High efficiency coding method and apparatus

Abstract: A high efficiency coding method and apparatus including carrying out quantization taking account of correlation of input signals of plural channels in compressing and/or expanding the input signals of the respective channels simultaneously. In the high efficiency coding method and apparatus, spectrum data from MDCT circuits or coefficient data are transmitted to another channel via an other-channel transmission circuit, and data from another channel are transmitted to correlation adaption circuits. The data from correlation adaption circuits are transmitted to adaptive bit allocation circuits, respectively, wherein bit allocation in coding taking account of input signals of another channel is carried out.

Description: This invention relates to a high efficiency coding method and apparatus for compressing and quantizing input signals, and more particularly, to a high efficiency coding method and apparatus for simultaneously compressing input signals of plural channels and quantizing them. A variety of high efficiency coding techniques for audio signals or speech signals are known. For example, a technique of carrying out fixed bit allocation by utilizing auditory sense masking to provide the necessary signal-to-noise ratios for a number of frequency bands is disclosed in M.A. Kransner MIT, The critical band coder -- digital encoding of the perceptual requirements of the auditory system, ICASSP 1980. With this technique, however, since the bit allocation is fixed in the case of measuring the characteristics with a sine wave input, it is not possible to have good characteristic values. In order to solve this problem, there is proposed a high efficiency coding apparatus in which all the bits usable for bit allocation are divided to be used into bits of a fixed bit allocation pattern determined in advance for each of a number of small blocks of data and bits for carrying out bit allocation dependent upon the magnitudes of signals in respective blocks, so as to allow the division ratio to be dependent upon a signal related to an input signal, thus allowing the division ratio for the fixed bit allocation pattern to be greater as the spectrum of the signal becomes smoother. According to this method, when the signal energy is concentrated on a specific spectrum component as in the case of a sine wave input, allocation of a larger number of bits to a block including that spectrum component makes it possible markedly to improve the entire signal-to-noise characteristics. Generally, the human auditory sense is extremely sensitive to a signal having a steep spectrum component. Therefore, the improvement in the signal-to-noise characteristics using such a method not only leads to an improvement in the measured numerical values, but also is effective to improve the sound quality from the viewpoint of the auditory sense. However, if the allocation of bits dependent upon input signals is carried out simply on the basis of an improvement in the signal-to-noise characteristics, when attempting to compress a signal including a large number of steep spectrum components, such as the sound of a triangle, at a low bit rate, sufficient bits cannot be allocated to blocks corresponding to respective spectrum components, thus failing to provide sufficiently high sound quality from the viewpoint of the auditory sense. In view of the above, the present Assignee has proposed, in the US Patent Application S.N. 08/011,376, filed on January 29, 1993, a technique in which all the bits usable for bit allocation are divided for use into bits of a fixed bit allocation pattern determined in advance for any short time interval and bits for carrying out bit allocation dependent upon the magnitudes of signals of respective blocks, thus carrying out the bit allocation such that weighting is conducted in accordance with corresponding bands of blocks for the bit allocation dependent upon magnitudes of signals of respective blocks as well as the fixed bit allocation pattern. However, in all the above techniques, quantization is carried out only for input signals of the corresponding channel. Therefore, if the above techniques are applied to an apparatus having plural channels, at the same compression rate as in the case of a single channel, the results will be as follows. That is, while preferable results may be obtained when input signals of the respective channels are strongly correlated, differences in quantization errors may be generated between channels in which the signals are weakly correlated, thus affecting stationary quality of a sound image. ICASSP '89, vol. 3, May 1989, NY, USA, pages 1993-1996, Johnston "Perceptual Transform Coding of Wideband Stereo Signals" discloses a stereo signal coder in which input L and R channel signals are converted into L+R and L-R signals which are then windowed, Fourier transformed and quantized. The L+R and L-R signals are each quantized using the same perceptual threshold. In view of the above-mentioned status of the art, it is an object of the present invention to provide a high efficiency coding method and apparatus, by which the quantization noise volume between respective channels is controlled when input signals are applied to plural channels, thereby optimally realizing the stationary quality of the sound image. One aspect of the present invention provides a high efficiency coding method for compressing and quantizing input signals of plural channels, the method comprising: carrying out quantization with bit allocation determined by taking account of correlation of the input signals during compression of the input signals of the respective channels, characterised in that: said method further comprises the step of adapting the correlation to implement modification of spectrum power for maintaining optimal correlation on the basis of the spectrum data on the frequency axis of said respective channels. A second aspect of the invention provides a high efficiency coding method according to the first aspect, wherein the input signals are speech signals, and the method utilizes characteristics of human auditory sense. A third aspect of the invention provides a high efficiency coding apparatus for compressing and quantizing input signals of plural channels, the apparatus comprising: means for carrying out quantization with bit allocation determined by taking account of correlation of the input signals during compression of the input signals of the respective channels, characterised in that: said apparatus further comprises means for adapting the correlation to implement modification of spectrum power for maintaining optimal correlation on the basis of the spectrum data on the frequency axis of said respective channels. A fourth aspect of the invention provides a high efficiency coding apparatus according to the third aspect, wherein the input signals are speech signals, and the apparatus utilizes characteristics of human auditory sense. The correlation between the channels used in this bit allocation is preferably given as a numerical value dependent upon the input signals. However, the correlation may take a fixed value, or may take a value produced having regard to a value dependent upon the input signals and the fixed value, at a constant or variable ratio. In addition, preferable results may also be obtained by changing respective coefficients for a bit allocation. The correlation used in this change of coefficients may be found from spectrum data on the frequency axis of the input signal or coefficient data at the time of orthogonal transformation, or may be found from floating information as part of quantization results. According to the present invention, in realizing efficient bit allocation conforming to the auditory sense in encoding input signals of plural channels, the deterioration in stationary quality of a sound image due to the difference in quantization errors between the respective channels can be reduced or avoided. In addition, masking effects can be utilized more efficiently than before because of the tendency for the generated quantization noise to be concentrated in a position wherein the stationary quality of the sound image is also concentrated. Accordingly, it is possible to encode music signals of high quality at a lower bit rate. The invention will be further described by way of non-limitative example with reference to the accompanying drawings, in which:- Fig.1 is a block circuit diagram showing an embodiment of a high efficiency coding apparatus according to the present invention. Fig.2 is a view showing frequency and time division of a signal of the apparatus. Fig.3 is a view showing an example of algorithm of a correlation adaption circuit of the apparatus. Fig.4 is a view showing an example of bit allocation calculated separately for each channel for explaining effects of the correlation adaption circuit of the apparatus. Fig.5 is a view showing an example of bit allocation calculated having regard to correlation information between channels for explaining the effects of the correlation adaption circuit of the apparatus. A preferred embodiment of the high efficiency coding method and apparatus according to the present invention will now be described with reference to the accompanying drawings. First, a technique of high efficiency coding of input digital signals such as audio PCM signals, using techniques of so-called Subband Coding (SBC), Adaptive Transform Coding (ATC) and Adaptive Bit Allocation (APC-AB), is described with reference to Fig.1. In the high efficiency coding apparatus of the present embodiment, quantization taking account of correlation between respective channels is carried out in compressing input audio PCM signals of plural channels simultaneously. The correlation between the respective channels may be found by constantly calculating it from the input signals, or may be found from spectrum data of the input signals on the time axis or coefficient data upon orthogonal transformation, or may be found from floating information of another channel. Also, quantization may be carried out on the basis of constant correlation regardless of the input signals. Further, quantization may be carried out in consideration of the correlation dependent upon the input signals and the constant correlation, at a variable ratio. In the high efficiency coding apparatus, concretely shown in Fig.1, an input digital signal is divided in frequency into plural frequency bands by using filters, and orthogonal transformation is carried out for each of the respective frequency bands, thereby implementing adaptive bit allocation and encoding of resulting spectrum data on the frequency axis for each so-called critical band, as later described, taking account of the characteristics of the human auditory sense. It is a matter of course that the frequency band divisional width of non-block division by means of filters may be an equal divisional width. Further, in the embodiment of this invention, the block size (block length) is adaptively changed in accordance with an input signal before orthogonal transformation, and floating processing is carried out for every critical band or for every block formed by further subdividing the critical band in the higher frequency band. This critical band is a frequency band divided having regard to the characteristics of the human auditory sense, that is, a band of a narrow band noise at the time when a pure sound is masked by that noise having the same intensity as that of the pure sound in the vicinity of the frequency thereof. In this critical band, the bandwidth becomes broader on a higher frequency band side, and the entire audio frequency band of 0 ~ 20 kHz is divided into, for example, 25 critical bands. In Fig.1, an input terminal 10 is supplied with, e.g., an audio PCM signal of 0 ~ 20 kHz. This input signal is divided in frequency into a frequency band of 0 ~ 10 kHz and a frequency band of 10 k ~ 20 kHz by using a band division filter 11 such as a so-called QMF filter, and the signal in the frequency band of 0 ∼ 10 kHz is further divided into a frequency band of 0 ∼ 5 kHz and a frequency band of 5 k ~ 10 kHz by using a band division filter 12 such as the so-called QMF filter. The signal in the frequency band of 10 k ~ 20 kHz from the band division filter 11 is sent to a Modified Discrete Cosine Transform (MDCT) circuit 13 which is an example of an orthogonal transformation circuit, and the signal in the frequency band of 5 k ~ 10 kHz from the band division filter 12 is sent to an MDCT circuit 14, while the signal in the frequency band of 0 ~ 5 kHz from the band division filter 12 is sent to an MDCT circuit 15. Thus, those signals are individually processed by MDCT processing, respectively. A concrete example of block sizes of the respective MDCT circuits 13, 14, 15 is shown in Fig.2. In the concrete example of Fig.2, the frequency band is widened and the time resolution is enhanced (the block length is shortened) on the higher frequency band side of the spectrum. That is, in the example of Fig.2, MDCT processing is implemented twice in total for blocks b H1 , b H2 of the signal in the frequency band of 10 k ~ 20 kHz on the higher frequency band side, while MDCT processing is carried out for a block b L of a signal in the frequency band of 0 ~ 5 kHz on the lower frequency band side and a block b M of a signal in the frequency band of 5 k ~ 10 kHz in the medium frequency band. In this manner, the number of orthogonal transformation block samples in respective bands are made to be the same, thus simplifying the device, and at the same time, the frequency resolution is enhanced on the lower frequency band side where the critical bandwidth is narrow, while the time resolution is enhanced on the higher frequency band side where many components superior to a transient signal are included. In addition, in the present embodiment, the respective bands are allowed to be adaptively divided into two blocks or four blocks on the assumption that the temporal change of the signal is great. Returning to Fig.1, spectrum data or MDCT coefficient data on the frequency axis produced by MDCT processing in the respective MDCT circuits 13, 14, 15 are combined for every critical band or for each of the bands formed by further dividing the critical band. The data thus combined are transmitted to level correction circuits 22, 23, 24, correlation adaption circuits 28, 29, 30, and an other-channel transmission circuit 31. The correlation adaption circuits 28, 29, 30 implement modification for maintaining preferable correlation on the basis of the spectrum data on the frequency axis of another channel obtained from an input terminal 38 and the MDCT coefficient data, and then transmitting them to adaptive bit allocation circuits 16, 17, 18. For this modification, the correlation corresponding to the input signal or a fixed pattern may be used. The other-channel transmission circuit 31 outputs from an output terminal 39 the spectrum data on the frequency axis of each band or the MDCT coefficient data as correlation information, in accordance with a data format for the use of the correlation adaption circuits and timing. The output terminal 39 is connected to the input terminal 38 of the other channel. The correlation information to be transmitted to the other channel may be constituted on the basis of floating information outputted from output terminals 35, 36, 37. The adaptive bit allocation circuits 16, 17, 18 allocate the number of bits to produce the bit rate necessary for each critical band or for each band produced by further dividing the critical band on the higher frequency band side, and transmit the data to the level correction circuit 22, 23, 24 and coding circuits. In the level correction circuits, quantization errors are found from the number of bits allocated by the adaptive bit allocation circuits, and according thereto, the spectrum data on the frequency axis or the coefficient data are level-corrected and transmitted to the coding circuits. The coding circuits re-quantize the level-corrected spectrum data on the frequency axis or MDCT coefficient data in accordance with the number of bits allocated by the adaptive bit allocation circuits. The data thus encoded are taken out via output terminals 32, 33, 34. At the same time, floating information indicating for what magnitude of the signal normalization is carried out, and bit length information indicating at what bit length quantization is carried out, are simultaneously transmitted by the output terminals 35, 36, 37. Outputs of the MDCT circuits 13, 14, 15 shown in Fig.1 are transmitted to a power adjustment circuit 308 and a correlation information calculation circuit 304 via an input terminal 302 shown in Fig.3. The information inputted to the input terminal 38 shown in Fig.1 is transmitted to the correlation information calculation circuit 304 via the input terminal 301 shown in Fig.3. The correlation information calculation circuit 304 calculates correlation information corresponding to the input signal by means of an input 301 and a coefficient provided by a coefficient table 306. An example of the calculation is explained on the assumption of using a normal 2-channel stereo. If the n'th spectrum power of the left channel and the n'th spectrum power of the right channel are represented by Pl(n) and Pr(n) , respectively, both being on the log scale, and the coefficient corresponding to the n'th spectrum provided by the coefficient table 306 is represented by K(n), correlation information of the respective channels ΔPl(n), ΔPr(n) can be found by the following formulae: ΔPl(n) = (Pr(n) - Pl(n)) x K(n) ΔPr(n) = (Pl(n) - Pr(n)) x K(n) As is apparent from the formulae (1) and (2), the correlation information indicates that 0 expresses the strongest correlation, and that a larger absolute value of the numerical value expresses weaker correlation. The coefficient K(n) is assumed to be based on the characteristics of the human auditory sense. For example, matters concerning directional perception which has been studied for a long time, and as factors thereof, intensity difference, phase difference and temporal difference in propagation of a sound to both ears are observed, as described in Dave, E.E. Jr., Gutman, N. and v. Bergeijk, W.A.: J. Acoust. Soc. Am., 31, pp.774-784 (1959). In addition to these factors, changes due to frequency and sound pressure levels are observed. Further, it is known, from every-day experience, to be difficult to identify the location of the sound source of an ultra-low or ultra-high sound. In this manner, the human directional perception changes, depending on the frequency and sound pressure of the sound. Therefore, despite a single spectrum power ratio, it is necessary to carry out correction corresponding to the frequency and sound pressure. In the calculation in this case, the correlation information may be found for each spectrum data on the frequency axis, or may be found for each floating band in quantization. Further, it is apparent that the same results can be obtained by using coefficients for floating instead of the spectrum powers. The correlation information found by the correlation information calculation circuit 304 is transmitted to a correlation information determining circuit 305. The correlation information determining circuit 305 synthesizes fixed correlation information obtained by a fixed correlation information table 307 and correlation dependent upon the input signals obtained from the correlation information calculation circuit 304 at a constant or variable ratio, and transmits the synthesized correlation information to the power adjustment circuit 308. On the assumption of the fixed correlation information being Pfix(n), and the ratio of synthesis being R(n), where 0 ≤ R(n) ≤ 1.0, the outputs Cl(n), Cr(n) of the correlation determining circuit 305 can be expressed by the following formulae: Cl(n) = ΔPl(n) x R(n) + Pfix(n) x (1 - R(n)) Cr(n) = ΔPr(n) x R(n) + Pfix(n) x (1 - R(n)) The fixed correlation information in this case means peculiar correlation information which can be predicted in an applied example, and for example, such a property as to indicate strong correlation for a portion of relatively low frequency in case of using a main channel and an ultra-low sound channel. Accordingly, the fixed correlation between channels may be unpredictable, depending upon the applied example. In this case, R(n) = 1.0, that is, equivalent to the above-mentioned case in which the correlation information determining circuit 305 and the fixed correlation information table 307 are omitted. On the other hand, if the correlation is clearly determined in advance, R(n) = 0, that is, equivalent to the case in which the correlation information calculation circuit 304 and the coefficient table 306 are omitted. Also, the ratio of synthesis R(n) may be constant, or may be variable in accordance with the input signals, for example, the magnitude of spectrum power and frequency. The power adjustment circuit 308 adjusts spectrum power information used in bit allocation, on the basis of the output of the correlation information determining circuit 305, and transmits the adjusted spectrum power information as an output 308 to the adaptive bit allocation circuits 16, 17, 18 shown in Fig.1. On the assumption of information from an input terminal 302 being Pil, Pir, and information to an output terminal 309 being Pol(n), Por(n), the operation in the power adjustment circuit 308 can be expressed by the following formulae: Pol(n) = (Cl(n) + 1.0) x Pil(n) Por(n) = (Cr(n) + 1.0) x Pir(n) Through this operation, the spectrum of strongly correlated signals between channels is subject to a small volume of adjustment, while the spectrum of weak correlation is subject to a large amount of adjustment. In Figs.4 and 5, the function of the correlation adaption circuit is expressed in simplified models with the number of quantization bands being 4, the number of channels being 2, and the difference between the channels being only of the quantization band. In case of separately calculating bit allocations as shown in Fig.4, the spectrum power of an input signal on the frequency axis is different only in the third quantization band between 1ch and 2ch, thus having different volumes of quantization noise. In this state, for example, if the two channels are extended and reproduced as a 2-channel stereo, there is a possibility that portions different in quantization noise affect stationary quality. The correlation adaption circuit takes account of correlation between 1ch and 2ch, as in the case in which the correlation is taken into account as shown in Fig.5. Then, the correlation adaption circuit operates so as to control the volume of quantization noise in the third quantization band, and to reproduce high stationary quality.

Claims: A high efficiency coding method for compressing and quantizing input signals of plural channels, the method comprising: carrying out quantization with bit allocation determined by taking account of correlation of the input signals during compression of the input signals of the respective channels, characterised in that : said method further comprises the step of adapting the correlation by adjusting spectrum power for maintaining optimal correlation on the basis of the spectrum data on the frequency axis of said respective channels. A high efficiency coding method according to claim 1, wherein the input signals are speech signals, and the method utilizes characteristics of human auditory sense. The high efficiency coding method as claimed in claim 1 or 2, wherein the correlation between the respective channels is determined by constantly implementing calculation from the input signals. The high efficiency coding method as claimed in claim 1 or 2, wherein the correlation between the respective channels is determined from spectrum data on a time axis of the input signals or coefficient data produced by orthogonal transformation. The high efficiency coding method as claimed in claim 1 or 2, wherein the correlation between the respective channels is determined from floating information of other channels. The high efficiency coding method as claimed in claim 1 or 2, wherein quantization is carried out on the basis of constant correlation regardless of the input signals. The high efficiency coding method as claimed in any one of claims 1 to 6, wherein quantization is carried out by taking account of a variable ratio of: (i) correlation dependent upon the input signals; and (ii) constant correlation independent of the input signals. A high efficiency coding apparatus for compressing and quantizing input signals of plural channels, the apparatus comprising: means (16, 17, 18, 25, 26, 27) for carrying out quantization with bit allocation determined by taking account of correlation of the input signals during compression of the input signals of the respective channels, characterised in that : said apparatus further comprises means (28, 29, 30) for adapting the correlation by adjusting spectrum power for maintaining optimal correlation on the basis of the spectrum data on the frequency axis of said respective channels. A high efficiency coding apparatus according to claim 8, wherein the input signals are speech signals, and the apparatus utilizes characteristics of human auditory sense. The high efficiency coding apparatus as claimed in claim 8 or 9, wherein the correlation between the respective channels is determined by constantly implementing calculation from the input signals. The high efficiency coding apparatus as claimed in claim 8 or 9, wherein the correlation between the respective channels is determined from spectrum data on a time axis of the input signals or coefficient data produced by orthogonal transformation. The high efficiency coding apparatus as claimed in claim 8 or 9, wherein the correlation between the respective channels is determined from floating information of other channels. The high efficiency coding apparatus as claimed in claim 8 or 9, wherein quantization is carried out on the basis of constant correlation regardless of the input signals. The high efficiency coding apparatus as claimed in any one of claims 8 to 13, wherein quantization is carried out by taking account of a variable ratio of: (i) correlation dependent upon the input signals; and (ii) constant correlation independent of the input signals.

Inventors: AKUNE MAKOTO, SUZUKI HIROSHI, AKUNE, MAKOTO, SUZUKI, HIROSHI, AKUNE, MAKOTO, C/O SONY CORPORATION, SUZUKI, HIROSHI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1993
Main classification: H04B1
IPC codes: H03M7, H04H5, G10L19, H04B1, H04H20