Band compression apparatus for a video signal

A band compression apparatus for transmitting a video signal having three color component signals sampled at a sampling frequency of 2Fs and a gradation of N bits per picture element by a transmission path of a gradation of N bits per sample and a sample transmission frequency 2Fs includes a prefilter for restricting the band of each color component signal; a sub-sample circuit for compressing the respective band restricted color component signal to a sampling frequency Fs; a signal separation switching circuit for separating data of 3n bits comprising the upper n bits of the sub-sampled color component signals EQU 3n=2N+k (k<N) into two main signals as transmission samples of N bits and a fraction signal of k bits; a fraction signal storage RAM for storing one line video signal portions of said fraction signal; and a signal outputting switching circuit for outputting said stored fraction signal to the transmission path by time compressing the same into a signal of N bits as a first transmission sample during the horizontal blanking period of the video signal, or outputting said sub-sampled signal of N bits as a second transmission sample during the video signal period.

FIELD OF THE INVENTION 
The present invention relates to a band compression apparatus for a video 
signal, and more particularly to an apparatus capable of reducing the 
transmission information quantity of the video signal. 
BACKGROUND OF THE INVENTION 
FIG. 4 shows a conventional transmission path for video signals without 
band compression. In FIG. 4 the reference numerals 1, 2, and 3 designate 
R, G, and B signal digital video input terminals, respectively. The 
reference numerals 13, 30, and 31 designate data transmission paths for 
transmitting the signals from the input terminals 1, 2, and 3, 
respectively. The reference numerals 22, 23, and 24 designate R, G, and B 
signal digital video output terminals, respectively, for outputting the 
signal from the data transmission paths 13, 30, and 31 to the outside. 
FIG. 5 shows a conventional band compression apparatus for video signals 
using the PASS (Phase Alternative Sub-Nyquist Sampling) system as a band 
compression method. In FIG. 5, the same reference numerals designate the 
same or corresponding elements as those shown in FIG. 4. The reference 
numeral 4 designates a sub-sampling prefilter for restricting the band of 
the signal from the R signal digital video input terminal 1 before the 
sub-sampling. The reference numeral 6 designates a sub-sampling prefilter 
for restricting the band of the signal from the B signal digital video 
input terminal 3 before the sub-sampling. The reference numeral 50 
designates a switch for switching the output between the sub-sampling 
prefilter 4 and the output of the sub-sampling prefilter 6 at each one 
picture element. The reference numeral 13 designates a data transmission 
path for transmitting the output of the switch 50 to the receiver's side. 
The reference numeral 51 designates a switch for switching the output from 
the data transmission path 13 at each one picture element. The reference 
numeral 19 designates a sub-sampling interpolating filter for generating 
an interpolating value from the output of the switch 51. The reference 
numeral 21 designates a sub-sampling interpolating filter for generating 
an interpolating value from the output of the switch 51. The reference 
numeral 30 designates a transmission path for transmitting the signal from 
the G signal digital video input terminal 2. The reference numeral 22 
designates an R signal digital video output terminal for outputting the 
output from the sub-sampling interpolating filter 19 to the outside. The 
reference numeral 23 designates a G signal digital video output terminal 
for outputting the output from the transmission path 30 to the outside. 
The reference numeral 24 designates a B signal digital video output 
terminal for outputting the output from the sub-sampling interpolating 
filter 21 to the outside. 
FIGS. 7 and 8 show examples of a sub-sampling prefilter and a sub-sample 
interpolating filter of FIG. 5, respectively. 
In FIG. 7, the reference numeral 71 designates a digital video input 
terminal, the numeral 72 designates a 1H delay line for delaying the 
output from the digital video input terminal 71 by one line. The numeral 
73 designates a 1H delay line for delaying the output of the 1H delay line 
72 by one line. The numeral 74 designates a one picture element delay line 
for delaying the output of the 1H delay line 73 by one picture element. 
The numeral 75 designates a one picture element delay line for delaying 
the signal from the digital video input terminal 71 by one picture 
element. The numeral 76 designates a one picture element delay line for 
delaying the output of the 1H delay line 72 by one picture element. The 
numeral 77 designates a one picture element delay line for delaying the 
output of the one picture element delay line 76 by one picture element. 
The numeral 78 designates an adder for adding the output of the 1H delay 
line 72, the output of the one picture element delay line 77, the output 
of the one picture element delay line 74, and the output of the one 
picture element delay line 75. The reference numeral 79 designates a 
divider for dividing the output of the one picture element delay line 76 
by 2 (or a multiplier for multiplying the output of the one picture 
element delay line 76 by 1/2). The numeral 80 designates a divider for 
dividing the output of the adder 78 by 8 (or a multiplier for multiplying 
the output of the adder 78 by 1/8). The numeral 81 designates an adder for 
adding the output of the divider 79 and that of the divider 80. The 
reference numeral 82 designates a digital video output terminal for 
outputting the output of the adder 81 to the outside. 
In FIG. 8, the reference numeral 83 designates a digital video input 
terminal, the numeral 84 designates a 1H delay line for delaying the 
output of the digital video input terminal 83 by one line, the numeral 85 
designates a 1H delay line for delaying the output of the 1H delay line 84 
by one line. The numeral 87 designates a one picture element delay line 
for delaying the signal from the digital video input terminal 83 by one 
picture element. The numeral 86 designates a one picture element delay 
line for delaying the output of the 1H delay line 85 by one picture 
element. The numeral 88 designates a one picture element delay line for 
delaying the output of the 1H delay line 84 by one picture element. The 
numeral 89 designates a one picture element delay line for delaying the 
output of the one picture element delay line 88 by one picture element. 
The numeral 90 designates an adder for adding the output of the 1H delay 
line 84, the output of the one picture element delay line 86, the output 
of the one picture element delay line 87, and the output of the one 
picture element delay line 89. The numeral 91 designates a divider for 
dividing the output of the adder 90 by 4 (or a multiplier for multiplying 
the output of the adder 90 by 1/4). The numeral 92 designates an adder for 
adding the output of the one picture element delay line 88 and the output 
of the divider 91. The numeral 93 designates a digital video output 
terminal for outputting the output of the adder 92 to the outside. 
The device operates as follows. 
First of all, the operation of the conventional system shown in FIG. 4 will 
be described. A color video signal is usually processed and transmitted by 
being decomposed into three original colors of light, that is, an R signal 
(red), a G signal (green), and a B signal (blue). In FIG. 4, the R signal 
video information input to the R signal digital video input terminal 1 is 
transmitted by the transmission path 13 to the receiver's side, and is 
output from the R signal digital video output terminal 22. Similarly as 
above, the G signal video information input to the G signal digital video 
input terminal 2 is transmitted by the transmission path 30 to the 
receiver's side, and is output from the G signal digital video output 
terminal 23. Similarly as above the B signal video information input to 
the B signal digital video input terminal 3 is transmitted by the 
transmission path 31 to the receiver's side, and is output from the B 
signal digital video output terminal 24. Now suppose that a video signal 
is sampled with a gradation of 8 bits per picture element and a sampling 
frequency of 2Fs=10 MHz, three 80 MBPS (Bit Per Second) transmission paths 
are required in this transmission system. 
A band compression is conducted so as to reduce the number of the required 
transmission paths. The conventional system utilizing the PASS method as a 
band compression method will be described with reference to FIGS. 5 to 8. 
First of all, the R signal video information input to the R signal digital 
video input terminal 1 is band restricted by the sub-sampling prefilter 4. 
On the other hand, the B signal video information input to the B signal 
digital video input terminal 3 is band restricted by the sub-sampling 
prefilter 6. The R signal and B signal video information, both band 
restricted, are switched by the switch 50 at each one picture element to 
be sub-sampled as shown in FIG. 6. Herein, the mark O designates the 
position on the screen of the resampling point of the R signal, and the 
mark X designates the position on the screen of the resampling point of 
the B signal. Accordingly, the transmission information quantity of the R 
signal and the B signal become 1/2, and the output of the switch 50 can be 
transmitted to the switch 51 at the receiver's side by only a transmission 
path 13. The signal transmitted to the switch 51 is switched at one 
picture element, and is decoded to the R signal and the G signal. The 
decoded R signal is input to the sub-sampling interpolating filter 19. In 
this filter 19, an interpolating value is generated at the non-sampling 
point shown by the mark X in FIG. 6 by that data 0 inserted at the 
non-sampling point. Thus, an R signal digital output is output from the R 
signal digital video output terminal 22 to the outside. 
Similarly as above the decoded B signal is input to the sub-sampling 
interpolating filter 21. In this filter 21, data 0 is inserted at the 
non-sampling point, an interpolating value is generated at the 
non-sampling point shown by the mark O in FIG. 6 by that data 0 inserted 
at the non-sampling point. Thus, a B signal digital output is output from 
the B signal digital video output terminal 24 to the outside. 
On the other hand, the G signal is input to the G signal digital video 
input terminal 2, and is transmitted to the receiver's side by the 
transmission path 30. The transmitted G signal is output as a G signal 
digital video output from the G signal digital video output terminal 23 to 
the outside. Two 80 MBPS transmission paths are required in the above 
described PASS system. 
The sub-sampling prefilters 4 and 6 shown in FIG. 5 will be described in a 
greater detail with reference to FIG. 7. When a zigzag-grid shaped 
sub-sampling shown in FIG. 6 is conducted, the characteristics in a two 
dimensional space spectrum as shown in FIG. 9 is obtained. In FIG. 9, the 
alias centers formed by the zigzag-grid shaped sub-sampling appear at 
positions designated by double circles, and therefore alias noises are 
usually generated at the regions where the alias signal originated from 
the alias center and the base-band signal overlap with each other on the 
spectrum plain. Therefore, it is possible to reproduce picture images 
accurately on a screen by band restricting the base-band signals into 
rhombus shaped regions without the alias signals being overlapped with 
base-band signals. The video signal input to the digital video input 
terminal 71 of the sub-sampling prefilter shown in FIG. 7 is transmitted 
in accordance with the transmission characteristics represented by the 
following formula (1) before it reaches the digital video output terminal 
82, whereby the base-band signals are band restricted into the hatched 
regions shown in FIG. 9 without overlapping with the alias signals 
originating from the alias centers. 
##EQU1## 
Herein Z.sup.-l : two line delay on the screen (1H delay of field video 
signal) 
Z.sup.-1 : one picture element delay on the screen 
Next, in the sub-sampling interpolating filter shown in FIG. 8, a video 
signal into which the data 0 is inserted at the dropped sampling point 
(non-sampling point) is input to the digital video input terminal 83. 
Accordingly, the dropped sampling point is interleaved by the sub-sampling 
interpolating filter which realizes the characteristics of the formula (1) 
before it reaches the digital video output terminal 93. 
In this conventional band compression system for a video signal using the 
PASS method, the compression ratio is 1/2 because the video signals are 
compressed at a frequency of 1/2 of the sampling frequency, and for 
example, number of the transmission paths is only reduced to two from 
three which corresponds to the number of R, G, and B signals obtained by 
decomposing the video information. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved band 
compression apparatus for a video signal capable of reducing number of the 
required transmission paths from 3 corresponding to the number of the 
decomposed R, G, and B signals to 1. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given hereinafter; it should be understood, 
however, that the detailed description and specific embodiment are given 
by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent to those 
skilled in the art from this detailed description. 
According to the present invention, there is provided a band compression 
apparatus for a video signal for transmitting a video signal comprising 
three color component signals sampled at a sampling frequency of 2Fs and a 
gradation of N bits per picture element by a transmission path of a 
gradation of N bits per sample and a sample transmission frequency 2Fs: a 
prefilter for restricting the band of each color component signal; a 
sub-sampling means for compressing the respective band restricted color 
component signal to a sampling frequency Fs; a signal separation means for 
separating the data of 3n bits comprising the upper n bits of the 
sub-sampled color component signals 
EQU 3n=2N+k (k&lt;N) 
into two main signals as transmission samples of N bits and a fraction 
signal of k bits; a fraction signal storage means for storing one line 
video signal portions of said fraction signal; and a signal outputting 
means for outputting said stored fraction signal to the transmission path 
by time compressing the same into a signal of N bits as a first 
transmission sample during the horizontal blanking period of the video 
signal, or outputting said sub-sampled signal of N bits as a second 
transmission sample during the video signal period.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In order to explain the present invention in detail, reference will be 
particularly made to FIG. 1. 
In FIG. 1, the reference numerals 1, 2, and 3 designate an R, G, and B 
signal digital video input terminals, respectively. The reference numerals 
4, 5, and 6 designate sub-sampling prefilters for band restricting the 
video information from the R, G, and B signal digital video input terminal 
1, 2, and 3, respectively. The numeral 7 designates a latch for 
compressing the output of the sub-sampling prefilter 4, 5, and 6 to one 
half of the sampling frequency of the input signal. The numeral 8 
designates a switch for switching a sequence comprising the 6 bit R signal 
and the upper 2 bits of the B signal and a sequence comprising the whole 6 
bits of the G signal and the middle 2 bits of the B signal from the latch 
7 at each picture element. The numeral 9 designates a 1H delay line for 
delaying the output of the switch 8 by one line. The numeral 10 designates 
a shift register for producing an 8 bit signal from four clock portions of 
the lower 2 bits of the B signal from the latch 7. The numeral 11 
designates a RAM for storing a 1H period portion of the signal from the 
shift register 10. The numeral 12 designates a switch for outputting the 
output of the RAM 11 in a horizontal blanking period (hereinafter referred 
to as "H blank period") and the output of the 1H delay line 9 in the video 
signal period. The numeral 13 designates a transmission path of a 
gradation of 8 bits per sample and a sample transmission frequency of 2Fs 
for transmitting the output of the switch 12. 
The reference numeral 14 designates a switch for outputting a sequence 
comprising the whole 6 bit of the R signal and the upper 2 bits of the B 
signal (BH signal) and a sequence comprising the whole 6 bits of the G 
signal and the middle 2 bits of the B signal (BM signal) from the signal 
output from the transmission path 13. The numeral 15 designates a one 
picture element delay line for delaying the output of the switch 14 by one 
picture element. The numeral 16 designates a latch for latching the output 
of the switch 14 and the output of the one picture element delay line 15. 
The numeral 17 designates a RAM into which the BL signal, which is 
produced by time compressing the lower 2 bits of the B signal, is written 
in during the H blank period, and from which the BL signal is read out 
during the video signal period. The numeral 18 designates a shift register 
for generating a BL signal (the lower 2 bit of the B signal) for each two 
picture elements from the output of the RAM 17. The numeral 19 designates 
a sub-sampling interpolating filter for obtaining an interpolating value 
from the output of the latch 16. The numeral 20 designates a sub-sampling 
interpolating filter for obtaining an interpolating value from the output 
of the latch 16. The reference numeral 21 designates a sub-sampling 
interpolating filter for obtaining an interpolating value using the upper 
2 bits and the middle 2 bits of the B signal obtained from the latch 16 
and the lower 2 bits of the B signal obtained from the shift register 18. 
The reference numeral 22 designates an R signal digital video output 
terminal for outputting the output of the sub-sampling interpolating 
filter 19 to the outside. The numeral 23 designates a G signal digital 
video output terminal for outputting the output of the sub-sampling 
interpolating filter 20 to the outside. The numeral 24 designates a B 
signal digital video output terminal for outputting the output of the 
sub-sampling interpolating filter 21 to the outside. 
The operation of the device will be described with reference to FIGS. 1 and 
2. 
The R signal, G signal, and B signal are analog-to-digital converted at the 
gradation of 6 bits per picture element and a sampling frequency of 2Fs, 
and each of them is input to the R, G, and B signal digital video input 
terminals 1, 2, and 3, respectively. These correspond to the signals a, b, 
and c in the timing chart of FIG. 2. In these signals in usual analog form 
the H blank period occupies about 1/5 of 1 line. In the digitized signals 
a large portion of the H blank period is empty because the digitized 
horizontal synchronous signal (H sync) has a short code as shown in FIG. 
2. 
The R signal input to the R signal digital video input terminal 1 is band 
restricted by the sub-sampling prefilter 4. Similarly as above, the G 
signal input to the G signal digital video input terminal 2 is band 
restricted by the sub-sampling prefilter 5, and the B signal input to the 
B signal digital video input terminal 3 is band restricted by the 
sub-sampling prefilter 6. Each band restricted signal is sub-sampled by 
the latch 7 with the use of the clock of frequency Fs having an inverted 
phase at each line, and an 8 bit signal d comprising the whole 6 bits of 
the sub-sampled R signal and the upper 2 bits of the B signal (BH signal), 
and an 8 bit signal e comprising the whole 6 bits of the sub-sampled G 
signal and the third and the fourth bits of the B signal (BM signal) are 
obtained. These signals d and e are switched by the switch 8 with the use 
of the clock of frequency 2Fs to be time multiplied, whereby a signal g as 
a first transmission sample is obtained. On the other hand, four picture 
element portions of the signal f which is the lower 2 bits of the 
sub-sampled B signal (BL signal) are stored at the shift register 10, and 
one line portions of an 8 bit signal are stored at the RAM 11 as time 
compressed signals. Next, this one line portion of the BL signal stored at 
the RAM 11 is read out as a second transmission sample at a portion after 
the H sync signal of the horizontal blanking period of the next scanning 
line, and is input to the switch 12. The signal g is delayed by 1 line by 
1H delay line 9, and is input to the switch 12. The switch 12 selects the 
output of the RAM 11 (the second transmission sample) during the 
horizontal blanking period, and selects the output of the 1H delay line 9 
(the first transmission sample) during the video signal period. 
Accordingly, the output of the switch 12 becomes a signal h as shown in 
FIG. 2 at the receiver's side. This signal h is sent out to the 
transmission path 13. 
Next, at the receiver's side the signal h from the transmission path 13 is 
separated into an 8 bit signal comprising the 6 bit R signal and the BH 
signal and an 8 bit signal comprising the 6 bit G signal and the BM signal 
by the switch 14, which signals are hereinafter designated by i and j, 
respectively. The signal i is delayed by one picture element by the one 
picture element delay line 15 to become the same phase as that of 
frequency the signal j, and is latched with the clock of Fs by the latch 
16, the 6 bit R signal and the BH signal are both output at the sampling 
frequency Fs, and the 6 bit G signal and the BM signal are both output at 
the sampling frequency Fs. This 6 bit R signal is hereinafter designated 
by l, and the 6 bit B signal is hereinafter designated by m. 
On the other hand, the BL signal from the transmission path 13 is written 
into the RAM 17 during the horizontal blanking period, read out from the 
RAM 17 during the video signal period, and this 8 bit signal is decomposed 
into four picture element portions by the shift register 18, and a time 
extension is conducted thereto. The B signal becomes a 6 bit signal having 
a sampling frequency Fs comprising the signals BH, BM, and BL. The signals 
l, m, and n are sub-sampled signals of sampling frequency Fs, and 
interpolating values therefore are generated by the sub-sampling 
interpolating filters 19, 20, and 21, and signals o, p, and q of sampling 
frequency 2Fs are output from the R, G, and B signal digital video output 
terminal 22, 23, and 24, respectively. 
The signal multiplexing method of the present embodiment will be described 
with reference to FIG. 3. In FIG. 3, the sub-sampled R, G, and B signals 
are only required to be transmitted as 6 bit signals one time for each 2 
picture elements, and therefore it is only required that 3.times.6=18 bits 
are transmitted for each 2 picture elements. However, it is only possible 
to transmit 16 bits for each 2 picture elements by the transmission path 
of the gradation of 8 bits per picture element and of a sample 
transmission frequency 2Fs. Therefore the remaining 2 bits are transmitted 
by being time compressed into a horizontal blanking period, and these 
remaining 2 bits are obtained from the horizontal blanking period by the 
time extension at the receiver's side. These decoded R, G, and B signals 
become a 6 bit signal effective as a picture element in the period of 2 
picture elements. 
In the above embodiment with such a construction a portion of the B signal 
is transmitted during the H blank period by being time compressed, and 
therefore it is possible to transmit a sufficient quality video 
information signal by only a single transmission path compared with that 
the conventional PASS method which requires two transmission paths. 
Although the R, G, and B signals are arranged in a special order for 
simplification in the illustrated embodiment, the input positions of the 
R, G, and B signal digital video input terminals can be arbitrarily 
changed. 
Although the BH, BM, and BL signals are generated by selecting particular 
bits of the B signal, but the BH signal may be an arbitrary 2 bit signal 
among the whole 6 bits of the B signal, the BM signal may be an arbitrary 
2 bits signal among the remaining 4 bit of the B signal, and the BL signal 
may be the remaining 2 bit signal. Furthermore, the bit number per 
transmission sample in the illustrated embodiment may be a value other 
than 8. 
In the illustrated embodiment, the present invention is applied to a 1H 
type PASS, but any type of sub-sampling method can be used by selecting a 
sub-sampling prefilter and a sub-sampling interpolating filter which have 
characteristics appropriate to the sub-sampling system and establishing 
the sub-sampling timing of the both filters and the latch. 
As is evident from the foregoing description, according to the present 
invention, a portion of the video information is sub-sampled, and is time 
compressed and time multiplied to be transmitted during the horizontal 
blanking period which is usually empty, whereby only a single transmission 
path is required without any deterioration in the picture quality.