Digital processing color camera using digital delay lines and an analog encoder

A digital processing color camera includes a matrix circuit for arithmetically processing digital R, G and B video signals, which have been sampled by a clock signal, to provide a digital brightness signal, a first digital color difference signal and a second digital color difference signal. Those signals are subsequently converted by associated digital-to-analog converters into an analog brightness signal, a first analog color difference signal and a second analog color difference signal. Both the brightness signal and the first color difference signal, are delayed a predetermined period of time by digital delay lines inserted between the matrix circuit and the digital-to-analog converter circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to a television camera and, more 
particularly, to a digital processing color TV camera having digitized 
signal processing units. 
2. Description of the Prior Art 
A solid-state camera utilizing a solid-state imaging element which is 
compact and light-weight, and has a low power consumption and is highly 
reliable is today largely used as a color television camera not only in 
the television broadcasting industry, but also in homes. Particularly in 
the field of the television broadcasting industry, demands for a digital 
color camera are increasing because improvements in reliability, handling 
capacity and quality of images can readily be accomplished and also 
because it can readily be connected with other equipment which have been 
digitized. 
One example of such prior art digital color cameras comprises a processing 
circuit for effecting black-balance, white-balance and gamma corrections, 
a Y color difference matrix circuit for providing a brightness signal and 
a color difference signal, low pass filters for limiting bands, delay 
lines for time adjustment and an encoder circuit for effecting a 
quadrature two-phase amplitude modulation of two color difference signals 
and combining them with the brightness signal. 
Assuming that three primary colors of red, green and blue contained in a 
video signal are expressed by R, G and B, respectively, the brightness 
signal Y can be expressed by the following equation. 
EQU Y=0.30R+0.59G+0.11B 
The two color difference signals, denoted I and Q signals, respectively, 
can be expressed by the following respective equations. 
EQU I=0.60R-0.28G-0.32B 
EQU Q=0.21R-0.52G+0.31B 
An NTSC output signal which is a standard television system is generally 
processed in the following manner by a color encoder to combine R, G and B 
sync signals so as to thereby provide a single signal (NTSC) signal. 
The two color difference signals, that is, the I and Q signals, emerging 
from the Y color difference circuit have their respective bands restricted 
according to the NTSC standards to 1.5 MHz and 0.5 MHz, respectively. 
Specifically, the I signal must have such frequency characteristics that 
(1) the amount of the I signal attenuated at 3 MHz is smaller than 2 dB 
and (2) the amount of the I signal attenuated at 6 MHz is greater than 20 
dB. On the other hand, the Q signal must have such frequency 
characteristics that (1) the amount of the Q signal attenuated at 0.4 MHz 
is smaller than 2 dB, (2) the amount of the Q signal attenuated at 0.5 MHz 
is smaller than 6 dB and (3) the amount of the Q signal attenuated at 0.6 
MHz is greater than 6 dB. 
Thus, as compared with the I signal, the Q signal is subjected to a steep 
band restriction. Therefore, if the band restriction is carried out by the 
use of a low-pass filter of a simple construction having few stages, the 
phase characteristic tends to be deteriorated and ringing such as 
overshoot and undershoot tends to occur. Conversely, if a low-pass filter 
of a complicated construction having an increased number of stages is 
employed, a favorable phase characteristic can be attained, but a problem 
tends to occur in that a delay time increases. 
In view of the foregoing, the use has been made of a low-pass filter in 
which an induction m-type is combined with a window trap for providing a 
phase characteristic and a steep cut-off characteristic. However, the use 
of such low-pass filter tends to increase a delay time undesirably. 
Thus, since the respective bands of the I and Q signals are restricted to 
1.5 and 0.5 MHz, respectively, by means of the low-pass filter, the Q and 
I signals tend to be delayed about 2.35 .mu.sec. and about 0.18 .mu.sec., 
respectively, as compared with the brightness signal Y. 
In order to compensate for those delay times, delay lines are employed to 
compensate for a difference between the delay time of the Q signal and 
that of any one of the brightness signal and the I signal, so as to 
thereby equalize the delay times of the brightness signal Y, the I signal 
and the Q signal. 
The delay time added to the brightness signal Y and the delay time added to 
the I signal are 1.4 .mu.sec. and about 1.2 .mu.sec., respectively. 
It is, however, to be noted that the brightness signal Y as well has its 
band restricted by a low-pass filter to 6 MHz as a transmission bandwidth. 
Therefore, if a delay time attributable to this low-pass filter and any 
other processing such as an aperture correction is added, the delay time 
thereof can be subtracted, but the brightness signal Y is generally given 
a delay time of about 1 .mu.sec. (See, "NHK Terebi Gijutsu Kyokasho (Jou)" 
(NHK TV Technical Text), pages 17 to 19, published by Nippon Hoso Kyoukai, 
Apr. 10, 1989.) 
The I and Q signals to which the respective delay times have been added in 
the manner described above are supplied to a quadrature two-phase 
amplitude modulator in which they are combined together with no cross-talk 
to provide a single color signal. This color signal is subsequently added 
to the brightness signal having the delay time adjusted by means of the 
delay line, so as to thereby provide an NTSC output signal. 
Since the circuitry so constructed as described hereinabove is an analog 
circuit, the delay line to be inserted in each of the brightness signal 
and the I signal is employed in the form of a delay circuit having a 
concentrated constant such as, for example, a resistor, a capacitor or a 
coil, for example, a delay circuit of induction m-type capable of 
exhibiting a linear phase characteristic, or a delay cable. 
Accordingly, the phase characteristic and the amplitude characteristic tend 
to be adversely affected as a result of a change in temperature and/or a 
change with time and, therefore, the image quality tends to be adversely 
affected considerably. Where the delay cable is employed, a reflection of 
a high frequency occurs, thereby adversely affecting the image quality. 
In order to suppress the reduction in characteristic resulting from the 
change with time, it is a recent trend to digitize the camera. Although 
the camera wherein both of a signal processing circuit and an encoder are 
digitized is today available (See, "Signal Processing LSI for Totally 
Digitalized Color Camera", Technical Report of the Society of Television, 
1984 Vol. 8, No. 3, pages 27 to 32.), a digital camera wherein only the 
signal processing circuit is digitized to reduce power consumption while 
an analog encoder is employed is rather widely used and, therefore, the 
use of the analog delay line cannot be avoided, failing to make best use 
of the digital features. 
SUMMARY OF THE INVENTION 
The present invention is intended to provide a digital processing color 
camera wherein, even if an analog encoder is employed for an encoding 
circuit, a high quality television signal can be obtained without both the 
phase characteristic and the amplitude characteristic being adversely 
affected. 
In order to accomplish the foregoing object, the digital processing color 
camera according to the present invention comprises a matrix circuit for 
arithmetically processing digital R, G and B video signals, which have 
been sampled by a clock signal, to provide a digital brightness signal, a 
first digital color difference signal and a second digital color 
difference signal; a a brightness signal delay circuit for delaying the 
digital brightness signal outputted from the matrix circuit; a color 
difference signal delay circuit for delaying the first digital color 
difference signal outputted from the matrix circuit; a digital-to-analog 
converter for converting the digital brightness signal, the first digital 
color difference signal and the second digital color difference signal 
into an analog brightness signal, a first analog color difference signal 
and a second analog color difference signal, respectively; a low-pass 
filter for imposing a predetermined band restriction on the analog 
brightness, first and second analog color difference signals so as to 
thereby provide a band-restricted brightness signal, a first 
band-restricted color difference signal and a second band-restricted color 
difference signal, respectively; a quadrature two-phase amplitude 
modulator for modulating the first and second band-restricted color 
difference signals to provide a color signal; and a synthesizer circuit 
for combining the band-restricted brightness signal and the color signal 
to provide a composite video signal. 
Preferably, each of the brightness delay circuit and the color difference 
delay circuit employed in the digital processing color camera of the 
present invention is employed in the form of an adjustable delay line 
capable of selecting one of a plurality of delay times each being equal to 
N/2 times the cycle of a clock signal, wherein N represents a positive 
integer. 
According to the present invention, the matrix circuit processes the 
digital R, G and B image video signals descriptive of red, green and blue 
images, respectively, to provide the brightness signal and the first and 
second color difference signals. Where the respective bands of the first 
and second color difference signals are equally restricted such as 
practiced in the scheme, brightness signal is delayed a predetermined 
time by the digital delay line. However, where they are restricted 
differently such as practiced in the NTSC scheme, both of the brightness 
signal and the first color difference signal are delayed a predetermined 
time by the digital delay line. By the provision of these digital delay 
lines, the delay incurred by the second color difference signal in the 
encoder can be substantially matched with the delay time by which both of 
the brightness signal and the first color difference signals are delayed. 
After the foregoing digital processing, the brightness signal and the 
first and second color difference signals are converted by the 
digital-to-analog converter into the respective analog signals. Each of 
the analog brightness signal and the first and second analog color 
difference signals are then restricted in band by respective low-pass 
filters, and the band-restricted first and second color signals are 
subsequently modulated by the quadrature two-phase amplitude modulator to 
provide a color signal. This color signal is then combined by the 
synthesizer circuit with the band-restricted brightness signal thereby to 
provide a composite video signal. 
Also, the delay circuit is capable of selecting one of a plurality of delay 
times each being equal to N/2 times the cycle of a clock signal, wherein N 
represents a positive integer, so that it can cope accurately with a 
difference in delay time resulting from a difference in band-restricting 
characteristic of the color difference signals.

DETAILED DESCRIPTION OF THE EMBODIMENT 
With reference to FIG. 1, a digital processing color camera embodying the 
present invention comprises a processing circuit 1 for effecting 
black-balance, white-balance and gamma corrections to R, G and B color 
video signals, respective waveforms thereof being shown in FIGS. 2(a), 
2(b) and 2(c) a Y color difference matrix circuit 2 for synthesizing a 
brightness signal Y and color difference signals C1 and C2, and 
analog-to-digital converters 10 inserted between the processing circuit 1 
and the matrix circuit 2 for converting the R, G and B color video signals 
into respective digital signals. Reference numeral 3 represents a low-pass 
filter for the brightness signal and reference numerals 4 and 5 represent 
respective low-pass filters for the color difference signals. The low-pass 
filter 3 is connected directly to a synthesizer circuit 9 operable to 
combining the brightness signal and the color difference signal together 
and the low-pass filters 4 and 5 are connected to the synthesize circuit 9 
through a quadrature two-phase amplitude modulator 8 for converting the 
two color difference signals into a single signal. 
Reference numeral 13 represents a brightness signal delay circuit for 
delaying the brightness signal Y for a predetermined delay time, reference 
numeral 14 represents a color difference signal delay circuit for delaying 
the first color difference signal C1 for a predetermined time, and 
reference numerals 15, 16 and 17 represent digital-to-analog converters 
for converting the digital signals into respective analog signals. 
The low-pass filters 3, 4 and 5 and the quadrature two-phase amplitude 
modulator 8 altogether constitute an analog encoder comprised of the 
analog circuits. 
Thus, the illustrated embodiment of the present invention represents a 
digital processing color camera wherein the encoder is constituted by 
analog circuits. 
The operation of the digital processing color camera embodying the present 
invention will now be described with particular reference to FIGS. 2 to 4. 
FIGS. 2(a) to (j) shown waveforms of signals appearing at respective points 
(a) to (j) in the circuit shown in FIG. 1. 
FIGS. 3(a) and 3(b) show respective examples of an internal structure of 
the brightness signal delay circuit 3 while FIGS. 4(a) and 4(b) show 
respective examples of an internal structure of the color difference 
signal delay circuit 14, wherein reference numeral 18 represents a delay 
flip-flop and reference numeral 19 represents a data selector. 
After the R, G and B color video signals have been subjected by the 
processing circuit 1 to the black level adjustment, the white level 
adjustment and the gamma correction, the R, G and B color video signals 
are converted by the associated analog-to-digital converters 10, 11 and 12 
into the respective digital video signals, each having a predetermined 
number, for example, eight, bits, in synchronism with a predetermined 
sampling clock. 
The digital R, G and B color video signals emerging from the associated 
converters 10 to 12 are supplied to the Y color difference matrix circuit 
2 from which the brightness signal Y and the first and second color 
difference signals C1 and C2 emerge. 
Assuming that the first and second color difference signals C1 and C2 are 
represented by the I and Q signals, respectively, each of the brightness 
signal Y and the I signal, that is, the first color difference signal, 
must be delayed a predetermined period of time which is enough to 
compensate for a delay time of the Q signal (the second color difference 
signal C2) caused by the low-pass filter 5. In the illustrated embodiment, 
the digital delay circuits 13 and 14 are employed to accomplish a delay of 
the brightness signal Y and the first color difference signal C1. This 
will now be described in detail. 
Assuming that the R, G and B color video signals applied to the processing 
circuit 1 have the respective waveforms shown in FIGS. 2(a), 2(b) and 2(c) 
and after they have been processed by the processing circuit 1 and then by 
the Y color difference matrix circuit 2, the digital-to-analog converters 
15 and 16 output the brightness signal delayed a predetermined time TYd by 
the digital delay circuit 13 (the waveform of which is shown in FIG. 2(d)) 
and the I signal delayed a predetermined time TId by the digital delay 
line 14 (the waveform of which is shown in FIG. 2(e), respectively. On the 
other hand, the digital-to-analog converter 17 outputs the Q signal which 
is not delayed, the waveform of which is shown in FIG. 2(f). 
Thereafter, the brightness signal is supplied to the low-pass filter 3 by 
which the band thereof is restricted to represent such a waveform as shown 
in FIG. 2(g) and, similarly, the I and Q signals are supplied to the 
associated low-pass filters 4 and 5 by which the corresponding bands 
thereof are restricted to represent such waveforms as shown in FIGS. 2(h) 
and 2(i), respectively. At this time, the brightness signal and the I and 
Q signals are brought in an equally timed relationship with each other. 
In other words, the delay times TYd and TId, both shown in FIG. 2, are so 
chosen and so fixed that, by adding to the brightness signal a sum of the 
delay time TYd and the delay time of the low-pass filter associated with 
the brightness signal and also by adding to the I signal a sum of the 
delay time TID and the delay time of the low-pass filter associated with 
the I signal, both of the brightness signal and the I signal are 
synchronized with the delay time of the Q signal. The selection of the 
respective delay times required to accomplish the foregoing 
synchronization is accomplished by the use of the digital brightness 
signal delay circuit 13 and the color difference delay circuit 14. If a 
delay time of the quadrature two-phase amplitude modulator 8 is neglected, 
the synthesizing circuit 9 modulates the brightness signal and the I and Q 
signals, which have been matched in timing with each other, thereby to 
synthesize a color signal having a waveform shown in FIG. 2(j), which is 
utilized as a composite video signal (NTSC signal). It is to be noted 
that, in the waveform shown in FIG. 2(j) a hatched area represents a 
modulated color signal. 
The details of the digital delay circuits 13 and 14 will now be described 
with particular reference to FIGS. 3(a)-3(b) and 4(a)-4(b) respectively. 
In the example shown in FIGS. 3(a) and 3(b), the delay circuit 13 
associated with the brightness signal Y comprises a delay circuit having N 
delay flip-flops and the delay circuit 14 associated with the first color 
difference signal C1 (the I signal) comprises a delay circuit having M 
delay flip-flops. 
Thus, when the clock to be applied to the delay flip-flops is the same as 
the clock applied to the analog-to-digital converters, the brightness 
signal Y can be delayed a predetermined period of time which is N times 
the cycle of sampling clocks SP applied to the analog-to-digital 
converters 10, 11 and 12, and the first color difference signal C1 (the I 
signal) can be delayed a predetermined period of time which is M times the 
cycle of sampling clocks SP applied to the analog-to-digital converters 
10, 11 and 12. It is to be noted that N and M are each positive integers. 
Thus, in the illustrated embodiment, TYd=N.times.t and TId=M.times.t. 
In the practice of the present invention, by properly selecting a 
particular value for each of the parameters N and M, the predetermined 
delay time can be obtained and, due to the digital signal, no 
deterioration in phase and amplitude characteristics which would otherwise 
occur as a result of a change in temperature and/or a change with time 
occurs. 
FIGS. 4(a) and 4(b) illustrate alternative delay circuits which can be 
employed for the delay circuits 13 and 14, respectively. The delay circuit 
shown in each of FIGS. 4(a) and 4(b) is so designed that one of a 
plurality of, for example, three, delay times can be selected by a control 
signal outputted from, for example, a delay time selector switch, which 
control signal corresponds to a band-restriction of the color signal 
within the camera. With this arrangement, the delay time can have a 
freedom of choice. More specifically, assuming that three characteristics 
are chosen for the low-pass filter used to restrict the band of the second 
color difference signal C2 (Q signal) and due to a difference in 
characteristic thereof, an A input of the data selector 19 is selected 
since the delay time is prolonged in the case of the low-pass filter 
having a narrow band, or a B input or a C input is selected since the 
delay time decrease with an increase of the band. Nevertheless, in order 
for the respective delay times of each of the brightness signal and the 
first color difference signal C1 (I signal) to match with that of the 
selected delay time of the filter for the second color difference signal 
(Q signal), the number of stages of delay flip-flops for the delay time of 
a signal inputted to each of the A, B and C inputs are determined in 
consideration of a system of both of the brightness signal and the first 
color difference signal C1 (I signal). 
Thus, according to the present invention, the brightness signal Y and the 
first color difference signal C1 (the I signal), which have been 
accurately delayed by the associated digital delay circuits 13 and 14, and 
the second color difference signal C2 (the Q signal) which has not delayed 
are converted by the corresponding digital-to-analog converters 15, 16 and 
17 into the respective analog signals which are in turn supplied to the 
associated low-pass filters 3, 4 and 5. During the passage of these 
signals Y, C1 and C2 through the associated low-pass filters 3, 4 and 5, 
the respective bands thereof are restricted (It is to be noted that a 
difference in delay time resulting from the band restriction has been 
compensated for by the digital delay circuits 13 and 14.), and the two 
color difference signals C1 and C2 (the I and Q signals) are combined by 
the quadrature two-phase amplitude modulator 8 to provide a single color 
signal. Thereafter, this single color signal emerging from the quadrature 
two-phase amplitude modulator 8 is combined by the synthesizing circuit 9 
with the brightness signal to provide the NTSC output signal. 
Although the present invention has been described in connection with the 
preferred embodiment thereof with reference to the accompanying drawings, 
it is to be noted that various changes and modifications are apparent to 
those skilled in the art. By way of example, although the 
analog-to-digital converter 10 to 12 have been described as being inserted 
between the processing circuit 1 and the Y color difference matrix circuit 
2, they may be disposed in a front stage of the processing circuit 1 
remote from the matrix circuit 2. 
Also, each of the digital delay circuits 13 and 14 may be employed in the 
form of a memory. For example, the circuit such as shown in FIG. 5 is 
constructed. Assuming that the clock applied to the delay circuit is a 
sampling clock of 2 times (2SP), the selection of a select signal S (which 
is supplied from a delay time selector switch in the camera) makes it 
possible to select one of arbitrary delay times to, 5/2, 2t/2, 3t/2, . . . 
, (n-1)/2 and nt/2at intervals of t/2, wherein t represents the cycle of 
the sampling clock SP. Similarly, although reference has been made to the 
use of the color difference signals for the I and Q signals, the system of 
the present invention can work satisfactorily with R-Y and B-Y signals. 
Where the delay circuits 13 and 14 are of the construction shown in FIGS. 
4(a) and 4(b), each of the data selectors 19 may be in the form of a 
switching circuit having a plurality of switching positions equal to the 
number of delay times that can be selected. 
Moreover, if desired for connection with a digital encoder or any other 
digital equipment, arrangement may be made so that a component digital 
brightness signal YD and digital color difference signals C1D and C2D can 
be obtained from the output of the Y color difference matrix circuit 2. 
Accordingly, such changes and modifications are to be understood as being 
included within the scope of the present invention as defined by the 
appended claims, unless they depart therefrom.