtain all the individual transfer functions.
21. A method according to claim 19, wherein the room impulse response is determined by simulating the room.

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