1. Field of the Invention
This invention relates in general to a video signal processing circuit using a comb filter for obtaining and processing chrominance and luminance signals.
2. Description of the Prior Art
In the prior art it is well known to utilize a comb filter to separate the luminance and chrominance signals from the composite color video signals as, for example, in NTSC color video system utilizing vertical correlation and phase inversion of the chrominance signals. When the NTSC composite color video signals are to be processed, the comb filter must have a delay line of 1H where H is the horizontal period of 63.5 .mu.sec so as to accomplish the vertical correlation between two adjacent scanning lines. Thus, the signals processed by the comb filter are delayed by 1H. Thus, if the comb filter is incorporated in a video tape recorder and dubbing (copy) occurs repetitively, the delay time will accumulate so as to deteriorate the quality of the signals.
When the luminance signals are separated by using the comb filter, the chrominance signals in the portions without the vertical correlation remain in the luminance signal so that the high frequency components of the separated luminance signals must be limited with a low pass filter. Also, the luminance signals in the portion without the vertical correlation will be subjected to distortion and false signals tend to arise. Particularly, the edges of the input signals will be deteriorated. Thus, high frequency components of the luminance signals must be passed through a low pass filter to limit them which results in a degraded picture.
FIG. 1 illustrates a conventional comb filter of the prior art and FIGS. 2A and 2B are vector diagrams of a chrominance signal C and a luminance signal Y that has been processed through the circuit of FIG. 1 for each horizontal line. The symbol O in the vector diagrams represent the state in which no signals are present.
As shown in FIG. 1, a NTSC color video signals a is fed to an input terminal 1 and is passed through a one horizontal period delay line 2. The output b of the delay line 2 and the input signals a are added in an adder 3 so as to obtain vertical correlation. The level of the output of the adder 3 is cut in half by a one-half attenuator 4 and in the output of the attenuator 4 the luminance signals Y are separated. If a subtractor is used instead of the adder 3, the chrominance signals will be separated.
As shown in the vector diagram of FIG. 2A, the phase of the chrominance signal is inverted by the horizontal delay 2. Therefore, if the input signal a and the delay signal b are added together, the resulting output does not in principle contain the chrominance signal as illustrated by c in FIG. 2A. As represented by lines H3 and H6 in FIGS. 2A-C, however, false or spurious signals of the one-half level infiltrate as residual chrominance signals into the luminance signals Y. By adding the input a and the delayed signals b luminance signals Y which do not contain chrominance components can be obtained as shown by c in FIG. 2B provided vertical correlization of the signals occurs. At the leading and trailing edges of the signals, however, the vertical correlation is destroyed by the delay 1H which is indicated by lines H3 and H6 of c in FIG. 2B and the signal level is half and distortion results.
FIG. 3 shows a conventional circuit for processing video signals a utilizing a comb filter in which an 1H delay signal b and an input signal a in which the phase has been inverted by an inverter 5 or combined in the adder 3 and the level of the added output is cut in half by the one-half attenuator 4 so as to separate chrominance signals C. Also, the input signal a and the output c of the one-half attenuator 4 are combined in an adder 6 so as to separate the luminance signals Y. The, thus, separated chrominance signals C and the luminance signals Y are supplied to processing circuits 7 and 8 such as emphasis circuits, noise removing filters, noise cancelling circuits and the like and are again added in the adder 9 to form the color video signals C'+Y'.
FIG. 4 comprises a vector diagram of a chrominance signal processed by the circuit of FIG. 3 for each of the horizontal lines. As represented by c in FIG. 4, the level of the leading and trailing portions of the chrominance signal C separated by a+b in the adder 3 is halved and is distorted relative to the original signal as indicated by lines H3, and H13. Also, the luminance signal Y separated by c+a in the adder 6 contains a chrominance signal of one-half level as residual components as is indicated on lines H3 and H13 of d in FIG. 4. Therefore, the chrominance signal in the added results C'+Y' produced by the adder 9 will have been corrected for the level distortion at the signal edges as indicated by c+d in FIG. 4, but there will be a delay by 1H relative to the original signal A and it is also inverted in phase.
FIG. 5 illustrates a conventional circuit for processing video signals equipped with the comb filter illustrated in FIG. 3. In this circuit, a delay signal b is inverted with an inverter 10 and the input signal a are added in the adder 3 and the separated chrominance signal C and a signal a which is obtained by inverting the input signal a with an inverter 11 are added in an adder 6 to obtain luminance signals Y.
The processing circuit of FIG. 5 operates almost the same as that of FIG. 3. The process signals C'+Y' contain chrominance signals with the same phase as that of the input signals a as is indicated by c+d in FIG. 6 but the signal edges are delayed by 1H. Therefore, the color of the picture is shifted in the vertical direction by one line which deteriorates the quality of the picture.