CA1156369A - Adaptive differential pulse code modulation system for speech or like signals - Google Patents

Abstract

Abstract of the Disclosure An adaptive differential pulse code modulation system is described in which the problems of instability and deterioration of signal to noise ratio are mitigated. The transmitter of the system includes a subtractor for obtaining the difference between an input signal and a predicted value, a quantizer for quantizing the output signal given from said subtractor, and a decoder which stores the output signal given from said quantizer, multiplies at least one past signal of those output signals given from said quantizer by at least one coefficient, calculates the sum of the products as results of the multiplications, adaptively corrects the coefficient so as to lessen the output signal given from the quantizer, and produces the sum of the products as a predicted value or a part of the predicted value. The receiver of the system includes a decoder which receives and stores the output signal given from the quantizer in the transmitter, multiplies at least one past output signal given from the quantizer by at least one coefficient, calculates the sum of the results of the multiplications, adaptively corrects the coeffici-ent so as to lessen the output signal given from the quantizer, and produces the sum of the multiplication results as a predicted value or part of the predicted value, and an adder for calculating the sum of the predicted value and the output signal given from the quantizer and producing it as a repro-duction signal.

Description

This is a divisional application of copending Canadian Patent application serial NoO 336532 filed SeptO 27, 1979 filed in the names of Araseki and Ochiai and subsequently assigned to Nippon Electric CoO, Ltd.
The present invention relates to a differential pulse-code modulation ~DPCM) srstem and, more particularly, an adaptive DPCM ~ADPCM~ system for performing frequency band compression of speech or like signals.
The DPCM s~stem utilizing the redundancy of a speech signal is a band compression system in which the prediction of an amplitude of each sample of the speech signal at the present time point is made on the basis of the past speech signal sampleO This is because the speech signal samples have a great correlation with each otherO The simplest method of the DPCM is to give as a predicted Yalue a sample preceding the present sample or the product of that preceding sample and a value slightly smaller than lo The DPCM system improves b~ about 6 dB of signal to noise ~S/N) ratio o~er the PCM system when speech signals are transmitted with the same number of bits. In other words, the DPCM system can save about 1 bit per sample compared with the PCM system when the signals are transmitted with the same S/N ratioO
As a practical matter, a plurality o the past samples as well as one past sample ma~ be used for the purpose of the band compression. In greater detail, a predicted value ~j of a speech signal ~sample) Xj at a time point L is gi~en by:

j-2 + A2 Xj-2 + o o o o An ~ Xj o~o~1) ~ere Al, A2, OOO~ An are called the prediction coefficients and are so selected as to lessen the difference between Xj and~j, iOe~ a prediction .

1 15~36g errorO Once the optimum prediction coefficients for the speech signal are selected, an adequate increase of n (about 5 to 8) improves S/N ratio by approximately 10 dB compared with the PCM system. The characteristic of the speech signal varies with time, so that the optimum values of the co-efficients also changeO Therefore, if the optimum prediction coefficients are seiected adaptively to the time-variation of the speech signal, the S/N
ratio can be improved by approximately 14 dB. This improvement can be similarly achieved for other signals lying within the bandwidth of a speech signal, such as signals given from a date modem tmodulator-demodulator) equipment by using the DPCM system.
The prediction coefficients are obtained b~ the following two methods: one is to analyze a speech signal for the optimum prediction co-efficients and the other is to adaptively correct the prediction coefficients so as to lessen the prediction error while the prediction error is being observedO The former method must transmit the quantitized prediction error signal and the prediction coefficients obtained. The latter method need not transmit the prediction coefficients, resulting in simplifying the circuit structure in the system. An ADPCM system using the latter method is discussed by DA~ID L. COHN et alO in his paper entitled "The Residual Encoder - An Improved ADPCM System for Speech Digitization", IEEE TRANSACTIONS ON COMMUNICA~
TIONS, VOL, COM-23, No. 9, September issue, 1975, pp. 935-9410 However since the ADPCM system is vulnerable to transmission errors, the system needs extra hard~are to eliminate these errors, which causes deterioration of the S/N ratio and makes the system complicated and costly to manufactureO
Accordingly one object of the invention is to provide a transmitter for an ADPCM system which has a simple circuit construction and which is stably .

115~36g operable with a great improvement of S/N ratioO
According to one aspect of the invention, there is provided a transmitter for an adaptive differential pulse code modulation system compris-ing: a subtractor for obtaining as output signals Ej the differences between input signals Xj and predicted values Xj; a quantizer for quantizing the output signals Ej from said subtractor to obtain quantized output signals Ej; and a transmit decoder receiving said quantizer output signals and generating there-from said predicted values, said transmit decoder comprising a predictor, having no feedback loop, for receiving the output of said quantizer and generating therefrom said predicted valuesO
The present invention and that of copending Canadian application serial No. 336532 will now be described in greater detail with reference to the accompanying drawings, in ~hich:
Figure lA is a schematic block diagram of a conventional ADPCM
system;
Figure lB is a representat~on of waveforms for describing the system sho~n in Figure lA;
Figure 2 is a schematic block diagram of a first embodiment of the invention; and Figure 3 is a schematic block diagram of a second embodiment of the invention~
In the drawings, like reference numerals represent like structural elementsO

~3 . ~ .. . . .

, 1 15B36g A conventional ADPC~I system will be describcd with referencc to Fi~ures lA and lB. ~efore commencing the description, it should be under-stood that, although the wavefor~s are expressed in analog form in Figure lB, digital signals are used in the systems sho~ in Figures lA, ~, and 3.
Althou~h not shown, analog to digital con~erters are used at appropriate locations, such as the preceding stage of the ADPCM systems for converting the analog signals into digital signals.
r~eferring to Figure lA, a speech signal Xj to be transmitted is applied to a terminal 1 of a transmitter at a time point ~. A difference signal Ej between the input signal and the output signal Xj given from a predictor 30 is obtained by a subtractor 10, is quantized by a quantizer 20 and is outputed from a terminal 2. The output signal Ej of the quantizer 20 and the predicted value Xj are added to each other in adder 40 and the result of the addition is sent to the predictor 30. The predictor 30 produces the predicted value Xj using a past input signal Xj 1 inputed to the predictor 30. The predicted value is given by:

N

Xj = ~ Ai Xj-i ''' (2) i=l where Ai(i = 1 to N) are prediction coefficients. The coefficients Ai are adaptively corrected in accordance with equation (3).

i i g Fl (X~-i ) F2 (~j) ........................ (3) where g is a positivc small value, which is about ~ 3, and Fl and F2 are non-dccrcase functions.
The predictor 30 and tlle addcr 40 servc as a local decoder.

..~i 115B36~

A recciver, that is, decoder reccives thc signal transmitted from the transmitter at the terminal 3. An addcr 140 calculates the sum of the incoming signal and the output signal X; given from a predictor 1~0. The adder 140 then prodlices a reproduction signal Xj through a terminal 4. The decoder opcrates in the same manner as th~t of the transmitter. ~hen a predictor 130 and the adder 140 are identical, respectively to predictor ~0 and adder 40 of the transmitter, the reproduction signal X~ in the receiver is e~actly the same as signal X~ given from the adder 40 of the transmitter.
In this manner, without transmitting the prediction coefficients, the pre-diction coefficients can be obtained on the basis of only the quantized pre-diction error signal for reproduction of an original signal. The predictor 30 or 130 may be composed of the type shown in Figure 1 on page 936 in the above-mentioned article by David L. Cohn et al.
In an actual transmission line, since a transmission error takes place frequently, however, the above-mentioned discussion cannot be applied to the practical system. To be more specific, the prediction errors produced are different from each other at the transmitter and receiver and therefore a reproduction signal is greatly different from an original signal. For the gratual elimination of the adverse effect of the transmission error once produced, the following equation to correct the prediction coefficients is used:
A i = Ai (1-~) + g Fl (Xj-i) F2 ~j) where i = 1 to N and, C is a positive value much smaller than 1, and g is a propcr positive constant. As ~ becomcs larger, the adverse effect of the tr~nsmission error disappears more rapidly, resulting in degracling thc prediction perforl~ance. For e~mple, when ~ is sclectcd to be a practical valuc, the im~rovement of S/.~ ratio is 10 dB or less. This restricts the ~.

36~

selection of thc value of ~ so as not to bc largcr in value. The constraint of said selection allows the case where an error produced beyond the error eliminating ability greatly degrades the speech ~uality. The most serious problem involved in the construction shown in Figure lA is the instability of the operation in the decoder on the receiver side having a feedback loop when a transmission error takes place. In such a situation, since the pre-dictor 130 and the adder 140 form a closed circuit, some of the selected prediction coefficients might cause the receiver to oscillate or to be un-stable in operation. In fact, it has been easily established in experiments that intentional causing of the transmission error results in occurrence of the oscillation or an unstable operation at the receiver. Once the operation becomes unstable, a long time is needed until the operation settles down to be stable. A countermeasure is taken for this problem as follows: Namely, by monitoring the prediction coefficient on the receiving side, an unstable operation is detected and some measure for its instability is taken on the basis of the detection. The countermeasure, however, encounters a difficulty in ehecking the stability of the operation, resulting inherently in the necessity of a large scale of the system.
In the present invention, the predicted value of the speech signal Xi is obtained from the output signal ~j of the quantizer, not from Xj 1' in the following manner:

M

Xj = ~ Bi Ej-i where Bji represent the prediction cocfficients in the predictors 50 and 150, situated respectively in the transmitter and rcceiver.
This apyroach avoids thc adoption of a closed circuit in both the ~6-115~36g transmitter and receiver, so that even occurrencc of the transmission error never renders its operation unstable.
Here, the coefficients Bi may be adaptively obtained by equation (5):

B ~ ) B i + g E j ~

wherein i = 1 to M, and ~ is a positive value much smaller than l to be used to erase a detrimental effect of the transmission error. In the absence of the quantizer 20, that is, when Ej = Ej, the transmitter serves as a filter performing the following operation:
~1 Ej = Xj = ~ Bi Ej_i .... (6) i=l with the transfer function of: .

.... (7) 1 + ~ Bi Ej-i i=l An implementation of the just-mentioned invention, which is a first embodiment, is illustrated in Figure 2 in block form. The transmitter quantize~ the difference between a signal Xj and its predicted value Xj by a quantizer 20 for transmission. The difference signal is obtained by a substractor 10 as before. The quantizer 20 may be easily realized by utiliz-ing techniques discussed in a paper "Adaptive Quantization in Differential PCM Coding of Spcech" by P. Cummiskey et al., The Bell Systcm Technical Journal, Vol. 52, No. 7, Septembcr issue, 1973, pp. 1105 to 111~. No detailed description of thc quantizer will be given hereunder. The output signal Ej of the quantitizer 20 is fed to a prcdictor 50. The prcdictor 50 calculates 7 _ . f ., ~......

1 15~3S~

predicted valuc ~j at a time point J by:

X. = ~ B~ E. . .... (8) ] 1 ~-1 i=l The coefficient B~i are adaptively corrected depending on equation ~5). On the receiver side, the quanti~ed prediction error Ej is applied to the predictor 150 which in turn produces a predicted value Xj in accordance with equation (8). An adder 160 adds the prediction error signal Ej to the predicted value ~j to produce a reproduction signal Xj. The reason why the output signal Xj of the adder 160 is used as an output signal will be apparent from the fact that if the quantizer 20 is not used, Xj = Xj. In the present invention, when the number M of the prediction coefficients is selected to be approximately 7 with a practical value of ~ ~i.e., about 2 6), S/N ratio for an incoming speech signal may improve by at least 10 dB compared with the PCM system.
Figure 3 shows a second embodiment of the invention using a pair of predictors, which further improves the performance over the first embodiment.
In the present embodiment, the predicted value ~?j of the signal Xj is express-ed by:
~j = Yj ~ Zj .--- (9) where Yj and Zj are given by:

j i ~j-i .... (10) i=l z3 = Ai Xj-l . ... (11) Bji arc corrected depcnding on equ~tion (53, and Ai rep~esentative of the prediction cocfficients in thc predictors 30' and 130' are corrected ~8-. .~ .

115~36g dependinT on equation (12).
j+l j "~ ~
l = A- (1-8~ ~ g Ej ~j-l .... (12) wllere ~T' = 2-3 ,_ Xj in equation (11) is calucated by the adder 60 in accordance ~ith the following e~uation (13):
Xj - Xj + Ej .... (13) Also, on the receiver side, the Yj and Zj are calculated by the predictors 130' and 150 and the predicted value ~j is produced from the adder 170. A reproduction signal is the output X; given from the adder 160.
In this embodiment, the predictor 130' and the adders 160 and 170 form a closed loop, so that there is a concern that the transmission error renders the operation of the decoder unstable. Since the number of the prediction coefficients Al in the predictor 130~ is 1, however, it is readily seen that ¦A1 ¦ <1 is a condition for the stability of the operation.
Actually, the instability may be eliminated by adjusting both the transmitter and receiver so as to have 0 ~ Aj ~ 0.9. In experiments carried out by c 1 the inventors, it was observed that such adjustment provides no degradation of the pcrformance.
In the present embodiment, if the coefficients of the predictors 50 and 150 are each 3, the S/N improvement of 14 dB is attained as compared with the PCM system. ~hen the signal to be transmitted includes only the speech signal, if Ajl is fixed at about 0.9, the performance never deteriorates.
The second embodiment having the two predictors appears complicated in structure. Ilowever, if pairs of ~j 1 and Ej i~ and A3 and Bi (i = l, 2, and 3~ are subj~ctcd to the sum of prodl~cts as is apparently understood from equations tlO) and ~11), each opcration of the predictors 30 and 50 and thc .

115636~

adder 70 is performcd at a time. Thc st~lcture of the embodiment shown in Figure 3 is thcrcfore comp.~rable to that of the first embodiment shown in Figure 2; rather, f~vorable rcsults are expected since the number of the prediction coefficients is reduced.
l~hen it is desired to increase the number of the prediction co-efficients in the predictors 30' and 130' of Fi~ure 3, there is no concern that the transmission error causes the operation to be unstable, provided each coefficient is fixed so as to stabilize the system as mentioned above.
On the other hand, when the number of the coefficients in the predictors 30' and 130' is small, such as 1 or 2, the judgement of the stability of the operation is performed easily despite the adaptive correction of the pre-diction coefficients in the predictors 30' and 130'. The predictors 30' and 130' used in the embodiment of the invention have the same constructions as those of the predictors 30 and 130 in Figure lA.
~ As mentioned above, the ADPCM system of the invention can ensure a perfect stability of the invention, even if transmission errors take place, with improved S/N ratio and a simple circuit construction.

_~ o_ , . . .

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A transmitter for an adaptive differential pulse code modulation system comprising: a subtractor for obtaining as output signals Ej the differences between input signals Xj and predicted values ?j; a quantizer for quantizing the output signals Ej from said subtractor to obtain quantized output signals ?j; and a transmit decoder receiving said quantizer output signals and genera-ting therefrom said predicted values, said transmit decoder comprising a pre-dictor, having no feedback loop, for receiving the output of said quantizer and generating therefrom said predicted values.

2. A transmitter for an adaptive differential pulse code modulation system comprising: a transmitter having a subtractor for obtaining as output signals Ej the differences between input signals Xj and predicted values ?j, a quantizer for quantizing the output signals Ej from said subtractor to obtain quantized output signals ?j, a first prediction means, having no feedback loop, for receiving the outputs of said quantizer and generating therefrom first inter-mediate transmit prediction values Yj, a first transmit adder receiving second intermediate transmit prediction values Zj and said first intermediate predic-tion values and providing said predicted values Xj to said subtractor, a second transmit adder receiving said predicted values and said quantizer out-puts and providing output signals, and a second transmit prediction means re-ceiving the output signals from said second transmit adder and generating therefrom said second intermediate transmit prediction values Zj.