The present invention relates to apparatus for processing binaural signals. Historically, the term stereophonic was coined in the 1950s to apply to sound reproduction over two or more transmission channels. In the 1960s, there was a resurgence of interest in recording using dummy-head microphone techniques, and the expression "binaural" was coined exclusively for recordings made by such means and for electronic equivalents wherein the acoustic processing effects of the human head and external ear are synthesized. In the present specification the term binaural is intended to cover both dummy-head recordings and synthesized recordings.

The first demonstration of a stereophonic effect is believed to have taken place in Paris in the 1890s, when multiple microphones situated in an array across the front of a stage were each connected to individual earpieces in an adjacent room, and listeners found that the use of adjacent pairs of earpieces (and hence microphones) provided very realistic sound reproduction with spatial properties. The first explicit report of a dummy-head type of sound reproduction method appears in US- A- 1,855, 149, dated 1927, in which the purpose was to record sounds in such a way that the natural, head-related time-of-arrival and amplitude differences between L and R signals were convolved acoustically on to the sounds, and then replay was achieved using either earphone reproducers or equi-distant loudspeakers, placed directly to the left and right of the listener, such that the virtual sound origins were secured. GB-A-394325 filed in 1931 by Blumlein, relates to conventional, present-day stereo in which the use of two or more microphones and appropriate elements in the transmission circuit were used to provide directional-dependent loudness of the loudspeakers, together with means to cut discs and thus record the signals. Stereo sound recording and reproduction was not commercially exploited until the 1950s. At the present time, the commonest forms of stereo are the following.

(i) Amplitude-based stereo: where a number of individual, monophonic recordings are placed in the sound-stage between the audition loudspeakers by pan-potting alone (to create L-R loudness differences).

(ϋ) Enhanced version of (i): where artificial reverberation and other effects are added to enhance the spatial aspects (room acoustics, and distance).

(iii) Live recordings: where stereo microphone pairs are used, so as to be either (a) coincident, or (b) spaced-apart (by about one head- idth, or thereabouts).

Only the latter (ϋi(b)) goes part-way to the reproduction of a natural acoustic image of a performance, but there have been several intermittent periods since the 1950s when the use of the dummy-head recording method for producing binaural signals has been experimented with for improving the quality of the stereo image.

Dummy-head (binaural) recording systems comprise an artificial, lifesize head and sometimes torso, in which a pair of high-quality microphones are mounted in the ear canal positions. The external ear parts are reproduced according to mean human dimensions, and manufactured from silicon rubber or similar material, such that the sounds which the microphones record have been convolved acoustically by the dummy head and ears so as to possess all of the natural sound localization cues used by the brain.

It has long been recognized that binaural recordings possess remarkable properties when listened to via headphones: sounds are localized outside the head, rather than inside it, and in three dimensions - even above and behind die listener's head. However, it has long been recognized ti at the tonal qualities of binaural recordings are not true-to-life, and this is especially noticeable when listening to music, where a wide bandwidth is present. This is caused by the sounds passing - in effect - serially through two pairs of ears: first those of dummy head, and secondly, those of the listener. Generally speaking, there is a resonance associated with the main cavity in the external ear (the concha) which occurs at a frequency of several kHz and boosts the mid-range gain of the system, hence the consequence of two passages through the ears is that the sounds appear to lack both low- frequency and high-frequency content, and thus are perceived as thin and "shelly".

In order to compensate for the "twice-through-the-ears" effect, it is known to use audio filters to shape the spectral response. Essentially, what is required is a filter which is the complement of the air-to-ear transfer function, which can be one of many, as follows.

1. Headphone-to-ear: the functions differ from one headphone manufacturer and type to another.

2. Loudspeaker-to-ear: the functions are dependent both on the angle of incidence and distance from the loudspeakers. 3. Free-field vs diffuse-field conditions: the transfer functions can be measured under both free-field (anechoic) and diffuse-field (echoic) conditions: this applies to 2, above.

4. Compromise: some have attempted to provide a single equalization means suitable for both headphone and loudspeaker auditioning. All of the above comments on equalization relate to the tonal quality of the perceived sounds, but there is a second important correction factor which is applicable to the loudspeaker reproduction of binaural signals, namely transaural cross-talk cancellation.

In binaural reproduction, it is desirable that the recorded information is transferred efficiently to the listener without the detrimental effects usually associated with such a process, including die acoustic crosstalk which is present between the ears ("transaural" crosstalk). Efficient transfer of the sound signals is especially important for binaural recordings in order to maintain the effectiveness of the various aural localization "cues" - the various attributes which the brain uses to estimate the spatial position of individual sound sources, such as the time-of-arrival differences between left- and right-ear signals, and also spectral information. In order to ensure that the right ear of the listener hears only signals from the right loudspeaker alone, it is necessary to cancel out, at the right ear, those signals which arrive at the right ear from the left loudspeaker. A similar requirement must be met for the left ear such that no sounds are heard from the right loudspeaker. One of the first to recognize the need for crosstalk cancellation was B. B. Bauer, J. Audio Eng. Soc 9, (2), 1961, pp 148 - 151, who described an analogue circuit for this purpose.

US-A-3,236,949 describes transaural crosstalk cancellation by the inclusion of a pair of crossfeed filters, each coupling one of the binaural pair of signals to the other. There exist, however, problems with such crosstalk compensation. If the loudspeakers are substituted by a pair of headphones, then crosstalk (i.e. the left ear hearing sound from the right loudspeaker, and vice-versa) does not occur to such a significant extent because the loudspeakers are cupped over the ears, yet compensation is still being applied. This gives the effect of the headphone sound image being foreshortened somewhat so that it does not

appear as "deep" as it might be otherwise. Furthermore in the case if discrete loudspeakers (as opposed to headphones) movement of the listener's head within the sound field may cause a distortion of the binaural effect and in some cases the effect may even be lost. US-A-5, 136,651 discloses transaural crosstalk cancellation by means of low pass filters with a cut-off of 10 kHz or minimum phase filters in compensation channels between the left and right reproduction channels. The stated object of such construction is to make the cancellation effect independent of the position of the listener's head.

Nevertheless, further improvements are desirable in making the cancellation effect independent of the precise position or orientation of the listener's head. In a general aspect, the invention provides apparatus for processing binaural signals for subsequent reproduction, comprising transducer means for deriving a pair of binaural signals, a left channel for receiving a left binaural signal and a right channel for receiving a right binaural signal, each channel including a branch node, a summing junction and channel filter means, and left and right cross channels each connected between a respective left and right branch node and a respective right and left summing junction, each cross channel including a cross channel filter, with outputs of the left and right channels being coupled to reproducing or recording means, characterized in that the signal attenuations introduced by die left and right channels relative to the signal attenuations introduced by d e crossfeed channels are such that in the recorded signal, significant residual crosstalk signals remain so that when the recorded signal is reproduced, there is, in an optimum region for a listener's head, a significant amount of crosstalk signal remaining such that movement and rotation of the listener's head is permitted within the region without significantly changing the binaural effect experienced by the listener.

In a further aspect the invention provides apparatus for processing binaural signals, the apparatus comprising transducer means for deriving a pair of binaural signals, a left channel for receiving a left binaural signal and a right channel for receiving a right binaural signal, each channel having a branch node, a summing junction, channel filter means and an output coupled to recording or reproducing means, and left and right cross channels each connected between a respective left and right branch node and a respective right and left summing junction, characterized in that the signal attenuations introduced by the left and right channels relative to the signal attenuations introduced by the crossfeed channels are such that crosstalk signals exist at the ear of the listener in an optimum

listening position, the magnitude of the crosstalk signal being a function of GA(l-x), where G is the transfer function of said channel filter means, A is the acoustic transmission function from a transducer to the far ear of the listener, and x is a factor determined by the relative channel attenuations, wherein x<0.95. Thus the effect of adjusting the attenuation values of the channels is to change the signals experienced at a listener's head from ideal values, in which perfect crosstalk cancellation is achieved at the ear of a listener, to a value in which only partial crosstalk cancellation is achieved. It has however, been found from careful observation that where the remaining crosstalk signal is represented as GA(1 - x), where 0.5 < x < 0.95, the imperfect crosstalk cancellation is not significant in that it is not significantly noticeable for the average listener, whereas the space in which maximum crosstalk cancellation occurs and thus acceptable reproduction occurs is significantly enlarged.

As will be shown below, with a value of x in the region of 0.95 and with die listener about 7 feet from loudspeakers, head movement of me order of inches is permitted, in any direction, but principally in a direction lateral to which that in which the listener is facing. This region also permits substantial rotation of the head, which is sufficient for normal movement of the head while listening. In addition, because there is only a relatively small amount of crosstalk signal present, the apparent direction of sound at die listener's ears can be made to appear to be at normal incidence to the ears, and thus a true three-dimensional sound effect can be produced. By the term "normal incidence to d e ears", it is meant that the sound direction appears to originate in the horizontal plane, and on d e right hand side at an azimuth angle of 90° (where 0° azimuth corresponds to the direction directly ahead of die listener, and 180° directiy behind).

With a value of x in the region of 0.5, a very substantial movement is permitted, of the order of 1 foot, for example, permitting the listener to change seat position and to move and rotate the head relatively freely without changing the quality of the perceived sound. However as the value of x decreases then d e three-dimensional sound effect achieved by die binaural signals is degraded so that, for example, sound which is intended to appear to be coming from a direction normal to the listener's head, in fact appears to be at an azimuth angle of about 50° At this point, the expert listener would appreciate that the reproduction quality of the sound has been degraded to a point at which the binaural effect is spoilt. Naturally, these quantitative restrictions are somewhat subjective, since

one listener may be more tolerant than another listener, but we have found in practice that valuable effects are achieved witii an x factor between 0.5 and 0.95.

As preferred, x is not dependent on frequency for the audible frequency range; nevertheless some dependence on the frequency may be tolerated provided tiiat x stays within the range mentioned above.

In an analogue implementation, the simplest and most effective method of achieving the desired crosstalk cancellation factor is to insert a potential divider of such value as to give attenuation x in the crossfeed paths between left and right channels, or alternatively, to insert a potential divider into a signal path of the crossfeed filters. In a digital implementation, the attenuation can be introduced as a scaling factor in a signal path witiiin the filter.

The crosstalk cancellation and other correction means are preferably located in circuit between the transducers for producing binaural signals and die means for recording such signals. Otiier arrangements are possible; for example the cancellation could be provided in die sound reproduction system subsequent to recording. In other arrangements the binaural signals once corrected are not recorded, but transmitted direcdy for reproduction, for example to an adjacent room or over a radio link.

The summing junction in the left and right channels may be of any convenient form, for example, a simple wire connection, or an operational amplifier wherein the inputs are applied to selected non-inverting and inverting inputs. Where it is desired to subtract the values of two signals, one signal may either be applied to d e summing junction as a negative quantity, or alternatively the signal may be applied as a positive quantity to an inverting input of an amplifier. In a digital implementation, d e summing nodes may be incorporated in the digital representations of the crossfeed and channel filters. Brief Description of the Drawings

A preferred embodiment of the invention will now be described with reference to the accompanying drawings wherein:-

Figure 1 illustrates a known arrangement for cancelling crosstalk in a binaural reproduction system; Figure 2 is a graph illustrating the frequency dependence of die acoustic transmission functions of Figure 1 ;

Figure 3 is a schematic view of a preferred embodiment of the invention;

Figure 4 are schematic views of a crossfeed filter and a main channel filter for the embodiment of Figure 3;

Figure 5 is a graph of crosstalk cancellation as a function of A/S ratios for various values of the variable x; Figure 6 and 7 are graphs showing how "sweet-spot" size and apparent placement angle of a 90° azimuth reproduced sound vary with different crosstalk cancellation factors. Description of the Prior Art

Referring now to Figure 1, this shows the system described in US-A-3,236,949 and comprises a left transmission channel 2L and a right transmission channel 2R. Each channel has a respective input 4L, R for receiving binaural signals derived from dummy head microphones 5L, R. Each channel has sequentially in its path, a branch node 6L, R, a summing junction 8L, R, a correction filter 10L, R, a gain adjustment filter 12L, R and a recording means 13L, R. The recorded signals are subsequentiy reproduced by reproducing means 14L, R and applied to loudspeaker transducers 15L, R. These loudspeakers provide sound to d e head of a listener 16 via direct signal paths from the transducer to the adjacent ear of a listener 18L, R, such transmission paths having a transmission function S, and via indirect signal transmission path 20L, R from a loudspeaker to the far ear of a listener and having a transmission function A.

In addition, crossfeed channels 22L, R are provided extending between a branch node 6 and a summing junction 8 in the other transmission channel 2. Each crossfeed channel includes a crossfeed filter 24L, 24R.

The listener 16 as shown faces loudspeakers 15 in a direction represented as 0° azimuth. The direction opposite to this behind his head is 180° azimuth, and d e directions at normal incidence to die ears are 90° azimuth (with positive values representing the Right Hand Side of die listener, and negative values the Left Hand Side). Loudspeakers for stereo listening are placed so as to subtend angles of 30° with respect to the vertex of the triangle they form with the listener 16 (at the apex), and hence A and S can be established by direct measurement, ideally from a dummy head having physical features and dimensions representative of the mean human counterparts. The amplitude components of typical transmission functions A and S, measured in such a way, are shown in Figure 2, using a logarithmic frequency scale. It will be seen there is a pronounced maximum

in both functions at about 5 kHz; this corresponds to the resonance of the major cavity in the external ear (the concha).

Considering Figure 1 again, it will be appreciated that, if there were no crosstalk across the head at all, a transmission function of 1 from the right source to the right ear, and 0 from the right source to the left ear, can be achieved by simply adding a serial (1/S) function (the inverse of the "same-side" transmission function) between source and loudspeaker. The presence of crosstalk, however, requires a cancellation signal to be provided - by the other loudspeaker - but this cancellation signal also crosstalks to the first ear, and this too, must be cancelled (and so on, ad infinitum). The crosstalk cancellation can be achieved by feeding die R input 4R via crossfeed filter 24R having a transmission function C, which is made equal to -(A/S), and adding it to die left channel 2L at summing junction 8L; die subsequent, serial correction filters 10 having a function 1/(1 -C ² ) deal witii the multiple cancellation problem. By inspection, it can be seen that this scheme provides a theoretically ideal solution. The overall transmission function from the right input (R) to the right ear (r), R^f) is:

and die overall transmission function from the same (R) input to the left ear (l),R,(f) is:

* ( / )= μ_ i cp-Viy-o (2)

At low frequencies, A and S are almost identical (both in amplitude and phase). There are two important points about this prior art configuration: firsdy, that it can perform very well; but secondly, tiiat it is very listener-position dependent (if the listener turns his head by more men 10° from the frontal direction, the realistic illusion disappears, frequently changing into an "inside-ti e-head" sensation).

However, it has been found that the headphone image is somewhat degraded by me effects of the cancelling crosstalk signals when the crosstalk is not actually present in the same degree when using headphones. The effect is to foreshorten the sound image somewhat, so that it is not as deep as it might appear to be otherwise. The image does, however, retain an "out-of-the-head" effect, and in tiiis respect is a considerable improvement on conventional stereo. The Preferred Embodiment

It is die purpose of the present invention to provide a binaural reproduction system which (a) is more tolerant of listener position, and (b) provides a better headphone image, than the prior art. This has been achieved following our observation that it is not essential to cancel all of the transaural crosstalk in order to achieve the requisite three-dimensional effects via loudspeaker auditioning. If the crosstalk is only cancelled partially, dien there are two benefits: firsdy, the slight position-dependent artifacts of the prior art schemes are eliminated, and secondly, because there is less cancellation taking place, then the headphone image is more representative of a full binaural recording, witii consequent enhanced image depth. The partial cancellation scheme is in the preferred embodiment applied over the whole bandwidtii, and d e degree of partial cancellation has an optimal range. This will be described below by reference to Figure 3, wherein parts similar to Figure 1 are denoted by the same reference numerals. In Figure 3, crossfeed filters 30L, R have functions xC, i.e. an attenuation factor x has been introduced into the filters as compared with that of Figure 1. Delay elements (not shown) may be inserted in the crossfeed channel paths between junctions 6 and summing junctions 8 in order that the phase relationships between the signals in the main channels and die crossfeed channels are preserved such that when the sound is reproduced die cancellation signal arrives simultaneously with the primary signal. In digital implementations, however, it is possible to incorporate die time delays into the filter blocks themselves, in which case extrinsic delay elements become superfluous. In addition, a single filter 34L, R is introduced into die main channel path, which encompasses the functions of filters 10, 12 of Figure 1 but has a new filter function, G. It is possible to define G explicitiy in terms of x, A and S, in order to provide die requisite goal of precise, partial crosstalk cancellation, whilst dealing with die multiple

cancellation problem correctly, as before, such that there is unity gain between the right source and die right ear (i.e. R _r (f) = 1).

The overall transmission function from the right input (R), to the right ear (r), R,(f) is:

and this function must be equal to 1 for unity gain, as described above, hence:

GS + G H-i> = ι and therefore G (4)

A consequence of this is that G becomes a function of A, S and x. Thus, a relatively high degree of crosstalk cancellation occurs when A<S, and die crosstalk cancellation reduces when A approaches S (e.g. at very low frequencies). This creates an intrinsically stable system: another important advantage of the invention. Prior art systems, in which full crosstalk cancellation is implemented at low frequencies, are impractical because the A and S functions converge at low frequencies.

The overall transmission function from the right input (R), to the left ear (I), R,(f) is:

Rff) = GA+J--)GS = GA (1 - X) (5)

substituting from (4) this gives:

The crosstalk factor, R,(f), as a function of (A S) (which always lies between 0 and 1), using several different values of the crossfeed gain factor, x. is shown in Figure 5.

As shown in Figure 2, the difference between A and S is, for the most part, greater than 10 dB above 2 kHz, and 5 dB above 700 Hz. Consequently, even a modest crossfeed cancellation factor (x) of 0.8 yields corresponding crosstalk suppression values of -17 dB and -12 dB respectively. (For x = 0.9, these figures are improved further, to -20 dB and - 15 dB, respectively). In practice, we have found tiiat excellent results are achieved using a value of x = 0.9.

It will be noted from equation (6) that even where S and A are approximately equal, there is nevertheless a stable crosstalk factor, since (S ² - xA ² ) will be finite.

Referring now to Figure 4 diis shows schematic views of the filters 30 and 34 of Figure 3. Crossfeed filter 30 shown in Figure 4a comprises an input signal path 40 witii a series of one sample time delays Z ^"1 42, witii tapping paths 44 coupled between nodes between the delay elements and a summing junction 46. Each tapping path has a multiplier 48 where an appropriate scaling factor C _n is applied to die signal in the patii. The output of the summing junction 46 has an attenuation element 50 tiierein of value x. It will be seen that such filter is a finite impulse response filter. The attenuation factor introduced by die element 50 may be introduced into the input path 40, or alternatively, it may be introduced by modification of the scaling factors C _n .

Referring to Figure 4b showing a schematic view of filter 34, similar parts to those of Figure 4a are represented by die same reference numeral. In this filter, scaling factors D _n are applied to multipliers 48, and tiiese scaling factors are derived from equation (4).

Thus the effect of the various scaling factors D _n is to produce the filter function shown in equation (4).

Referring now to Figures 6 and 7, diese show two graphs which illustrate the effect of partial crosstalk cancellation on a listener. The effect is necessarily to an extent subjective, but the graphs have been derived experimentally from the listening experiences of experts in the art. The experts in the art listen to speakers arranged as indicated in Figure 3 at the 30° angle of azimuth and at a distance of about 7.5 feet (2.3 metres). The experimental results actually recorded are indicated as filled rectangles in the graphs.

Figure 6 is a graph showing the degree of crosstalk cancellation along the ordinate and apparent "placement angle" or azimuthal angle Θ along the abscissa. In a perfect binaural system, perceived sound is truly three dimensional and can be made to appear to arrive at the ears from directions outside die angle of die loudspeakers. It is possible to

make sound appear to arrive from a direction normal to the listener's ear (at 90° azimuth), and Figure 6 shows the effect for degrees of crosstalk cancellation on perceived sound which is intended to arrive normal to the listener's ear. For one hundred per cent cancellation, the azimuthal angle of arrival is at 90° to the listener's ear, as intended. The graph slowly and continuously curves down to a 30° value (the angle of the loudspeakers) with zero cancellation.

It will be noted tiiat the placement angle decreases relatively quickly to a value of 50° at x = 0.5, whereas the angle decreases slowly between x = 0.5 and x = 0. Thus, x = 0.5 represents an approximate lower limit below which, for listeners who are expert in the art, the binaural effect collapses or becomes so degraded that the true binaural effect is not apparent, this corresponding to sound which is intended to arrive normal to the listener's ear arriving at an angle of 50° to the direction in which the listener is facing. Clearly, such figures are to an extent subjective, but die graph shown represents an averaged mean for a set of experts in the art. Figure 7 is a similar graph wherein abscissa represents the "sweet-spot" size, namely the region in which the listener may position his head and experience the optimum binaural effect. For 100% crosstalk cancellation, the system corresponds to die prior art system of Figure 1 wherein there is only one particular position in which the listener can position his head, and if he moves from that position, then die binaural effect is degraded. It will be noted tiiat with only a very small amount of residual crosstalk, the size of the sweet-spot rapidly increases to x = 0.9. It has been found diat with crosstalk cancellation of 95%, tiien a reasonable size of sweet-spot, some inches either side of a listener's head is generated. This sweet-spot size actually exists in three dimensions so tiiat a listener may move his head backwards, forwards or vertically, rotate the head, as long as he remains within the sweet-spot region either side of die mean position. As shown between the values x = 0.2 and x = 0.8, the sweet-spot size increases slowly, just below 10 inches, whereas below x = 0.2, the size increases again rapidly. It has been found by averaging die results for a set of experts in the art, that with cancellation of 0.95, a sufficiently large sweet-spot is provided which will accommodate normal movement of a listener's head while listening to a performance. As shown, the size of the sweet-spot increases continuously with decreasing cancellation such that at 50%, the sweet-spot size is of die

order of 10 inches (25 cm) so that a listener may, for example, move chair position while still preserving the optimum binaural effect.

Thus combining the results of the observations represented by Figure 6 and 7 it may be seen that good results are obtained witii 0.5 < x < 0.95. It is important to note that in certain circumstances, it is desirable to separate out the two principal elements of the transaural crosstalk cancellation schemes which have been described. These two elements are (a) the crosstalk cancellation itself, and (b) spectral equalization, to compensate for the "twice-through-the-ears" effect. The schemes deal witii both of tiiese factors simultaneously by virtue of the incorporation of the (second) air-to-ear transmission factor, S, as part of the whole system, such that the equations for providing ideal transfer characteristics form the source to the listener, when solved, generate filter networks which implement crosstalk cancellation and spectral equalization simultaneously.

It is sometimes convenient, however, to provide a system which only provides crosstalk cancellation, without spectral equalization.

This alternative embodiment of die present invention (i.e. crosstalk cancellation without spectral equalization) can readily be achieved, simply by solving the appropriate equation for an overall transfer function into the primary (same-side) ear to be equal to S (radier than 1). The overall transmission function from die right input (R) to the right ear (r), R-if)

R _r (f) =GS+x(--)GA (3)

must be set equal to S, as described above, hence:

GS+GAx (7)

It will be noted tiiat this is the product of previous equation (4) and S, and that the implementation involves simply substituting the solution of (7) for G in Figure 3.