Compression of video signals

A digital video signal is compressed by spatial sub-band filtering to form data sets constituting respective sub-bands of the two-dimensional spatial frequency domain. The data sets for a field or frame are stored. A first sequencer controls writing, in accordance with a desired sequence, of the stored data to a quantizer in which they are quantized in accordance with respective values, those values being such that the amount of quantization of at least a data set constituting a sub-band to which dc luminance information of the signal is at least predominantly confined is less than the average of the amounts of quantization of the remaining data sets. The quantized data sets are then encoded in an entropy encoder which has a first coding portion for coding quantized data representative of dc luminance information and a second coding portion for coding quantized data representative of ac luminance information. A second sequencer, which may be the same sequencer as the first sequencer, controls operation of the quantizer so that each datum (sample) written thereto is appropriately quantized, and controls operation of the entropy encoder so that each quantized sample is directed to the appropriate one of the first and second coding portions.

CROSS-REFERENCE TO RELATED APPLICATION 
Reference is made to copending U.S. patent application Ser. No. 07/810,337, 
which corresponds to UK Patent Application No. 9100591.8 filed Jan. 11, 
1991 and is assigned to the assignees hereof, which was filed on the same 
day as the present application, and which includes claims directed to the 
following disclosure. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the compression of video signals. 
2. Description of the Prior Art 
Compression of video signals on an intra-image basis (for example, 
compression on an intra-field or intra-frame basis) makes use of the 
redundancy present in pictures or images represented by the signals to 
reduce the amount of information needed to represent the pictures or 
images. The compression can be used to reduce bandwidth, in the case of 
transmission of a video signal, or to reduce storage capacity, in the case 
of storage of a video signal. 
Intra-image compression can, as is known, be effected in the time domain by 
the use of differential pulse code modulation, in which a predictor is 
used to predict the values of samples representing pixels based on 
previous pixel values. Since the image pixels are highly correlated, the 
prediction is accurate and results in a small and uncorrelated error (that 
is, a difference between the predicted and actual values). The error 
samples are encoded and, since they can be encoded using fewer bits than 
the samples representing the original pixels, compression can be achieved. 
FIG. 1 of the accompanying drawings shows a known apparatus or system for 
effecting intra-image compression of a video signal in the two-dimensional 
spatial frequency domain. A video signal, which is in digital form and 
comprises successive multi-bit (for example 8-bit) samples or words each 
representing a respective pixel of an scanned image or picture, is applied 
via an input 10 to a decorrelator 12. A decorrelated version of the video 
signal is outputted by the decorrelator 12 to a quantizer 14 and then to 
an entropy encoder 16, which together compress the decorrelated version of 
the video signal outputted by the decorrelator 12 to produce a compressed 
signal of an output 18. The compressed signal can then be transmitted or 
stored. (Note that, although the decorrelator 12, quantizer 14 and entropy 
encoder 16 are shown for clarity as being separate items, they may in 
practice be embodied in an at least partially combined form.) After 
transmission or storage, the compressed signal can be restored 
substantially to its original form by expansion by way of entropy 
decoding, dequantizing and correlation operations which employ parameters 
converse to those used for decorrelation, quantization and entropy 
encoding, respectively, upon compression. 
The operation of decorrelation performed in the decorrelator 12 relies upon 
the fact that neighboring pixels of an image are highly correlated, 
whereby processing an image (for example, a field or frame of a video 
signal) to form decorrelated signal portions representing different 
components of the image in the two-dimensional spatial frequency domain 
enables a reduction in the amount of information needed to represent the 
image. Specifically, the decorrelated signal portions represent different 
spatial frequency components of the image to which the human psychovisual 
system has respective different sensitivities. The different decorrelated 
signal portions are subjected to different degrees of quantization in the 
quantizer 14, the degree of quantization for each signal portion depending 
upon the sensitivity of the human psychovisual system to the information 
in that portion. That is, each of the decorrelated signals is quantized in 
accordance with its relative importance to the human psychovisual system. 
This selective quantization operation, which is a lossy operation in that 
it involves deliberate discarding of some frequency data considered to be 
redundant or of little importance to adequate perception of the image by 
the human psychovisual system, in itself enables some signal compression 
to be achieved. The quantizer 14 enables compression to be achieved in two 
ways: it reduces the number of levels to which the data inputted to it can 
be assigned, and it increases the probability of runs of zero value 
samples on the data it outputs. Note that, in video signal compression 
apparatus described in detail below, the ability to achieve signal 
compression provided by the operation of the quantizer 14 is not used to 
produce a bit (data) rate reduction in the quantizer itself. Instead, in 
that case, the ability to achieve signal compression provided by the 
operation of the quantizer is carrier into effect in the entropy encoder 
16 in that the reduction in information content achieved in the quantizer 
14 enables a consequential bit (data) rate reduction to be achieved in the 
entropy encoder. 
Further (non-lossy) compression, and bit (data) rate reduction, is provided 
in the entropy encoder 16 in which, in known manner, using for example 
variable length coding, the data produced by the quantizer 14 is encoded 
in such a manner that more probable (more frequently occurring) items of 
data produce shorter output bit sequences than less probable (less 
frequently occurring) ones. In this regard, the decorrelation operation 
has the effect of changing the probability distribution of the occurrence 
of any particular signal level, which is substantially the same as between 
the different possible levels before decorrelation, into a form in which 
in which it is much more probable that certain levels will occur than 
others. 
The compression/coding system or apparatus as shown in FIG. 1 can be 
embodied in a variety of ways, using different forms of decorrelation. An 
increasingly popular form of implementation makes use of so-called 
transform coding, and in particular the form of transform known as the 
discrete cosine transform (DCT). (The use of DCT for decorrelation is in 
fact prescribed in a version of the compression system of FIG. 1 described 
in a proposed standard prepared by JPEG (Joint Photographic Experts Group) 
and currently under review by the ISO (International Standards 
Organization).) According to the transform technique of decorrelation, the 
signal is subjected to a linear transform (decorrelation) operation prior 
to quantization and encoding. A disadvantage of the transform technique is 
that, although the whole image (for example, a whole field) should be 
transformed, this is impractical in view of the amount of data involved. 
The image (field) thus has to be divided into blocks (for example, of 
8.times.8 samples representing respective pixels), each of which is 
transformed. That is, transform coding is complex and can be used on a 
block-by-block basis only. 
A recently proposed approach to compression/coding in the frequency domain 
is that of sub-band coding. In this approach, the decorrelator 12 in the 
system of FIG. 1 would comprise a spatial (two-dimensional) sub-band 
filtering arrangement (described in fuller detail below) which divides the 
input video signal into a plurality of uncorrelated sub-bands each 
containing the spatial frequency content of the image in a respective one 
of a plurality of areas of a two-dimensional frequency plane of the image, 
the sub-bands then being selectively quantized by the quantizer 14 in 
accordance with their positions in the sensitivity spectrum of the human 
psychovisual system. That is, decorrelation is achieved in this case by 
putting the energy of the overall image into different sub-bands of the 
two-dimensional spatial frequency domain. Sub-band filtering is believed 
to provide better decorrelation than the transform approach. Also, unlike 
the transform technique, there is no restriction to operation on a 
block-by-block basis: the sub-band filtering can be applied directly to 
the video signal. 
The proposed JPEG standard mentioned above requires that data decorrelated 
by means of a block-by-block DCT transformation operation be quantized and 
then entropy encoded. The data is required by the standard to be quantized 
and encoded in a particular sequence dictated by the order in which it is 
outputted by the decorrelator, which order is in turn dictated by the way 
in which the input digital video signal is divided into blocks for 
transformation. The sequence is such that each successive one of a 
sequence of (for example) 8.times.8 arrays (blocks) of data each resulting 
from transformation of an 8.times.8 block of data of the input signal is, 
after quantization, entropy encoded in such a manner that one of the 64 
data in the array is entropy encoded in a manner different than the other 
63 data in the array. This requires switching of the entropy encoding at 
the block frequency, that is once every 64 data (samples). Moreover, the 
sequence is such that the 64 data in each block (in turn) are quantized 
and encoded in a particular order specified in the standard. In the case 
of sub-band filtering, the data to be quantized and encoded is of a very 
different format than that obtained in the case of block-by block DCT 
transformation. On the face of it, the JPEG standard approach not only 
excludes the use of sub-band filtering, but, on the face of it, is wholly 
incompatible with the use of sub-band filtering, which does not require a 
block-by-block approach. This is unfortunate because, as mentioned above, 
sub-band filtering is believed superior to block-by block DCT 
transformation. 
OBJECTS AND SUMMARY OF THE INVENTION 
An object of the invention is to provide a technique for compressing a 
video signal which is of a flexible nature. 
Another object of the invention is to provide a technique for compressing a 
video signal which enables the use of sub-band filtering (as opposed to 
block-by-block orthogonal transformation) for decorrelation, and yet also 
allows the compression to follow (if desired) the general format 
prescribed by the above-mentioned JPEG standard. 
The invention provides a method of compressing a video signal, in which a 
digital video signal is subjected to spatial two-dimensional sub-band 
filtering to form a plurality of data sets constituting respective 
sub-bands of the two-dimensional spatial frequency domain. These data sets 
for a field or frame of the video signal are stored, and the stored data 
sets are quantized in accordance with respective values, these values 
being such that the amount of quantization of one of the data sets 
constituting a sub-band to which dc luminance information of the signal is 
at least predominantly confined is less than the average of the amounts of 
quantization of the remaining data sets. The stored data are written to a 
quantizer, for carrying out the quantizing, in a desired sequence, and the 
quantizing is controlled in accordance with that desired sequence such 
that each datum written to the quantizer is appropriately quantized. The 
quantized data sets are entropy encoded such that quantized data 
representative of dc luminance information is coded by a first coding 
technique and quantized data representative of ac luminance information is 
coded by a second coding technique. The entropy encoding is controlled in 
accordance with the above-mentioned desired sequence such that each 
quantized datum is subjected to the appropriate one of the first and 
second coding techniques. 
The flexibility resulting from the fact that the data sets for a field or 
frame are stored and then written to the quantizer in a desired sequence, 
and a realization that, while their respective formats are very different, 
the data obtained in the respective cases of sub-band filtering and 
block-by block DCT transformation have sufficient resemblance (at least in 
terms of information content) to one another that the former can be 
rearranged to resemble the latter, to a greater or lesser degree, means 
that the apparent incompatibility between sub-band filtering and the JPEG 
standard is not as great as at first appears. On the contrary, the same 
general approach as stipulated in the JPEG standard can, to a greater or 
lesser degree, be adopted. 
The extent to which the JPEG standard is followed depends upon the 
application. If, for example, strict compliance with the standard is not 
required, for example if a proprietary piece of recording equipment is to 
be produced in which the designer can specify at his discretion the way in 
which the signal is to be compressed and subsequently expanded, there is 
no need to follow the above-described sequence of data quantization and 
entropy encoding laid down in the standard. The designer can use a 
sequence that seems best in the circumstances. 
Thus, according to one form of implementation of the invention described 
hereinbelow: 
the desired sequence in which the stored data is written to the quantizer 
is such that 
(i) all the data of said one of the stored data sets constituting the 
sub-band to which the dc luminance information is at least predominantly 
confined are quantized, 
(ii) after which, upon each occurrence of an operation carried out for a 
number of times equal to the number of data in each stored data set, those 
data corresponding to a respective one of the spatial positions in each of 
the remaining stored data sets are quantized in a predetermined order; 
the quantizing is controlled such that all the data written to the 
quantizer in step (i) are quantized in the same amount, namely in the 
amount appropriate to the sub-band to which the dc luminance information 
is at least predominantly confined, and each of the data written to the 
quantizer in each of the operations of step (ii) is quantized in an amount 
appropriate to the data set of which it forms a part; and 
the entropy encoding is controlled such that all the data quantized upon 
being written to the quantizer in step (i) are subjected to the first 
coding technique and all the data quantized upon being written to the 
quantizer in all the operations of step (ii) are subjected to the second 
coding technique. 
The use of the two steps or stages set out at (i) and (ii) for quantizing 
the data means that the format of the data to be quantized is very 
different than in the case of the JPEG (DCT) standard; and that the 
entropy encoding has to be switched at the field or frame frequency rather 
than at the much higher frequency (block frequency) used in the case of 
the JPEG (DCT) standard. However, this form of sequencing is believed 
superior to the JPEG sequence at least in some cases, in that it groups 
the dc and ac information together rather than intermingles it; and the 
invention enables this form of sequencing to be used if desired. 
Even if the above form of sequencing is used, the ac data can be treated in 
a generally similar manner to that specified in the JPEG standard. Thus, 
for example, the aforesaid predetermined order may comprise successive 
groups of the remaining stored data sets in a sequence, as between the 
groups, of sub-bands containing ac luminance information of increasing 
spatial frequency. In fact, to accomplish a form of zig-zag sequencing 
generally similar to that specified in the standard, said groups may 
comprise legs of a zig-zag pattern connecting the data of the same spatial 
position in the different stored data sets. 
According to another form of implementation of the invention described 
hereinbelow: 
the aforesaid desired sequence in which the stored data is written to the 
quantizer is such that, upon each occurrence of an operation carried out 
for a number of times equal to the number of data in each stored data set, 
those data corresponding to a respective one of the spatial positions in 
each of the stored data sets are quantized in a predetermined order; 
the quantizing is controlled such that each of the data written to the 
quantizer in each of said operations is quantized in an amount appropriate 
to the data set of which it forms a part; and 
the entropy encoding is controlled such that, for each of said operations, 
that one of the quantized data forming part of the data set constituting 
the sub-band to which the dc luminance information is at least 
predominantly confined is subjected to the first coding technique and all 
of the other data are subjected to the second coding technique. 
In this case, the sequencing is very similar to that specified in the JPEG 
standard, in particular if the aforesaid predetermined order comprises 
successive groups of the stored data sets in a sequence, as between the 
groups, of sub-bands containing ac luminance information of increasing 
spatial frequency; and if said groups comprise legs of a zig-zag pattern 
connecting the data of the same spatial position in the different stored 
data sets. 
Thus, as explained in more detail below, the format of the data to be 
quantized is very similar to that in the case of the JPEG (DCT) standard, 
which has the advantage that quantization can be sequenced in a very 
similar or even identical way to that used in the case of the standard. 
Thus, it may be possible to use an "off the shelf" quantizer chip or 
assembly intended for use in a JPEG compression apparatus (possibly with 
changes in the quantization values). Also, in this case, the entropy 
encoding has to be switched at the frequency of carrying out the 
operations in which those data corresponding to a respective one of the 
spatial positions in each of the stored data sets are quantized, that is 
at a frequency determined by the number of data sets (sub-bands), rather 
than at the field or frame frequency. If, as in the case of embodiments of 
the invention described below, the number of data sets (sub-bands) is the 
same (8.times.8=64) as the number of samples per block as specified in the 
JPEG standard, the frequency of switching the entropy encoding is the same 
as the frequency (block frequency) used in the case of the JPEG (DCT) 
standard. Thus, it may be possible to use an "off the shelf" entropy 
encoder chip or assembly intended for use in a JPEG compression apparatus. 
As is well known, a color video signal can be in component or composite 
form. A component color video signal comprises three separate signal which 
together represent the totality of the video information. The three 
separate signals may, for example, be a luminance signal and two color 
difference signals (Y, Cr, Cb) or three signals each representing a 
respective color (R, G, B). A composite color video signal, on the other 
hand, is a single signal comprising all the luminance and chrominance 
(color) information. 
Previously proposed color video signal compression systems as described 
above all operate on component signals only. That is, taking the example 
of the system of FIG. 1, three separate systems as shown in FIG. 1 are 
needed, one for each of the three components. Also, if the signal is in 
composite form, there is a need for means to convert it into component 
form prior to compression. Further, three expansion systems are needed to 
convert the transmitted or stored signals back to their original form, 
together with (if appropriate) means to convert the component signals back 
into composite form. The need to process the video signal in component 
form thus involves the expense and inconvenience of considerable hardware 
replication. 
While the invention is applicable in the case of component (or monochrome) 
video signals, a preferred feature of the invention is that it can be used 
also to compress composite color video signals. This preferred feature 
takes advantage of a realization by the inventors that, due to the way in 
which luminance and chrominance information are combined in conventional 
broadcast standard (for example, NTSC and ) composite color video 
signals, such a signal can be spatially sub-band filtered such that the 
chrominance information can be (as is explained in detail below) 
concentrated in a certain area of the two-dimensional spatial frequency 
domain (that is, in certain of the sub-bands), whereby if the data sets to 
which the dc chrominance information and dc luminance information are at 
least predominantly confined are quantized more lightly than the other 
data sets (which contain wholly or largely only the ac luminance 
information) are on average quantized, then since the dc information is 
more important to satisfactory appreciation of the image by the human 
pyschovisual system than the ac luminance information it is in fact 
(surprisingly) possible satisfactorily to compress a composite color video 
signal directly, that is without first converting it to component form and 
compressing each component individually. 
Another advantage of the sub-band approach to signal decorrelation is that 
(as is also the case for the DCT approach) the sub-band approach is 
separable between two orthogonal spatial directions. Thus, the digital 
video signal is preferably separately spatially sub-band filtered in 
respective orthogonal spatial directions. The separable approach 
simplifies design. 
A further advantage of the separable approach is that it enables the method 
to be performed such that a field or frame of the digital video signal is 
sub-band filtered in one of the orthogonal directions in a first 
one-dimensional sub-band filter arrangement, stored, and then transposed 
and sub-band filtered in the other of the orthogonal directions in a 
second one-dimensional sub-band filter arrangement which is of 
substantially the same construction as the first one-dimensional sub-band 
filter arrangement. 
To further minimize hardware requirements, according to a preferred form of 
the method, in a first stage of the filtering in each of the orthogonal 
directions, the digital video signal is subjected to low pass filtering 
followed by decimation by two and also to high pass filtering followed by 
decimation by two, thereby to produce two intermediate outputs, and, and 
in at least one subsequent stage of the filtering in each of the 
orthogonal directions, each of the intermediate outputs produced in the 
previous stage is subjected to low pass filtering followed by decimation 
by two and also to a high pass filtering followed by decimation by two. 
The use of such a "tree" or "hierarchial" structure of so-called 
quadrature mirror filters (QMFs), as opposed to the alternative 
possibility of a band of filters operating in parallel, is preferred in 
that it reduces hardware requirements and also enables better 
reconstruction of the signal to be achieved on subsequent expansion. In 
this regard, the aliasing that will of necessity be introduced in the 
hierarchial QMF filtering or decomposition during the course of 
compression can in principle be removed completely during the course of a 
converse composition operation performed upon expansion (after 
transmission or storage of the compressed signal). 
According to an alternative to the storage and transposition approach, the 
digital video signal may be sub-band filtered in a first one-dimensional 
sub-band filter arrangement configured to sub-band filter the signal in 
one of the orthogonal directions, and then sub-band filtered in a second 
one-dimensional sub-band filter arrangement configured to sub-band filter 
the signal in the other of the orthogonal directions. This approach may be 
preferable if the filter structure is constructed as a unit on silicon 
rather than by wiring together separate integrated circuits. 
The location in the two-dimensional spatial frequency domain of the dc 
chrominance information is determined by the relationship between the 
frequency at which an analog composite color video signal has been sampled 
to form the digital composite color video signal, and the frequency of a 
color sub-carrier frequency of the composite color video signal. According 
to a preferred form of the method, the digital composite color video 
signal has been formed by sampling an analog composite color video signal 
at a frequency equal to four times the frequency of a color sub-carrier 
frequency of the composite color video signal. 
According to the preferred form of the method disclosed below, the 
plurality of sub-bands make up a square array in the two-dimensional 
spatial frequency domain. The array may, for example, be a 4.times.4 array 
or an 8.times.8 array. However, it is equally feasible, and may in some 
cases be appropriate, to have different numbers of sub-bands in the two 
orthogonal directions, that is to employ a non-square, rectangular array. 
For instance, the sub-bands may make up, in the two-dimensional spatial 
frequency domain, a rectangular array having a dimension of 8 in the 
direction of scanning of the video signal and a dimension of 4 in the 
direction orthogonal thereto. 
The invention also provides apparatus for compressing a video signal. The 
apparatus comprises a spatial two-dimensional sub-band filtering 
arrangement that filters a digital video signal to form a plurality of 
data sets constituting respective sub-bands of the two-dimensional spatial 
frequency domain, a store for storing the data sets for a field or frame 
of the video signal, a quantizer that quantizes the stores data sets in 
accordance with respective values, those values being such that the amount 
of quantization of one of the data sets constituting a sub-band to which 
dc luminance information of the signal is at least predominantly confined 
is less than the average of the amounts of quantization of the remaining 
data sets, an entropy encoder that encodes the quantized data sets, the 
entropy encoder comprising a first coding portion for coding quantized 
data representative of dc luminance information and a second coding 
portion for coding quantized data representative of ac luminance 
information, and sequencing means that controls writing of the stored data 
from the store to the quantizer in a desired sequence, controls operation 
of the quantizer in accordance with that desired sequence such that each 
datum written thereto is appropriately quantized, and controls operation 
of the entropy encoder in accordance with the aforesaid desired sequence 
such that each quantized datum is directed to the appropriate one of the 
first and second coding portions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A method and apparatus for compressing a digital video signal will now be 
described with reference to the drawings. The basic construction of the 
apparatus is in accordance with FIG. 1 (described above). The decorrelator 
12 of the present apparatus is constituted by a sub-band filtering 
arrangement which, according to one form of implementation as shown in 
outline form at 12A in FIG. 2, comprises a horizontal filter arrangement 
20A, an intermediate field store 22, a transpose sequencer (address 
generator) 24, a vertical filter arrangement 26A, an output field store 
(FS) 28 and an output sequencer (address generator) 29A. As explained 
above, sub-band filtering can be effected on a separable basis. Thus, in 
FIG. 2, filtering in the two orthogonal image directions, namely the 
horizontal direction (the direction of image scanning in the case of 
conventional video) and the vertical direction, is effected entirely 
independently and separately of one another by respective one-dimensional 
filtering operations performed in the horizontal and vertical filter 
arrangements 20A and 26A, respectively. 
The horizontal filter arrangement 20A and vertical filter arrangement 26A 
can be of substantially the same construction as one another. Thus, the 
construction of the horizontal filter arrangement 20A only will be 
described in detail. 
It will be assumed that the filtering is to achieve 8 sub-bands in each of 
the horizontal and vertical directions, that is to say that a square array 
of 64 (8.times.8) sub-bands is to be produced. It will further be assumed 
that the 64 sub-bands are (as is preferred) to be of equal extent to one 
another. 
The horizontal filter arrangement 20A is preferably of a tree or 
hierarchical structure as shown in FIG. 3, comprising three successive 
filter stages 30, 32 and 34. 
The first stage 30 comprises a low pass filter (LPF) 36 and a high pass 
filter (HPF) 38, each of which is followed by a respective decimator (DEC) 
40. The LPF filter 36, HPF filter 38 and the decimators 40 together make 
up a quadrature mirror filter (QMF). Each of the filters 36 and 38 can be 
a finite impulse response (FIR) filter of conventional form. In use, a 
line of a field of the input digital video signal is applied, 
sample-by-sample, to the first stage 30, to be low pass filtered and high 
pass filtered by the LPF 36 and HPF 38, respectively. Thus, the LPF 36 and 
HPF 38 produce outputs comprising low pass filtered and high pass filtered 
versions of the input line, respectively, the outputs representing the 
spatial frequency content of the line in the upper and lower halves of the 
horizontal spatial frequency range. That is, the first stage 30 divides 
the input line into two sub-bands in the horizontal direction. The 
decimators 40 decimate (sub-sample) the respective outputs by a factor of 
two, whereby the total number of samples outputted by the decimators 40 
(together) is the same as the total number of samples in the line. 
The second stage 32 is of similar construction to the first stage 30, 
except that there are two QMFs each as in the first stage and the output 
from each of the decimators 40 of the first stage is passed as an input to 
a respective one of the two QMFs. Thus, the second stage 32 produces four 
outputs representing the spatial frequency content of the line in four 
equal quarters of the horizontal spatial frequency range. That is, the 
second stage 32 further divides the two sub-bands, into which the input 
line was divided in the first stage 30, into four sub-bands in the 
horizontal direction. The four decimators of the second stage 32 decimate 
(sub-sample) the respective outputs by a factor of two, whereby the total 
number of samples outputted by the decimators of the second stage 
(together) is the same as the total number of samples in the line. 
The third stage 34 is of similar construction to the first stage 30, except 
that there are four QMFs each as in the first stage and the output from 
each of the four decimators of the second stage 32 is passed as an input 
to a respective one of the four QMFs. Thus, the third stage 34 produces 
eight outputs representing the spatial frequency content of the line in 
eight equal one-eighths of the horizontal spatial frequency range. That 
is, the third stage 34 divides the four sub-bands into which the input 
line was previously divided into the required eight sub-bands in the 
horizontal direction. The eight decimators of the third stage 34 decimate 
(sub-sample) the respective outputs by a factor of two, whereby the total 
number of samples outputted by the decimators of the third stage 
(together) is the same as the total number of samples in the line. 
The eight outputs of the third stage 34, that is of the horizontal filter 
arrangement 20A, are passed to a intermediate field store 22 and stored at 
positions corresponding to respective one-eighths of a first line thereof. 
The above process of horizontal filtering is then repeated for all the 
other lines of the field of the input digital video signal. This results 
in the intermediate field store 22 containing a version of the field of in 
the input digital video signal that has been filtered into eight sub-bands 
in the horizontal direction (only). Each line of the field stored in the 
intermediate field store 22 is divided into eight portions each containing 
the horizontal spatial frequency information in a respective one of eight 
sub-bands of the horizontal spatial frequency range of the image that the 
original field represented. Thus, the horizontally filtered field stored 
in the intermediate field store 22 can be considered to be divided into 
eight columns. 
Referring back to FIG. 2, the horizontally filtered field stored in the 
intermediate field store 22 is then fed (under the control of the 
transpose sequencer 24) into the vertical filter arrangement 26A, in which 
it is filtered into eight sub-bands in the vertical direction in similar 
manner to that in which filtering into eight sub-bands in the horizontal 
direction was achieved in the horizontal filter arrangement 20A. The 
horizontally and vertically filtered field is fed on a line-by-line basis 
into the output field store 28 to be passed from there to the quantizer 
14A. The store 28 can be considered to have been partitioned into an array 
of 64 (8.times.8) storage regions, in each of which a respective on of the 
64 sub-bands is stored. Thus, successive fields of in the input digital 
video signal are sub-band filtered and passed, duly filtered, to the 
quantizer 14 after a delay of two field intervals. 
The transpose sequencer 24 produces read addresses for the intermediate 
field store 22, to control reading of the contents thereof into the 
vertical filter arrangement 26A, as follows. As will be recalled, the 
signal as stored in the intermediate field store 22 comprises the lines of 
the original field, each divided horizontally into eight sub-bands. That 
is, the signal as stored in the intermediate field store 22 can, as 
mentioned above, be considered to comprise eight columns. To enable the 
signal stored in the intermediate field store 22 to be vertically filtered 
by hardware of the same construction (the vertical filter arrangement 26A) 
used to horizontally filter it, it must be transposed, that is rotated 
through 90 degrees, as it is read to the vertical filter arrangement 26A, 
so that it comprises eight rows (as opposed to columns). The transpose 
sequencer 24 addresses the intermediate field store 22 in such a manner as 
to accomplish this. 
The horizontally and vertically filtered field stored in the output field 
store 28, which has been sub-band filtered by a factor of eight in both 
directions, can thus be considered as having been divided into eight rows 
and eight columns, that is into an 8.times.8 sub-band array. The 
horizontally and vertically sub-band filtered field, as stored in the 
output field store 28 of the sub-band filtering arrangement 12 ready for 
quantization, can be represented (subject to the qualification mentioned 
below concerning sub-band scrambling) on a two-dimensional frequency plane 
as shown in FIG. 4. In conventional manner for considering image 
(two-dimensional) signals, frequency is represented in normalised form in 
FIG. 4, the symbol pi being equivalent to half the Nyquist limit sampling 
frequency. For the time being, it is assumed that the input digital video 
signal is a component (luminance) signal, or even a monochrome signal, 
rather than a composite signal. Thus, the 64 sub-bands comprise a single 
sub-band, referred to hereinafter as the dc (zero spatial frequency) 
sub-band, which contains most or all of the dc information image intensity 
data, namely the sub-band (shown shaded) in the upper left hand corner of 
FIG. 4, together with 63 ac sub-bands which contain edge data, that is 
components of the two-dimensional frequency spectrum of the image in 
respective sub-bands higher than dc (zero spatial frequency). In this 
regard, if the filtered signal in the output field store 28 were viewed on 
a monitor, it would be intelligible. Thus, a very heavily filtered version 
of the original signal would be seen in the upper left hand corner picture 
area (dc sub-band) and higher frequency components could be observed in 
the other 63 picture areas (ac sub-bands). 
The sub-band filtering arrangement structure described above with reference 
to FIG. 3 (unlike an alternative arrangement described below with 
reference to FIG. 5), because of its hierarchical QMF structure, 
"scrambles" the order or sequence of the sub-bands. That is, due to a 
frequency inversion that takes place in each of the QMFs, if a field of 
the filtered signal in the output field store 28 were viewed on a monitor, 
there would not be a one-to-one correspondence between the field as viewed 
and the showing of FIG. 4. Thus, while the dc sub-band would remain in the 
upper left-hand corner, the frequency plane locations of the 63 ac 
sub-bands would be different from (that is, scrambled with respect to) 
their locations in FIG. 4. The locations would of course by the same for 
successive fields and can readily be determined from the structure of FIG. 
3. In other words, while each of the 64 storage regions into which the 
store 28 is partitioned stores a respective one of the 64 sub-bands, the 
relative positioning of the 63 storage regions containing the ac sub-bands 
is scrambled (in a known manner) with respect to the relative positioning 
of the ac sub-bands as shown in FIG. 4. 
In order that the scrambled locations of the 63 ac sub-bands are 
descrambled (that is, put into the pattern shown in FIG. 4) before the 
sub-band filtered signal is passed to the quantizer 14A, the outputs 
sequencer 29A (which can be located, as shown, in the sub-band filtering 
arrangement 12A, though it could be located elsewhere, for example in the 
quantizer 14), which is connected to the output field store 28 to produce 
read addresses therefor to cause the data therein to be read out to the 
quantizer 14A, is so designed that the data is read out in a descrambled 
manner, that is in such a manner that the sub-bands as supplied to the 
quantizer conform to FIG. 4. (The operation of the sequencer 29A in this 
regard is described in more detail below with reference to FIGS. 9 and 
10.) 
FIG. 5 shows at 12B a form of implementation of the sub-band filtering 
arrangement which can be used instead of that (12A) described above with 
reference to FIGS. 2 and 3. The sub-band filtering arrangement 12B 
comprises a horizontal filter arrangement 20B, a vertical filter 
arrangement 26B, and output field store 28, and an output sequencer 29B. 
As in the case of the sub-band filtering arrangement 12A of FIGS. 2 and 3, 
filtering in the horizontal and vertical directions is in this case also 
effected entirely separately of one another, namely by respective 
one-dimensional filtering operations performed in the horizontal and 
vertical filter arrangements 20B and 26B, respectively. 
The horizontal filter arrangement 20B is of a conventional FIR structure, 
comprising a chain of an appropriate number of one-sample delay elements 
40a, 40b, . . . 40n tapped off to multipliers 42a, 42b, . . . 42n+1 
(supplied with respective appropriate weighting coefficients WC) whose 
output signals are summed by adders 44a, 44b, . . . 44n to produce a 
horizontally sub-band filtered output signal 45; at the output of the 
final adder. Similarly, the vertical filter arrangement 26B is of a 
conventional FIR structure, comprising a chain of an appropriate number of 
one-line delay elements 46a, 46b, . . . 46m tapped off to multipliers 47a, 
47b, . . . 47m+1 (supplied with respective appropriate weighting 
coefficients WC) whose output signals are summed by adders 49a, 49b, . . . 
49m to produce a horizontally and vertically sub-band filtered output 
signal 48 at the output of the final adder, which signal is stored on a 
field-by-field basis in the output field store 28. The output sequencer 
29B (which can be located, as shown, in the sub-band filtering arrangement 
12B, though it could be located elsewhere, for example in the quantizer 
14A), is connected to the output field store 28 to produce read addresses 
therefor to cause the data therein to be read out to the quantizer 14A. 
It should be noted that the intermediate field store 22 and the transpose 
sequencer 24 used in the sub-band filtering arrangement 12A of FIGS. 2 and 
3 are not necessary when the sub-band filtering arrangement 12B of FIG. 5 
is used. It should however be noted that the above-described sub-band 
frequency scrambling that occurs in the sub-band filtering arrangement 12A 
of FIGS. 2 and 3 also takes place in the sub-band filtering arrangement 
12B of FIG. 5. Thus, the output sequencer 29B of the sub-band filtering 
arrangement 12B of FIG. 5 has to perform descrambling. 
Before the quantizer 14A is described in more detail, the principle on 
which it operates will be explained with reference to FIGS. 6 and 7. FIG. 
6 is a graph representing an empirically determined equation approximately 
representing the response of the human psychovisual system to different 
spatial frequencies, the vertical axis representing the sensitivity of the 
human psychovisual system, the horizontal axis representing spatial 
frequency, and the frequency value fs representing the Nyquist limit 
sampling frequency. As can be seen from FIG. 6, the human psychovisual 
system is most sensitive to lower frequencies, peaking at a value just 
above dc (zero spatial frequency), and the sensitivity rapidly drops as 
the frequency increases. It is therefor readily possible for the quantizer 
14A to achieve compression of the sub-band filtered video signal by 
selectively removing information, in conformity with the graph of FIG. 6 
(possibly also taking into account the amount of aliasing introduced into 
each sub-band by the sub-band filtering), to which the human psychovisual 
system is effectively insensitive. This is done by quantizing the 64 
sub-bands of the sub-band filtered video signal by respective appropriate 
amounts. Specifically, it is assumed that circular symmetry extends the 
(one-dimensional) response curve of FIG. 6 to two dimensions. (This 
assumption is believed justified in that the human psychovisual system is 
less sensitive to diagonal frequencies than to horizontal and vertical 
frequencies.) The resultant generated surface is then integrated under 
each of the 64 sub-band regions to produce an array of 64 numbers (values) 
which act as thresholds for the purpose of quantization of respective ones 
of the sub-bands in the quantizer 14A. As will be evident, the numbers 
determine the extent of quantization for their respective sub-bands. If, 
as in the example described below, the numbers are used to achieve 
quantization by virtue of their being used to divide data arriving from 
the sub-band filtering arrangement 12A or 12B, then the greater the 
number, the greater the quantization threshold and the greater the 
probability of a sample in the relevant sub-band having a zero or near 
zero value after quantization. 
It should be appreciated that the above-described technique of establishing 
the 64 numbers to be used for quantizing the different sub-bands 
represents one possible approach only and, even if this approach is used, 
the numbers derived by the somewhat theoretical method described above may 
be modified. In more detail, the quality or viewer-acceptability of a 
picture represented by a video signal which has been compressed by the 
present (or any other) technique and thereafter expanded by a converse 
technique is, in the final analysis, a matter of subjective opinion. Thus, 
a final determination of the numbers to used for quantizing the different 
sub-bands might well best be achieved by selecting rough initial or 
starting point values by the theoretical method described above and then 
refining those initial values by viewer testing (trial and error) to 
produce values judges subjectively to be optimum. 
The above-described 64 numbers can be stored in the form of a quantization 
matrix (naturally an 8.times.8 matrix in the case of an 8.times.8 sub-band 
filtered signal), for example in a look-up table in a programmable read 
only memory (PROM). FIG. 7 shows an example of an 8.times.8 quantization 
matrix produced for a particular design of sub-band filtering arrangement. 
The positioning of the numbers in the matrix of FIG. 7 corresponds to the 
positioning of the sub-bands in FIG. 4. That is, for example, the number 
68 applies to the dc sub-band and the number 8192 applies to the ac 
sub-band in the bottom right-hand corner in FIG. 4. It will be seen that 
the dc sub-band is only lightly quantized (number=68). Although the two ac 
sub-bands horizontally and vertically adjacent to the dc sub-band are 
quantized marginally even more lightly than the dc sub-band (number=64), 
the amount of quantization (quantization threshold) of the dc sub-band is, 
as can clearly be seen from FIG. 7, considerably less than the average of 
the amounts of quantization (quantization thresholds) of the ac sub-bands. 
The following two factors must be borne in mind concerning the quantization 
matrix. 
(a) The relative values of the numbers, rather than their absolute values, 
are of importance. In this regard, as explained below, the numbers in the 
quantization matrix may be scaled before they are used to effect 
quantization of the sub-bands in the quantizer 14A. 
(b) Since, as mentioned above in the description of FIG. 4, it is being 
assumed for the time being that the input digital video signal is a 
component (luminance) signal, rather than a composite signal, the numbers 
represented in FIG. 7 apply to a component (luminance) signal. (The 
modifications made to the quantization matrix of FIG. 7 in the case of 
processing a composite signal are explained below.) 
In the light of the foregoing explanation of its principle of operation, 
the quantizer 14A will now be described with reference to FIGS. 8 to 11. 
FIG. 8 shows the quantizer 14A in block diagram form. The quantizer 14A 
comprises a divider 50 that receives data read thereto from the output 
field store 28 of the sub-band filtering arrangement 12A or 12B under the 
control of the output sequencer 29A or 29B, and outputs quantized data 
from the quantizer 14A to the entropy encoder 16A (FIG. 1). 
The above-mentioned quantization matrix, referenced 52 in FIG. 8, and 
stored for example in a look-up table in a PROM, is connected to one input 
of a multiplier 54. A scale factor generator 56 is connected to another 
input of the multiplier 54. A sequencer (address generator) 58 is 
connected to the quantization matrix 52 to control it so that it outputs 
the appropriate one of the 64 numbers stored in the matrix at the correct 
time, that is so that each sample supplied to the quantizer is quantized 
in accordance with the sub-band in which it is located, and is connected 
to the entropy encoder 16A to supply thereto a timing signal that 
indicates to the entropy encoder whether data being supplied by the 
quantizer 14A to the entropy encoder results from quantization of the dc 
sub-band or quantization of the ac sub-bands. 
The scale factor generator 56 multiples each of the 64 numbers outputted by 
the quantization matrix 52 by a scale factor, whereby the samples of the 
stored field supplied to the quantizer 14A are divided in the divider 50 
by the product of the scale factor and the number currently outputted by 
the quantization matrix 52. The scale factor is usually kept constant 
throughout the period during which the same stored field is supplied to 
the quantizer 14A from the sub-band filtering arrangement 12A or 12B, 
whereby the values for the different sub-band samples as applied by the 
multiplier 54 to the divider 50 maintain the same relationship relative to 
one another over the field as do the numbers (shown in FIG. 7) in the 
quantization matrix 52. However, the absolute values applied by the 
multiplier 54 to the divider 50 are determined by the value of the scale 
factor. Variation of the scale factor therefore can vary the output data 
(bit) rate of the entropy encoder 16A, that is of the entire compression 
apparatus, and can therefore be employed, for example, to keep the data 
rate (which can vary with image content) constant. 
The quantizer 14A reads and processes a field of data stored in the output 
field store 28 of the sub-band filtering arrangement 12A or 12B, and 
passes it on after processing to the entropy encoder 16. The processing 
comprises, as explained above, and as described in more detail below, a 
selective quantization operation used to achieve compression of the video 
signal. In addition, as explained below, the processing involves 
arrangement of the data outputted to the entropy encoder in a format that 
readies it for entropy encoding and bit rate reduction. 
Since, in the quantizer 14A described above with reference to FIG. 8, the 
quantization is effected by dividing the input data (in the divider 50), 
the numbers (FIG. 7) in the quantization matrix 52 must be such that those 
for sub-bands that are to be quantized by a relatively large amount are 
greater than those for sub-bands that are to be quantized by a relatively 
small amount. Instead, the quantization could be effected by multiplying 
the input data (in a multiplier taking the place of the divider 50), in 
which case the numbers in the quantization matrix 52 would be such that 
those for sub-bands that are to be quantized by a relatively large amount 
are smaller than those for sub-bands that are to be quantized by a 
relatively small amount. (For example, in the latter case the numbers in 
the quantization matrix 52 could be reciprocals of those shown in FIG. 7.) 
It will be appreciated that, in both cases, the amount of quantization of 
the dc sub-band is considerably less than the average of the amounts of 
quantization of the ac sub-bands. 
FIG. 9 shows a part (the upper left-hand corner) of FIG. 4 on an enlarged 
scale. More accurately, FIG. 9 is a map of a sub-band filtered field as 
supplied to the quantizer 14A from the output field store 28 of the 
sub-band filtering arrangement 12A or 12B, each sub-band being stored (as 
mentioned above) in a respective one of an 8.times.8 array of regions into 
which the store 28 can be considered to be partitioned. In this regard, 
the stored field comprises an 8.times.8 array of sub-bands filtered from 
the corresponding field of the input video signal. 
A field of, for example, an NTSC digital video signal has a horizontal 
extent of 910 samples and a vertical extent of 262 samples. The sub-band 
filtering described above is however carried out on the active part only 
of the field, which part comprises 768 samples in the horizontal direction 
and 248 samples in the vertical direction. (In fact, there are 243 active 
samples, corresponding to the number of active lines, in the active part 
of an NTSC field. In order to produce numbers of active samples in both 
directions that are integrally divisible by 8, 5 blank lines are added to 
make the number of active samples in the vertical direction equal to 248.) 
Thus, each of the 64 sub-band areas in the active sub-band filtered field 
comprises (768/8).times.(248/8)=2976 samples, that is an array of 
96.times.31 samples (as shown in FIG. 9). (The whole active field 
comprises, of course, 64 times that number of samples.) The output 
sequencer 29A or 29B of the sub-band filtering arrangement 12A or 12B is 
operative to output the samples of the active field stored in the output 
field store 28 of the sub-band filtering arrangement 12A or 12B as 
follows. 
The sequencer 29A or 29B first causes all of the 2976 samples forming the 
dc sub-band (the upper left-hand sub-band area in FIG. 9), namely those in 
that one of the 64 regions of the output store 28 of the sub-band 
filtering arrangement 12A or 12B containing the data constituting that 
sub-band, to be fed in turn to the quantizer 14A. This can be done by 
addressing the relevant regions of the output store 28 in an order akin to 
the raster scan employed to form the full active field, though in this 
case the area (and the number of samples) is reduced by a factor of 64 as 
compared to a full field. The process is represented schematically by the 
arrowed lines drawn in the upper left-hand sub-band area in FIG. 9. The 
resulting 2976 samples are supplied in turn to the divider 50. While this 
process is taking place, the sequencer 58 (which, though shown as a 
separate item, could be combined with the output sequencer 29A or 29B of 
the sub-band filtering arrangement 12A or 12B) causes the quantization 
matrix 52 to output to the multiplier 54 the number (68) for the dc 
sub-band. Thus, all the 2976 samples of the dc sub-band are quantized (by 
the same amount) by being divided in the divider 50 by the product of the 
number (68) for the dc sub-band and the scale factor (from the scale 
factor generator 56), and passed on as a run or sequence of 2976 samples 
to the entropy encoder 16A. Also, while the above process is taking place, 
the sequencer 58 causes the timing signal that it supplies to the entropy 
encoder 16A to be such as to indicate to the entropy encoder that the 
quantized samples that it is receiving relate to the dc sub-band. 
When the dc sub-band samples have been processed through the quantizer 14A 
as just described, the sequencer 58 causes the timing signal that it 
supplies to the entropy encoder 16A to be such as to indicate to the 
entropy encoder that the quantized samples that it is about to receive 
relate to the ac sub-bands. Thus, the timing signal is changed once per 
field; that is, it has a frequency equal to the field frequency. The 
output sequencer 29A or 29B then causes writing to the quantizer 14A of 
the ac sub-band data, and the sequencer 58 causes a corresponding 
selection of the numbers to be outputted by the quantization matrix 52, in 
a manner now to be described. 
The ac sub-band data is processed through the quantizer 14A in a rather 
different manner than the dc sub-band data. An operation is carried out 
2976 times, under the control of the output sequencer 29A or 29B, in each 
of which the respective 63 samples having a respective one of the 2976 
spatial positions (pixel sites) in the 63 sub-bands are passed to the 
divider and multiplied by their respective coefficients. This operation 
may be more readily understood by referring to FIG. 9. 
In the first of the above-mentioned 2976 operations, as a first step the 
first stored sample accessed is the top left-hand one (indicated by a dot) 
in the ac sub-band numbered 1 in FIG. 9. That sample is divided by the 
product of the scale factor and the number in the quantization matrix 52 
relating to that sub-band, that is the number 64: see FIG. 7. Next as a 
second step, the same process is repeated for the top left-hand sample 
(again indicated by a dot) in the ac sub-band numbered 2 in FIG. 9, the 
number outputted by the quantization matrix 52 in this case being the 
number 64. As a third step, the process is repeated for the ac sub-band 
numbered 3 in FIG. 9, the number outputted by the quantization matrix 52 
in this case being the number 84. The process is repeated until it has 
been carried out 63 times, that is for all of the 63 ac sub-bands. The 
order in which the sub-bands are accessed is in accordance with the 
sequence 1 to 63 in which the ac sub-bands are designated in FIG. 10 (and, 
for some only of the ac sub-bands, in FIG. 9). It will be seen from FIG. 
10 that the order of processing or scanning of the ac sub-bands is a 
zig-zag order (shown partially by arrowed chain-dotted lines in FIG. 9 for 
the top left-hand samples) in that it involves scanning the ac sub-bands 
in a diagonal direction and in opposite senses. (Thus, the legs of the 
zig-zag comprise successive ones of a series of groups of the 63 ac 
sub-bands in a sequence as between the groups (legs of the zig-zag) of ac 
luminance information of increasing spatial frequency.) The 
above-explained zig-zag scanning technique is based upon, though 
considerably modified with respect to, a zig-zag scanning technique 
(described below) that has been proposed as part of the above-mentioned 
JPEG (Joint Photographic Experts Group) standard, which (rather than 
sub-band filtering) requires the use of DCT coding with 8.times.8 sample 
blocks, to each of which an 8.times.8 DCT transform is applied, as 
mentioned at the beginning of this description. 
The remaining ones of the above-mentioned 2976 (63-step) operations are 
carried out in the same manner as the first one, except that, in each 
case, a respective different one of the 2976 sample sites is used. Thus, 
for example, in the second operation the samples that are processed are 
those having the spatial positions indicated by crosses in FIG. 9, those 
being those immediately to the right of those, indicated by dots, that 
were processed in the first of the operations. 
It will be understood from the foregoing explanation that the data inputted 
to and outputted by the quantizer 14A for the ac sub-bands (only) has a 
format as represented in FIG. 11. That is, 2976 successive series 
(hereinafter referred to as "scans")--represented in FIG. 11 by horizontal 
strips--of 63 quantized samples are sent to the entropy encoder 16A, each 
such scan relating to a respective one of the 2976 sub-band pixel sites 
and each such scan having employed the zig-zag technique of scanning the 
63 ac sub-bands as described above. The total number of samples sent to 
the entropy encoder 16A per field (including the dc sub-band and the ac 
sub-bands) is the same as the number of samples in the stored sub-band 
filtered field written to the quantizer. However, as will be evident from 
the foregoing explanation, the data sent to the entropy encoder no longer 
has any resemblance to a video field. 
During the writing of the dc and ac data from the field store 28 to the 
quantizer 14A under the control of the sequencer 29A or 29B, the sequencer 
58 is operative to control the quantization matrix 52 such that each 
sample supplied to the quantizer is appropriately quantized. Specifically, 
the matrix 52 first continuously outputs the number (68) for the dc 
sub-band for a period having a duration of 2976 samples, and then outputs 
the 63 numbers for the ac sub-bands in a 63-stage sample-by-sample zig-zag 
manner corresponding to the manner in which the samples are written from 
the field store 28 to the quantizer 14A. 
The aim of reducing information in the video field by the quantizing 
operation performed in the quantizer 14A, and therefore enabling 
compression to be achieved by virtue of the quantizing operation, is 
achieved by the division operation performed in the divider 50. Thus, 
particularly for the higher frequency sub-bands, and particularly for 
image positions that contain little ac spatial frequency information, the 
sample outputted by the divider 50 will have a zero or very low value, 
being constituted wholly or mostly by bits of the value zero. It should, 
however, be noted that, at least in the apparatus presently being 
described, no reduction in bit (data) rate is carried out in the quantizer 
14A. That is, the bit length of each sample outputted by the divider 50 is 
the same as that of the sample inputted to it. However, the presence of 
long runs of zero value samples in the data outputted by the quantizer 
14A, and the reduction in the number of levels to which the data inputted 
thereto can be assigned, enables a consequential bit rate reduction to be 
effected in the entropy encoder, as described below. 
The entropy encoder 16A of the video signal compression apparatus may be 
embodied in the form shown in FIG. 12. The entropy encoder 16A shown in 
FIG. 12 complies with a so-called "baseline" version of the 
above-mentioned JPEG standard, which version sets out minimal requirements 
for complying with the standard, whereby it is in many respects of known 
form or based on known technology and will therefore not be described in 
great detail. 
The entropy encoder 16A shown in FIG. 12 comprises a switch 60 controlled 
by the above-mentioned timing signal provided to the entropy encoder 16A 
by the sequencer 58 (FIG. 8) of the quantizer 14A. When the timing signal 
indicates that the data emerging from the quantizer 14A relates to the ac 
sub-bands, that is when such data is one of the 2976 successive scans 
(each having a length of 63 samples) represented in FIG. 11, the switch 60 
directs the data to a run length detector/data modeller 62. When, on the 
other hand, the timing signal indicates that the data emerging from the 
quantizer 14 relates to the dc sub-band, that is when such data is the run 
or sequence of 2976 samples of the dc sub-band preceding the 2976 
successive scans represented in FIG. 11, the switch 60 directs the data to 
a differential pulse code modulator (DPCM) 64. The switch 60 is thus 
changed over once per field. 
The detector/modeller 62 is connected to a PROM 66 containing a variable 
length code (VLC) look-up table and to a PROM 68 containing a fixed length 
code (FLC) look-up table. An output of the detector/modeller 62 is 
connected via a multiplexer 70 to the output 18 of the apparatus. 
An output of the DPCM 64 is connected to a data modeller 72, an output of 
which is in turn connected va the multiplexer 70 to the output 18 of the 
apparatus. In similar manner to the detector/modeller 62, the modeller 72 
is connected to a PROM 74 containing a VLC look-up table and to a PROM 76 
containing an FLC look-up table. The VLC PROMs shown at 66 and 74 may in 
fact be the same PROM: they are shown as being separate in FIG. 12 largely 
for the sake of clarity. Similarly the FLC PROMs shown at 68 and 76 may in 
fact be the same PROM. Further, rather than being (as shown) a separate 
item, the modeller 72 can be a part (sub-set) of the detector/modeller 62. 
The operation of the entropy encoder 16A shown in FIG. 12 will now be 
described, considering first the case in which the data arriving from the 
quantizer 14A relates to the ac sub-bands and is therefore directed by the 
switch 60 to the detector/modeller 62. 
The detector/modeller 62 examines each of the 2976 63-sample scans (FIG. 
11) arriving from the quantizer 14A and looks for runs of consecutive zero 
value samples each preceded and followed by a sample of non-zero value. 
The detector/modeller 62 models the incoming data by converting each such 
run of zero consecutive value samples to a word pair of the following 
form: 
EQU [RUNLENGTH,SIZE][AMPLITUDE]. 
The two components or "nibbles" (RUNLENGTH and SIZE) of the first word of 
the pair each have a length of 4 bits. The bit pattern of the first nibble 
(RUNLENGTH) represents in binary form the number of consecutive zero value 
samples in the run and is generated by a counter (not shown) that counts 
the number of consecutive zero value samples following a previous non-zero 
value. (Run lengths from 0 to 15 are allowed and a runlength continuation 
is indicated by a code [F,O].) The bit pattern of the second nibble (SIZE) 
represents the number of bits to be used to indicate the amplitude of the 
sample of non-zero (value) amplitude that follows the consecutive run of 
zero value samples and is looked up from the table--represented in FIG. 
13--contained in the FLC PROM 68, the left hand part of FIG. 13 
representing ranges of actual values (in decimal form) and the right hand 
part representing values of SIZE for the different ranges. The second word 
(AMPLITUDE) of the pair represents the amplitude of the sample of non-zero 
value in the form of a number of bits determined by the value of SIZE. For 
a positive non-zero value, AMPLITUDE is the result of truncating the 
non-zero value (in binary form) to have only the number of bits specified 
by SIZE. For a negative non-zero value, the non-zero value is decremented 
by one and the same truncation procedure is followed. To illustrate the 
nature of the word pair by way of an example, suppose that the 
detector/modeller 62 detects a run of 4 samples of zero value followed by 
a sample having a value (amplitude) of +7. In this case, the word pair 
will be as follows: 
EQU [4,3][111]. 
The number 4 (or, more accurately, its binary equivalent, namely 0100) for 
RUNLENGTH indicates that the length of the run of zero value samples is 4. 
The number 3 (or, more accurately, its binary equivalent, namely 0011) for 
SIZE indicates (as can be seen from FIG. 13) that 3 bits are used to 
represent the number +7, namely the amplitude (in decimal form) of the 
sample of non-zero value (amplitude). The number 111 is in fact the 
amplitude (+7) of the sample of non-zero value expressed in binary form 
and truncated to 3 bits. 
It will be appreciated that the above operation will be carried out for the 
whole of each scan and that a sequence of word pairs will be generated for 
each scan. The number of word pairs (that is, the length of the sequence 
of word pairs) generated for each scan will depend upon the picture 
content. In general, the greater the number and length of runs of zero 
value samples, the lesser the number of word pairs. 
The operation of the detector/modeller 62 as so far described represents 
only the first of two stages of data (bit) rate reduction carried out in 
the detector/modeller. This first stage represents a reduction in bit rate 
resulting from the above-described reduction of information effected in 
the quantizer 14A that results (without perceptible degradation in picture 
content) in a large number of samples of zero value (and, more especially, 
runs thereof) emerging from the quantizer, especially in the data relating 
to the ac sub-bands. 
The second stage of data rate reduction effected in the detector/modeller 
62 is achieved as follows. The first of each of the above-mentioned word 
pairs is replaced in the data outputted from the detector/modeller 62 with 
a code therefor looked up in the VLC PROM 66. The VLC PROM 66 stores a 
respective such code for each possible value of the first word. The codes 
are of different lengths, and their lengths are selected such that the 
length of each code is, at least approximately, inversely proportional to 
the probability of the associated word value occurring. In this way, a 
further reduction in the data (bit) rate, resulting from entirely 
loss-free compression, is achieved. 
The operation of the entropy encoder 16A shown in FIG. 12 will now be 
described for the case in which the data arriving from the quantizer 14A 
relates to the dc sub-band and is therefore directed by the switch 60 to 
the DPCM 64. The dc sub-band (unlike the ac sub-bands) is subjected to 
DPCM treatment. Since the dc sub-band contains the intensity information 
of the original image (field), it has similar statistics to the original 
image. The ac sub-bands, on the other hand, contain sparse image edge 
information separated by zero value data and thus have completely 
different statistics to the dc sub-band. Consequently, it is believed 
desirable to entropy encode the ac and dc sub-band data separately and in 
respective different manners to minimize the overall data rate. 
Specifically, the dc sub-band data is treated, firstly, in the DPCM 64, 
prior to entropy encoding proper. The DPCM 64 uses a previous sample 
predictor with no quantization of the error data, because the fact that 
the dc sub-band data represents only a small proportion of the overall 
data means that high complexity DPCM treatment is difficult to justify. 
The DPCM 64 decorrelates (adjusts the probability distribution of) the dc 
sub-band samples so that a greater degree of compression can be achieved 
in the modeller 72. 
Next, entropy encoding proper, resulting in a reduction in the data rate, 
is carried out in the data modeller 72. The modeller 72 operates similarly 
to the detector/modeller 62, except that there is no detection of runs of 
zero value samples, such runs being much less likely in the dc sub-band. 
The modeller 72 models the incoming data by converting the incoming data to 
a sequence of word pairs of the following form: 
EQU [SIZE][AMPLITUDE]. 
As in the case of the ac sub-band data, SIZE is looked up from the FLC 
table of FIG. 13 (in the FLC PROM 76) and indicates the number of bits 
used to represent AMPLITUDE. The bits used to represent AMPLITUDE are 
determined in the same way (truncation) as in the case of ac sub-band 
data. The word SIZE is the encoded in that it is replaced in the data 
outputted from the modeller 72 with a code therefor looked up in the VLC 
PROM 74. The VLC PROM 74 stores a respective such code for each possible 
value of the word. The codes are of different lengths, and their lengths 
are selected such that the length of each code is, at least approximately, 
inversely proportional to the probability of the associated word value 
occurring. In this way, a further reduction in the data (bit) rate, 
resulting from entirely loss-free compression, is achieved. 
FIG. 14 is a graph, corresponding to FIG. 4, showing, on the 
two-dimensional frequency plane, what the inventors have discovered 
happens when a field of a digital NTSC composite video signal, sampled at 
a frequency equal to four times the color sub-carrier frequency fsc (fsc 
is approximately equal to 358 MHz), is sub-band filtered in a video signal 
compression apparatus as described above. The dc and ac luminance data is 
distributed among the 64 sub-bands in substantially the same way as 
described above for a component (luminance) signal. Surprisingly, however, 
it was found that the chrominance data, or at least the chrominance data 
that is needed, is largely (substantially) restricted to two only of the 
sub-bands (shown shaded in FIG. 14), namely to those two adjacent 
sub-bands (hereinafter referred to as "dc chrominance sub-bands") at the 
bottom centre in FIG. 14. Attempts have been made on an ex post facto 
basis to explain this phenomenon. 
As regards the horizontal positioning of the dc chrominance information, 
this seems on consideration to be appropriate since it should be centred 
around the position pi/2 along the horizontal axis of FIG. 14 by virtue of 
the use of sampling frequency equal to 4. fsc. Thus, if a sampling 
frequency of other than 4. fsc were used, the dc chrominance information 
would be displaced horizontally from the position shown in FIG. 14. If 
this were the case, the horizontal positioning of the sub-bands to be 
treated as the dc chrominance sub-bands would differ from that described 
above. 
As regards the vertical positioning of the dc chrominance information in 
FIG. 14, this can be explained as follows. FIG. 15 is a graph showing the 
two-dimensional frequency content of a field of an analog NTSC composite 
color video signal, the horizontal axis being in units of MHz and the 
vertical axis being in units of cycles per picture height (cph). It is of 
course known that analog NTSC is characterized by a luminance bandwidth of 
5.5 MHz and a chrominance bandwidth of 1.3 MHz modulated about the color 
sub-carrier frequency of 3.58 MHz. It is also known that the number of 
sub-carrier cycles per line is 227.5, as a result of which the phase of 
the sub-carrier is shifted by 180 degrees for each line. This is 
responsible for a modulation of the chrominance signal vertically, which, 
as shown in FIG. 15, leads to the chrominance being centered at a spectral 
position of 131.25 cph. This appears to explain the vertical positioning 
of the chrominance information in FIG. 14. Thus, the process of modulation 
generates lower and upper sidebands. Since the vertical carrier frequency 
is at the Nyquist limit frequency, the upper sidebands are on the other 
side of the Nyquist limit and thus do not form part of the frequency plane 
of FIG. 14. Therefore, for NTSC, the dc chrominance data will appear at 
the bottom of FIG. 14. 
As regards the horizontal extent of the dc chrominance information, the 
fairly harsh filtering (horizontal bandwidth restriction) to which the 
color (chrominance) information is subjected before it is modulated onto 
the luminance information appears to explain why the horizontal extent of 
the chrominance is restricted as shown in FIG. 14, namely so that it falls 
largely within two horizontally adjacent ones of the 64 sub-bands employed 
in this case, that is so that the horizontal extent is equal to about 
pi/4. (In fact, as explained below, the dc chrominance data in fact 
"spills over" somewhat into the two sub-bands in the bottom row of FIG. 14 
that are horizontally adjacent to those shown shaded.) 
It seems on reflection that the vertical extent of the needed color 
information in FIG. 14 is restricted to about the height of one of the 
sub-bands, namely about pi/8, for the following reason. It is probable 
that the dc chrominance information is wholly or largely restricted to the 
two sub-bands shown shaded at the bottom of FIG. 14. It is likewise 
probable that ac chrominance appears in at least some of those sub-bands 
above the two shown shaded at the bottom of FIG. 14. However, since the 
human psychovisual system has a low sensitivity to high frequency (ac) 
chrominance information, it appears to produce subjectively acceptable 
results if any such sub-bands that are co-occupied by ac luminance and ac 
chrominance information are treated as if they are occupied only by ac 
luminance information. 
However, whatever the explanation, the restricted bandwidth (in both 
directions) of the needed color information has proven very fortunate 
because, as is explained below, it leads to the advantageous effect that, 
with very minor modification, the apparatus as described above can handle 
an NTSC composite color video signal. Thus, conversion of the signal to 
component form, and tripling of the hardware to handle the three 
components separately, is not necessary, leading to a large saving in 
expense. 
The only modification that has to be made to the apparatus as described 
above to enable it to handle an NTSC color composite signal is to change 
the numbers in the quantization matrix 52 that determine the amount of 
quantization of the sub-bands that contain the dc chrominance data, namely 
the two dc chrominance sub-bands as shown shaded in FIG. 14. Specifically, 
instead of being heavily quantized as high frequency ac luminance 
sub-bands of relatively little importance, the two sub-bands should be 
relatively lightly quantized so as to preserve the dc chrominance 
information. The amount of quantization is in fact desirably reduced to 
about the same level as applied to the dc luminance sub-band. The 
necessary effect can therefore be achieved by changing the two bottom 
center numbers in the quantization matrix as represented in FIG. 7 from 
their values of 1856 and 2491, for a component (luminance) signal, to 68 
(or thereabouts) for an NTSC composite signal. This is shown schematically 
in FIG. 7. 
In principle, no changes other than the above-described change to two 
numbers in the quantization matrix 52 are necessary to enable the 
apparatus to handle a digital NTSC composite color video signal. In 
particular, it is to be noted that the (now lightly quantized) dc 
chrominance sub-bands can be handled in the quantizer 14A and entropy 
encoder 16A together with, and in the same manner as, the ac luminance 
sub-bands. 
Although, in principle, only the above-described change in the quantization 
is necessary to enable the apparatus to handle a digital NTSC color 
composite signal, another change that can advantageously be made is as 
follows. The zig-zag sequence or order in which, for a component 
(luminance) signal, the 63 sub-bands other than the dc luminance sub-band 
are quantized and then entropy encoded is, as explained above, shown in 
FIG. 10. It will be seen that, in the case of a digital NTSC color 
composite signal, the dc chrominance sub-bands have the positions 49 and 
57 in the sequence. This could result in a decrease in the efficiency of 
compression in that the dc chrominance sub-bands are much more likely than 
the adjacent sub-bands in the sequence to contain non-zero value samples: 
that is, they could break up runs of zero value samples. (This is even 
more likely in the case of than NTSC because, as explained below, in 
the case of there are four dc chrominance sub-bands positioned in the 
center of the frequency plane as shown in FIG. 14.) Thus, preferably, the 
apparatus is further modified in that the sequencer 29A (or 29B) is 
modified to change the zig-zag sequence so that the dc chrominance 
sub-bands occupy (in any specified order) the first positions in the 
sequence and the remaining sub-bands occupy the remaining positions in the 
sequence in the same order as before. That is, in the case of an NTSC 
signal, and using the same numbering system for the sub-bands as shown in 
FIG. 10, the sequence will comprise, in the following order, sub-band 49 
(or 57), sub-band 57 (or 49), sub-bands 1 to 48, sub-bands 50 to 56, and 
sub-bands 58 to 63. (The changed sequence that would be adopted in the 
case of a signal, as will be clear from the description given below 
with reference to FIG. 17, will be sub-bands 24, 31, 32 and 39 (in any 
order), sub-bands 1 to 23, sub-bands 25 to 30, sub-bands 33 to 38, and 
sub-bands 40 to 63.) The sequencer 58 in the quantizer 14A (if separate 
from the sequencer 29A or 29B) is modified in correspondence with the way 
in which the sequencer 29A or 29B is modified in order to ensure that each 
sub-band is appropriately quantized. That is, instead of outputting the 63 
numbers for the sub-bands other than the dc luminance sub-band as shown in 
FIG. 7 in the same zig-zag order as that in which the sub-bands other than 
the dc luminance sub-band are numbered 1 to 63 in FIG. 10, the sequencer 
58 is modified so that it outputs those numbers in an order which is 
modified in the same way in which the zig-zag sequence of quantizing the 
sub-band filtered samples is (as was just explained) modified. 
Further consideration was given to the phenomenon of spectral concentration 
of the color information by examining the two-dimensional frequency plane 
for a frame (as opposed to a field) of a digital NTSC composite color 
video signal sampled at 4. fsc, as shown in FIG. 16. It will be seen that 
the composite data in the center of the frequency plane is composed of 
four distinct regions due to modulation of the negative frequencies. These 
four regions are identical except for frequency inversion and a phase 
shift. Ideally, as explained below, the chrominance data should be 
restricted to a small number of the sub-bands. FIG. 16 indicates that the 
use of 64 (8.times.8) sub-bands is a good choice in this respect. 
Ideally, the horizontal extent or span of the sub-bands should equal the 
baseband chrominance bandwidth for efficient compression. This is because, 
in this case, the chrominance information falls exactly within the 
relevant sub-bands, that is it occupies the whole of those sub-bands and 
does not occupy parts of adjacent sub-bands, so that all of the dc 
chrominance information is lightly quantized and no substantial amount of 
adjacent ac luminance information is lightly quantized. In other words, a 
smaller span would lead to the chrominance data falling into a greater 
number of sub-bands (which is in conflict with the above-mentioned 
requirement of keeping the number of chrominance sub-bands as small as 
possible) and a greater span would lead to the adjacent luminance data not 
being appropriately quantized. 
It will be seen from FIG. 16 that there is in fact a small overlap or 
"spill over" of chrominance data into adjacent sub-bands which are treated 
as ac luminance sub-bands, whereby the overlapping parts of the 
chrominance will be (heavily) quantized in accordance with the 
quantization thresholds set for those adjacent sub-bands. In practice, it 
is believed that the results will nonetheless be subjectively acceptable. 
The overlap occurs in the horizontal direction because, as can be seen 
from FIG. 16, the horizontal extent of each sub-band is approximately 
equal to 0.9 MHz, whereas the chrominance data has a bandwidth (two 
sidebands) of 1.3 MHz, which is slightly larger. Provided, of course, that 
the overlap is not so large that a significant amount of low-frequency 
chrominance information spills over into adjacent sub-bands which are 
treated in the quantization process as ac luminance sub-bands, the overlap 
will generally be tolerable because, as explained above, it will comprise 
higher frequency chrominance information to which the human psychovisual 
system is not very sensitive. However, the overlap could be avoided, in 
theory, by slightly increasing the size of the sub-bands in either or both 
directions, that is by slightly decreasing the total number of sub-bands. 
Thus, an inspection of FIG. 16 indicates that the overlap would be reduced 
if a 7.times.7 or a 6.times.6 array were used. While such an array is 
realizable in theory, it could not be realized in the case of the "tree" 
or "hierarchical" QMF structure described with reference to FIGS. 2 and 3 
because this can only produce, in each direction, a number of sub-bands 
which is an integral power of two. Thus, if the tree structure is to be 
used, the overlap described above could be avoided only by going down to a 
4.times.4 array. While a 4.times.4 array is usable and produces acceptable 
results, it would result in the extent of the sub-bands that would have to 
be used as chrominance sub-bands (which, similarly to FIG. 14, would be 
the two at the bottom center of the 4.times.4 array) being substantially 
greater than the extent of the dc chrominance data. Also, it would reduce 
the efficiency of compression by virtue of the fact that the number of 
sub-bands would be greatly reduced. The reason for this is as follows. 
The amount of compression achievable by virtue of the quantization step 
decreases, up to a certain extent, as the number of sub-bands decreases. 
This is because the ratio between the number of ac luminance sub-bands and 
the number of dc (luminance and chrominance) sub-bands will increase with 
the total number of sub-bands and the ac sub-bands are on average more 
heavily quantized than the dc sub-bands. Thus, for example, in 
above-described case in which there are 64 sub-bands, of which one is a dc 
luminance sub-band and two (for NTSC)--or four (for , see below)--are 
dc chrominance sub-bands, either 61 (for NTSC)--or 59 (for )--of the 64 
sub-bands are ac luminance sub-bands. That is, either 61/64 or 59/64 of a 
field can be relatively heavily quantized on average, thereby enabling a 
higher degree of compression to be achieved than would be the case if the 
number of sub-bands were less than 64. (Thus, for example, if 16 
(4.times.4) sub-bands were used, only 13/16 of a field (for NTSC) would be 
ac luminance sub-bands.) Therefore, it is in general desirable to use as 
large a number of sub-bands as is practical, bearing in mind, however, 
that hardware realization will become impractical if too many sub-bands 
are used. Also, if a large increase (over an 8.times.8 array) is made in 
the number of sub-bands, there will be no net benefit (or at least not a 
greatly increased benefit) because more than two of the sub-bands (for 
NTSC) or more than four of the sub-bands (for ) may have to be treated 
(due to extensive overspill of chrominance information) as dc chrominance 
sub-bands. At present, the use of an 8.times.8 square array (or a 
non-square array of similar size) is believed to provide a good compromise 
between the above constraints, though, as mentioned above, a 4.times.4 
array is usable. Also arrays having horizontal and vertical extents of 4 
and 8, and 8 and 4, respectively, are usable, the latter being considered 
promising. At the very least, it is highly preferable for the number of ac 
luminance sub-bands to exceed the number of dc luminance and chrominance 
sub-bands. 
As an alternative to ignoring limited overspill or increasing the size of 
the sub-bands to reduce or remove overspill, it is possible to take 
account of the fact that some chrominance information appears in bands 
adjacent to these treated (in the quantization operation) as dc 
chrominance sub-bands by quantizing the adjacent sub-bands to an extent 
intermediate that to which they would be quantized if considered as ac 
luminance sub-bands only, and that to which the sub-bands treated as dc 
chrominance sub-bands are quantized. The actual extent of quantization of 
the adjacent sub-bands might well have to be established empirically. 
As mentioned above, the use of sampling frequency equal to four times the 
color sub-carrier frequency is preferred since it has the effect of 
centering the dc chrominance sub-bands about pi/2 in the horizontal 
direction, that is locating them in the horizontal sense where shown in 
FIG. 14. However, other sampling frequencies can be used. 
The foregoing description with reference to FIGS. 14 to 17 has concentrated 
on NTSC composite color video signals. It is to be noted, however, that 
the technique outlined above can be applied to other broadcast standard 
composite color video signals. The application of the technique to 
composite color video signals will now be described. 
FIG. 17 is a view corresponding to FIG. 4, but showing on the 
two-dimensional frequency plane both the sub-band filtered field of an 
NTSC composite color video signal, and a sub-band filtered field of a 
composite color video signal, each sampled at four times its color 
sub-carrier frequency. It will be seen that, in the case of , the 
chrominance information occupies (in the case of an 8.times.8 array of 
sub-bands) the four sub-bands (shown shaded) clustered at the center, 
rather than, as in the case of NTSC, the two at the bottom center, namely 
those numbered 24, 31, 32 and 39 in FIG. 10. 
The only modification that has to be made to the apparatus as described 
above to enable it to handle a color composite signal is to change the 
numbers in the quantization matrix 52 that determine the amount of 
quantization of the sub-bands that contain the chrominance data in the 
case of , namely the four dc chrominance sub-bands as shown shaded 
in the center of FIG. 17. Specifically, instead of being heavily quantized 
as high frequency ac luminance sub-bands of relatively little importance, 
the four sub-bands should be relatively lightly quantized so as to 
preserve the dc chrominance information. As in the case of NTSC, for 
also the amount of quantization is in fact desirably reduced to about the 
same level as applied to the dc luminance sub-band. The necessary effect 
can therefore be achieved by changing the four numbers clustered in the 
center of the quantization matrix as represented in FIG. 7 from their 
values of 260,396,396 and 581, for a component (luminance) signal, to 68 
for a composite signal. This is shown schematically in FIG. 7. 
Further, in the case of also, the apparatus is desirably further 
modified (as already indicated above) to change the zig-zag sequence of 
treatment of the 63 sub-bands other than the dc luminance sub-band so that 
the four dc chrominance sub-bands come first. 
Since, in the case of , the chrominance data occupies 4 of the 64 
sub-bands, whereas in the case of NTSC the chrominance data occupies only 
2 of the 64 sub-bands, there is a slightly lower potential for compression 
(as compared to NTSC) for . Specifically, as indicated above, only 
59/64 of a field in the case of , as opposed to 61/64 of a field in the 
case of NTSC, is occupied by ac luminance sub-bands and therefore can be 
relatively heavily quantized on average. 
The invention can, of course, be embodied in other ways than that described 
above by way of example. For instance, although the above-described 
apparatus operates on a field-by-field basis, which will generally be more 
convenient, it could instead operate on a frame-by-frame basis. In this 
case the sub-bands would have twice the number of samples in the vertical 
direction and the various field stores would be replaced by frame stores. 
Further, although the above-described apparatus operates only on an 
intra-field basis, whereby sub-band filtering is effected in two 
dimensions or directions only, namely the horizontal and vertical spatial 
directions, it could in principle be extended to operate also on an 
inter-field or inter-frame basis, whereby sub-band filtering would in this 
case be effected in three dimensions or directions, namely the horizontal 
and vertical spatial directions and the temporal dimension or direction. 
As alluded to in the introduction to this specification, the format 
(described above with reference to FIGS. 9 to 11) of the data written from 
the field store 28 to the quantizer 14A, under the control of the 
sequencer 29A or 29B, for quantization and entropy encoding, and the 
consequential outputting of the numbers of the quantization matrix 52 
(FIG. 7), under the control of the sequencer 58, are very different than 
in the case of the JPEG (DCT) standard. The same applies to the timing 
signal supplied to the switch 60 of the entropy encoder 16A by the 
sequencer 58, in that the entropy encoder has in the above case to be 
switched at the field or frame frequency rather than at the much higher 
frequency (block frequency) used in the case of the JPEG (DCT) standard. 
While this form of sequencing is believed superior to the JPEG sequence at 
least in some cases, in that it groups the dc and ac information together 
rather than intermingles it, it is subject to the disadvantage that 
difficulty might arise if the compression apparatus must operate in close 
conformity with the JPEG standard and/or if there is a desire to construct 
the apparatus using a quantizer 14A and/or an entropy encoder 16A designed 
specifically for use in accordance with the JPEG standard. As will now be 
described, this difficulty can be overcome by modifying the operation as 
described above to operate in close accordance with the JPEG standard. The 
modification is possible by virtue of a realization of an extent of 
commonality between decorrelation effected by the technique of sub-band 
filtering and decorrelation effected by way of the very different 
technique of linear block transformation, for example by way of a DCT 
technique. The commonality will now be explained, commencing with a brief 
review of the linear block transformation method. 
FIG. 18 shows how the samples making up a field or frame of a digital video 
signal are divided into blocks or arrays of samples which are each to be 
processed by a linear transform, for example a DCT. It is assumed, by way 
of example, that the item depicted in FIG. 18 is a frame of a 4:2:2 
component (luminance or color difference) signal according to CCIR 
Recommendation 601. If DCT were employed in a compression apparatus like 
those embodying the invention as described above, it might instead be the 
case that a field (or frame) of an NTSC or composite signal would 
instead in fact be processed. The only difference in that case is that 
there would be a different number of blocks since a field (or frame) of an 
NTSC or composite signal has a different extent (number of samples) in 
both the horizontal and vertical directions than a CCIR 601 4:2:2 frame. 
The frame shown in FIG. 18 has a horizontal extent of 720 samples and a 
vertical extent of 576 samples. Prior to being processed by a linear 
transform, for example a DCT, the frame is divided by suitable hardware 
into blocks BL of (for example) 8.times.8 samples. Since 720 and 576 are 
each integrally divisable by 8, the frame is divided into an array of 
(720/8.times.576/8=) 6480 blocks, the array having a horizontal extent of 
(720/8=) 90 blocks and a vertical extent of (576/8=) 72 blocks. 
FIG. 19 shows a linear transform decorrelator 12C for carrying out the 
above-outlined operation. A digital input video signal is applied via an 
input 10 to a blocking circuit 80 that divides each field or frame of the 
signal into 8.times.8 sample blocks. In a manner analogous to a raster 
scan, the blocking circuit 80 sequentially outputs the blocks to a linear 
transform circuit 82 which transforms each block. For convenience, it will 
be assumed that the linear transform circuit 82 performs a DCT transform; 
and the circuit will thus hereinafter be referred to as a DCT circuit. 
However, as indicated above, other suitable linear block transforms known 
in the art can be used. 
The transformation performed by the DCT circuit 82 on each 8.times.8 block 
BL of samples results in the circuit outputting an 8.times.8 block BL(T) 
of transformed samples which are (somewhat confusingly) referred to in the 
art as "coefficients". Each coefficient is a sample or measure of the 
frequency content of the video signal at a respective one of an 8.times.8 
array of positions in the two-dimensional frequency domain or plane 
corresponding to a respective one of the samples inputted to the DCT 
circuit 82. The coefficient blocks BL(T) are supplied from the DCT circuit 
82 to a quantizer 14A and entropy encoder 16A for compression of the 
signal, as described above with reference to FIG. 1. As explained below, 
the decorrelator 12C causes the coefficients to be supplied to the 
quantizer 14A in a rather different manner than that in which the 
sequencers 29A and 29B of FIGS. 2 and 5, respectively, cause the sub-band 
filtered samples in the stores 28 of FIGS. 2 and 5 to be written to the 
quantizer. 
FIG. 20 shows a DCT decorrelator 12D that employs DCT correlation followed 
by a coefficient reordering process that emulates sub-band filtering. The 
decorrelator 12D of FIG. 20 is essentially the same as the decorrelator 
12C of FIG. 19, except that: (i) the DCT circuit 82 is followed by a 
reorder circuit (address generator) 83 followed by a store 84, the reorder 
circuit 83 being operative, as described below, to cause the coefficients 
making up the blocks BL(T) emerging from the DCT circuit 82 to be written 
into the store 84 in a very different manner to that in which they are 
outputted from the DCT circuit 82 in the case of FIG. 19; and (ii) the 
writing of data from the store 84 to the quantizer 14A is controlled by an 
output sequencer (address generator) 29D, which can operate similarly to 
the output sequencer 29A (29B) of FIG. 2 (FIG. 5). 
In the case of FIG. 20, the store 84 can be considered to be partitioned 
into a number of regions equal to the number of coefficients per block 
(64, that is 8.times.8, in the above example), each such region having a 
capacity equal to the number of blocks (6480, that is 90.times.72, in the 
above example). In this case, the 64 coefficients making up each 
coefficient block BL(T) are spread out over the whole of the store 84 
rather than being outputted as a unit to the quantizer 14A as in the case 
of FIG. 19. More specifically, each of the 64 coefficients making up each 
coefficient block BL(T) is written into a respective one of the 64 regions 
into which the store is partitioned. The exact positioning of each 
coefficient within its respective region of the store 84 will now be 
explained with reference to FIGS. 21 and 22. 
FIG. 21 shows one of the 8.times.8 coefficient blocks BL(T). FIG. 22 shows 
the store 84 of FIG. 20 partitioned, as mentioned above, into 64 
(8.times.8) regions R each having a capacity equal to 6480 (90.times.72) 
coefficients. The way in which each coefficient is positioned by the 
reorder circuit 83 in the store 84 is as follows. Assume that the 
coefficient block BL(T) is that corresponding to the first input sample 
block BL, namely that shown in the upper left-hand corner of FIG. 18. 
Employing a convention in which the coefficients in the block (BL(T) are 
identified as c(m,n), where m varies from 0 to 7 and represents the 
horizontal position of the coefficient within the block and n varies from 
0 to 7 and represents the vertical position of the coefficient within the 
block, the origin being the coefficient c(0,0) in the upper left-hand 
corner in FIG. 21, and employing an identical convention to identify the 
regions R(m,n) of the store 84 as shown in FIG. 22, each coefficient 
c(m,n) of the coefficient block BL(T) corresponding to the input sample 
block BL shown in the upper left-hand corner of FIG. 18 is store din the 
upper left-hand one of the 90.times.72 array of storage positions in that 
one of the regions R identified by the same values of m and n as the 
coefficient. These coefficients are thus stored in positions represented 
(for some only of the regions R) by dots in FIG. 22. 
A similar process is then carried out for the coefficients c(m,n) of the 
second coefficient block BL(T), namely that corresponding to the input 
sample block BL which is horizontally adjacent to and on the right of the 
input sample block shown in the upper left-hand corner of FIG. 18. The 
coefficients of the second block BL(T) are stored in the next set of 
storage positions in the regions R of the store 84, namely those 
horizontally adjacent to and on the right of the positions in which the 
coefficients of the first block (BL(T) were stored. The coefficients of 
the second block BL(T) are thus stored in positions represented (for some 
only of the regions R) by crosses in FIG. 22. 
The foregoing process is then repeated, in a manner analogous to a raster 
scan, for the coefficients c(m,n) of each of the remaining coefficient 
blocks (BL(T), until the coefficients of the final block BL(T), namely 
that corresponding to the input sample block BL which is shown in the 
bottom right-hand corner of FIG. 18, are stored in the final set of 
storage positions in the regions R of the store 84, namely those in the 
bottom right-hand corners of the regions R. That is, the coefficients of 
the final block BL(T) are stored in positions represented (for some only 
of the regions R) by circles in FIG. 22. 
In more general terms, employing a convention in which the coefficient 
blocks BL(T) are identified as BL(T)(p,q), where p varies from 0 to 90 and 
represents the horizontal position of the block and q varies from 0 to 72 
and represents the vertical position of the block, the origin being the 
coefficient block corresponding to the input sample block BL in the upper 
left-hand corner in FIG. 18, and employing an identical convention to 
identify the storage positions s(p,q) of each of the regions R(m,n) of the 
store 84, each coefficient c(m,n) of each coefficient block BL(T)(p,q) is 
stored in that one of the regions R identified by the same values of m and 
n as the coefficient and, within that region, in that one of the storage 
position s of that region having the same values of p and q as the 
coefficient block. 
If the structure of the data content of the store 84 as represented in FIG. 
22 is analyzed, it will be seen that, starting from the region R(0,0) in 
the upper left-hand corner, the content of each region increases in 
horizontal spatial frequency as one goes right (horizontally) and 
increases in vertical spatial frequency as one goes down (vertically). 
That is, for example, the region R(0,0) will contain dc spatial frequency 
information (that is, the coefficients c(0,0) of all of the blocks BL(T)), 
the region R(0,7) will contain the highest vertical frequency information 
and dc horizontal frequency information, the region R(7,0) will contain 
the highest horizontal frequency information and dc vertical frequency 
information, and the region R(7,7) will contain the highest diagonal 
frequency information. Thus, the reordering process effected by the 
reorder circuit 83 results in the content of the store 84 being such that 
the contents of the different regions R thereof are data sets which are in 
substance the same as would have been obtained if, instead of being 
decorrelated in the decorrelator 12D of FIG. 20, the video signal had been 
decorrelated in a decorrelator in the form of a sub-band filtering 
arrangement, for example either the arrangement 12A described above with 
reference to FIGS. 2 and 3 or the arrangement 12B described above with 
reference to FIG. 5. That is, the content of the store 84 of the 
decorrelator 12D of FIG. 20 as read out to the quantizer 14A is 
substantially the same as the contents of the stores 28 of the 
decorrelators (sub-band filtering arrangements) 12A and 12B of FIGS. 2 and 
3, and FIG. 5, respectively, as read out to the quantizer 14A. 
Thus, the decorrelator 12D of FIG. 20 emulates the sub-band filtering 
carried out in the decorrelators (sub-band filtering arrangements) 12A and 
12B, as a consequence of which the decorrelator 12D could (though this is 
not done in the present invention, since the emulated sub-band filtering 
technique does not preserve the advantages of actual sub-band filtering 
over the DCT approach) be used in direct substitution for the decorrelator 
12A or 12B in a video signal compression signal apparatus which can handle 
a digital composite color video signal (or a component video signal). In 
this regard, assuming that the decorrelator 12D is configured to process, 
for example, an NTSC signal on a field-by-field basis, the content of the 
store 84 will correspond to FIG. 14. Thus, the region R(0,0) of the store 
84 will be quantized in the quantizer 14A (as described above with 
reference to FIGS. 7 and 8) on the basis that it contains dc luminance 
information, the contents of the regions R(3,7) and R(4,7) will be 
quantized on the basis that they contain dc chrominance information, and 
the contents of the other 61 regions will be quantized on the basis that 
they contain ac luminance information. Likewise, if the decorrelator 12D 
is configured to process a signal on a field-by-field basis, the 
content of the store 82 will correspond to the relevant parts of FIG. 17. 
Thus, the region R(0,0) of the store 84 will be quantized on the basis 
that it contains dc luminance information, the contents of the regions 
R(3,3), R(3,4), R(4,3) and R(4,4) will be quantized on the basis that they 
contain dc chrominance information, and the contents of the other 59 
regions will be quantized on the basis that they contain ac luminance 
information. 
The foregoing description with reference to FIGS. 18 and 22 substantiates 
the above suggestion of a commonality or duality between sub-band 
filtering and linear transformation in that the two-dimensional spatial 
frequency information obtained in the case of the former is present also 
in the coefficients obtained in the case of the latter and can be 
recovered by data reordering. Thus, it is possible to compress a digital 
composite color video signal (without splitting the composite signal into 
its components), not only by using sub-band filtering for decorrelation, 
but also by emulating sub-band filtering by using linear transform (for 
example, DCT) decorrelation followed by data reordering. However, as will 
now be described, realization of the commonality leads to the further 
development that merely by altering the operation of the sequencer 29A (or 
29B) of the decorrelator 12A (or 12B) of FIGS. 2 and 3 (or FIG. 5), and 
correspondently modifying the operation of the sequencer 58 of the 
quantizer 14A, a video signal compression apparatus embodying the 
invention, in which the data format outputted by the decorrelator 12A (or 
12B) is substantially the same as that of the JPEG standard, can be 
achieved. 
In the foregoing regard, in the apparatus described above with reference to 
FIGS. 2 to 17, the sub-bands stored in respective ones of the 64 regions 
into which the output store 28 of the decorrelator 12A or 12B is 
partitioned comprise respective data sets representing dc luminance 
information, ac luminance information and (if a composite color video 
signal is being compressed) dc chrominance information of the video signal 
in the two-dimensional frequency domain. It was demonstrated above that 
there is a commonality or duality between sub-band filtering and transform 
decorrelation in that, in the case of transform decorrelation as described 
with reference to FIGS. 18 and 19, the data sets obtained in the case of 
sub-band filtering are still present (in that each coefficient block BL(T) 
contains a respective member of each of the data sets) and the data sets 
can be put into storage (in the store 84 of FIG. 20) in the same manner as 
in the case of sub-band filtering so as thereby to emulate sub-band 
filtering whereby the stored data sets can be treated after outputting 
from the store 84 in substantially exactly the same way as if they had 
been obtained by sub-band filtering. Pursing the commanality further, it 
is in fact the case that, since the data sets obtained in the case of 
sub-band filtering are still present in the case of transform 
decorrelation, it is possible to reorder the data outputted by the 
decorrelator 12A or 12B to conform to the format obtained by the use of 
transform coding per se, that is as would be obtained if the DCT 
decorrelator 12C of FIG. 19 were used. 
This is accomplished as follows. Instead of the sequencer 29A or 29B being 
operative (as described above with reference to FIGS. 9 to 11) to first 
scan or output all of the 2976 samples (for NTSC) of the storage region of 
the store 28 holding the dc sub-band and then to zig-zag scan the storage 
regions holding the remaining 63 sub-bands (each made up of 2976 samples) 
2976 times, each time scanning 63 of the samples having a common one of 
the 2976 possible spatial positions, the sequencer 29A or 29B zig-zag 
scans all 64 of the storage regions (each made up of 2976 samples) 2976 
times, each time scanning all 64 of the samples having a common one of the 
2976 possible spatial positions. That is, the operation of the sequencer 
29A or 29B is modified with respect to that described with reference to 
FIGS. 9 to 11 in that the samples in the field store 28 are outputted by 
zig-zag scanning them in an order which is the same as that shown in FIG. 
10, save that the order in the present case is a 64-stage one (rather than 
a 63-stage one) starting with the area in FIG. 10 that is not numbered and 
then carrying on in the order of the areas numbered 1 to 63. The 64-stage 
zig-zag would be as shown by arrowed chain-dotted lines in FIG. 21 if the 
dots in FIG. 21 were considered to represent (instead of the coefficients 
of a transformed coefficient block BL(T) in the DCT decorrelator 12D of 
FIG. 20) those pixels represented by dots in FIG. 9. 
The data that in this case is inputted to and outputted from the quantizer 
14A has a very different form than that described with reference to FIG. 
11. Instead of there being a run of 2976 samples relating to the dc 
sub-band followed (as shown in FIG. 11) by 2976 scans or sequences of 63 
samples (one for each ac sub-band) each relating to a respective one of 
the 2976 sub-band spatial positions, there are 2976 successive scans or 
sequences of 64 samples (one for each of the 64 sub-bands) each relating 
to a respective one of the 2976 sub-band spatial positions. It is 
therefore necessary for the sequencer 58 of the quantizer 14A to operate 
in a correspondingly different manner. That is, instead of first 
outputting the same number from the quantization matrix 52 as shown in 
FIG. 7 (that for the dc sub-band) continuously for a period having a 
duration of 2976 samples, and then cyclically outputting the 63 numbers 
for the other sub-bands, 2976 times, in a 63-stage, sample-by-sample 
zig-zag manner, as described above, so as to conform with the manner of 
reading the output field store 28 as described above, in this case the 
sequencer 58 always cyclically outputs all 64 numbers of the quantization 
matrix 52 in a zig-zag manner which is the same as the 64-stage zig-zag 
scanning of the 64 samples of the same spatial position in the 64 regions 
of the store 28 containing the respective sub-bands. 
Also, the timing signal supplied to the entropy encoder 16A of FIG. 13 by 
the sequencer 58 must be altered to reflect the fact that there is a 
difference in the timing of receipt by the entropy encoder of data 
relating to dc frequency information (which is switched by the switch 60 
to the DPCM 64) and ac frequency information (which is switched by the 
switch 60 to the run length detector/data modeller 62). As described above 
with reference to FIGS. 9 to 11, in that case the switch 60 switches the 
data to the DPCM 64 for the initial run of samples (duration of 2976 
samples, in the case of NTSC) of the dc sub-band, and then switches the 
data to the detector/modeller 62 for the other 63 sub-bands (duration of 
63.times.2976 samples). That is, the switch 60 is changed over once per 
field (or frame). In the present case, the sequencer 58 must be operative 
to change over the switch 60 rather more frequently. Specifically, the 
switch 60 is changed over every 64 samples, that is once per 64-stage 
zig-zag scan, to supply the first sample of each scan (namely that in the 
region of the store 28 containing the dc sub-band), after quantization, to 
the DPCM 64, and to supply the remaining 63 samples of each zig-zag scan 
(namely those in the other 63 storage regions of the store 28), after 
quantization, to the detector/modeller 62. In short, while the switch 60 
has to be changed over (by the sequencer 58) once per field (or frame) in 
the case of FIGS. 9 to 11, in the present case the switch 60 has to be 
changed over every 64 samples, that is once every 64-stage zig-zag scan. 
Thus, in summary of the above-described modification, the format of the 
data to be quantized is very similar to that in the case of the JPEG (DCT) 
standard, which has the advantage that quantization can be sequenced in a 
very similar or even identical way to that used in the case of the 
standard. Thus, it may be possible to use an "off the shelf" chip or 
assembly intended for use in a JPEG compression apparatus (possibly with 
changes in the quantization values, that is the numbers in the 
quantization matrix 52 as shown in FIG. 7) for the quantizer 14A. Also, 
the entropy encoder 16A is switched at the frequency of carrying out the 
64-stage zig-zag scanning operations in which those samples corresponding 
to a respective one of the spatial positions in each of the store data 
sets are quantized, that is at a frequency determined by the number of 
data sets (sub-bands), rather than at the field or frame frequency. If, as 
described, the number of data sets (sub-bands) is the same (8.times.8=64) 
as the number of samples per block as specified in the JPEG standard, the 
frequency of switching the entropy encoding is the same as the frequency 
(block frequency) used in the case of the JPEG (DCT) standard. Thus, it 
may be possible to use an "off the shelf" chip or assembly intended for 
use in a JPEG compression apparatus for the entropy encoder 16A. 
Although illustrative embodiments of the invention have been described in 
detail herein with reference to the accompanying drawings, it is to be 
understood that the invention is not limited to those precise embodiments, 
and that various changes and modifications can be effected therein by one 
skilled in the art without departing from the scope and spirit of the 
invention as defined by the appended claims.