Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to a television signal coder for en- ~
coding a television signal making use of correlations to compress the amount ~ -
of information to be transmitted.
An example of a prior art system for the transmission of a tele-
vision signal using reduced amounts of information based on the frame-to-
frame correlation is shown in an article by J. C. Candy et al entitled
"Transmitting Television as Clusters of Frame-to-Frame Differences" published
in the "THE BELL SYSTEM TECHNICAL JOURNAL" July-August, 1971, pp 1189-1917.
In this transmission system, the frame-to-frame difference, i.e., the
difference in signal level between a picture element of one frame and that
of the immediately preceding frame is derived and transmit~ed only when it
is significant compared with a predetermined threshold value. Because of
the high frame-to-frame correlation inherent in a television picture signal,
this system makes it possible to reduce the amount of information to be
transmitted significantly. However, when the signal is picked up from a
fast changing subject, the frame-to-frame correlation is lowered, resulting
in an increase in the amount of information to be transmitted. This may be
overcome by quantizing the frame-to-frame differences and making the quan-
tizing levels variable depending on the rate of change in the image to be
picked up.
However, the broadening of the quantization width results in the
degradation of the quality of the reproduced picture with a granular noise
appearing over the entire picture. This problem is avoided only by increasing
the number of quantizing levels.
Therefore, an object of the present invention is to provide a
television signal coder capable of reducing the amount of information to be
transmitted without causing degradation of picture quality.
In the television signal coder according to the present invention,
_ fields are selected for every _ fields (m, n being positive integers;
_>_~, and a predicted signal level for each picture element in the _ fields
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is represented by an interpolated value of the respective
spatially corresponding picture elements in the preceding and
succeeding fields. The difference between the interpolated
value and each corresponding picture element, i.e. the predic-
tion error, is coded and transmitted as a generated information
among the _ fields. The signal for the remaining (m-_) fields
is coded and transmitted according to any one of the coding
systems based on frame-to-frame difference, field-to-field
difference and inter-frame difference. ~ith regard to the
coding system to be employed, no limitation is imposed.
According to the present invention, for the selected
n fields, a prediction value is derived from the preceding and
succeeding fields by interpolation. Compared with the conven-
tional frame-to-frame prediction based on the signal in the
past, the prediction error is reduced and the coding efficiency
is enhanced. In addition, the coding system according to the
present invention finds a broad practical application because
the invention is applicable regardless of whether the input
television signal is a monochromatic signal or an NTSC (Natural
Television System Committee)color signal.
Broadly stated, the present invention provides a
television signal coder for coding a television signal by making
use of correlation, said television signal consisting of a
series of fields each representing a number of two-dimensionally
arranged picture elements of an optical image to be transmitted,
said coder comprising: means for selecting n fields of said
television signal among every m fields of said television
signal, said m and n being positive integers/ said n being less
than said m; means for deriving an array of interpolated values
corresponding to the interpolation between said picture elements
of preceding and succeeding fields adjacent to said selected n
fields; first coding means for coding the difference between
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the television signal level in said selected n fields and said
interpolated values; second coding means for coding the tele-
vision signal in the (m-n) fields consisting of said m fields
excluding said selected n fields; and means for code-converting
the outputs of said first and second coding means.
The present invention will now be described in
greater detail with reference to the accompanying drawings,
in which:
Figure 1 shows the principle of the technique for
deriving one field from two adjacent fields by interpolation;
Figure 2 shows field-to-field and line-to-line phase
relations of a color subcarrier of a television picture signal
for the NTSC system;
Figure 3 is a block diagram of one preferred embodiment
of the present invention;
Figure 4 shows a mode of switching between frame-to-
frame coding (system-A) and coding employing an average value
between two fields (system-B);
Figure 5 shows a first modification of the preferred
embodiment of Figure 3; and
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Figure 6 shows another modification of the first preferred embodi-
ment adapted to an NTSC television picture signal.
For simplicity of explanation, in Figure 1 it is assumed that m=2
and n=l, where for every other field an interpolated value is used as the
prediction value. As a prediction value for a picture element bjk (indicat-
ing that the element is the k-th picture element in the j-th line) in the
(i+l)-th field Fi+l, ~ajk + cjk)/2 is used, and after quantization has been
executed on the difference bjk-(ajk + cjk)/2 as a prediction error and code-
conversion has been achieved, transmission is carried out while adding
positional and synchronizing information, for example, to every line. Since
the phase of the color sub-carrier for an NTSC television picture signal is
reversed for every two successive frames and for every two neighboring
horizontal lines as shown in Figure 2, two horizontal lines having their
line numbers deviated by 1 in the field Fi and Fi+2, respectively, such as
the ~j-l)-th line in the field Fi and the j-th line in the fied Fi+2, the
j-th line in the field Fi and the (j+l)-th line in the field Fi+2, ... are
employed to derive prediction values for the lines in the field Fi+l where
the phase of the respective color sub-carrier is coincident with that on
first-mentioned two horizontal lines. Accordingly, a prediction value for
bjk is equal to (ajk + c(j+l)k)/2. It is a matter of course that this pre-
diction value of ~ajk + c~j+l)k)/2 with regard to the NTSC color signal is
equally applicable to a block and white television signal. It is to be
noted in Figure 2 that the symbol o indicates that the phase af the color
sub-carrier is a while, the symbol indicates that the phase is ~0 + 180).
Now a first preferred embodiment of the present invention assuming
m=2 and n=l will be explained with reference to Figure 3.
A digitized television signal is applied to a field memory 102
~FM2) which is capable of storing at least one field of television signal,
and to side-b of a switch 112. The output of the memory 102 is applied to
side-a of the switch 112 and to subtractor 107 via lines 212 and 207, re-
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spectively. Side-a inputs of switches 112, 113 and 114 are selected upon
frame-to-frame coding which makes use of a field memory 101 (FMl), whereas
side-b inputs thereof are selected upon the coding which makes use of a pre
diction value derivecl from two adjacent fields by interpolation.
The output of the switch 112 is applied to a subtractor 103, where
the difference between said output and the output of memory 101 fed through
a line 1103 is derived. The output of the subtractor 103 is fed to a
quantizer 104 to be quantized, and the quantizing characteristics are appro-
priately varied in accordance with the command signal supplied from a buffer
memory monitor 111 through a line 1114. The output of the quantizer 104 is
applied to side-a of the switch 114 and to an adder 105 via lines 414 and
405, respectively. In the switch 114, either the signal applied to the
side-b after the quantization of the output of the subtractor 107 by the
quantizer 108 or the signal applied to the side-a through the line 414 is
selected depending upon which one of the frame-to-frame coding and the coding
employing interpolated values between two fields is executed.
The adder 105 produces a locally decoded signal in the frame-to-
frame coding by taking the sum of the signals applied through lines 405 and
1105, and the output is applied to the side-a of the switch 113 and to an
interpolator circuit 106 through lines 513 and 506, respectively. In the
switch 113, the side-a input is selected for the frame-to-frame coding,
while the side-b input is selected for the coding employing interpolated
values between two fields. The output of the switch 113 is fed to the
memory 101. In the interpolator 106, interpolated values are calculated
from the signals of two fields fed through lines 1106 and 506, respectively,
and the result is applied to the subtractor 107.
The description of the switching between the frame-to-frame coding
~hereinafter referred to as system-A) and the coding employing interpolated
values between two fields ~hereinafter referred to as system-B) will now be
given with reference to Figure 4.
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When the input signal to the memory 102 is for the i-th field Fi
(hereinafter abbreviated simply as Fi), the signal at a point Y in Figure 3
is a signal Fi which is derived by executing the coding of system-A between
the output Fî 2 of the memory 101 (the symbol '~ ' representing a signal
processed and thereafter locally decoded) and Fi and thereafter locally
decoding the same, so that the interpolator circuit 106 is supplied with
Fi and Fi 2. At this point in time, i.e., when the system-B coding is to
be executed, all the switches 112, 113 and 114 select the side-b. Subsequent-
ly, when the input signal is turned to Fi+l, the output of the memory 102
is Fi, then the system-A coding is executed between this output Fi and the
output Fi 2 of the memory 101, and the switch 113 is controlled so that the
locally decoded signal ~i at this point in time may be applied to the memory
101. In other words, upon execution of the system-A coding, all the switches
112, 113 and 114 select the side-a. Thereafter, the same operation is re-
peated for the input signals Fi+2, Fi+3, ... .
Referring again to Figure 3, the output of the switch 114 is fed
to a code converter lO9 for coding the generated information. The output
of the code-converter 109 is fed to a buffer memory 110 for the purpose of
transmission speed matching with a transmission path. The state of the
buffer memory 110 is monitored by the buffer memory monitor 111, and the
quantization characteristics of the quantizers 104 and 108 are modified in
accordance with the state of the buffer memory. The modification command is
fed to the respective quantizers 104 and 108 through lines 1114 and 1108,
respectively. In addition, information representing quantization character-
istics then used is fed to the code-converter 109 through line 1109. It is
to be noted that the quantization characteristics of the quantizers 104 and
108 may be of the same type. The construction of the coder may be simplified
accordingly. More particularly, as shown in Figure 5, the outputs of the
subtractors 103 and 107 are applied to the side-a and side-b, respectively,
of the switch 114. The output of the switch 114 is applied to the quantizer
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104, where output is fed to the adder 105 and the code-converter 109. By
such modification, the quantizer 108 in Figure 3 can be omitted.
Referring again to Figure 3, a switching signal for selecting
either the frame-to-frame coding or the coding making use of interpolated
values between two fields in the switches 112, 113 and 114, is fed from a
switching signal generator 115 to these switches through lines 1512, 1513
and 1514, respectively. At the same time, the same signal is fed to the
code-converter 109 via a line 1509 to designate whether the used coding is
of system-A or of system-B.
10In the above description of the first preferred embodiment, frame-
to-frame coding is employed for the coding of the (_-_) fields. However,
inter-frame coding can be used also.
Now description will be made of a second preferred embodiment of
the present invention corresponding to the case where m=4 and _=2. With
re&ard to the coding in which frame-to-frame coding and coding making use
of interpolated values between adjacent two frames are executed alternately -
in each of the successive frames, the above description is also applicable
by merely substituting the term "frame" or "frame memory" for the term
"field" or "field memory" in the description of the first preferred embodi-
ment and in Figure 3~ and by substituting the term "between-every-second-
frame" for the term "frame-to-frame" in Figure 4.
Next, description will be made of a third preferred embodiment of
the present invention corresponding to the case where the input signal in the
first preferred embodiment is an NTSC color signal. In this case, since
the phase of the color signal subcarrier is reversed with respect to two
neighbouring lines and between successive frames as shown in Figure 2, the
j-th line in Fi, and the (j+l)-th line in Fi+2 are employed in the coding of
system B upon calculating interpolated values for the j-th line in Fi+l.
In Pigure 2, the markings o and represent the lines on which the phase of
the color signal sub-carrier is 0 and ~0~180 ), respectively. More particu-
larly, it is only necessary to use the j-th line in Fi and the ~j+l)-th
line in Fi+2, and so, in this case, the addition of one circuit element as
shown in Figure 6 is effected to the circuit shown in Figure 3. More par-
ticularly, at the left of the point X in Figure 3 and Figure 6 there is
additionally provided a one-line delay circuit 200. With regard to the
coding of system-A according to Figure 2, since the (j-l)-th line in Fi
which has been delayed by the delay circuit 200 at the point X corresponds
to the j-th line in Fi+2, the phase of the color signal sub-carrier on these
lines coincide with each other, whereby the frame-to-frame coding can be
executed. The same applies to the subsequent lines.
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