Predictor stage for a digit rate reduction system

A predictor stage for a digital processing system for sampled and PCM coded signals in the form of a sequence of samples x.sub.n, having first transmission means and first reception means for forming a predicted sample x.sub.pn. Second transmission means are also provided for subtracting said sample x.sub.pn predicted from said sample x.sub.n coming in at the transmission side, and deriving therefrom a difference sample d.sub.n representing the prediction error. The first transmission means has a recursive structure, with each predicted sample x.sub.pn being derived from the samples d.sub.n, d.sub.n-1, d.sub.n-2 . . . etc. previously obtained at the output of the second transmission means. The first reception means has a transverse structure, with each sample x.sub.pn being derived at the reception side from the difference samples d.sub.n, d.sub.n-1 . . . , previously received. The samples x.sub.pn predicted at the reception side are added to the difference samples within the second reception means.

The present invention relates to the digital transmission of PCM coded 
information and more particularly to the digital processing for reducing 
the digit rate on the transmission line. 
Signals capable of being processed by the system may be telephone signals 
such as speech, data, sound, telegraphy, multifrequency signals, etc . . . 
, image signals and, generally, all digital coded information presenting a 
redundancy. 
Systems for reducing digit rates are already known. They achieve a 
compression of the digit rate by exploiting, for the transmission of a 
signal sample the knowledge acquired upon the transmission of the 
preceding samples. They usually consist in a cascaded arrangement of three 
stages; the first stage, called predictor stage, makes it possible to 
replace the input PCM signal by a signal d.sub.n representing the 
difference between the input PCM signal and the predicted value of this 
sample calculated from the preceding samples. In a second stage, called 
automatic gain compression stage, the amplitude of the difference signal 
d.sub.n derived from the first stage is divided by an estimator of the 
mean power. A third stage, called quantizer, effects coding of the samples 
derived from the second stage and furnishes, at the output, a digital 
signal of reduced redundancy formed by words of fixed length or of 
variable length. An equivalent device at reception enables the PCM signal 
to be recovered. 
It is an object of the invention to provide a digital processing device 
which is stable even in the presence of transmission errors. 
The invention consists in providing a digital processing system comprising 
a predictor stage with recursive structure at transmission and a predictor 
stage with transverse structure at reception. The reception predictor in 
this case having a finite memory, the trailing of the error is limited and 
does not affect the convergence of the system. 
The invention is applied to all systems with linear prediction, for example 
with matchable prediction or fixed prediction.

With reference to FIG. 1, the predictor stage of the transmission part of 
the known digit rate reduction devices is usually composed of an 
adder-subtractor circuit 1 which receives the samples x.sub.n of the input 
PCM signal S and furnishes at the output a signal d.sub.n representing the 
difference between the value of the incoming sample x.sub.n and the 
predicted sample x.sub.pn. This predicted sample x.sub.pn is furnished by 
a predictor 2 from a sample x.sub.n furnished by an addition circuit 3. 
The predictor 2 is usually a fixed predictor, or an adaptative predictor 
as in the case of Applicant's earlier French Patent Application No. 79 20 
445. 
The addition circuit 3 receives the sample x.sub.pn predicted by the 
predictor 2 and adds it to the estimated sample d.sub.n furnished by a 
reverse quantizer 4. The difference sample d.sub.n furnished by the 
addition circuit 1 is applied to the input of a quantizer 5 which 
furnishes, at the output, samples y.sub.n with reduced rate. 
At reception, the signal y.sub.n is received by the quantizer 5' which 
furnishes a difference signal d.sub.n at the input of a predictor stage. 
This predictor stage is essentially composed of an addition circuit which 
adds the difference signal d.sub.n derived from the reverse quantizer 5', 
to the sample x.sub.pn furnished by the predictor 2. 
A recovered signal x.sub.n is available at the output of the addition 
circuit 1'. This signal x.sub.n is, on the one hand, applied to the input 
of the predictor 2', on the other hand is available at the output of the 
rate reducing device. 
FIG. 2a schematically shows the predictor stage of the prior art thus 
described when the noise ascribable to the quantizer is disregarded. The 
predictor stage is usually in the form of a transverse filter at 
transmission making a prediction from the incoming signal x.sub.n and 
furnishing samples x.sub.pn predicted from the preceding samples; to this 
end, a predictor 11 furnishes coefficients a.sub.i, fixed or optimized 
according to the systems, such that 
##EQU1## 
where N is the number of coefficients of the predictor. 
When the coefficients are reactualized by means of any algorithm, for 
example the gradient algorithm or the Kalman algorithm, the rate reducing 
device is better adapted to the different statistics of the signals, but 
is unstable at reception in the presence of transmission errors. 
In fact, the reception predictor stage of the known systems as shown in 
FIG. 2b recalculates the signal x.sub.n from the erroneous signal, for 
example d.sub.n, received. The reactualized coefficients used by the 
predictor 2' are then erroneous and the new sample x.sub.n+1 is also 
erroneous. The error is propagated in the system, the algorithm 
reactualizing the coefficients at reception diverges from that of the 
transmission device (FIG. 2a), and may cause instabilities. Such a 
phenomenon is explained mathematically by observing that the structure of 
the predictor stage (FIG. 2b) is recursive at reception and that the 
transfer function of the reception device presents poles. 
Such a design of the predictor stages at transmission and at reception 
requires heavy protection in the case of using the system on a noise 
channel. 
The transmission and reception predictor stages according to the invention 
are shown schematically in FIGS. 3a and 3b respectively. With reference to 
FIG. 3a, the incoming signal S is a signal sampled and coded in digital 
code. From each incoming sample x.sub.n is subtracted a predicted sample 
x.sub.pn furnished by a predictor 11 by means of a subtracter circuit 10 
which furnishes difference samples d.sub.n at the output representative of 
the difference between the value of the incoming sample x.sub.n and of the 
predicted sample x.sub.pn furnished by the predictor 11. The predictor 11 
receives, according to the invention, at its input, the difference signal 
d.sub.n transmitted in line and furnishes the signal x.sub.pn to the 
subtraction circuit 10. The difference signals d.sub.n are thus formed 
from the preceding sample d.sub.n-1 according to the recurrent form of the 
type: 
##EQU2## 
where a.sub.i are the N coefficients furnished by the predictor 11. 
The predictor 11 may be of the type with fixed coefficients (a.sub.i) or of 
the type with optimized coefficients (a.sub.i).sub.i=1,N. In this latter 
case, the coefficients are reactualized, for example at each sampling 
period according to the gradient algorithm, according to the Kalman 
algorithm or by any algorithm well known in the art. 
Such a predictor stage presents a recursive structure at transmission. The 
transfer function of the transmission device is of the type: 
##EQU3## 
where the (a.sub.i).sub.i=1,N are the coefficients of the predictor 11. 
FIG. 3b shows the predictor stage of the reception device. It consists 
essentially in a predictor 11', identical to that of the transmission 
device, furnishing from the difference signal d.sub.n received a predicted 
signal x.sub.pn, which is added to the signal d.sub.n by means of an adder 
circuit 10', to form the reconstructed signal x.sub.n. Such a predictor 
stage is, at reception, of transverse structure. It makes at reception the 
prediction from the difference signal d.sub.n. However, the transfer 
function is of the type 
##EQU4## 
at reception and does not present any pole. Therefore, whatever the values 
of the coefficient a.sub.i of the predictor 11' at reception, this digit 
rate reducing device according to the invention will be stable as it will 
have at reception a predictor stage of transverse structure. Therefore, 
even in the presence of transmission errors, the device of the invention 
will be stable. 
FIG. 4 is a more complete representation of the transmission and reception 
devices for the digital processing of the coded signals according to the 
invention. The PCM incoming signals S received by the addition circuit 10 
is converted into a difference signal d.sub.n by subtraction of the signal 
x.sub.pn predicted by the predictor 11. The difference signal d.sub.n from 
the addition circuit d.sub.n is converted into a reduced rate signal 
y.sub.n by means of a quantizer 12 well known in the art. Such a quantizer 
has already been described in Applicant's French Patent Application No. 79 
20445. It essentially converts the high rate samples d.sub.n into a series 
of low rate samples y.sub.n as a function of the conditional probability 
distribution of the signal d.sub.n to be quantized. 
The choice of the quantizing curve adopted, according to whether the signal 
to be processed is a data signal or a speech signal, may, in the same way 
as in the above-mentioned Application, be determined by the knowledge of 
the vector of the coefficients (a.sub.j).sub.i.ltoreq.j.ltoreq.N of the 
predictor 11. 
A reverse quantizer 13 mounted on a negative feedback loop receives at its 
input the signal y.sub.n transmitted in line and furnishes a signal 
d.sub.n identical to the one which would be received at reception in the 
absence of transmission errors. This magnitude d.sub.n is applied to the 
input of the predictor 11. 
At reception, the low rate signal y.sub.n is received by the reverse 
quantizer 13' which furnishes at the output a difference signal d.sub.n. 
This difference signal d.sub.n is applied on the one hand to the input of 
the predictor 11', and on the other hand to the input of the adder circuit 
10'. 
This adder circuit 10' adds the signal d.sub.n from the inverse quantizer 
13' to the predicted signal x.sub.pn furnished by the predictor 11', and 
furnishes at the output a reconstructed signal x.sub.n whose rate is equal 
to that of the input signal S. 
The predictor 11 and 11' of the transmission and reception devices may for 
example be able adaptative predictors applying the gradient algorithm. 
There is then a recurrent equation connecting the N coefficients A(n), 
enabling them to be reactualized 
EQU A(n+1)=A(n)+K(n) 
where A(n)=[a.sub.1 (n) - - - a.sub.N (n)] is the vector of the 
coefficients, and where K(n) represents the correction vector at instant 
n,K(n) may be calculated by any algorithm used in prediction techniques, 
for example the gradient algorithm, Kalman algorithm or the like. 
However, in order to avoid any instability at transmission and to ensure 
convergence of the transmission and reception devices, in particular of 
the transmission and reception predictor stages, a leakage term (1-.beta.) 
is added to the coefficient reactualization algorithm, .beta. being a 
constant of the order of 10.sup.-3. The equation of reactualization of the 
coefficients reads then: 
EQU A(n+1)=(1-.beta.)[A(n)+K(n)] 
FIG. 5 shows a modified embodiment of the invention, in which the 
prediction for the speech signals is improved. A filter 14 is disposed at 
the input of the transmission predictor stage which makes a prefiltering 
promoting the prediction of the system according to the invention. 
This filter 14 receives the incoming signal S composed of the samples 
x.sub.n and introduces a delay T amounting to a sampling period, said 
delay being equivalent to that of a conventional fixed predictor of the 
first order optimized for speech. The signal x.sub.pn furnished by the 
filter 14 is subtracted from the signal x.sub.n by means of the 
subtraction circuit 15. The signal S' furnished at the output of this 
subtraction circuit 15 is a signal having the same rate as S, but 
presenting a minimisation in energy for the low speech frequencies. 
This signal S' is then processed in the same way as in the devices of the 
invention described with reference to FIGS. 3a and 4. In fact, a predictor 
11 of a type known per se, for example an adaptative predictor functioning 
according to the gradient algorithm as described in Applicant's French 
Patent Application Ser. No. 79 20445, furnishes a prediction from the 
difference signals d.sub.n furnished by the preceding samples at the 
output of the addition circuit 10. 
The predictor 11 furnishes at its output a predicted signal x.sub.p2n which 
is added to the sample x'.sub.n contained in the signal S'. 
The difference samples d.sub.n furnished at the output of the transmission 
predictor stage of FIG. 5 enable properties of the data signals to be 
substantially unaltered and, for speech signals, and a gain to be obtained 
comparable to that obtained at the output of an adaptative predictor with 
the same number of coefficients. 
The transmission digital processing device of the invention, and 
particularly its predictor stage, is illustrated in detail in FIG. 6. The 
subtracter circuit 10 furnishes, from the incoming samples x.sub.n and the 
predicted samples x.sub.pn, differences samples d.sub.n representing the 
difference between the value of the incoming sample x.sub.n and predicted 
sample x.sub.pn. This outcoming sample d.sub.n is converted into a low 
rate signal y.sub.n by means of a quantizer 12 well known in the art. A 
reverse quantizer 13 mounted on a negative feedback loop receives at its 
input the signal y.sub.n transmitted in line and furnishes, at the output, 
a signal d.sub.n identical to the one which would be received at reception 
in the absence of transmission errors. This signal d.sub.n is applied, on 
the one hand, to the input of the predictor 11, on the other hand, to the 
input of the calculating device 16 reactualizing the coefficients 
(a.sub.i).sub.1.ltoreq.i.ltoreq.4 of the predictor 11. This device 16 is 
well known in the art. It applies the gradient algorithm, the Kalman 
algorithm or any algorithm known per se. This device 16 furnishes the 4 
coefficients a.sub.1, a.sub.2, a.sub.3, a.sub.4 in the embodiment of FIG. 
6 to the predictor 11 at the input of the four multiplier circuits 110, 
111, 112, 113; each of these multiplier circuits 110, 111, 112, 113 
receives the signal d.sub.n furnished by the reverse quantizer 13 
respectively delayed by a time T, 2T, 3T, and 4T by means of four shift 
registers 114, 115, 116, 117 respectively. The two signals obtained at the 
output of the multiplier circuits 110, 111 are applied to the input of an 
adder circuit 118. The result obtained at the output of this adder circuit 
118 is added to the result of the multiplier circuit 112 by means of the 
adder circuit 119. The result obtained at the output of the adder circuit 
119 is applied to the input of the adder circuit 120 by means of which it 
is added to the result obtained at the output of the multiplier circuit 
113. The result obtained at the output of the adder circuit 120 is 
predicted signal x.sub.p2n. This prediction is improved according to the 
invention by inserting a transverse filter 14 constituted by a shift 
register 140 followed by a multiplier circuit 141. To this end, an adder 
circuit 17 adds the predicted sample x.sub.pn, furnished at the input of 
the adder circuit 10, to the sample d.sub.n obtained at the output of the 
reverse quantizer 13 to form the reconstructed sample x.sub.n. It is this 
sample x.sub.n which is successively delayed by a time T by means of the 
shift register 140 then multiplied by a fixed coefficient b.sub.o by means 
of a multiplier circuit 141. The sample x.sub.p1n furnished at the output 
of the transverse filter 14 is added to the predicted sample x.sub.p2n 
furnished by the predictor 11 by means of the adder circuit 15. At the 
output of the adder circuit 15, a predicted sample x.sub.pn is obtained 
which is used at the input of the adder circuit 10. 
The samples d.sub.n furnished at the output of the adder circuit 10 are 
converted into a signal y.sub.n with low sample rate by means of the 
quantizer 12. 
In this case, if the quantizing levels are sufficiently fine (the bit 
number of the words of signal y.sub.n is greater than or equal to 3 bits), 
and if the quantizer does not clip the signal, the induced quantizing 
noise is white. 
Referring now to FIG. 7, another mode of insertion of the transverse filter 
14 is provided according to the invention. The pre-filtering operation is 
effected directly at the input of the transmission predictor stage, from 
the incoming samples x.sub.n. The filter 14 furnishes samples, at the 
output, which are added to the predicted samples, derived from the 
predictor 11, by means of an adder circuit 15. The sample from the circuit 
15 is applied to the input of the subtracter circuit 10 to be subtracted 
from the incoming sample x.sub.n. The sample d.sub.n from the subtracter 
circuit 10 is applied to the input of the quantizer 12. On the negative 
feedback loop, from the samples y.sub.n, the reverse quantizer 13 
furnishes reconstructed samples d.sub.n at the input of the predictor 11. 
The predictor 11 associated with its coefficient reactualization device 
(not shown) furnishes predicted samples at each instant at the input of 
the adder circuit 15. Such a mode of insertion of the transverse filter 14 
outside the negative feedback loop on which the inverse quantizer is 
inserted, procures a more coloured transfer function noise reading 1/1- 
B.sub.o (z) (white noise filtered by the fixed predictor), which may be 
interesting subjectively in the case of speech processing. 
FIG. 8 schematically shows the reception device corresponding to the 
transmission device of FIGS. 6 and 7. The incoming signal y.sub.n is 
applied to the input of the reverse quantizer 13' which furnishes, at the 
output, samples at the input of the adder circuit 10' and at the input of 
the predictor 11'. This predictor 11' associated with its coefficient 
reactualization device (not shown), furnishes at the input of the adder 
circuit 15' predicted samples, which are added to the samples furnished by 
the transverse filter 14'. The prefiltering operation carried out by this 
filter 14' is effected from the reconstructed sample x.sub.n furnished at 
the output of the addition circuit 10'. The adder circuit 15' furnishes at 
the output samples which are added to those derived from the reverse 
quantizer 13' by means of the adder circuit 10'.