Multistage selective differential pulse code modulation system

A differential pulse code modulation system, in which an input signal is at first approximated to a stepwise waveform having level changes at a constant number n of sampling points selected in the order of level magnitude from a predetermined number N of samples in the input signal. The remainder of the above approximating operation is secondary followed-up by delta modulation, which uses a variable step level controlled in accordance with the variation feature of the remainder. The differential pulse code modulation output is provided by the above two approximating operations.

FIELD OF THE INVENTION 
This invention relates to a DPCM (Differential Pulse Code Modulation) 
system, in which the difference between a predictive value and an input 
information value is coded. 
BACKGROUND OF THE INVENTION 
The PCM transmission system has widely been studied because of its many 
advantages on the picture transmission, and its typical examples are the 
DPCM coding system and the orthogonal transform coding method, both of 
which utilize the statistic property of a picture. 
The DPCM method makes use of statistic and visual properties of a level 
variation but is considered to have a difficulty in that an abrupt level 
variation and a gentle slope of a picture, which are extremely different 
in nature from each other, are processed by a certain signal operation. 
Meanwhile, the orthogonal transform coding system starts from an idea of 
determining an orthogonal transform in such a manner as to minimize a 
square mean error by a linear operation based on a correlation matrix of a 
picture and is the most effective with respect to a continuous Gaussian 
process or the like. However, a picture has a great feature in a contour 
portion having abrupt level variations. In this point, the orthogonal 
transform presents a problem. 
Further, when the picture is subjected to Hadamard transform, not only 
Gaussian noise but also irregular movements of an oblique straight contour 
portion, and defocusing of a contour out of synchronization with an 
orthogonal function may appear in some cases, making the picture 
unnatural. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a multistage selective DPCM 
system which overcomes the abovesaid defects of the prior art and is 
capable of coding with less quantizing distortion which is suitable for 
any states of variation of input information, that is, abrupt level 
variations approximate to stepwise waveforms and level variations like 
gentleslope waveforms. 
The multistage selective DPCM system of this invention is a system which 
achieves in a multiple stage operations of achieving DPCM on the basis of 
a certain selection principle in such a manner as to approximate a picture 
signal or like input information in the order of the magnitude of feature 
and further approximating the remainder of the above approximation in 
accordance with a standard based on its statistic property. 
In accordance with this invention, there is provided a multistage selective 
differential pulse code modulation system comprising: 
a first encoding stage for approximating an input signal to a stepwise 
waveform having constant n level changes at sampling points selected in 
the order of level magnitude from a predetermined adjacent N samples in 
the input signal by developing a first coded output indicative of said 
stepwise waveform and a remainder corresponding to a difference between 
said input signal and said stepwise waveform; a second encoding stage 
connected to said first stage for receiving said remainder to encode said 
remainder by delta modulation, which uses a variable step level controlled 
in accordance with the level variation feature of said remainder so as to 
provide a second coded output indicative of successive values of said 
variable step level of said delta modulation; and output means connected 
to said first stage and said second stage to send out said first coded 
output and said second coded output to indicate said input signal.

DETAILED DESCRIPTION OF THE INVENTION 
With the present invention, it is possible to achieve a substantially 
constant-speed scanning of a picture and transmit its coded output within 
a relatively narrow band. 
This invention will hereinafter be described in detail but, in order to 
facilitate a better understanding of the invention, a description will be 
given first in connection with a picture signal for use as input 
information to which this invention is applied. 
As is well-known, the correlation function of a picture signal such as a 
television signal is well approximate to an exponential function in the 
vicinity of the origin, as follows: 
EQU .rho.(x)=e .sup.-.alpha.x (1) 
The remainder of that portion of the correlation function which is 
expressed by the exponential function undergoes a small and gentle change, 
and this belongs to a low-frequency component in the television signal, so 
that a high-frequency component can be considered to be composed only of 
the portion expressed by the exponential function. 
As regards the stochastic process in which the correlation function becomes 
an exponential function, the following has been well-known. 
It is the process in which a code change follows the Poisson process, as 
shown in FIG. 1, that is, the stochastic process in which the probability 
P.sub.n (t).DELTA.t that a code change occurs n times in a time interval t 
to (t+.DELTA.t), is given by 
##EQU1## 
In this case, the mean value E(t) and dispersion V(t) of the number of 
code changes in the time t are as follows: 
EQU E(t)=V(t)=at (3) 
The correlation function .rho.(t) of this process becomes as follows: 
EQU .rho.(t)=e .sup.-2at (4) 
Accordingly, the power spectrum G(f) becomes as follows: 
##EQU2## 
Where .sigma. is the amplitude. In the high-frequency region (f&gt;&gt;a), the 
spectrum G(f) decreases at 6 dB/oct along with the frequency. A similar 
relationship also holds in the case of a stepwise waveform such as 
indicated by the broken line in FIG. 2, in which each amplitude is random 
and the mean square value is .sigma..sup.2. This is believed to be a 
completely integrated waveform of the Poisson pulse. In practice, 
measurement of the power spectrum indicates that is attenuates at 6 dB/oct 
along with the frequency, as typically shown in FIG. 3. In FIG. 3, the 
curves A, E and C respectively indicate a portrait of a person, a picture 
of the entire body of a person and a distant view. 
In picture information, a contour or outline attracts the man's most 
attention. This is also evident from the fact that a man sees a scene in 
terms of the mutual relation of boundaries of individual, independent 
objects. Mainly from the above facts, it is deduced that the 
high-frequency component of a picture results from discontinuity of 
differentiation in the neighborhood of the origin of the correlation 
function and that this is the most striking feature. The following will 
describe the multistage selective DPCM system of this invention under the 
condition of constant-speed scanning. 
(1) Approximation by stepwise waveform (first stage) 
Since it is believed that attenuation of frequency power spectrum of a 
picture signal at 6 dB/oct occurs because the signal is principally 
composed of such a waveform as shown in FIG. 2, it is the first-stage 
processing of the multistage selective DPCM of this invention is achieved 
by approximation of the input signal to a stepwise waveform from the 
standpoint of constant-speed scanning as follows: 
The picture signal is divided into groups at every N picture elements, and 
the features forming a contour are sampled in the order of magnitude. This 
feature is evaluated by the absolute value of the level difference between 
adjacent picture elements. The constant-speed scanning is made possible by 
selecting n points from N successive picture elements in the order of the 
absolute value of the level difference and transmitting information of 
each of the points within a certain number of bits. 
The above concept is shown in FIG. 4. In FIG. 4, the full line indicates a 
picture signal S(t), which is approximated to such a stepwise waveform 
S.sub.p (t) as indicated by the broken line. In the signal composed of N 
picture elements S(O) to S(N), the values 
EQU .vertline.S(t)-S(t-1).vertline. (where 1.ltoreq.t.ltoreq.N) 
are arranged in the order of magnitude, and n points are selected in this 
order while their times are selected as follows: 
EQU t.sub.0 .ltoreq.t.sub.1 .ltoreq.t.sub.2 .ltoreq.t.sub.2 .ltoreq. . . . 
t.sub.n .ltoreq.t.sub.N 
Then, by transmitting the following time intervals: 
EQU T.sub.i =t.sub.i -t.sub.i-1 , (where i=1, 2, 3 . . . N) 
the points forming the contour can be transmitted. Each sampled point is 
coded into "0" or "1", "0" indicating that no feature is selected, and "1" 
further indicating that a level variation is selected. The notation 
T.sub.i indicates the duration of "0" between the points of "1". The 
coding is achieved by a coding method for the construction of minimum 
redundancy, such as the Huffman coding method as shown in Table 1. In this 
case, zero occurrence probability P(0)=0.76 is optimum. In Table 1, 
"0.sup.5 " indicate a sequence of consecutive five "00000". 
______________________________________ 
ORIGINAL 
BINARY CODE 
SEQUENCE WORD 
______________________________________ 
0.sup.5 00 
1 01 
01 100 
001 101 
0001 110 
00001 111 
______________________________________ 
T.sub.i =8 indicates that "0.sup.5 001" follows the preceding bits "1", and 
this is coded into information "00101"; and T.sub.i =12 corresponds to 
"0.sup.5 0.sup.5 01" and coded into "0000100". As mentioned above the 
number of bits necessary for coding each time interval T.sub.i is as 
follows: 
If N=64 in a case where n=16, the number of bits is less than fifty-one 
bits. In an extreme case, there is a train of consecutive forty-six "0" 
accompanying with a train of consecutive eighteen "1" and three blocks of 
"01". In this case, 2.times.9+3+2.times.11+3.times.3=52 bits are 
sufficient for coding this train. Similarly, when n=12, forty-four bits 
will suffice. 
In order to perform it by a real time system to arrange level variations in 
the order of the magnitude of absolute value and to select n points in the 
period of N points, it is necessary to effect the operation in a time 
substantially equal to N. In this case, a method called "radix sort" 
enables linear classification and, in this case, by a selective level 
extracting circuit using a detector such as shown in FIG. 5, the absolute 
value of the level variation is detected by the detector and accumulated 
on some threshold values S.sub.k, S.sub.k-1, . . . S.sub.1, as in the case 
of a pulse height analyzer, whereby to obtain a level corresponding to n. 
The detector for absolute value of level difference can be formed as shown 
in FIG. 15 by a delay circuit of one sampling period 30, a difference 
detector 31 and an absolute value circuit 32. 
Next, the coding operation of S(t.sub.i)-S(t.sub.i-1) is also achieved in 
accordance with the Huffman coding method. A value n=15.5.apprxeq.16 is 
optimum corresponding to the entropy H(p) of zero occurence probability 
P(O)=0.76. 
The value of an input represented by 8 bits or more is coded into 6 bits, 
and this coding is achieved as shown in Table 2, where the probability 
distribution of the level variation is used as an exponential function. 
In this coding, bit allocation is based on the assumption that the 
probabilities of the absolute value of the level variation being less than 
3, less than 7 and less than 15 are 1/2, 3/4 and 7/8, respectively. In 
this case, the required number of bits is 4.5 bits on an average but 5 
bits are necessary for representing the level variation in anticipation of 
an operation margin. However, since there is substantially no possibility 
of selecting +1, it is omitted. 
______________________________________ 
LEVEL CODE LEVEL 
CHANGE WORD CHANGE CODE WORD HEADER 
______________________________________ 
2 000 -2 010 0 
3 001 -3 011 
4 10000 -4 10100 
5 10001 -5 10101 
6 10010 -6 10110 10 
7 10011 -7 10111 
______________________________________ 
______________________________________ 
LEVEL 
CHANGE .+-. 8 .about. .+-. 1 5 (HEADER 1 1 0) 
CODE 
WORD 1 1 0 0 0 0 0 .about. 1 1 0 1 1 1 1 
LEVEL 
CHANGE .+-. 1 6 .about. .+-. 3 2 (HEADER 1 1 1) 
CODE 
WORD 1 1 1 0 0 0 0 0 .about. 1 1 1 1 1 1 1 1 
______________________________________ 
The time intervals between adjacent points of level variations and the 
level variation value are transmitted in pairs. For example, if T.sub.i =7 
and if S(t.sub.i)-S(t.sub.i-1)=13.4, the data are transmitted in the form 
of 0010011000101. 
In such a case, if the points of level variation are assumed to have the 
sharpest slope, when each of such points is positioned at a middle point 
of a sampling period, most of them can be each correctly expressed by 
level variations at two points. 
The above is the processing operation of the first stage of the multistage 
selective DPCM system of this invention, by which the contour, which is 
the most striking feature, can be transmitted by a step-shaped waveform. A 
block diagram for such operation is illustrated in FIG. 6. In FIG. 6, such 
an input signal S(t) as shown at the top of FIG. 4 is sampled and 
quantized by a sampling and quantizing circuit 2 into a pulse train such 
as shown at the bottom of FIG. 4. The output pulses of the circuit 2 are 
applied to a extractor 3 of large level variations and a delay circuit 4. 
The extractor 3 selects a predetermined number of pulses, for instance, n 
pulses P.sub.t1, P.sub.t2, . . . P.sub.tn from the pulse train 1, 2, . . . 
N to provide gate pulses each corresponding to the timing t.sub.i of each 
pulse shown in FIG. 4. The sampled and quantized N pulses, delayed by the 
delay circuit 4 by substantially N picture elements, are selected by the 
gate pulses in a gate 6 and temporarily stored in a quantized output 
holding circuit 7 comprising a voltage hold circuit. A level difference 
between each output pulse of the quantized output holding circuit 7 and 
the immediately succeeding pulse is obtained by a level difference 
detector 8 such as a differential amplifier. Further, the time intervals 
of the output pulses of the selection level extractor 3 are detected by a 
time interval detector 5, which is formed as shown in FIG. 16 by a counter 
5-1, used for counting sampling pulses from a sampling pulse generator 
(not shown) after resetting by each of the pulses from the extractor 3, 
and an AND gate for reading out the count of the counter 5-1 in response 
to each of the pulses from the extractor 3. The pulse time intervals 
detected by the time interval detector 5 and the pulse level difference 
detected by the level difference detector 8 are separately coded and then 
combined in a first encoder 9 to provide codewords by the first-stage 
processing. The output pulses indicative of a difference between the 
output of the delay circuit 10 and the output pulse of the quantized 
output holding circuit 7 are subjected to the second-stage and subsequent 
processing. 
(2) Approximation for slant level change 
As a result of studies of picture transmission by the orthogonal transfer, 
it has been recognized that slant orthogonal transform is substantially 
optimum, that the form of function of the optimum orthogonal transfer is 
not so much varied for the moving picture, and that the slant orthogonal 
transform improves S/N about 3 dB with the same number of bits, as 
compared with the Hadamard transform. 
The above can be considered to indicate that there is no appreciable 
difference between the Hadamard orthogonal transform and the slant 
transform in the representation of the positional accuracy of the large 
level change point, that the slant orthogonal transform is more excellent 
than the Hadamard transfer in the ability of expression of the slant 
component, and that the indication of the slant component is of 
importance. 
Accordingly, most of the remainder provided after approximation to the 
contour portion by the step-shaped waveform are slope components and are 
considered to be further added with unselected contour portions of small 
level variations. The spectrum of the remainder obtained by subtracting 
the original signal from the stepwise waveform provided from the abovesaid 
first-stage processing operation attenuates at a rate of 8 to 12 dB/oct 
along with frequency, as shown in FIG. 7. A typical example of attenuation 
at a rate of 12 dB/oct along with the frequency is a double integral of a 
random poisson pulse process, and there is a trapezoidal wave as a wave 
whose envelope attenuates at a rate of 12 dB/oct. It is believed in that 
these are the principal part of the remainder. Moreover, it is considered 
that if an attenuation along with frequency is less than that at a rate of 
12 dB/oct at a part, small contour portions of 16 dB/oct are mixed with 
the part. The above can be ascertained in an actual waveform. Such a 
property is a secondary feature of the original picture signal, which is 
masked by the primary property of the step-shaped waveform having a 
frequency attenuation characteristic of 6 dB/oct and does not appear in 
the primary property until the primary property is removed. Accordingly, 
the multistage processing is significant for the remainders, which are 
successively obtained by the operations of approximating to the processes 
indicative of statistical properties arranged in the order of magnitude. 
In view of the above, the second-stage processing operation is required to 
be capable of following the slant of a waveform and approximating to level 
variations which are not so large and occur at times. 
The .DELTA. modulation and the .DELTA.-.SIGMA. modulation are suitable for 
transmitting gentle-slant waveforms and provide a certain degree of 
approximation but cannot well follow relatively large level variations and 
abrupt changes of the slope. Consequently, it is necessary to adopt such a 
method that the variation width is dynamically changed and, if follow-up 
to the level variation has been completed, the unit variation width is 
shortened. The same is true of the .DELTA.-.SIGMA. modulation method. 
FIG. 8 is a state transition diagram of the dynamic .DELTA. modulation 
system. The following up to a gentle slant waveform is achieved by moving 
among the states F, G, A and B. If a difference between the present value 
of a signal r(t), which is a difference between the value of the original 
signal and the approximated value obtained by the first-stage processing, 
and the output value of the preceding dynamic modulation has the plus 
sign, the operation proceeds in the direction of plus indicated by the 
arrow, by which a value shown under an oblique line is added to the 
preceding output value to obtain the present value. If the abovesaid 
difference has the minus sign, the operation proceeds in the direction of 
minus. 
In each case of the states A and F, a linear operation takes place 
regardless of whether the difference has a sign of plus or minus. However, 
in the cases of the states B and G, if the difference signals has 
respectively signs of minus and plus, the operation returns to the states 
F and A belonging to the linear region, or when plus and minus difference 
signals have been applied in the cases of the states B and G, 
respectively, it is decided that this is not a gentle slope but a 
non-selected level variation or a steep slope, which cannot be followed 
up. 
At a selected sampling point in the period including N picture elements 
extracted by the first-stage processing operation, an amplitude error is 
lower than a minimum quantizing level, so that the state of this point is 
returned to an initial state S.sub.0 and the operation is started 
therefrom, ensuring to remove a residual error and to prevent the state in 
which a large variation width occurs at this point. 
In a case where not a gentle slant but a level variation having a certain 
degree of magnitude occurs, the operation is brought out of the states G, 
F, A and E and the variation width is successively increased from the 
minimum variation width .DELTA. to other widths a.DELTA., 2a.DELTA. and 
3a.DELTA.. 
For example, when the operation is in the state C and the amount of output 
predicted in the dynamic .DELTA. modulation is x.sub.i, if the output of 
the residual component in the first-stage processing operation at the next 
sampling point is larger than x.sub.i, the operation shifts to the state D 
to produce a quantized output 2a.DELTA.. On the other hand, if smaller, 
the operation shifts to the state S.sub.0 to provide a quantized output 
-a.DELTA.. 
When the distribution of the non-selected level variation is the 
exponential one and na.DELTA. is chosen at present, if the probability 
that the larger level difference than na.DELTA. corresponds to 1/2.sup.n, 
a binary search method is suitable therefor. 
The state transition diagram is designed so that if a difference between an 
actual level and a coded output becomes smaller, the operation is as close 
to its initial state as possible. The reason is that since the operation 
is the second-stage one, a variation following a certain level variation 
may be considered to be gradual. Values .+-.a.sup.n .DELTA. may also be 
used in place of .+-.an.DELTA. and, if a.gtoreq.2, this is suitable for 
use in a case where a large level variation or a steep slope exists. 
When a&lt;2, it is suitable for use in a case where the level variation is 
limited to a certain range. 
Many modifications may be derived from the state transition of such a 
dynamic .DELTA. modulation system. The simplest one is such that when a 
level difference of a sign different from the preceding one, the operation 
is returned to its initial state. Even with this method, an appreciably 
dynamic effect can be obtained. 
Such a second-stage processing follows the first-stage processing and is 
achieved with such a circuit construction as shown in FIG. 9A. This 
selects the state in a manner to minimize a transitional error in 
accordance with the statistic property of a picture, and then alters the 
variation width in accordance with the selected state. In the case of 
a=1.5, such an operation as shown in FIG. 10 is performed. 
In FIG. 9A, a difference between the output of a preceding value prediction 
circuit 11 and the residual component of the first-stage processing 
operation is applied to a quantizer 12 and a state transition circuit 
(i.e. a sequential logic circuit 13.) The quantizer 12 quantizes an input 
difference signal under the control of the sequential logic circuit 13 
having transition states shown in FIG. 8 and supplies the quantized output 
to an encoder 14 of .DELTA. modulation. The sum of the outputs of the 
circuits 12 and 11 is stored as the next predictive value of preceding 
value in the circuit 11 comprising a register. The encoder 14 produces 
code words of the second-stage operation from the quantized output of the 
quantizer 12. 
In each period, values a and .DELTA. are selected in accordance with the 
mean square value of an error resulting from the first-stage processing 
operation, and information (a, .DELTA.) is classified with 2 to 3 bits to 
transmit at the beginning of each period, which gives a better 
approximation. Further, it is also possible to employ such a modification 
in which a secondary signal is successively divided into blocks of 
information for every two picture elements in place of setting up the 
state in the second-stage processing operation, so that if the level 
variation between the two picture elements, that is, the absolute value of 
a slope, is regarded as indicating a flat portion or a slope depending 
upon whether the abovesaid absolute value is smaller or larger than a 
certain value. 
(3) Approximation using correlation between scanning lines or frames (third 
stage) 
It can be considered that redundancy on one scanning line has been removed 
by the first- and second-stage processing operations and that the 
remaining error signal is white noise, and there does not exist any 
particular statistic property to be extracted. 
Accordingly, the property which should be selected at the next third stage 
is high correlation of signals between adjacent scanning lines. The main 
causes of high correlation between adjacent scanning lines are that almost 
all positions having large level variation along adjacent scanning lines 
occur within a slight deviation of points and that the value of level 
variation is rather small. The differences between the value of the 
preceding scanning line used as a predictive value and the value of the 
present scanning line are composed of random pulses and gentle 
low-frequency components, or components having no correlation because of 
switching in a picture. This is classified by use of the mean square 
error. 
From scanning line blocks which are composed of N.sub.3 lines in which 
block signals are arranged in the order of magnitude of the mean square 
error relative to the preceding scanning line, n.sub.3 blocks are picked 
up in the order of magnitude of the mean square error by the use of a line 
block selector 21 and, in connection with the selected blocks n.sub.3, the 
first- and second-stage processing operations are achieved. In the 
remaining blocks (N.sub.3 -n.sub.3) of small error electric power, an 
error relative to the point selected on the preceding scanning line is 
very small. For the correction of this, in the case of a signal using "0" 
or "1" as the header, a corrected value at a sampling point immediately 
following or preceding the selected point on the preceding scanning line, 
shown in FIG. 11, is coded by the Hoffman coding method as in the case of 
Table 2. However, the coding method is such that in the case of "0", a 
code word of amplitude "0", that is, of no change, is added. After this, 
the second-stage processing is applied thereto and, for the signal value 
obtained as a result of those processings, the first-stage processing is 
applied to obtain the selected points. The corrected points are used for 
the comparison with the next scanning line. 
Thus, in this case, the upper limit of the code length is defined by 
n.sub.3 in the N.sub.3 blocks, so that scanning of substantially constant 
speed is possible. 
Alternatively, there is a scheme such that n points are chosen in the 
selected n.sub.3 blocks including N picture elements among N.sub.3 line 
blocks at the first stage of the multistage selective DPCM system and that 
n.sub.4 points (n.sub.4 &lt;n) are chosen in the unselected (N.sub.3 
-n.sub.3) blocks from the first stage, while all blocks are applied to the 
second stage of the multistage selective DPCM system. 
An example of the third stage is shown in FIG. 14, in which a line block 
selector 21 selectively activates the first stage 22 and the second stage 
23 mentioned above to process n.sub.3 blocks from N.sub.3 lines of the 
input signal. A coder 24 having the same circuit construction as that 
shown in FIG. 9A or 9B selectively codes (N.sub.3 -n.sub.3) blocks. 
Outputs of the stages 22, 23 and the coder 24 are applied to a combiner 25 
to obtain an output digital signal. The digital output is converted to an 
analog signal substantially equal to the original signal by a 
digital-analog converter 26. The output of the converter 26 is applied to 
a subtractor 28 through a one-line delay 27. A mean square error of a 
difference between adjacent lines is obtained at the output of the 
subtractor 28 and applied to the selector 21 and the coder 24. The 
selector 21 is so formed to select n.sub.3 lines having larger mean square 
differences from N.sub.3 successive lines. 
The same procedures can be used for interframe correlation. 
Simulation is achieved by the A-D conversion of a luminance signal obtained 
from a flying spot scanner and by reading the converted signal in a 
computer. 
In the flying spot scanner (FSS), the result of the A-D conversion is 
applied to a computer, such as NEAC 3200-50, through an FSS control device 
and a universal input device, and the output is connected to the FSS 
control device through a universal output device for the intensity 
modulation of a CRT. In this case, positions of spot are applied to the 
FSS control device from the computer. FIG. 12 is a block diagram of the 
computer for achieving the above simulation. 
In accordance with data and a control signal from a computer, the output 
control section transmits the positional data of the spot to FSS and a 
luminance signal at its coordinates can be produced by a 12-bit D-A 
converter. 
The scanning modes are (1) raster scanning and (2) random scanning, and 
have a boundary limiting function. This enables boundary setting with a 
register switch. 
The input control section is to transfer the luminance signal to the 
computer while FSS is used and the luminance of the scanning CRT having 
A-D converters in two channels can be corrected. Further, the A-D 
converter has an overflow detecting function and enables a decision with a 
lamp and a program. 
By FSS described above, a slide of a face used as a sample was cut off and 
simulation was achieved with N.sub.1 =128. In relation to the dynamic 
range, the part corresponding to 9 bits is caused to correspond to white 
and black levels in the computer, by which the luminance varies over 
substantially the entire amplitude range. The simulation program subjects 
such a picture signal to the first-stage processing of the multistage 
selective DPCM system. In this case, the minimum level interval is one 
corresponding to 6 bits PCM. A comparison operation of the result of 
simulation in the case of 6 bits PCM with that in the case of 7 bits 
indicates that the minimum level corresponding to 7 bits PCM is better by 
about 1 dB in S/N. A difference signal between a 9-bit luminance signal 
and the output resulting from the first-stage processing operation in the 
case of 6 bits PCM is subjected to the second-stage processing operation. 
To perform this, simulation was effected in connection with the ordinary 
.DELTA. modulation, that is, the case of a=1.8 in FIG. 8, and the cases of 
.DELTA.=1.0 and 1.5, with a=1.2, 1.5 and 2.0. A typical example of the 
results of simulation is shown in FIG. 13. However, since the results 
obtained with .+-.a.sup.n .DELTA. are better than those with 
.+-.an.DELTA., the former is shown. The value .DELTA. is classified into 
groups of the ratio S/N respectively being lower than 20 dB, 20 dB to 24 
dB and higher than 24 dB, and the value a is set to be 1.5, 1.0 and 0.5, 
respectively. For this purpose, the circuit example shown in FIG. 9A can 
be modified as shown in FIG. 9B, in which a mean square error detector 
13a, which is formed by a cascade connection of a square circuit and an 
average circuit, is provided to select a suitable value of .DELTA. in the 
quantizer 12. As a result of this simulation when the number selected at 
the first stage is small, the S/N ratio has the decreasing tendency, but 
indicates the tendency of recovering S/N by using the value of a little 
larger than one. However, there is still the possibility of S/N being a 
little enhanced by adaptive changing a and .DELTA. in each block. The S/N 
ratio obtained as described above is S.sub.p-p /N.sub.rms which does not 
include a synchronizing signal portion, and the weighted signal to noise 
power ratio can be obtained by adding thereto about 14 dB. From this, a 
weighted signal to noise power ratio of about 50 dB can be obtained with 3 
bits/picture element. The results of simulation shows that appreciably 
excellent S/N is obtained with a small number of points selected, and 
there exists the possibility of dividing the selective DPCM of the second 
stage into two parts. 
The multistage selective DPCM of this invention is a method in which 
signals of the most striking feature are selected and are approximated by 
utilizing the following order feature, a feature of its difference signal 
is obtained so that it gives better signal approximation, and successive 
approximation to signals to thereby decrease redundancy. As a result of 
application of this concept to a picture signal, it has been found that 
good results can be obtained. The features of the system used are as 
follows: 
(1) When a signal is compressed by the selective DPCM of the two-stage 
construction to 3 bits/picture element, weighted S/N of 50 dB is obtained 
to provide for appreciably excellent transmission performance. 
(2) Constant-speed scanning is possible. 
(3) Since a signal is divided into blocks, the influence of a bit error can 
be confined within each period.