JP2610416B2 - NTSC digital chroma demodulation circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital chroma demodulation circuit of a composite color signal for directly separating an NTSC composite color signal digitally.

(Prior art)

FIG. 1 is a block diagram showing a digital chroma demodulation circuit of an NTSC composite color signal generally considered in the prior art. In the figure, 1 is a signal line for supplying a composite color signal, 2 is an A / D converter, 3 is a comb filter for separating the digitized composite color signal into a luminance signal and a chrominance signal, 4 and 5 are buffers connected to the color signal output section of the comb filter 3, and 6 and 7 are chrominance signals. This is a signal line that outputs two orthogonal components. Reference numeral 8 denotes a clock pulse generating circuit which generates clock pulses C1, C2, and C3 as described below.

That is, the A / D converter 2 is supplied with a clock pulse C1 having a repetition frequency four times that of the chrominance subcarrier and phase-controlled so that the rise coincides with the zero phase of the color burst. A clock pulse C2 having a repetition frequency equal to the chrominance subcarrier and having a phase controlled so that the rise coincides with the zero phase of the color burst is supplied, and the buffer 5 has the same repetition frequency as the clock pulse C2, It is configured to supply a clock pulse C3 whose phase is delayed by 90 ° from the clock pulse C2. FIG. 2 shows a color burst and clock pulses C1, C2, C3.
FIG. 4 is a timing chart showing the relationship of FIG.

Next, the operation will be described. The composite color signal is converted from an analog signal to a digital signal by the A / D converter 2 at the moment when the clock pulse C1 rises. Next, the color signal components separated by the comb filter 3 are supplied to the buffers 4 and 5. Clock pulse
As described above, since the phase of C1 is controlled so that the pulse rises at the zero phase of the color burst, the color signal output from the comb filter 3 includes the clock pulses C2 and C3.
Are the two orthogonal components of the color signal at the moment when the signal rises. Since the buffers 4 and 5 output the color signals to the signal lines 6 and 7 at the moment when the clock pulses C2 and C3 rise, respectively, two orthogonal components of the color signals are obtained as digital signals on the signal lines 6 and 7. .

Since the conventional digital chroma demodulation circuit is configured as described above, the quadrature component of the chrominance signal is detected only at every cycle of the chrominance subcarrier, and has a drawback that it cannot cope with a sudden change in the image. Was.

[Summary of the Invention]

The present invention has been made to eliminate the above-mentioned drawbacks of the prior art, and has a repetition frequency four times that of the chrominance subcarrier, and a color burst for each sampling point of a fixed period determined by the repetition frequency. Sampling means for sampling the NTSC composite color signal with sampling pulses phase-controlled to match the zero-phase position of the signal; and a luminance signal obtained by alternately outputting the signal sampled at the quadruple repetition frequency. Separating means for separating into color signals comprising first and second signal trains; and alternately and serially inputting the color signals comprising the first and second signal trains outputted by the separating means. First and second delay means for giving a constant delay time to each of the color signals; and an averaging means for inputting an output signal of the second delay means and an output signal of the separation means, averaging and outputting The output of the averaging means and the first The output of the delay means is input from the first and second input terminals, the output signal of the averaging means is interpolated between successive output signals of the first delay means, and Color signal
Output means for outputting from a second output terminal, wherein the output means includes the first and second input terminals and the first and second input terminals.
An NTSC digital chromatograph comprising switching means for alternately switching between coupling with a second output terminal, wherein the switching is performed based on a pulse which is inverted every 90 ° phase with respect to the zero phase position of the color burst. With the demodulation circuit, with a simple configuration in which the increase in circuit scale is stopped insignificantly,
It is an object of the present invention to detect two orthogonal component values of a color signal at a repetition frequency four times as high as that of a color subcarrier, thereby enabling more precise color reproduction.

(Example of the invention)

An embodiment of the present invention will be described below with reference to the drawings. In FIG. 3, reference numeral 11 denotes a signal line for supplying an NTSC composite color signal, 12 denotes an A / D converter for sampling the supplied NTSC composite color signal at a predetermined sampling frequency, and 13 denotes an A / D converter. A comb filter for separating the composite color signal digitized by the device 12 into a luminance signal and a chrominance signal; 14 a sign inverter connected to the color signal output of the comb filter;
5, 16 are delay circuits having a delay time equal to one sampling period, 17 is an adder that adds the output of the sign inverter 14 and the output of the delay circuit 16, and 18 is the output of the adder, And can be constituted by a shift register, for example. Reference numeral 19 denotes a switch circuit that switches and outputs the output of the arithmetic circuit 18 and the output of the delay circuit 15 at a predetermined timing.
Reference numerals 0 and 21 denote signal lines for outputting two orthogonal components of the color signal. A clock pulse generating circuit 24 generates clock pulses C4, F1, and F2 as described below.

That is, the clock pulse generation circuit 24
2 is supplied with a clock pulse C4 having a repetition frequency four times that of the color subcarrier and having a phase controlled so that the rise coincides with the zero phase of the color burst.
14 has a repetition frequency equal to the color subcarrier, has a theoretical value of “1” at the zero phase and 90 ° phase of the color burst, and has a logical value “1” at the 180 ° phase and 270 ° phase of the color burst. A clock pulse F1 whose phase is controlled to have 0 "is supplied, and the switch circuit 19 has a frequency twice as high as the color subcarrier and has a logical value" 1 "at the zero phase of the color burst, Logical value “0” at 90 ° phase of color burst
And supplies a clock pulse F2 that is phase-controlled to have The sign inverter 14, delay circuits 15, 16, adder 17, arithmetic circuit 18, switch circuit 1
9 and a clock pulse generation circuit 24, a color signal separation circuit is formed, which thins out every other sample value from the color signal, and interpolates the thinned out part by the previous and next sample values to orthogonalize the color signal. To separate the two components.

FIG. 4 shows the color burst and the clock pulses C4, F1,
FIG. 6 is a timing chart showing a phase relationship of F2.

Next, the operation and effect will be described. The composite color signal is A /
The D converter 12 converts the analog signal to a digital signal at the rising instant of the clock pulse C4,
It is input to the comb filter 13. Then, the color signal components are separated by the comb filter 13 and output. Since the phase of the clock pulse C4 is controlled such that the rising edges coincide with each other when the color burst has a zero phase, the digital color signal extracted by the above operation is a repetition of two orthogonal components except for the sign. That is, in the A / D converter 12, the color signal sampled at the zero phase of the color burst is a “red color difference signal”, and the color signal sampled at the 90 ° phase of the color burst is a “blue color difference signal”. The color signal sampled at the 180 ° phase of the color burst is “− (red color difference signal)”, and the color signal sampled at the 270 ° phase of the color burst is “− (blue color difference signal)”. It is.

The color signal output from the comb filter 13 is then input to the sign inverter 14. In this sign inverter 14,
Since the sign is inverted only when the clock pulse F1 is "1", as a result, the digital values of the red difference signal and the blue difference signal which are alternately repeated at a cycle determined by twice the frequency of the color subcarrier are converted to the delay circuit 15 and It is supplied to the input of the adder 17. The signal supplied to the delay circuit 15 is delayed by two samples by the delay circuit 15 and the delay circuit 16 at the next stage, and the adder receives the signal.
Entered in 17. Therefore, the adder 17 adds the current sample value of the color signal in consideration of the sign and the sample value two samples before. This added value is halved by an arithmetic circuit 18 including a shift register or the like, and as a result, the average value of the two sample values is supplied to the first input unit 22 of the switch circuit 19. The output of the delay circuit 15, that is, the sample value one sample before the current color signal in consideration of the sign is supplied to the second input unit 23 of the switch circuit 19.

The switch circuit 19 connects the output unit 20 to the input unit 22 and the output unit 21 to the input unit 23 when the clock pulse F2 is “1”,
When the clock pulse F2 is “0”, the output unit 20 is connected to the input unit 23,
The output unit 21 is switched to the input unit 22.

As a result, the output unit 20 of the switch circuit 19 is always output at a cycle determined by four times the frequency of the color subcarrier of the red difference signal, and the output unit 21 receives the blue difference signal at the same cycle as above. Is output. Therefore, it is possible to follow a sharp change in the image, and to perform more precise color reproduction as compared with the conventional one.

In the above description, for the sake of simplicity, all the time delays of signal propagation occurring in the arithmetic circuit including the delay circuit 15 and the comb filter 13 have been ignored.

〔The invention's effect〕

As described above, according to the present invention, the repetition frequency is four times that of the color subcarrier, and the phase is set so as to coincide with the zero phase position of the color burst for each sampling point of a fixed period determined by the repetition frequency. Sampling means for sampling the NTSC composite color signal with a controlled sampling pulse; and a luminance signal obtained by sampling the signal sampled at the quadruple repetition frequency, from the first and second signal trains output alternately. Separation means for separating the color signals into a plurality of color signals, and color signals consisting of the first and second signal strings output by the separation means are alternately and serially input, and the color signals have a predetermined delay time, respectively. First and second delay means for inputting an output signal of the second delay means and the output signal of the separation means, averaging means for averaging and outputting, and an output of the averaging means and The output of the first delay means and the first and second input terminals , And the output signal of the averaging means is interpolated between successive output signals of the first delay means to obtain a color signal composed of two new signal trains.
1, output means for outputting from a second output terminal, wherein the output means includes the first and second input terminals and the first and second input terminals.
It is configured by switching means for alternately switching the coupling between the second output terminal and the second output terminal, and the switching is performed based on a pulse that is inverted every 90 ° phase with respect to the zero phase position of the color burst, With a simple configuration in which the increase in the circuit scale is stopped insignificantly, the values of two orthogonal components of the color signal are detected at a repetition frequency four times the color subcarrier, and can follow steep changes in the image. There is an effect that an NTSC digital chroma demodulation circuit capable of color reproduction can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a conventional NTSC digital chroma demodulation circuit, FIG. 2 is a timing diagram of clock pulses used in the above circuit, and FIG.
FIG. 4 is a block diagram showing an SC digital chroma demodulation circuit, and FIG. 4 is a timing chart of clock pulses used in the above circuit. 12 A / D converter, 13 Comb filter, 14 Sign inverter, 15, 16 1-sample delay circuit, 17 Adder, 18 Operation circuit, 19 Switch circuit , 24 ... Clock pulse generation circuit.

Claims (1)

(57) [Claims] 1. A sampling pulse having a repetition frequency four times that of a chrominance subcarrier and having its phase controlled so as to coincide with a zero phase position of a color burst at each sampling point of a fixed period determined by the repetition frequency. A sampling means for sampling the NTSC composite color signal, and a signal which is sampled at the quadruple repetition frequency into a luminance signal and a color signal comprising first and second signal strings output alternately. And first and second color signals consisting of the first and second signal strings output by the separation means are alternately and serially input to give a constant delay time to each of the color signals. Averaging means for receiving an output signal of the second delay means and an output signal of the separation means as inputs, averaging and outputting, and an output of the averaging means and an output of the first delay means. And output from the first and second input terminals. The output signal of the averaging means is interpolated between the continuous output signals of the first delay means, and the first and second color signals are formed as two new signal sequences.
Output means for outputting from a second output terminal, wherein the output means is provided by switching means for alternately switching coupling between the first and second input terminals and the first and second output terminals. An NTSC digital chroma demodulation circuit, wherein the switching is performed based on a pulse that is inverted at every 90 ° phase with respect to the zero phase position of the color burst.