Surround signal processing apparatus

Abstract: A surround signal processing apparatus is proposed whereby, in the reproduction of surround sound based on a monaural rear surround signal input, where a sound image localization process is used, good surround sound field control is achieved by minimizing the correlation between the left and right rear sound signals. The result is natural surround sound in which the listener perceives a sense of expansiveness. In addition to performing the signal processing required to render mutually non-correlative, a left-right pair of rear surround signals based on a monaural rear surround sound signal input, the apparatus also performs the signal processing required to localize sound images to locations behind and substantially laterally symmetrical with respect to a listener, based on input of a left-right pair of rear surround signals that have been signal processed.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to surround signal processing apparatus for applications such as Dolby surround sound or high- definition television (HDTV) sound reproduction. 2. Description of Prior Art Recent years have seen widespread use of multi-channel stereophonic sound in audio/visual systems. The trend in the technology has been away from conventional stereo sound reproduction systems, and toward "surround sound" techniques where the sound field is dynamically (and intentionally) shifted to the sides of and behind the listener, in concert with the video scene. Known in the art as sound field control methods, are reproduction methods such as the "Dolby surround" and HDTV "3-1" techniques, in which, through sound image localization, sound sources that would normally require rear speakers can be reproduced by conventional stereo sound systems having only two front speakers, with results equivalent to those of multi- channel stereo systems. The Dolby surround active matrix technique employed in the sound field control systems described in U.S. Patent No. 3,746,792, for example, is one such known system. In the above conventional sound field control systems, however, the rear sound was single-channel (monaural) sound only. Consequently, the system lacked the ability to adequately represent the sound field to the rear of the listener, or to clearly represent movement of the sound image. Accordingly, a weakness of these systems was that sound reproduced by only two loudspeakers positioned to the left and right in front of the listener failed to evoke the desired feeling of expansiveness. In multi-channel audio systems having surround signal processing systems such as the "3-1" (three front - one rear channel) technique used in HDTV and Dolby surround sound reproduction systems, in particular, since the rear surround sound was monaural, the surround sound reproduction equipment simply split this roar channel into two identical rear (SR and SL) channels. Consequently, w hen these signals were reproduced by the speakers, for a listener in the center of the sound system, the (virtual) location of the sound source tended to be localized inside the listener's head, thus defeating the surround effect of the original signal. Even in standard five-speaker sound reproduction, when the localized sound image locations are laterally symmetrical with respect to the two rear speakers, the localized location often ends up inside the head. Accordingly, to prevent this inside-the-head localization, the signals for the monaural rear sound image being localized need to be made different in some way. In Japanese patent "kokai" (laid open) document No. H5 (1993) - 207597, for example, measures such as adding reflected sound by inverting the phase on one side, or changing delay times were used. However, adding different amounts of delay to the SL and SR signals of a two-channels system had the disadvantage that it skewed the sound image to one side or the other. Also, while inverting the phase between the left and right sides improved the left-right separation it also evoked a strong perception of the phase inversion (a disagreeable and unnatural characteristic in the sound). It is known in the art to perform signal processing of a monaural signal to render the monaural signal pseudo-stereophonic using comb filter to form surround sound. Such surround signal processing apparatus is taught in Patent Abstracts Japan Vol., 95, No. 1, Feb. 28, 1995 and JP 06289858 A (YAMAHA), 10 Oct. 1994; and in the article "A New Sound Image Broadening Control System Using a Correlation Coefficient Method in Electronics and Communications in Japan", Vol. 67, No. 6, June 1985, Silver Spring, Maryland, USA, pages 33-41. BRIEF SUMMARY OF THE INVENTION 1. Objects of the Invention It is an object of this invention to obtain surround signal processing apparatus in which, when the rear sound signal is a single monaural signal, the correlation between the left and right rear signals is reduced, to thereby achieve improved surround sound field control that enhances the naturalness of the sound and creates a feeling of expansiveness. It is a further object of this invention to obtain surround signal processing apparatus in which, when the rear sound signal is a signal monaural signal, if a virtual sound image reproduced by the image localization process is localized to laterally-symmetrical locations behind the listener (a situation conducive to inside-the-head localization), inside-the-head localization is avoided by enhancing an acoustic effect occurring in pseudo-stereo processing, and a surround space with naturalness and a heightened sense of expansiveness can thereby be created. 2. Brief Summary According to the invention, a surround signal processing apparatus is provided for reproducing surround sound, from a pair of loudspeakers placed in front of and substantially laterally symmetrical with respect to a listener, based on the input of a rear monaural surround signal input, with said apparatus comprising: a signal processing means including a comb filter for producing a left-right pair of rear surround signals that are based on said rear monaural sunound signal input, characterized in that said apparatus further comprises: a sound image localization circuit for performing signal processing required to localize a sound image at locations behind and substantially laterally symmetrical with respect to a listener, based on an input of said left-right pair of rear surround signals that have been signal processed by said signal processing means; wherein said sound image localization circuit comprises; a first adder for adding sum signals and difference signals as output pairs of surround signals from said comb filter; a first subtractor for subtracting said sum signals and difference signals; a first filter P for receiving an output from said adder and performing thereon a convolution process; a second filter N for receiving an output from said subtractor and performing thereon a convolution process; a second subtractor for subtracting the outputs of said first and second filters; and a second adder for summing of the outputs of said first and second filters. Preferred embodiments of the invention are disclosed in the dependent claims. The above and other related objects and features of the invention will be apparent from a reading of the following description of the disclosure found in the accompanying drawings, and the novelty thereof pointed out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of an example of a surround signal processing apparatus. Fig. 2 is a graph of the filter characteristics of the comb filters shown in Fig. 1. Fig. 3 is a block diagram of an embodiment of the surround signal processing apparatus of the present invention. DETAILED DESCRIPTION OF THE INVENTION Fig. 1 is a block diagram of an example of a surround signal processing apparatus. In Fig. 1, item 1 is a comb filter that, in a multi-channel sound reproduction system that employs monaural rear surround sound reproduction, functions as a signal processing means that adds a delay to a monaural rear surround signal S (hereinafter referred to simply as a "surround signal") which is supplied to it after its level has been adjusted by a master level controller (ndt shown), and produces therefrom, signals that are the sum and difference signals of the "base" (pre-delay) signal and the delayed signal, which it outputs as two- channel rear surround signals. The comb filter1 has a delay circuit 1a that adds a delay (in a range, for example, of 0 - 20 ms) to the input surround signal S, and outputs it as delay signal T; a summing circuit 1b that obtains a sum signal (S + T) by adding delay signal T to base surround signal S, and outputs the result as the L channel rear surround signal SL; and a subtraction circuit 1c that obtains a difference signal (S - T), by subtracting delay signal T from base surround signal S, and outputs the result as the R channel rear surround signal SR. Fig. 2 is a graph of output signal amplitude characteristics, that explains the effect obtained in comb filter 1, which delays surround signal S and produces sum and difference signals from the delayed and base signals, and outputs the resulting signals as 2-channel rear surround signals, as described above. As shown in Fig. 2, by passing the base rear surround signal S through comb filter 1, left-right separation in the frequency spectrum is effected, thus producing 2-channel rear surround signals SL and SR which have low left-right correlation, and good surround effect is thereby achieved. Referring again to Fig. 1, item 2 is a sound image localization circuit for performing a process to localize the sound image for each side to specific locations to the side and rear of the listener. It does this by performing additional summing of the filter processing results of each of the Z-channel rear surround signals SL and SR from comb filter 1. Items 3L and 3R in the drawing represent loudspeakers placed to the left and right in front of the listener. Provided in sound image localization circuit 2, as previously proposed by this inventor in Japanese patent application H5(1993)-208871, are, in each of two separate rear surround signal channels SL and SR, an L-channel sound image localization circuit having left- and right-side sound image localization filters 21a and 21b, each of which have one of a pair of convolvers defined to have transfer functions Hl and Hr based on human head-related transfer functions for locations to the rear of, and substantially laterally- symmetrical with respect to, the listener, a similar R- channel sound image localization circuit having right-and left-side sound localization filters 21c and 21d, an adder 21e that sums the outputs of left- side filters 21a and 21d, and an adder 21f that sums the outputs of right- side filters 21b and 21a. Also, the signals, having been output from sound image localization circuit 2 and localized to specific rear locations, are reproduced as sound by the pair of front speakers 3L and 3R. With the front left and right sound signals also being summed in adders 21e and 21f, the sound produced by this pair of speakers 3L and 3R, is, in fact, surround sound. Here, as in mentioned Japanese patent application H5(1993)-208871, Hl, the transfer function of left- side sound image localization filters 21a and 21d, and Hr, that of right-side sound image localization filters 21b and 21c, are as follows: Hl = (SF - AK)/(S 2 - A 2 ) and Hr = (SK - AF)/(S 2 - A 2 ) , where S is the transfer characteristic from one speaker of speaker pair 3L/3R to the listener's ear on the same side as the speaker; A is the transfer characteristic from one speaker of speaker pair 3L/3R to the listener's ear on the side opposite the speaker; F is the transfer characteristic from a location (on either side) to which the surround signal is to be localized, to the listener's ear on the same side; and K is the transfer characteristic a the location (on either side) to which the surround signal is to be localized, to the listener's ear on the opposite side. To obtain the above S, A, F, and K transfer characteristics, actual measurements are performed: Loudspeakers are placed at specific locations in an anechoic space, measurement data is taken from microphones positioned at both ears of a human (or dummy) head, and the measured data is then subjected to appropriate waveform processing. Also, in the above and following equations, the "+" sign indicates addition of transfer characteristics, "-" indicates subtraction of transfer characteristics, and "/" indicates inverse convolution. In addition, the term "same side" denotes, for example, the right ear for the right-hand speaker; while the term "opposite side" denotes, for example, the left ear for the right-hand speaker. Accordingly, with the above described system configuration, separation in the frequency spectrum of the left and right components of base surround signal S is accomplished by feeding base surround signal S through comb filter 1, thereby obtaining the 2-channel (left and right) rear surround signals SL and SR having low correlation between channels. In addition, in sound image localization circuit 2, the signals are subjected to further filter processing and the results summed, to localize each signal image to the rear. In addition, the system is arranged such that these sound-image-localized signals are reproduced as sound by a single pair of front speakers 3L and 3R. By so doing, the rear sound can be localized into left-rear and right-rear virtual sound images, and the correlation between the two can be made to range from no correlation to weak correlation, thus obtaining extremely good surround sound, and in particular, a surround space can be created, that is natural and evokes a heightened sense of expansiveness. Fig. 3 is a block diagram of an embodiment of the surround signal processing apparatus of the present invention. In this embodiment shown in Fig.3, sound image localization circuit 2 of the first embodiment, shown in Fig. 1, is simplified through the use of shuffler filters(see Duane H. Cooper and Jerald L. Bauck, "Prospects for Transaural Recording", J. Audio Eng. Soc., Vol. 37, No. 1/2, 1989 January/February, pp.3-9). That is, in the embodiment shown in Fig. 3, sound image localization circuit 24 comprises: adder 24a, which adds sum signal (S + T), the sum of the 2-channel rear surround signals SL and SR (sum signals obtained by adding versions of surround signal S that have been delayed by different amounts) received from comb filter 1, to the corresponding difference signal (S - T); subtractor 24b, which takes the difference between sum signal (S + T) and difference signal (S - T); first filter 24c, which has a transfer characteristic P (to be discussed later) and which receives the output of adder 24a as its input, on which it performs convolution, etc.; second filter 24d, which has a transfer characteristic N (to be discussed later) and receives the output of subtractor 24b as its input, on which it performs convolution, etc.; subtractor 24e, which outputs the difference of the outputs of first and second filters 24c and 24d; and adder 24f, which adds the outputs of first and second filters 24c and 24d. Finally, signals that have been sound-image-localized by this sound localization circuit 24 to specific substantially laterally symmetrical locations behind a listener, are reproduced by the pair of front speakers 3L and 3R. Here, P, and N, the transfer characteristics of first and second filters 24c and 24d, as previously proposed by this inventor, in Japanese patent application H5(1993)-208871, are given by the following equations: P = (F + K)/(S + A) N = (F - K)/(S - A) , where F, K, S, and A, are as defined earlier. In surround signal processing apparatus so constituted, separation in the frequency spectrum of base surround signal S is accomplished by feeding base surround signal S through comb filter 1, thereby obtaining the 2-channel (left and right) rear surround signals, SL and SR, having low correlation between channels, and a good surround effect is achieved In addition, when the 2-channel surround signals SL and SR are processed through sound image localization circuit 24, which has Shuffler filters, and the resulting signals are then reproduced by a pair of speakers 3L and 3R in front of a listener, the cross-talk, that is, the sound from left speaker 3L that circles into the listener's right ear, and that from the right speaker 3R that circles into the listener's left ear, will be canceled, with the result that only the sound from the left speaker can be heard in the listener's left ear, and only the sound from the right speaker can be heard in the listener's right ear, and in addition, the processing in accordance with transfer characteristics F and K will result in sound images being localized at specific substantially laterally symmetrical locations to the rear of the listener. In the above description the embodiment was discussed in terms of surround reproduction by two front loudspeakers. In addition to this, however, this system will also work well in systems having the normal 2- channel (left and right) signals, with signals split off for three front L, R, and C (left right and center), and one rear (monaural) channel, for a total of five speaker channels. Also, if a two channel surround signal is reproduced in the rear speakers of a normal 5-speaker surround system, the reduced correlation between channels provided by this invention will provide improved performance with respect to in- the- head localization problems, and excellent surround effect. One or more of the following beneficial effects may be realized by the use of the surround signal processing apparatus of this invention as described above: (1) In sound field control, it will be possible to effect extremely good surround sound field control, with little or no correlation between left and right rear surround components, and in particular, to create a highly natural surround space with a heightened sense of expansiveness. (2) In sound field control, with respect to the rear sound, it will be possible to localize virtual sound images in laterally symmetrical locations to the rear. (3) In sound field control, it will be possible to take a pair of surround sound signals that have been rendered non-correlative by a comb filter, and localize these surround signals, with a sound image localization circuit, to substantially laterally symmetrical locations to the rear of the listener, and thereby, with an extremely simple circuit configuration, to provide rear sound field representation and sound image motion with a high degree of clarity, thus achieving an entirely adequate surround effect. (4) In sound field control, it will be possible to take a pair of sound signals that have been localized to substantially laterally symmetrical locations to the rear of the listener by a sound image localization circuit, and, with a comb filter, render them non-correlative, and thereby, with an extremely simple circuit configuration, to provide rear sound field representation and sound image motion with a high degree of clarity, thus achieving a fully adequate surround effect. (5) It will be possible, by setting transfer characteristics, to give breadth to the image localization location, and set the range of surround reproduction. (6) When the rear sound signal is a single monaural signal, and when virtual sound images localized to laterally- symmetrical locations behind the listener are being reproduced, and pseudo- stereophonic processing is performed to avoid localization inside the listener's head, it will be possible to soften the perception of phase inversion [that accompanies such conditions] by manipulating the amplitudes of two rear surround signals to enhance the acoustic effect, and thereby create a natural surround space having a heightened sense of expansiveness. (7) It will be possible to create an amplitude differential between a pair of rear surround signals, and thereby reproduce sound fields with effective expansiveness. (8) It will be possible to set amplitudes and delay times as desired, and thereby reproduce sound fields with affective expensiveness. (9) It will be possible to individually set the amplitudes and delay times of a pair of rear surround signals as desired, to select values so as to realize the maximum effect, to thereby create virtual images with clarity and a variety of sound fields. (10) For rear sound, it will be possible to localize virtual images to laterally symmetrical locations to the rear, and create a natural surround space having a heightened sense of expansiveness. (11) It will be possible to add a sense of distance to the virtual sound images, to create the perception in the listener of being present in a concert hall or theater, and along with this, extremely good sound field reproduction for listening in a wide range of acoustic environments can be achieved.

Claims: A surround signal processing apparatus for reproducing surround sound, from a pair of loudspeakers (3L,3R) placed in front of and substantially laterally symmetrical with respect to a listener, based on the input of a rear monaural surround signal (5) input, with said apparatus comprising: a signal processing means including a comb filter (1) for performing signal processing required to render mutually non-correlative, a left-right pair of rear surround signals that are based on said rear monaural surround signal input, characterized in that said apparatus further comprises: a sound image localization circuit (24) for performing signal processing required to localize a sound image at locations behind and substantially laterally symmetrical with respect to a listener, based on the input of said left-right pair of rear surround signals that have been signal processed by said signal processing means; wherein said sound image localization circuit (24) comprises; a first adder (24a) for adding sum signals and difference signals output as pairs of surround signals from said comb filter; a first subtractor (24b) for subtracting said sum signals and difference signals; a first filter P (24c) for receiving an output from said adder and performing thereon a convolution process; a second filter N (24d) for receiving an output from said subtractor and performing thereon a convolution process; a second subtractor (24e) for subtracting the outputs of said first and second filters (24c,24d); and a second adder (24f) for summing of the outputs of said first and second filters (24c,24d). The surround signal processing apparatus of Claim 1, wherein said comb filter (1) in said signal processing means generates said sum and difference signals from delaye (T) and pre-delay versions of said rear monaural surround signal (5) and provides a pair of surround signals to said sound image localization circuit (24). The surround signal processing apparatus of Claim 1, wherein said comb filter (1) in said signal processing means provides a pair of surround signals to said sound image localization circuit (24) to be reproduced from said pair of loudspeakers (3L,3R). The surround signal processing apparatus of Claim 1, wherein the transfer characteristics of said first and second filter, P and N (24c, 24d) respectively, are set to P = (F+K)/(S+A), and N = (F - K)/(S-A), respectively, where S is the transfer characteristic from one speaker of a pair to a listener's ear on the same side of the speaker; A is the transfer characteristic from one speaker of a pair to a listener's ear on the side opposite the speaker; F is the transfer characteristic from a location to which a surround signal sound image is to be localized, to a listener's ear on the same side; and K is the transfer characteristic from a location to which a surround signal sound image is to be localized, to a listener's ear on the opposite side.

Inventors: IIDA TOSHIYUKI, MOURI TOMOHIRO, IIDA, TOSHIYUKI, MOURI, TOMOHIRO
Applicants: VICTOR COMPANY OF JAPAN, VICTOR COMPANY OF JAPAN, LIMITED
Application year: 1996
Main classification: H04S5
IPC codes: H04S3