Efficient coding/decoding apparatuses for processing digital image signal

Efficient coding decoding apparatuses are permitted to easily carrying out, without degradation of the picture quality, random access, high speed search or image editing necessary in media of the storage system in a processing system for recording, transmitting, and displaying a digital signal, and to efficiently encode/decode an image signal with a lesser quantity of codes. An original image signal from a terminal (1) is coded by an orthogonal transformer (4) and a quantizer (36). A subtracter (33) subtracts, from the original image signal, a reproduced image signal obtained by an inverse quantizer (7) and an inverse orthogonal transformer (10) to provide a negative error signal. An adder (30) adds a negative error signal delayed by one frame and multiplied by a predetermined coefficient (Ke) at a multiplier (34) and the original image signal. As a result, an operation is carried out to multiply an error of a frame by the predetermined coefficient (Ke) to subtract it from an original image of the next frame to encode the subtracted signal to subtract its error from an original image signal of the further next frame. The value of the predetermined coefficient (Ke) is varied depending upon matching between images.

BACKGROUND OF THE INVENTION 
This invention relates to an efficient encoding apparatus adapted for 
efficiently encoding an image signal with a less quantity of codes and a 
decoding apparatus adapted for decoding such a coded signal in a 
processing system for recording, transmitting, and displaying a digital 
signal. 
As an efficient coding technique utilizing image correlation, a "predictive 
coding" technology and a "orthogonal transform" technology are known as a 
most popular technology in recent years. In encoding a moving picture, the 
"prediction coding" technology is used for the interframe processing, and 
the "orthogonal transform" technology is used for the intraframe 
processing. Further, in the interframe prediction, there are many 
instances where a "motion compensation" to vary a predictive signal in 
correspondence with motion of a picture is carried out, and an 
orthogonally transformed and quantized predictive residual signal is 
replaced by a "variable length code". 
A coding apparatus and a decoding apparatus in this case will be described. 
FIG. 1 is a block diagram showing a conventional coding apparatus. 
In FIG. 1, an original image signal inputted from an image input terminal 1 
is delivered to a subtracter 2 and a motion vector detector 3. The 
subtracter 2 subtracts a predictive signal which will be described later 
from the original image signal to provide a predictive residual to deliver 
it to an orthogonal transformer or transform element 4. The orthogonal 
transformer 4 orthogonally transforms this predictive residual every block 
consisting of 8.times.8 pixels or so by the Discrete Cosine Transform 
(DCT) technique, etc. to deliver it to a quantizer 5. The quantizer 5 
quantizes an input signal with a suitable accuracy. Since most of input 
signals take a value originally nearly equal to zero, most of output 
signals from the quantizer 5 also take zero. 
An output signal from the quantizer 5 is delivered to a variable length 
encoder 6 and an inverse-quantizer 7. The variable length encoder 6 effect 
a processing such that when the input signal is equal zero, the encoder 6 
converts the number of succession of signals to a variable length code 
such as a Huffman code, etc., and when the input signal takes a value 
except for zero, the encoder 6 converts that value to a variable length 
code, thereafter to deliver the variable length code thus obtained to a 
buffer 8 as compressed data. At this time, the rate of data outputted from 
the variable length encoder 6 is not fixed. Accordingly, that data is 
delivered to the buffer 8 so that it has a fixed rate. The data passed 
through the buffer 8 is outputted from a compressed data output terminal 9 
to a decoding apparatus side. 
On the other hand, a predictive signal delivered to the subtracter 2 is a 
signal earlier by one frame. In order to allow this predictive signal to 
be the same as that on the decoding apparatus side, this predictive signal 
is processed as follows. 
The inverse-quantizer 7 inverse-quantizes a quantized signal which is an 
output signal of the quantizer 5 to replace it by a representative value 
of quantization to deliver it to an inverse orthogonal transformer 10. The 
inverse-orthogonal transformer 10 carries out an inverse transform 
processing of the orthogonal transformation to deliver its output signal 
to an adder 11. The adder 11 adds an output signal from the inverse 
orthogonal transformer 10 and a predictive signal delivered from a 
terminal 12c of a changeover switch 12 to provide the signal thus added as 
a reproduced image signal to deliver it to a frame memory 13. The frame 
memory 13 delays the reproduced image signal by one frame thereafter to 
deliver it to the motion vector detector 3 and a motion compensator 14. 
The motion vector detector 3 searches motion of an image every about 
16.times.16 pixels between an original image signal from the image input 
terminal 1 and a signal earlier by one frame from the frame memory 13 to 
obtain most accurate information to transmit it to the decoding apparatus 
side through a motion vector information output terminal 15, and to 
deliver it also to the motion compensator 14. 
The motion compensator 14 implements a motion compensative processing to an 
output signal from the frame memory 13 in correspondence with a motion 
vector value delivered from the motion vector detector 3 to obtain a 
predictive signal to deliver it as a subtraction signal to the subtracter 
2 through terminals 12b and 12c of the changeover switch 12. 
The changeover switch 12 serves to ensure a suitable interframe prediction. 
The operation of the changeover switch 12 will now be described. 
The motion vector detector 3 is adapted to output independent information 
for independently making coding without carrying out prediction in the 
case where a matching error between frames is large even if a motion 
vector is considered to be optimum to transmit such independent 
information to the decoding apparatus side through an independent 
information output terminal 16, and to deliver it also to the changeover 
switch 12. 
The changeover switch 12 is switched, by this independent information, not 
to the 12b side (output of the motion compensator 14), but to a fixed 
value (0) on the 12a side, thus to inhibit interframe prediction. 
FIG. 2 is a block diagram showing a conventional decoding apparatus. 
In FIG. 2, compressed data transmitted from the coding apparatus side shown 
in FIG. 1 is incoming through a compressed data input terminal 17 and a 
buffer 18, and is then delivered to a variable length decoder 19. The 
variable length decoder 19 converts a variable length code of the 
compressed data to a fixed length to deliver it to an inverse-quantizer 
20. The inverse quantizer 20 inverse-quantizes an input signal to deliver 
it to an inverse orthogonal transformer 21. 
The inverse-orthogonal transformer 21 implements an inverse orthogonal 
transform processing to that input signal to obtain a predictive residual 
signal to deliver it to an adder 22. The adder 22 adds the reproduced 
predictive residual signal and a predictive signal delivered from a 
changeover switch 23 to obtain a reproduced image signal to output it 
through a reproduced image output terminal 24, and to deliver it also to a 
frame memory 25. 
The frame memory 25 delays the reproduced image signal by one frame 
thereafter to deliver it to a motion compensator 26. The motion 
compensator 26 carries out motion compensation of the reproduced image 
signal by motion vector information transmitted from the coding apparatus 
side through a motion vector information input terminal 27 to obtain a 
predictive signal to deliver it as an addition signal to the adder 22 
through terminals 23b and 23c of the changeover switch 23. 
Further, the changeover switch 23 is switched, by independent information 
transmitted from the coding apparatus side through an independent input 
terminal 28, not to the 23b side (output of the motion compensator 26), 
but to the fixed value (0) on the 23a side, thus to inhibit interframe 
prediction. 
As an actual example of the previously described coding and decoding 
apparatuses standardized (H. 261) for use in a Television Conference, or a 
Television Telephone in the Consultive Committee of International Telegram 
& Telephone (CCITT). 
In an interframe predictive coding as described above, the efficiency is 
high because correlation between frames is effectively utilized. However, 
in order to realize application to storage (recording) system media, it is 
required for carrying out random access, high speed search or image 
editing to independently conduct coding within a frame without using 
prediction between several frames. 
While an employment of an increased rate of frames caused to be independent 
becomes easy to cope with editing, etc. by the increased rate, the 
efficiency is lowered, so that a quantity of codes generated increases. 
Particularly, when an attempt is made to carry out editing every one frame 
(replacement of image), it is necessary that respective frames are 
independent, giving rise to inconveniences such that the interframe 
prediction is unable to be used. 
Further, in the case of the interframe predictive processing, when an image 
correlation between frames is lowered to some extent, a quantity of codes 
of predictive residuals can become greater than that in the case where 
coding is independently carried out within a frame without using 
prediction. For this reason, there are instance where a method of 
independently carrying out coding within a frame is rather desirable. In 
these instances, it took place the inconvenience that it is necessary to 
make the respective characteristics of the intraframe coding with respect 
to a predictive residual and a raw image to be different from each other. 
SUMMARY OF THE INVENTION 
This invention has been made by drawing attention to the above-described 
problems, and its object is to provide efficient coding/decoding 
apparatuses adapted for independently coding respective frames within a 
frame to allow an error occurring between an original image signal and a 
coded reproduced image signal be exerted on other frames to carry out an 
addition between frames on the decoding side to lessen error signals so 
that respective frames can be independently handled, thereby making it 
possible to easily carry out, without degradation in picture quality, 
random access, high speed search or image editing necessary in media of 
the storage system, to provide a coding efficiency closer to that of the 
interframe predictive coding by reduction of an error, to improve 
efficiency to more degree rather than that in the predictive coding 
particularly in the case where a correlation between frames is low, to 
provide a reproduced image desirable from a visual point of view, and to 
employ a simple construction. 
In order to solve the above-described problems, this invention provides: 
(1) An efficient coding apparatus for use in a coding processing utilizing 
correlation between frames or fields of an image signal comprising; means 
for obtaining a signal component including a negative error signal 
provided by subtracting a reproduced image signal obtained by 
coding/decoding from an original image signal which has not undergone 
interframe/interfield coding processing, and means for adding the signal 
component including the negative error signal to input image signals of 
other frames or fields; 
(2) an efficient coding apparatus comprising; means for detecting the 
degree of matching every block or pixel of an image between frames or 
fields where interframe or interfield processing is carried out, and 
adding means such that when the degree of matching is high, the adding 
means is operative to increase the rate of a signal component including a 
negative error signal to add it to input image signals of other frames or 
fields, while when the degree of matching is low, the adding means is 
operative to decrease the rate of a signal component including a negative 
error signal to add it to those input image signals. 
(3) an efficient coding apparatus comprising; means for detecting the 
degree of matching every block or pixel of an image between frames or 
fields where interframe or interfield processing is carried out, and an 
quantization means such that when the degree of matching is high, the 
quantization means is operative to allow a quantization step to be coarse, 
while when the degree of matching is low, the quantization means is 
operative to allow the quantization step to be fine. 
(4) an efficient coding apparatus comprising; means for detecting the 
degree of matching every block or pixel of an image between frames or 
fields where interframe or interfield processing is carried out, means for 
controlling a quantization step, and adding means such that when the 
quantization step is caused to be coarse, the adding means is operative to 
increase the rate of a signal component including a negative error signal 
to add it to input image signals of other frames or fields, while when the 
quantization step is caused to be fine, the adding means is operative to 
decrease the rate of a signal component including a negative error signal 
to add it to those input image signals, 
(5) an efficient decoding apparatus comprising; means for adding reproduced 
image signals of other frames or fields to a current or present reproduced 
image signal in decoding coded data of an image in which error signals of 
other frames or fields are included, and 
(6) an efficient decoding apparatus comprising; means for detecting the 
degree of matching every block or pixel of a reproduced image between 
frames or fields where interframe or interfield processing is carried out, 
and adding means such that when the degree of the matching is high, the 
adding means is operative to increase the rate of reproduced image signals 
of other frames to add them to a current or present reproduced image 
signal, while when the degree of the matching is low, the adding means is 
operative to increase the rate of a current reproduced image signal to add 
it to reproduced image signals of other frames or fields. 
In this invention, a scheme is employed to independently carry out coding 
within respective frames to allow errors between an original image and a 
coded reproduced image, which takes place by quantization error, to be 
exerted on other frames, and to cancel such errors by additive processing 
between frames on the decoding side. 
In actual terms, when the coefficient of the coding side is assumed as Ke 
and the coefficient of the decoding side is assumed as Kd, an approach is 
employed to multiply an error of a frame by the coefficient Ke (0.about.1) 
to subtract it from an original image of the next frame to encode the 
subtracted image to further subtract an error of the coded image from an 
original image of the further next frame. On the decoding side, an 
approach is employed to multiply an image of a last frame by the 
coefficient Kd (0.about.0.8), and to multiply a reproduced image of the 
present frame by (1-Kd). 
The values of the coefficients Ke and Kd are changed depending upon degree 
of matching of respective images. Namely, in the case where correlation is 
high and respective images match with each other to much degree, those 
values are caused to be large. In contrast, in the case where correlation 
is low and respective images match with each other to less degree, those 
values are caused to be small. 
Further, in the additive/subtractive processing between frames, motion 
compensation is carried out in the same manner as in the case of the 
interframe prediction. 
Since respective frames are independently coded, random access, search or 
editing can be carried out with ease. Further, since an error of a frame 
is caused to be exerted on other frame, an error of the next frame becomes 
apt to have an opposite polarity if an image is unchanged. Accordingly, 
when an additive processing between frames is carried out on the decoding 
side, most of errors are canceled, resulting in extremely small quantity 
of remaining errors. In addition, since a quantity of additive/subtractive 
processing is reduced in dependency upon the degree of correlation at the 
portion where there is any change in an image, there is no possibility 
that blurring by movement or motion may take place, although a quantity of 
lessened errors is reduced. 
By using a motion compensation, high interframe correlation is provided at 
most portions of an image, so error is considerably lessened. Accordingly, 
in the case of attempting to obtain the same picture quality, quantization 
can be considerably coarse, so a quantity of data generated can be 
reduced. 
In this instance, the efficiency is improved to more degree than that in 
the case where coding is independently carried out even at the portion 
where interframe correlation of an image is relatively low. For this 
reason, in such a case, the efficiency becomes higher than that of the 
interframe predictive coding. Thus, the degree of changes of a data 
quantity, which takes place by the property of an image, becomes small. 
As stated above, in accordance with efficient coding/decoding apparatuses 
according to this invention, an approach is employed to independently 
encode respective frames within a frame to allow respective errors 
occurring between original image signal and corresponding coded reproduced 
image signal to be exerted on other frames to carry out an addition 
between frames on the decoding side to lessen error signals so that 
respective frames can be independently handled, thereby making it possible 
to easily carry out, without degradation in picture quality, random 
access, high speed search or image editing necessary in media of the 
storage system, to provide a coding efficiency closer to that of the 
interframe predictive coding by reduction of an error, to improve 
efficiency to more degree rather than that in the predictive coding 
particularly in the case where a correlation between frames is low, to 
provide a reproduced image desirable from a visual point of view, and to 
employ a simple construction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a block diagram showing a first embodiment of an efficient coding 
apparatus according to this invention wherein the same reference numerals 
are respectively attached to the same portions as those in FIG. 1, and 
their explanation will be omitted. 
In FIG. 3, an original image signal inputted from an image input terminal 1 
is delivered to an adder 30, a motion vector detector 31, and a frame 
memory 32. The adder 30 adds a negative error signal which will be 
described later to the original image signal to deliver an added signal to 
orthogonal transformer 4 and a subtracter 33. 
The operation at the orthogonal transform element 4 and components 
succeeding thereto is essentially the same as that in the prior art. The 
interframe prediction of the prior art is the processing for a predictive 
residual, whereas, in this embodiment, such interframe prediction is the 
processing substantially for an original image signal because an error 
signal is small although a negative error signal is added thereto. 
Accordingly, the quantization step, and the variable length coding, etc. 
somewhat differ from those of the prior art, and basically becomes 
equivalent to the case where coding is independently carried out within a 
frame. On the other hand, a quantized signal is delivered to an 
inverse-quantizer 7 in the same manner as in the prior art. Thus, a 
reproduced image signal is provided by the inverse-quantizer 7 and an 
inverse orthogonal transformer 10. 
An output signal of the inverse orthogonal transformer 10 was a reproduced 
image signal of a predictive residual in the prior art, but is a 
reproduced image signal of an original image in this embodiment. A 
subtracter 33 subtracts an output signal of the inverse orthogonal 
transformer 10 from an output signal of the adder 30 to obtain a negative 
error signal occurring in the intraframe coding/decoding processing to 
deliver it frame memory 13. The frame memory 13 delays the negative error 
signal by one frame to deliver it to motion compensator 14. 
Here, since the amplitude of the negative error signal is extremely smaller 
than that of the original image signal, when the amplitude of the negative 
error signal is limited before that signal is inputted to the frame memory 
13, the number of bits of the frame memory can be reduced. If the original 
image signal is 8 bits (i.e., 0.about.255) as an example, the number of 
bits of the frame memory can be reduced to 4 bits (-7.about.+7). Thus, the 
capacity of the frame memory 13 can be one half. 
The motion compensator 14 implements motion compensation processing to an 
output signal of the frame memory 13 in correspondence with a motion 
vector value delivered from the motion vector detector 31 to obtain a 
motion compensated negative error signal to deliver it to a multiplier 34. 
The motion vector detector 31 outputs matching information to deliver it to 
the decoding apparatus side through a matching information output terminal 
35, and to deliver it to a quantizer 36 and the multiplier 34. 
In the case where matching is good, an additive processing between frames 
is carried out on the decoding apparatus side, so an error is lessened, 
whereas in the case where matching is bad, a quantity of lessened errors 
becomes small, or there is no lessening of errors, so the picture quality 
is lowered. To improve this, the motion vector detector 31 carries out the 
above-mentioned operation. In the case where matching is bad, the 
quantizer 36 becomes operative to allow the quantization step to be fine 
to improve the picture quality so that the entirety thereof is 
well-balanced. 
It is to be noted that the portion where matching is bad is the portion 
where an image abruptly changes, and degradation in the picture quality is 
difficult to be visually conspicuous. Accordingly, it is unnecessary to 
entirely implement improvement of the picture quality by additive 
processing between frames. 
The degree of matching is determined by taking absolute values of 
differences between corresponding pixels of two images and averaging the 
absolute values. In this embodiment, setting is made such that the 
quantization step is caused to be fine relatively by about 15% every time 
the average value of the absolute values increments by 2 from zero, and is 
caused to be the same when the average value of absolute values is more 
than 6. 
The multiplier 34 multiplies the motion compensated negative error signal 
by the coefficient Ke (0.about.1) determined by information indicative of 
the degree of matching of an optimum motion vectors outputted from the 
motion vector detector 31 to deliver it to the adder 30. 
Setting of the above-mentioned coefficient Ke is made depending upon the 
degree of matching. Namely, when the average value of absolute values is 
less than about 3, the coefficient Ke is set to 1. When the average value 
of absolute values is above about 3, the coefficient Ke is set to a value 
less than 1. When the average value of absolute value is about 7, the 
coefficient Ke is equal to zero. 
It is to be noted that since the average of absolute values of pixel 
differences between two images is calculated in order to determine a 
motion vector, the motion vector detector 31 is only required to output, 
every block, an optimum vector corresponding to a minimum value. 
Accordingly, supplement of processing is not required in particular. 
The motion vector detector 31 searches, every about 16.times.16 pixels, 
motion of an image between an original image signal from the image input 
terminal 1 and a signal earlier by one frame from the frame memory 32 to 
obtain most accurate motion vector information to deliver it to the 
decoding apparatus side through the motion vector information output 
terminal 15, and also to the motion compensator 14. 
Here, a signal earlier by one frame is required at the motion vector 
detector 31. Since this signal must correspond to the original image, an 
output signal of the frame memory 13, which is obtained by delaying the 
negative error signal by one frame, is not used. Separately from this, it 
is necessary to provide frame memory 32 for an original image signal. For 
this reason, two frame memories in total are required. Since it is 
sufficient that the frame memory 13 for error signal has a capacity one 
half of that of the prior art, it is enough that the entire memory 
capacity is about 1.5 times that of the prior art. 
By processing as described above, an image signal in which an error signal 
of a last frame is subtracted at the portion where correlation of images 
is high is subjected to intraframe coding by the processing succeeding to 
the orthogonal transform processing. 
FIG. 4 is a block diagram showing an embodiment of an efficient decoding 
apparatus wherein the same reference numerals are respectively attached to 
the same portions as those in FIG. 2, and their explanation will be 
omitted. 
In FIG. 4, while the operation from the buffer 18 up to the inverse 
orthogonal transformer element 21 is the same as that in the prior art 
shown in FIG. 2, but parameter setting corresponding to the intraframe 
independent coding is made in correspondence with the coding apparatus. 
The inverse orthogonal transformer 21 implements an inverse orthogonal 
transform processing to an input signal to obtain a reproduced image 
signal to deliver it to a subtracter 37. The subtracter 37 subtracts the 
reproduced image signal which is an output signal of the inverse 
orthogonal transformer 21 from a motion compensated reproduced image 
signal of a last frame outputted from the motion compensator 26 to deliver 
it to a non-linear converter 38. 
On the other hand, matching information transmitted from the coding 
apparatus through a matching information input terminal 39 is delivered to 
the non-linear converter 38 and an inverse quantizer 40. 
The non-linear converter 38 is controlled by matching information to 
implement non-linear conversion to an input signal to deliver it to an 
adder 41. The adder 41 adds the reproduced image signal which is an output 
signal of the inverse orthogonal transformer 21 and an output signal of 
the non-linear converter 38 to output it as a reproduced image signal 
through reproduced image output terminal 24, and to deliver it also to 
frame memory 25. 
The frame memory 25 delays the reproduced image signal by one frame 
thereafter to deliver it to motion compensator 26. The motion compensator 
26 implements motion compensation processing to a reproduced image signal 
of a last frame by motion vector information transmitted from the coding 
apparatus side through the motion vector information input terminal 27 to 
obtain a motion compensated reproduced image signal of a last frame to 
deliver it to the subtracter 37. 
Here, the subtracter 37, the non-linear converter 38 and the adder 41 are 
provided for multiplying an original image signal by (1-Kd), and for 
multiplying a signal earlier by one frame by Kd to add them. The 
coefficient multiplied at the non-linear converter 38 is Kd. When Kd is 
equal to zero, i.e., an output signal of the non-linear converter 38 is 
equal to zero, an original image signal is outputted as it is. On the 
other hand, when Kd is equal to 1, i.e., an output signal of the 
non-linear converter 38 is the same as an input signal, the original image 
signal is canceled by subtraction and addition, so a signal earlier by one 
frame is provided as an output signal as it is. 
It is to be noted that the coefficient Kd will vary by an input signal of 
the non-linear converter, i.e., an interframe difference signal in 
correspondence with the non-linear conversion characteristic. 
FIG. 5 is a diagram showing an example of the characteristic of the 
non-linear conversion wherein the abscissa and the ordinate represent an 
input and an output, respectively. This Figure is used in common to both 
the coding apparatus and the decoding apparatus. Accordingly, the 
coefficient is represented by Ke in the case of the coding apparatus, and 
the coefficient is represented by Kd in the case of the decoding 
apparatus. When the absolute value of an input is small, the coefficient 
Ke (Kd) is equal to a value close to 1. When the absolute value of an 
input becomes large, the coefficient Ke (Kd) becomes small. When the 
absolute value of an input is above 8, the coefficient Ke (Kd) becomes 
equal to zero. The conversion characteristic is varied by matching 
information in correspondence with changes in the quantization step. 
Namely, in the case where matching is bad, so quantization step is fine (a 
side in FIG. 5), the coefficient Ke (Kd) is caused to be immediately 
small. 
It is to be noted that the reason why the coefficient Ke of the coding 
apparatus is roughly set every block is that error signals having a small 
amplitude are added in the coding apparatus, whereas the reason why the 
coefficient Kd of the decoding apparatus is changed every pixel is that 
since image signals of different frames are handled in the decoding 
apparatus, there is the possibility that images considerably differ 
positionally even within a block, with the result that degradation in the 
picture quality is apt to occur by the additive processing if the 
coefficient Kd is kept constant within the block. 
Comparison between the above-described technique of this invention and the 
conventional predictive processing will be made. The manners of utilizing 
correlation between frames with respect to the invention and the prior art 
are different as follows. Namely, in the case of the interframe 
prediction, attention is drawn to the fact that since a difference between 
frames is small, coding can be carried out with a less quantity of codes. 
On the contrary, in the case of the technique of the invention, the 
feature resides in that quantization is rather roughly carried out to 
allow an error to be exerted on other frames on the premise that the 
interframe additive processing can be carried out on the decoding side. 
FIG. 7 is a diagram showing coding efficiencies in the technique of this 
invention and the conventional predictive processing wherein the abscissa 
and the ordinate represent an image correlation and a quantity of data 
generated. 
In FIG. 7, in the case of both the technique of the invention and the 
conventional predictive processing, according as the interframe 
correlation becomes higher (the correlation becomes close to 1), the 
quantity of data becomes less. 
Since the predictive residual is equal to zero in the case where images are 
entirely the same, it is possible to reduce a quantity of data generated 
to a value close to zero. On the contrary, in the case of this technique, 
even if quantization is caused to be coarse, data is inevitably generated 
to some extent in order to encode an original image. In this respect, the 
technique of the invention is inferior to the predictive processing. 
On the other hand, in the case where correlation is low (correlation is 
close to zero), this is represented by a differential component in the 
predictive processing. For this reason, a quantity of data rather becomes 
greater than that in the case of the intraframe independent coding. 
However, in the case of this technique, a quantity of data is the same as 
that of the intraframe independent coding at the worst. If any correlation 
can be utilized, a quantity of data can be reduced accordingly. Rather, 
this technique is advantageous to the predictive coding. Accordingly, in 
the case where this technique is used, a change in the picture quality at 
the time of a fixed rate becomes small. 
By both the coding processing and the decoding processing, an error is 
lessened to much degree. Study of how an error is lessened by a single 
processing only the coding side or decoding side will be conducted. 
Initially, in the case of a single processing only on the coding side, 
there occurs a phenomenon such that an error is diffused to the high 
frequency band side in a time direction. This is in conformity with the 
visual characteristic, but noise may move even if an image is stationary. 
Further, in the case of a single processing only on the decoding side, 
errors occurring at random are lessened to some degree. However, since 
errors take the same value if images are the same, there results no 
improvement by adding them. 
On the other hand, execution of the additive processing on the decoding 
side in the predictive coding is no more than reduction of an interframe 
predictive residual. This is meaningless. In view of these discussions, it 
is considered that this invention holds both on the coding side and on the 
decoding side. 
FIG. 6 is a block diagram showing a second embodiment of an efficient 
coding apparatus according to this invention wherein the same reference 
numerals are respectively attached to the same portions as those of FIGS. 
1 and 3, and their explanation will be omitted. The major difference 
between the coding apparatus of this embodiment and the coding apparatus 
of FIG. 3 is as follows. Namely, in FIG. 3, only a negative error signal 
(coded error) is used for feedback. In contrast, in FIG. 6, signal 
component including a negative error signal, i.e., a signal in which a 
negative error signal is added to an original image signal is used. Thus, 
it is sufficient to use a single frame memory, and addition of errors can 
be carried out every pixel. 
In FIG. 6, an original image signal inputted from image input terminal 1 is 
delivered to adder 30, motion vector detector 3, and an activity detector 
43. The adder 30 adds an output signal of the non-linear converter 38 
which will be described later to the original image signal to deliver it 
to orthogonal transformer 4 and a doubler 44. 
The operation from the orthogonal transformer 4 to the inverse orthogonal 
transformer is the same as that of the prior art shown in FIG. 1. An 
output signal of the inverse orthogonal transform element 4 serves as a 
reproduced image signal of an original image, i.e., (original image 
signal+error signal). The doubler 44 amplifies an output signal of the 
adder 30 so that it becomes double to deliver it to subtracter 33. The 
subtracter 33 subtracts an output signal of the inverse orthogonal 
transformer 10 from an output signal of the doubler 44, i.e., carries out 
an operation of 2.times.original image signal-(original image signal+error 
signal) to obtain a signal component including a negative error signal, 
i.e., (original image signal+error signal) to deliver it to the frame 
memory 13. The frame memory 13 delays an input signal by one frame 
thereafter to deliver it to motion compensator 14 and motion vector 
detector 3. 
The motion vector detector 3 searches, every about 16.times.16 pixels, 
motion of an image between an original image signal from the image input 
terminal 1 and (original image signal-error signal) earlier by one frame 
from the frame memory 13 to obtain the likeliest motion vector information 
to deliver it to the decoding apparatus side through the motion vector 
information output terminal 15, and to deliver it also to the motion 
compensator 14. 
The motion compensator 14 implements motion compensation processing to an 
output signal of the frame memory 13 in correspondence with a motion 
vector value delivered from the motion vector detector 3 to obtain a 
motion-compensated (original image signal-error signal) earlier by one 
frame to deliver it to subtracter 45. The subtracter 45 subtracts the 
original image signal inputted from the image input terminal 1 from the 
motion-compensated (original image signal-error signal) earlier by one 
frame to deliver it to the non-linear converter 38. 
On the other hand, the activity detector 43 detects or determines an 
activity of an image every orthogonal transform block or motion 
compensation block to deliver it to a quantization step setter 46. 
The quantization step setter 46 is controlled by two parameters of 
information of a quantity of data delivered from the buffer 8 and activity 
to obtain information of the quantization step to deliver it to the 
decoding apparatus side through quantization step information output 
terminal 47, and to deliver it also to the non-linear converter 38 and the 
quantizer 36. 
The operation of the quantization step setter 46 is as follows. Namely, in 
the case where a quantity of data stored in the buffer is great, since it 
is necessary to reduce a quantity of data generated, the quantization step 
is caused to be coarse. Further, since degradation is difficult to be 
visually conspicuous at portions where the activity is high of respective 
blocks, the quantization step is caused to be coarse. 
The operations of the subtracter 45, the non-linear converter 38 and the 
adder 30 are the same as those of the subtracter 37, the non-linear 
converter and the adder 41 in the decoding apparatus shown in FIG. 4. 
Namely, original image signal and (original image signal-error signal) 
earlier by one frame are compared with each other. As a result, in the 
case where a difference therebetween is small, the (original image 
signal-error signal) earlier by one frame results in a signal to be coded. 
In contrast, in the case where that difference is large, the original 
image signal results in a signal to be coded. 
Namely, in the case of FIG. 3, only a negative error signal is added. On 
the contrary, in the case of FIG. 6, an image signal itself is replaced. 
Thus, a frame additive operation of an image is carried out. The circuit 
section including the non-linear converter 38 of FIG. 6 operates as a 
filter in a time direction so that the noise component is lessened. 
The non-linear converter 38 implements a non-linear conversion processing 
to an input signal in correspondence with a quantization step changing in 
dependency upon control of the data rate and the activity of an image to 
deliver it to the adder 30. 
The operation of the non-linear converter 38 is as follows. Namely, in the 
case where the quantization step is caused to be coarse, the range of an 
input value serving as a large coefficient Ke is widened. Thus, even if 
there is a difference to some extent, the time filter is caused to be 
effective to more degree (d side in FIG. 5). In contrast, in the case 
where the quantization step is fine, since an error produced by coding 
becomes small, the range of a difference signal subject to filtering is 
narrowed, thus allowing a change in an image not to be subjected to 
filtering (a side in FIG. 5). 
In this embodiment, the quantizer 36 quantizes an input signal at a 
quantization step corresponding to quantization step information delivered 
from the quantization step setter 46. 
As a decoding apparatus corresponding to the coding apparatus of FIG. 6, 
the decoding apparatus shown in FIG. 4 can be used as it is. This is 
because matching information in FIG. 3 may help to conduct control of the 
quantization step. 
While the processing of FIGS. 3, 4 and 6 is directed to the processing 
between respective adjacent two frames, it is conceivable, in the same 
manner as in the conventional predictive processing, also in this 
technique to adopt various processing between fames, or between fields. 
FIGS. 8(a) to 8(c) are views for explaining processing every frame of a 
non-interlaced signal. In these Figures, squares represents respective 
frames, and arrows represent frame pairs from which errors therebetween 
are given. FIG. 8(a) shows the basic case where each error between a frame 
and only a last frame (frame earlier only by one frame) is used. FIGS. 
8(b) and 8(c) show processing proposed by the standardization of ISO/IEC. 
Namely, FIG. 8(b) shows a processing of the first stage where each error 
between frames jumping by several frames (three frames in the Figure) is 
used. FIG. 8(c) shows a processing from frames of the first stage (frames 
indicated by slanting lines) with respect to frames caused to be jumped by 
the processing of the first stage. Since the processing of the second 
stage FIG. 8(c) is a processing from preceding and succeeding two frames, 
although a plurality of methods such as a method of using only the 
preceding frame, a method of using only the succeeding frame, and a method 
of using addition between preceding and succeeding frames, etc. are 
conceivable, alteration of the processing in that case is similar to 
alteration in the case of the predictive processing. 
FIGS. 9(a) to 9(d) are views for explaining the processing every field of 
an interlaced signal. In these Figures, squares represent respective 
fields, and arrows frame pairs from which errors therebetween are given. 
Because of the interlaced signal, there is a time shift of 1/2 frame 
between even fields and odd fields. 
FIG. 9(a) shows the processing from a last frame and a last field, which is 
proposed by the standardization of CCIR/CCITT. FIGS. 9(b) and 9(c) show 
the processing expanded to the processing between fields wherein 
processing between fields jumping by three fields is carried out with 
respective even and odd fields. Namely, in the processing of FIG. 9(b), 
errors between even jumping fields, errors between odd jumping fields, and 
errors between even and odd fields are used. Further, in the processing of 
FIG. 9(c), errors between even and odd jumping fields and those preceding 
and succeeding thereto are used. It is to be noted that since if only 
fields earlier by three fields are used in FIG. 9(b), there result solely 
fields having an even and odd relationship opposite to that of a current 
field, it is suitable to use fields earlier by three frames, which are 
earlier further by three fields. 
On the other hand, FIG. 9(d) shows a processing obtained by developing the 
processing of FIG. 9(c) wherein three fields before and after are used. In 
this case, since fields having the same odd and even relationship are 
present before and after, it is possible to maintain correlation between 
images at a high level. 
It is to be noted that while arrows in FIGS. 8(a) to 8(c) are represented 
by curves outside the squares, and arrows in FIGS. 9(a) to 9(d) are 
represented by straight lines inside squares, such an indication is 
adopted for convenience of drawing Figures. This is not particularly 
meaningful. 
As stated above, the interframe processing system and the interfield 
processing system of this technique are similar to the predictive 
processing. Accordingly, processing which can be used in the predictive 
processing may be basically used in this invention. Further, in the case 
of a system of two stages, an approach may be employed such that the 
processing of the first stage is carried out by this technique and the 
processing of the second stage is by the predictive processing, and vice 
versa. Particularly, in the case of interlaced interframe processing, the 
interframe correlation is easy to be lower. Accordingly, this technique is 
advantageous. 
On the other hand, in the case of the predictive processing from fields or 
frames before and after, correlation becomes high, so the prediction 
becomes effective. Accordingly, when an approach is employed such that the 
processing of the first stage is carried out by this technique and the 
processing of the second stage is carried out by the predictive 
processing, there is the possibility that coding efficiency rather becomes 
higher than that in the conventional all predictive coding methods while 
maintaining independency every several frames (fields). This is extremely 
advantageous. 
In the combination with the intraframe processing by the orthogonal 
transform processing, a quantization noise called a mosquito noise was apt 
to be conspicuous at the periphery of the edge. On the contrary, in the 
case of this technique, since there is no difference between frames at a 
flat portion, addition between frames is permitted to be carried out. 
Thus, noise is lessened to much degree. 
On the other hand, in the case where motion compensation processing is 
carried out, when the predictive processing is employed, there are 
instances where motion may become unnatural depending upon the degree of 
motion compensation. On the contrary, in the case of this technique, when 
a motion-compensated image shifts with respect to an original image, a 
value corresponding to that shift is not added, and the original image is 
instead adopted. Accordingly, the rate of noise reduction becomes small, 
but there is no possibility that motion becomes unnatural. For these 
reasons, a reproduced image desirable from a visual point of view can be 
provided. 
Since this technique is basically an intraframe independent coding, it is 
sufficient to prepare parameters for independent coding as the parameter 
for intraframe coding processing. Thus, the configuration becomes simple.