Color signal demodulation circuit and method therefor

A color signal demodulation circuit demodulates a color signal in correspondence with different chrominance subcarrier frequencies. The input color signal is delayed by .pi./2 using a so-called Hilbert transformer and the carrier signal for use in the demodulation is generated from a carrier generator. The input color signal and the carrier signal are selectively multiplied and added so as to demodulate a color difference signal of the desired baseband, to thereby correspond to the chrominance subcarriers having different frequencies while simplifying the hardware. A method for demodulating the chrominance signal, which method is particularly adapted to the demodulation circuit is also detailed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a color signal demodulation circuit and, 
more particularly, to a color signal demodulation circuit which can 
demodulate a modulated color signal corresponding to the different 
chrominance subcarrier frequencies. A corresponding demodulation method is 
also disclosed. 
Korean Patent Application No. 93-2762 is incorporated herein by reference 
for all purposes. 
2. Discussion of Related Art 
In the conventional color signal demodulation circuit, an input color 
signal (.omega..sub.sc +.omega..sub.m) are multiplied with the carrier 
signal for use in demodulation having the same frequency (.omega..sub.sc) 
with the subcarrier of an input color signal. Then, the unnecessary second 
harmonic signal is removed by a low-pass filter, to thereby demodulate the 
color difference signal R-Y and B-Y or baseband frequencies. This process 
can be explained in more detail with reference to FIG. 1. 
FIG. 1 is a block diagram of the conventional color signal demodulation 
circuit, wherein only the color burst signal is output via a burst gate 10 
among all of the input color signals after being separated into luminance 
and chrominance signals and sampled. 
The color burst signal output from burst gate 10 is applied to a phase 
difference detector 20, where its phase is compared with that of a carrier 
signal generated and fed back from a downstream carrier generator 30. The 
phase difference resulting from the comparison is output as a voltage 
signal to carrier generator 30, which oscillates at 3.58 MHz, i.e., the 
color subcarrier frequency, in the case of an exemplary NTSC system. As a 
result, first and second carrier signals (sin.omega..sub.sc t and 
cos.omega..sub.sc t) for use in the demodulation are output. 
In a color difference signal demodulator 40, the first and second carrier 
signals which are for use in demodulation which are output from carrier 
generator 30, are input. The fw1 color signal input via a modulated color 
signal input terminal is demodulated into color difference signals. In a 
first multiplier 41, the first carrier signal for use in the demodulation 
output from carrier generator 30 and the input modulated color signal (C) 
are multiplied, and the output signal of first multiplier 41 is output as 
the R-Y signal after filtering in first low-pass filter 43. 
In a second multiplier 42, the input modulated color signal (C) and the 
second carrier signal for use in the demodulation are multiplied, and the 
output signal of second multiplier 42 is output as the B-Y signal by a 
serially connected second low-pass filter 44. 
When various kinds of input color signals, whose carrier frequency and 
bandwidth are different, are to be processed, low-pass filters, which 
respectively correspond to the subcarriers of the color signals to be 
demodulated, are needed for use as a rear stage of the demodulation 
circuit. 
For example, the frequency characteristic of an input signal X is shown in 
FIG. 2A, assuming that the signal X has a center frequency .omega..sub.sc 
of 1 MHz and a bandwidth .omega..sub.m of 2 MHz. Furthermore, the 
frequency characteristic of an input signal Y is shown in FIG. 2B, 
assuming that the signal Y has a center frequency .omega..sub.sc of 2 MHz 
and a bandwidth .omega..sub.m of 4 MHz. 
When a 1 MHz carrier signal for use in demodulation is multiplied with the 
X-signal, the frequency characteristic shown in FIG. 2C is generated. 
Meanwhile, a 2 MHz carrier signal for use in the demodulation is 
multiplied with the Y-signal, and the frequency characteristic shown in 
FIG. 2D is generated. 
Accordingly, a low-pass filter whose cut-off frequency is 1 MHz is used to 
produce the final X-signal, and a low-pass filter whose cut-off frequency 
is 2 MHz is used to produce the final Y-signal, thereby demodulating an 
original signal. 
It will be noted that, when the frequencies of carriers of input color 
signals are different, other low-pass filters having different cut-off 
frequencies, respectively, are absolutely necessary. Therefore, component 
sharing between circuits is impossible. The number of circuits has to be 
enlarged in order to support a varied manufacturing facility. Furthermore, 
interchangeability with other systems is poor. 
An FM demodulation device is disclosed in EP 399758, where FM demodulation 
device delays the phase of an FM-modulated signal using a so-called 
Hilbert transformer, to thereby remove the undesirable high frequency 
component which is higher than the carrier signal by a predetermined 
multiple using a low-pass filter, while improving the S/N ratio. Since the 
low-pass filter, which removes the high frequency component higher than 
the carrier signal by two times, is used in demodulating the modulated 
signal in the above-mentioned device, this device also dictates the use of 
low-pass filters having varying cut-off frequencies depending on the 
frequency of the carrier signal. 
SUMMARY OF THE INVENTION 
Accordingly, an objective of the present invention is to provide a color 
signal demodulation circuit which demodulates a color signal without 
adjusting the hardware, while adapting to input color signals having 
different chrominance subcarriers. 
Another objective of the present invention is to provide a color signal 
demodulation method which can be achieved by the above-mentioned 
demodulation circuit. 
These and other objects and features of the present invention are provided 
by color signal demodulation circuit for demodulating an input modulated 
color signal using a carrier signal, including: 
a carrier generator for generating a first carrier signal having sine 
component and a second carrier signal having cosine component which are 
synchronized with a burst signal contained in the color signal; 
a phase delay for phase delaying the input color signal; 
a first demodulator for demodulating a B-Y color difference signal of the 
baseband using the result of multiplying the input color signal with the 
first carrier signal and the result of multiplying a delayed color signal 
output from the phase delay means with the second carrier signal; and 
a second demodulator for demodulating a R-Y color difference signal of the 
baseband using the results of multiplying the input color signal with the 
second carrier signal and the result of multiplying the delayed color 
signal with the first carrier signal. 
These and other objects, features and advantages of the present invention 
are provided by a color signal demodulation method for demodulating the 
input modulated color signal using a carrier signal, the method including 
steps for: 
(a) generating a first carrier signal having a sine component and a second 
carrier having a cosine component, each of which are synchronized with a 
burst signal included in an input color signal and a cosine wave-shaped 
second carrier signal; 
(b) phase delaying of the input color signal; 
(c) demodulating a B-Y color difference signal of the baseband using the 
result of multiplying the input color signal with the first carrier signal 
and the result of multiplying a delayed color signal output from the step 
(b) with the second carrier signal; and 
(d) demodulating an R-Y color difference signal of the baseband using the 
result of multiplying the input color signal with the second carrier 
signal and the result of multiplying the delayed color signal with the 
first carrier signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 is a block diagram showing a preferred embodiment of a color signal 
demodulation circuit according to the present invention. 
A burst gate 110 and a time delay compensator 150 are commonly connected to 
a modulation color signal input terminal 100. An input terminal of a phase 
value detector 120 is connected to an output terminal of burst gate 110, 
while an output terminal of the phase value detector 120 is connected to a 
first input terminal of a phase difference detector 130. 
An input terminal of a carrier generator 140 is, in turn, connected to an 
output terminal of phase difference detector 130, and a first output 
terminal of carrier generator 140 is connected to a second input terminal 
of phase difference detector 130, and second and third output terminals of 
carrier generator 140 are respectively connected to second and third input 
terminals of a color difference signal demodulator 160. 
A first input terminal of color difference signal demodulator 160 is 
connected to an output terminal of time delay compensator 150, while first 
and second output terminals of color signal demodulator 160 are 
respectively connected to the external terminals which respectively output 
R-Y and B-Y color difference signals. 
FIG. 4 is a detailed circuit diagram showing the color difference signal 
demodulator shown in FIG. 3. 
Color difference signal demodulator 160 includes a so-called Hilbert 
transformer 161 which is connected to the output terminal of time delay 
compensator 150, a first multiplier 162 whose first input terminal is 
connected to the output terminal of time delay compensator 150 and whose 
second input terminal is connected to the second output terminal of 
carrier generator 140, a second multiplier 163 whose first input terminal 
is connected to the output terminal of time delay compensator 150 and 
whose second input terminal is connected to the third output terminal of 
carrier generator 140, a third multiplier 164 whose first input terminal 
is connected to the output terminal of Hilbert transformer 161 and whose 
second input terminal is connected to the third output terminal of carrier 
generator 140, a fourth multiplier 165 whose first input terminal is 
connected to the output terminal of Hilbert transformer 161 and whose 
second input terminal is connected to the second output terminal of 
carrier generator 140. Preferably, a first adder 166, whose first input 
terminal is connected to the output terminal of first multiplier 162 and 
whose second input terminal is connected to the output terminal of third 
multiplier 164 is also included. An inverter 167 is advantageously 
connected to the output terminal of fourth multiplier 165. A second adder 
168 whose first input terminal is connected to the output terminal of 
second multiplier 163 and whose second input terminal is connected to the 
output terminal of inverter 167 is also provided. 
The operation of the color signal demodulation circuit shown in FIG. 3 will 
be explained below, with reference to FIGS. 3 and 4. 
Referring to FIG. 3, the modulated color signal C is input to time delay 
compensator 150 and to burst gate 110 simultaneously. The color signal 
input at this time is, for example, a balance-modulated signal. 
Time delay compensator 150 which advantageously may consist of a latch or 
delay, delays the input modulated color signal and outputs the delayed 
signal to color difference signal demodulator 160 so as to compensate for 
the signal processing delay time seen from the input terminal burst gate 
110 to the output terminals of carrier generator 140. 
Burst gate 110 outputs only the color burst signal loaded on the modulated 
color signal. Phase value detector 120 advantageously detects the standard 
point value of the color burst signal output from burst gate 110 while 
phase difference detector 130 compares the phase value of the color burst 
detected by phase value detector 120 with that of the carrier signal fed 
back from carrier generator 140, so that the phase difference 
corresponding to the difference between the phases is detected and 
provided to carrier generator 140. 
In carrier generator 140, the phase difference detected in phase difference 
detector 130 is added to the carrier signal currently being generated, and 
then, the carrier signals which are synchronized with input color signal 
C, is generated and output to color signal demodulator 160. 
In color signal demodulator 160, color signal C, i.e., an output signal of 
time delay compensator 150, is input and demodulated as described in 
detail below. 
The modulated color signal C input to color signal input terminal 100 can 
be expressed as follows. 
EQU C=Cb(sin .omega..sub.sc t)+Cr(cos .omega..sub.sc t) (1) 
wherein C is a color signal, Cb is the B-Y signal and Cr is the R-Y signal. 
Signal C', which results from delaying color signal C by .pi./2, can be 
expressed as follows. 
EQU C'=Cb(cos .omega..sub.sc t)-Cr(sin .omega..sub.sc t) (2) 
wherein C' is a color signal which is transformed by .pi./2 in a Hilbert 
transformer 161. Hilbert transformer 161 filters the entire band signal 
without attenuation, while only shifting the phase. 
First multiplier 162 multiplies color signal C by the carrier signal 
(sin.omega..sub.sc t) for use in the demodulation input from the second 
output terminal of carrier generator 140. This can be expressed as 
follows. 
##EQU1## 
Third multiplier 164 multiplies cos.omega..sub.SC t with color signal C' 
which is transformed by .pi./2 by Hilbert transformer 161. This can be 
expressed as follows. 
##EQU2## 
The output signals of first and third multipliers 162 and 164 are added in 
first adder 166, the operation of which can be expressed as follows. 
EQU 2C(sin 2.omega..sub.sc t)+2C'(cos 2 107 .sub.sc t)=2Cb (5) 
wherein Cb is the B-Y signal output by first adder 166. 
Second multiplier 163 multiplies color signal C by the carrier signal 
(cos.omega..sub.sc t) for use in the demodulation output from the third 
output terminal of carrier generator 140. This can be expressed as 
follows. 
##EQU3## 
The .pi./2-transformed color signal C', that is, an output signal of 
Hilbert transformer 161, is multiplied to the carrier signal 
(sin.omega..sub.sc t) for use in demodulation in fourth multiplier 165, 
which can be expressed as follows. 
##EQU4## 
The output signal of second multiplier 163 expressed in Expression (6) and 
the signal output via inverter 167 for inverting the output signal of 
fourth multiplier 165 expressed in Expression (7), are added in second 
adder 168. 
The result can be expressed as follows. 
EQU 2C(cos .omega..sub.sc t)+(-2C'(sin 2.omega..sub.sc t))=2Cr (8) 
Cr is the R-Y signal and, thus, the output of second adder 168 is R-Y. 
Accordingly, in the present invention, the low-pass filter used for the 
conventional color demodulation circuit is removed with regard to the 
color signals of the chrominance subcarriers having difference frequencies 
and a Hilbert transformer is used, to thereby demodulate corresponding to 
the color signals whose chrominance subcarriers differ in the broadcasting 
method only by changing the .omega..sub.sc value. 
In addition to this, the present invention is applicable in demodulating 
the analog or digital color signal of the video equipment, for example, a 
video cassette recorder or television. 
As described above, a color signal demodulation circuit of the present 
invention and method thereof demodulate the color signal in correspondence 
with the chrominance subcarriers having different frequencies, without 
necessitating the low-pass filter for removing the harmonics components of 
the modulation signal, thereby simplifying the hardware. 
Other modifications and variations to the invention will be apparent to 
those skilled in the art from the foregoing disclosure and teachings. 
Thus, while only certain embodiments of the invention have been 
specifically described herein, it will be apparent that numerous 
modifications may be made thereto without departing from the spirit and 
scope of the invention.