Coding transmission equipment for carrying out coding with adaptive quantization

lZ9Z~ -- 1 - CODING TRANSMISSION EQUIP~ENT FOR CARRYING OUT CODING WITH ADAPTI~JE QUANTIZATION BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to coding tranmission equipment in which an input signal, such as a voice signal or a picture signal, is coded and transmitted to a receiver side from a transmitter side and a received signal is decoded to become an original signal at the receiver side. In particular, the present invention relates to low bit rate coding equipment (bandwidth compression coding equipment), such as an ADPCM (Adaptive Difrerential Pulse Code Modulation) system CODEC (COder and DECoder), which carries out low bit rate coding of a voice or picture signal.

When ~he voice signal etc., is transmitted, bandwidth compression is required to ensure an efficient utilization of a transmission line. The low bit rate coding transmission equipment for the voice signal carries out information coinpression while maintaining the quality of the voice signal, which enables a reduction of line costs for the transmission of the voice information in equipment used for , for example, mobile radio communication, satellite communication, or office inter communication system, and a reduction of storage capacity needed, for example, for the storage of voice information in an audio response system.

2. Description of the Related Art ADPCM coding equipment is related to the present invention in that it carries out coding by adaptive quantization. In the ADPCM coding equipment, a prediction error E corresponding to a difference between a current input signal X and a predicted value X obtained on the basis of past input signals is produced and quantized at the transmitter side. At this time, ,, ~ ~Z9ZO~l --2- adaptive quantization, in which the size of a quantizing step is increased or decreased in accordance with a quantizing level of a signal sampled just prior to the current sampled signal, is carried out in order to realize a quick adaptation to a sudden change of the predicted error, instead of uniform quantization, whereby a high quality recovered signal is realized.

In this case, however, the sizes to which the quantizing step can increased or decreased are limited, and therefore, ~hen the input signals are voice signals, for example, a voiced sound, an unvoiced sound, or a voicelessness, etc., the nature of the signals changes from moment to moment and the coding system of the prior art cannot cope with these rapid variations in the voice signal. As a result, an optimum quantization is not realized, and thus the voice recovered at the receiver side has a poor quality.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide coding transmission equipment which can obtain a high quality recovered signal, for example, a voice signal recovered at a receiver sideO According to a one aspect of the present invention, there is provided coding transmission equipment for communication of an input signal, comprising: a transmitter device, comprising:

a plurality of adaptive coding units each including an adaptive quantizer, operatively connected to receive the input signal, for outputting a quantized value according to the input signal and an error calculating unit, operatively connected to said adaptive -quantizer, for calculating a quanti~ation error of said adaptive quantizer, said adaptive quantizer in each of said adaptive coding units having an individual updating speed of quantizing step size; an evaluating unit, operatively connected to said error calculating unit in lZ9~0~ -2aeach of said adaptive coding units, fox outputting decision information indicating which of said adaptive coding units should be selected as an optimum adaptive coding unit to carry out optimum quantization at every frame of the input signal in dependence upon the quantization errors output by said error calculating unit in each of said plurality of adaptive coding units;

and a sending unit, operatively connected to said adaptive quantizer in each of said adaptive coding units and said evaluating unit, for selecting an optimum quantized value output by the optimum adaptive coding unit in dependence upon the decision information output by said evaluating unit and transmitting the optimum quantized value together with the decision information identifying the optimum adaptive coding unit; a receiver device receiving the optimum quantized value and the decision information in a received signal, comprising: a dividing unit for dividing the received signal from said transmitter device into the quantized value and the decision information; an adaptive decoding unit, operatively connected to said dividing unit, for decoding the quantized value obtained by said dividing unit into the original input signal; and an optimum control unit, operatively connected to said dividing unit and said adaptive decoding unit, for controlling an updating speed of a quantizing step size of said adaptive decoding unit to coincide with the updating speed of the quantizing step size of the optimum adaptive coding unit in dependence upon the decision information obtained by said dividing unit~ This invention also provides a transmitter device, used in coding transmission equipment, for carrying out coding of an input signal with adaptive quantization, comprising: a plurality of adaptive coding units each including an adaptive quantizer, operatively connected to receive the input signal, for outputting a quantized value according to the input signal and an 1292Q~7~.

error calculating unit, operatively connected to said adaptive quantizer, for calculating a quantization error of said adaptive quantizer, said adaptive quantizer in each of said adaptive coding units having an individual updating speed of a quantizing step size; an evaluating unit, operatively connected to said error calculating unit in each of said adaptive eoding units, for outputting decision information indicating which of said adaptive coding units should be seleeted as an optimum adaptive eoding unit to earry out optimum quantization at every frame of the input signal in dependenee upon the quantization error output by said error ealeulating unit in each of said plurality of adaptive eoding unit; and a sending unit, operatively eonneeted to said adaptive quantizer in eaeh of said adaptive eoding units and said evaluating unit, for seleeting an optimum quantized value output by the optimum adaptive eoding unit in dependenee upon the decision information output by said evaluating unit and transmitting an optimum quantized value together with the decision information identifying the optimum adaptive eoding unit.

3LZ9~07~ --4- According to yet another aspect of the present invention, the receiver device further comprises a postprocessing filter disposed at a latter stage of the adaptive decoding unit, wherein the parameter of the post-processing filter is varied according to the parameter of the adaptive decoding unit.

BRIEF DESCRIPTION OF THE DR~WINCS Examples of coding transmission equipment in accordance with the present invention will be described with reference to the accompanying drawings, in which Fig. 1 shows a prior art ADPCM coder;

Fig. 2 shows a prior art ADPCM decoder;

Fig. 3 shows an embodiment of an ADPCM coder at a transmitter side of the coding transmission equipment according to the present invention;

Fig. 4 shows an embodiment of an ADPCM decoder at a receiver side of the coding transmission equipment according to the present invention;

Fig. 5 shows an adaptive differential coding unit of the ADPCM coder of Fig. 3 in more detail;

Fig. 6 shows another embodiment of the ADPCM coder according to the present invention;

Fig. 7 shows a further embodiment of the coding transmission equipment according to the present invention;

Fig. 8 shows an example of an error correcting code;

Fiq. 9 shows a format of the transmission signal including the error correcting code;

Fig. 10 shows an example of an error correction at the receiver side;

Fiq. 11 shows still another embodiment of the ADPCM decoder at the receiver side according to the present lZ9Z~ invention; and Fig. 12 is a view for ex?laining a post-process~ng filter in detail.

D~SC~IP~ION 5F THE P~EF~RR~D EMBODI~.ITS For a ~etter understand ng of the prefe_red e~bodiment, the problems in the prior art will be f rst explained with reference to Figs. 1 and 2.

Figure 1 shows an adaptive differential coder on the transmitter side in a prior art low bit rate coding transmission equipment for a voice signal. In Fig. 1, the coder comprises a subtractor 10 for calculating a predictlon error Eln) corresponding to a difference between an input voice signal X(n) and a predicted value X(n~ to be output; an adaptive quantizer 11 for adaptive quantizing of the prediction error E(n) to output the quantized value I(n) as a transmission signal; an adaptive inverse quantizer 12 for adaptive inverse quantizing of the quantized value I(n); an adder 13 for adding the inverse quantized value E(n) of the prediction error E(n) from the inverse quantizer 12 and the predicted value X(n); an adaptive zero predictor 14 for producing a zero predicted value X(n)z; an adaptive pole predictor 15 for producing a pole predicted value ~(nlp; and an adder 16 for adding the zero predic~ed value X(n)z and the pole predicted value X(n~p to produce the predicted value ~(n).

In this adaptive differential coder, the quantizer 11, the inverse quantizer 12, the zero predictor 14, and the pole predictor 15 are adaptivecontrolled. For example, in the quantizer 11 and theinverse quantizer 12, the adaptive quantization wherein the quantizing step size ~(n) is updated according to the following formula is carried out.

~(n~1) = a (n)Y x M(I(n)) ... ~1) where, y is a coefficient for gradually reducing the influence of the line error, which has a value close to and lower than l; and M(I(n)) is a step size updating 12~071 coefficient (or a step size updating speed).

When the quantize~ value I(n~ from the quan,izer 11 is, for e~ample, a l bit output signal, the updating coefficient M(I(n)) assumes a value ~ c10s2 to and lowe than 1, for example, ~ = 0.93, when the quantized value I(n) is "0", and a val-~e ~ close to and higher than 1, for e~ampie, ~ = 1.31, when the quantized value I(n) is "l". Also, when the quantized value Itn) is a plurality bit output signal, the step size updating coefficient M(I~n)) is increased in proportion to the increment or the numbe_ indicated by the plurality bit output signal.

Figure 2 shows an adaptive differential decoder on the receiver side in a prior art low bit rate coding transmission equipment.

In Fig. 2, the decoder comprises an inverse quantizer 71', an adder 73, a zero predictor 75, a pole predictor 76, and an adder 77.

In this decoder, the quantized value I(n) of the prediction error transmitted from the transmitter side is inverse-quantized by the inverse quantizer 71 to recover the prediction error, and the zero predicted value (n)z and the pole predicted value X(n)p are produced by the zero predictor 75 and the pole predictor 76 on the basis of the recovered past prediction errors respectively. Then, the predicted value X(n) obtained by adding this zero predicted value X(n)z and the pole predicted value X(n)p by the adder 73 is added to the inverse-quantized prediction error from the inverse quantizer 71' by the adder 73, ~o recover the voice signal S(n). The processing charactsristics of the inverse quantizer 71', the zero predictor 75, and the pole predictor 76 at the receiver side are the same as that of the inverse quantizer 12, the zero predictor 14, and the pole predictor 15 at the transmitter side, respectively.

In the differential coder of Fig. l, the step size lZ~Z(f~ updating coef~icient M(I(n)) of the quantizer 11 and the inverse quantizer 12 is a fixed value, since the ~alues a and ~ which dete~mine the updating coe ficient M(I(nJ~ - are fixed. The_efore, sometimes 'he optimum qu2nt-zct on is not car- ed out at the quantizer 11, since updat-ng coe_fic ent M(I~n)) OI the step size ~(n), which is a threshold for auantlzing th~ prediction error E(n), mav not sufficiently follow the change of the prediction error E(n).

In particular, when the input signal is a voice sisnal, the nature of the sïgnal, for example, a voiced sound, unvoiced sound, or silence, changes from moment to moment and frequently varies, and therefore, the optimum quantization is not carried out since the fixed step size updating coefficient M(I(n)) cannot follow the change of the voice signal.

- A preferred embodiment of the present invention will now be explained with reference to Figs. 3 to 5.

Figures 3 to 5 show a embodiment of the coding transmission equipment, according to the present invention, which is adapted to transmission equipment using the ADPCM coding system.

Figure 3 shows an ADPCM coder at the transmitter side, and Fig. 4 shows an ADPCM decoder at the receiver side.

In Fig. 3, 11 to lk are the k numbers of adaptive differential coding units in which the step size updating coefficients M(I(n))l to M(Itn))k of quantizers (in more detail, values a and ~ described above) are differentJ respectively.

The fundamental structure is the same as the adaptive di~ferential coder shown in Fig. 1, except that a subtractor is provided which calculates and outputs a quantization error e(n3 corresponding to a differential value between the prediction error E(n) and the inverse-quantized value E(n) of the prediction error E(n) from the inverse quantizer 12.

Figure 5 shows a more detailed structure of one of the adaptive differential coding units 11 to lk. In ~Z~21:~7~ Fig. 5, the subtractor 1~ calculates the prediction errcr E(n) corresponding to the differential value between the input voice signal X~n) and the predicted value Xln) and sends this prediction error E(nl to the adaptive quantizer 11. The adaptive quantizer 11 quantizes the prediction error E(n) and outputs a quantized value I(n). This quantized value I(n) is introduced to the inverse quantizer 12, which carries out the inverse quantization. At the same time, the quantized value I(n) is transmitted to the receiver side. The inverse quantized value E(n) of the prediction error E(n1 from the inverse quantizer 12 is introduced to the zero predictor 14 and the subtractor 17, and to the pole predictor 15 via the adder 13. The zero lS predictor 14 produces a zero predicted value X(n)z , and the pole predictor 15 produces a pole predicted value X(n)p. The zero predicted value X(n)z and the pole predicted value X(n3p are addea by the adder 16 to produce a predicted value X(n1, which is then sent to the subtractor 10. The subtractor 17 calculates the differential value between the prediction error E(n) and the inverse quantized value E(n~ thereof, and outputs the result as a quantization error e(nl.

The quantizer 11 and the inverse quantizer 12 are adaptive controlled type devices, and the quantizing step size ~(n) is updated by the quantiæing step adapter ~not shown~ according to the formula (1) described previously. This quantizing step size a (n) is a parameter for deciding inner conditions of the quantizer 11 and the inverse quantizer 12.

The zero predictor 14 and the pole predictor 15 are also adaptive control type devices, and are constituted by circuits comprising taps ~time delay elements) and coefficient multipliers, havinq the orders ~ and m, ys respectively. The zero predictor 14 and the pole predictor 15 hold prediction coefficients updated in sequence, and from the ~iewpoint of the transfer function ., ~29~ as a filter, hold tap data containing past records.

That is, in the zerc predictor 14, zero pre~ict-on coefficients Cz(l, n) ~ Cz(~, n) of each coe^~lcie~t - multlplie_ are updated by the predict on coe- -c en' adapter (not shown~ according to the following formula, respect-vely.

Cz(i, n-1) = Lz x Cz(i, n) f Dz x sgn~E(n)) x sgn(E(n-i)) ... (21 where, Lz and Dz are constant, and sgn( ) is a signature function which indicates a signature of "+ or "-".

The zero prediction coelicients Cz(l, n) to Cz(Q, n~ and tap datas E(n-l) to E(n-Q) stored at each tap are parameters for determining the inner condition of the zero predictor 14.

Also, in the pole predictor 15, pole prediction coefficients Cp(Q, n~ to Cp(m, n) of each coefficient multiplier are updated by the prediction coefficient adapter (not shown) according to the following formula, respectively.

~p(i, n+l) = Lp x Cp(i, n) ~ + Dp x sgn(E(n)) x sgn(S(n-i)) ..... (3) where, Lp and Dp are constant.

The pole prediction coeffic ents Cp(l, n) to Cp(m, n) and tap data S(n-l) to S(n-m) stored at each tap are the parameters for determining the inner condition of the pole predictor 15.

In Fig. 3, each component of Fig. 5 is represented in such a manner that the quantizer 11 is represented by 3~ Ql to Qk ~ the inverse quantizer 12 by Ql 1 to Qk 1, the zero predictor 14 by Hzl to Hzk , and the pole predictor 15 by Hpl to Hpk , respectively, In Fig. 3 quantization errors e(n)l to e~n)k are introduced to an evaluating unit 2. The evaluating unit 2 comprises a quantization error power calculating unit 21 and an optimum quantization deciding unit 22.

The calculating unit 21 calculates powers of the lZ~Z~71 -- 10 - quantizatlon errors e(n~l to efn~k ~ sample b~ sample respectively, during one frame of the VGiCe signal X(n) (for example, 16 sam?les), and integ~a es them, respec.lvely, at each frame, then OUtp'lt~ the summed powe-s of each of the quantization er-ors e~nj to e(n)k at the end of the frame t~ the de^-ding unit 22. ~he deciding unit 22 decides the opiimum coding unit 1 opt, which carries out the ~ptimum quantization, among the adaptive differential coding units 11 tG lk ~ by sPlect~ng the minimum calculated quantization error po~er from that of the quan~ization errors e(n)l to e(n)k . Then, the deciding unit 22 outputs an optimum coder number signal Nopt, indicating the number of the optimum coding unit 1 opt decided as aforementioned, to a selector 3 and a multiplexer 4.

The quantized values I(n)l to I(n)k from adaptive differential coding units 11 to lk are introduced to the selector 3, respectively. This selector is constituted by a memory such as a RAM, which can store quantized values I(n)l to I(n)k of one frame. When the coder number-signal Nopt is received at the end of one frame, the selector 3 sends one frame of the optimum quantized value I(n)opt of the optimum coding unit 1 opt, which is selected by using the coder number signal Nopt, as an address to the multiplexer 4. The multiplexer 4 adds the coder number signal Nopt from the evaluating unit to the optimum quantized value I(n~opt, and transmits a multiplexed signal to the receiver side after converting the signal to a transmission line code.

In the receiver of Fig 4, 6 is a demultiplexer, and 7 a decoder.

The decoder 7 comprises an adaptive inverse quantizer 71; a controller 72 capable of controlling the step size updating coefficient M(I(n)) of the inverse quantizer 71; an adaptive predictor 74 constituted by a zero predictor 75, a pole predictor 76, and an adder 77;

and an adder 73.

The operation of the above described coding trans 1~920~71 -- 11 - mission equipment is explained hereinaf~er.

The voice signal X(n) is input to each of adapt-ve differential ccding units 11 to lk in parallel, an adaptive d-r-erential coding process is carried ou- a_ each unit, and the quant-zed values I(n)l to I(~)k and tAe qlar.t-~at on erro-s e(n)l to e(n)k are output.

The quantization error power caiculating unit 21 calculates the power of the input quantization errors e(n)l , to e(n)k frame by frame, respectively, then sends the calculateZ results to the optimum quantizat~on deciding unit 22. The deciding unit 22 decides the optimum coding unit 1 opt for outputting the quantizat-on error e(n), having a minimum quantization error power, which carries out the optimum quantization and then sends the coder number signal Nopt indicating the coder number of the decided optimum coding unit 1 opt to the selector 3 and the multiplexer 4.

In response to this coder number signal Nopt, the selector 3 sends one frame of the optimum quantized value I(n)opt of the optimum coding unit lopt indicated by the coder number signal Nopt to the multiple~er 4.

The multiplexer 4 adds the coder number signal Nopt to the optimum quantized value I(n)opt and transmits the multiplexed signal to the receiver side.

In the receiver ~ide, the received signal is divided into the quantized value I(n)opt of the prediction error E(n) and the coder number signal Nopt, and input to the inverse quantizer 71 and the controller 72, respectively.

The inverse quantizer 71 inverse-quantizes the quantized value I(n)opt to obtain the original prediction error E(n), then sends the prediction error E(n) to the adder 73. The adder 73 adds the inverse-quantized prediction error E(n) and the predicted value (n) predicted by the predictor 74 on the basis of past recovered voice signals to recover the current voice signal S(n). The predictor 74 carries out the adaptive prediction according to the output signal of the inverse quantizer 71.

12~Z~7~ - 12 On the other hand, the controller 72 controls the step size upGating coefficient M(I(n~) of the ir.verse quantizer 71 to coincide with the step size updat-r.g coefficient M(I(n~)opt of the opt mum cod-ng unit 1 ?seiected as the coding unit carrylng out the ODt7mUmquantization for a relevant frame at the 1:ransmi'ter side. That is, the controller 72 reads out the ste size updating ccefficient M(I(n)) from a ROM br~ using the coder number signal Nopt as an address, then sends it to the inverse quantizer 71, and an inverse quantization step adapter in the inverse quantize~ 71 changes the updating coefficient M(I(n3).

As described above, in the codinq transmission equipment of the present invention, the coder of the transmitter side selects the optimum coding uni~ which carries out the optimum quantization from among a plurality of adaptive differential coding units having different step size updating coefficients, then transmits the quantized value thereof with the coding unit number, and the decoder of the receiver side recovers .he original voice signal from the received quantized value by using the optimum step size updating coefficient indicated by the received coding unit number, whereby an improvement of the quantity of the recovered voice signal is achieved.

Although a preferred embodiment has been described eretofore, various modifications and alterations are possible within the scope o~ the present invention.

For example, means for changing the optimum updating coefficient M~I(n)) in accordance with the received coder number signal Nopt at the receiver side is not limited to the above described example. The receiver can have a plurality of inverse quantizers having different step size updating coefficients corresponding to those of the transmitter side, and can recover the voice signal by selecting the optimum inverse quantizer by the selector according to the received coder number ~29ZQ71 - 13 signal. Also the receiver can have a plurality of decoders having difference processing characteristics corresponding to each coding unit at the transmitter side, and can select the optimum decoder according to the received coder number signal.

Figur~ 6 shows another embodiment of the coder according to the present invent-on. This coder is improved so that a better qu~iity recovered voice signal is obtained at the receiver side in comparison with the coder of Fig 3.

The quantizer, the inverse quantizer, the zero predictor, and the pole predictor in each adaptive differential coder 11 to lk are adaptive controlled, and therefore, the parameters deciding the inner condition of the coding unit, for example, the quantization step size ~(nJ of the quantizer and the inverse quantizer and the prediction coefficient and tap data of the predictor, etc., vary successively at each of the differential coding units ll to lk according to the processing procedure of the input signal, and become different at each differential coding unit.

Accordingly, when the prior selected differential coding unit is changed to the other differential coding unit currently selected at a boundary between frames, in order to select the optimum differential coding unit, the aforementioned parameter of the prior selected coding unit and that of the currently selected coding unit are different. As a result, when the differential coding unit is switched, a discontinuity in the process of the input signal arises at the boundary between frames due to the difference between these parameters, therefore the discontinuity of the recovered signal also arises at the boundary between frames in the receiver side, and thus the recovered voice has a poor quality.

The coder shown in Fig~ 6 is intended to solve this problem.

The difference between the coder of Fig. 6 and the ~,, 12~Z~7~ - 14 coder of Fig. 3 is that the coder of Fig. 6 is provided with a parameter copy controller 6. That is, parameter3 Pl to Pk from the differential coding units 11' to lk', which parameters determine the inner condition of the coding unit, and the coder number ~ignal Nopt from the evaluating unit 2 are input to the pa_amete_ copy controller 6. Here, parameters Pl to Pk are quantizing step sizes ~(n)l to ~n~k r zero prediction coefficients Czl to Czk , pole prediction coefficients Cpl to Cpk , tap data El to Ek ~ and tap datas Sl to Sk. The parameter copv controller 5 selects the parameter Popt of the optimum coding unit lopt according to the coder number sianal Nopt and then senas it to all of the differential coding units 11' to lk'.

The operation of the coder shown in Fig. 6 will be e~plained below.

The optimum coding unit lopt is decided by the evaluating unit at the end of one frame of the voice signal ~(n), and then the coder number signal Nopt is se~t to the selector 3, the multiple~er 4, and the parameter copy controller 5. The ~elector 3 sends one frame of the quantized value I(n)opt of the optimum coding unit lopt indicated by the signal Nopt to the multiplexer 4, and the multiplexer 4 then transmits this optimum quantized value I(n)opt with the coder number signal Nopt to the receiver side.

On the other hand, the parameter copy controller 5 selects the optimum parameter Popt of the optimum coding unit 1 opt corresponding to the coder number signal Nopt 3~ from among the input parameters Pl to Pk r and then sends that parameter Popt to all of the differential coding units ll' to lk'. Accordingly, all of the adaptive differential coding units copy the received optimum parameter Popt as an own parameter at the end of one frame.

That is, at each adaptive differential coding unit, the quantizing step size a (n)opt is written in the .

....

lZ9Z~7~ quantizer 11 and the inverse quantizer 12, the ze o pred_ction coerficient Cz. oDt and the tap data Eopt are written in the coefficient multiplier of the zero predictor 14 and the tap thereof, respective7~ ard 'he pole prediction coe'ficient Cp.opt and the ta~ data Co?~ are writ~en in the coef~lcient mult'?lier o. the pole predic_or 15 and the tap thereol, respe-'ive~ As a result, all of'the adaptive di~ e-entlal coding units 11' to lk' have the same inner conditlon as that of the currently selected adaptive differentizl coding unit 1' opt at the time of starting the nex~ frame. Accordingly, when, for e~ample, the adaptive differential coding unit ll' is selected as the optimum coding unit at current frame and then the adaptive differential coding unit 12' is selected at next frame, the parameter Pl of the different~al coding unit 1l' at the end point of the current frame coincides with the parameter P2 of the differential coding unit 12' at the starting point of the next frame, and therefore, no discontinuity arises at the boundary between the current and next frames.

' Figure 7 shows a further embodiment of the present invention. In the coding transmission equipment shown in Figs. 3 to 6, if the coder number signal Nopt indicating the optimum coding unit number is erroneous, the influence of the error extends over several frames, and thus the recovered voice at the receiver side has a poor quality.

The equipment of Fig. 7 is intended to solve this problem.

That is, the transmission equipment of Fig. 7 is provided with an error correction circuit for correcting the error arising in the transmitted optimum coder number signal Nopt.

In Fig. 7, the coder number signal from the evaluating unit 2 is input to the multiplexer 4 via an erro~ correcting coder 81, and then transmitted to the receiver side. On the other hand, at the receiver side, 1~9~(~7~ - 16 the coder number signal ~opt divided b~ the demultiplexer 6 is input, via an error correcting decoder 82, to the decoder 7, where-n the signal Nopt is inDu~ to the con_-olle~ 72 as shown in Fig. ~ for cont_olling '~e updating coefficient M(I(n)~ of the inverse quantize .

The error cor-ect ng code- 81 and the error c3r-ect~ng decoder 82 carry out the error correction as follows.

First, in this e~ample, the numoer OL differential coding units is four, and each coding unit is given the coder numbers l-00l-, "01", "lO", and "ll". Figure 8 shows an example of an error coding code, Fig. 9 shows 2 format of one frame of the transmission c-gnal, and Fig. 10 shows an e~ample of an error correction at the receiver side.

As shown in Fig. 8 the error correcting code ECC is obtained by converting "0" and "1" of each digit of the coder number into "00" and "11" respectively. The transmission code is configured by arranging the hlgh order digit error correcting code ECC(h1 of the coder number, the high order digit N(h) thereof, the low order digit error correcting code ECC(Q) the coder number, and the low order digit N(Q) thereof, in sequence.

Therefore, the coder number constituted by two digits is converted into the transmission code constituted by six digits and transmitted to the receiver side.

In the receiver side, the error correcting decoder 82 decides whether each of the high and low order digits of the received transmission code is "0" or "l" by a majority of "0" and "l" thereof, and corrects all digits of each of the high and low order digit of the received transmission code to nooo" or "ll~ n according to the majority decision, thereby recovering the correc~ high and low order of the digits of the coder number.

For example, Fig. lO shows the error correction of high order digits of the received transmission code at the receiver side.

As shown in Fig. lO, "001", "OlOn, .

~Z9Z~l - 17 and "100" of the high order digits wherein "1~ may be erroneous are correc~ed to ll000n.

Also, "011", "10 n, and "110" whe_eln "0" may be erroneous are correcte~ - ~o " 111" .

Various ~lnds of er-or correcting codes are kno~Jn, and a suitable e~~or c~_rec-ing co~e may ~e adopted according tO the number o. di-fer~ntial coding units and kind of coder number to be transmitted, etc.

Assuming that a sampling frequency of the input voice signal is 6.4 kHz, and the quantized value code o~ the prediction error is 2 bits at each sample point, an example of a bit allocation of the frame which transmits the quantized value codes of 16 sample points will be explained below.

One frame is constituted by 39 bits wherein 32 bits are the quantized value codes, each of which is 2 bits of I6 samples, 2 bits are the coder number, and 4 bits are error correcting codes ECC(h) & ECC(Q). Each frame is transmitted during 2.5 ms (= l/400 sec) correspondina to a 16 sample period at a sampling fre~uency of 6.4 ~Hz, therefore the transmission rate is 15.6 Kbit/sec.

~ Figure 11 shows still another embodiment of the receiver side of the coding transmission equipment according to the present invention. The difference between the embodiment of Fig. 5 and this embodiment of Fig. ll is that here a post-processing filter is provided. The post-processing filter carries out the signal process of the decoded signal in order to improve an auditory sense of the recovered voice. This postprocessing filter is disclosed in a paper entitled"Enhancement of ADPCM Speech by Adaptive Post-filtering" by V.

Ramamoorthy & N.S.

Jayant, published at the n ICC '85" held in the USA. According to this paper, when a transfer function A(z) of the pole predictor 76 of the predictor 74 and a transfer function B(z) of the zero predictor 75 thereof are as follows, ~z~z~ - 18 :2 B(z) = ~ b j=l ~ a transfer function Al(z1 of a pole post-~roczssing filter 86 of the post-processing filter 8 and a trans-e~ function Bl(z) of a zero post-processing filter 85 ls assumed to be 2S follows.

A'(z) = ~ i a Z j j=l ~ B'(z) = ~ bjZ i j=l ( O < u < 1, 0 < ~ < 1) The principal structure of th~ predîctor 74 and the post-processing filter 8 disclosed in this paper is shown in Fig. 12.

In Fig. 12, the reference symbols of the coefficient multipliers denote the coefficients thereof, respectively, for simplification of the explanation.

The predictor of the decoder 7 shown at left side of Fig. 12 comprises the zero predictor 75 including six time delay elements z 1, six coefficient multipliers bl to b6 r and an adder Al adding output signals of multipliers bl to b6 r which realizes the transfer function 8(z), and the pole predictor 76 comprising two time delay elements z 1, two coefficient multipliers al and a2 ~ and an adder A2 adding output signals of the multipliers al and a2 ~ which realizes the transfer function A(z). Output signals of the zero predictor 75 and the pole predictor 76 are added to the signal E(n) obtained by inverse quantizing the quantized value from the transmitter side at the inverse quantizer 71 (Fig. 11) by the adder 73, to obtain the recovered voice signal S(n~.

1~92~ -- 19 - A block at the right side in Fig. 12 is a postprocessing filter 8', which compris2s a zero postprocessing filter 85, a pole post-processing i7ter 86, and an adder 83. The zero post-processing 11ter 85 is rurther provided wi,h S coefficien. ~ultipliers ~ and 32 to ~ nse- ed be_ween coe-ficient ~ul ipliers bl to b2 and the adder Al, respectivelY, in co~p3risor.

with the zero predictor 75 in orde_ to car~y out the operation of 3 coefficient which is a powe_ of 3.

The pole pos.-processing fllter is provided with coefficient multiplier ~ and a2 inserted between coefficient multipliers al and a2 and the adder A2, respectively, in comparison with the pole predictor 76.

Output signals of post-processing filters 85 and 86 are added to the recovered voice output ~(n) at the adder 83, whereby t~e auditory sense of the recovered output is improved, and then output as an improved output Z.

The aforementioned coefficients bl to b6 ~ al and a2 are updated to adapt to the input signal, and must have the same values at the predictor and the postprocessing filter. Also, the coefficients a and ~ at the post-processing filter are updated to adapt to the input signal.

In Fig. 11, the post-processing filter 8, such as the filter sAown in Fig. 12, is disposed at Lhe output side of the decoder 7.

This post-processing filter 8 is provided with a coefficient weighting circuit 88 for controllin~ the coefficients ~ and ~ of the zero and pole post-processing filters 85 and 86.

The coefficients of the coefficients multipliers bl to b6 of the zero post-processing filter 85 and the coefficient multipliers al and a2 of the pole post-processing filter 86 are kept at the same values as those of the coefficients bl to b6 of the zero predictor 75' and coefficients al and a2 of the pole predictor 76', respectively, by the coefficient weighting circuit 88.

Also, the power coefficient ~ of the zero post-processing ~Z~2~7~L - 20 filter 85 and the powe- coefficient of the pole post-processirg filter 86 are ad,usted to suitab e values for car~ying cut a suitable post-processing bv - the coef icient weighting circuit 88. The coef~-cien a ~ and 3 are set to a value between 0 to 1. ~Jote in this example, a is 1.0 and 3 is ~ 2.

As described above, the recovered voice out?ut having a further improved quality can be obtained by adding the post-processing filter having a characteristic which is changed according to the characteristlc of the adaptive differential coding unit at the transmitter side.

In the embodiments explained hereinbefore, the presen~ invention is applied to the ADPCM system. But, of course, the present invention is not limited to the ADPCM system, namely, the present invention may be adapted to systems wherein the adaptive quantizaiion is carried out as, for example, APCM (Adaptive Pulse Code Modulation), ADM (Adaptive Delta Modulation), SBC (Sub Band Codinq), VRC (Variable Rate Coding), APC ~Adaptive Predictive Coding), and ATC (Adaptive Transform Coding), etc.