JPH0575236B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a color demodulation circuit, and in particular, when demodulating a carrier color signal subjected to double balance modulation, the reference subcarrier used for coherent detection is The present invention relates to a color demodulation circuit that synchronizes the phase with this burst signal in units of one burst signal period.

(Prior art) Composite video signals in NTSC and PAL systems include luminance signals and color signals (color difference signals RY, B).
-Y) is subjected to double-balanced modulation and superimposed, and as a result, in the above-mentioned NTSC system, PAL system, etc., a composite video signal can be transmitted over one transmission path. In order to demodulate the color signal superimposed on this composite video signal, the carrier color signal separated from the composite video signal is synchronously detected using a reference subcarrier.

This reference subcarrier is a signal whose frequency and phase are synchronized with the burst signal inserted into the back porch of the horizontal blanking period. below,
The case of NTSC system will be described.

FIG. 3 is a block system diagram showing a conventional color demodulation circuit.

In the figure, a composite video signal is supplied to a bandpass amplifier 2 via an input terminal 1, and is also supplied to a color synchronization circuit 3.

The band amplifier 2 separates the carrier color signal from the composite video signal, and the separated carrier color signal is supplied to the BY demodulator 4 and the R-Y demodulator 6.

The color synchronization circuit 3 is a circuit that extracts the burst signal and outputs a reference subcarrier whose frequency and phase are synchronized with the extracted burst signal, and this reference subcarrier is supplied to the BY demodulator 4.

The BY demodulator 4 demodulates the supplied carrier color signal on the BY axis using a reference subcarrier, and a BY signal is obtained from the output terminal 5.

The R-Y demodulator 6 converts the supplied carrier color signal into
RY axis demodulation is performed using a carrier wave whose phase is advanced by a 90° phase shifter 7 from a reference subcarrier wave, and an RY signal is obtained from an output terminal 8.

The circuit described above is a color demodulation system used in general television receivers.

(Problem to be Solved by the Invention) However, when directly converting a composite video signal into an image, if video processing is performed in which the same line (horizontal period), the same frame, etc. is repeated (in other words, when the composite video signal is digitized) (In cases where the data is stored in a line memory or frame memory, etc., and the same line, same frame data is read out and the same line, same frame, etc. is repeatedly visualized), the reference phase for synchronous detection of the carrier color signal The phase continuity of the burst signal cannot be ensured, and the normal color synchronization circuit (indicated by 3 in Figure 3)
In this case, the phases did not synchronize instantly, and until synchronization occurred, normal color demodulation could not be performed.

In particular, in the case of an NTSC composite video signal, the burst phase is reversed by 180° for each line and for each frame.

This is shown as a model in FIG. As shown in the figure, the phase of the burst signals between adjacent lines (horizontal periods) is the same when the horizontal synchronization signals are aligned.
Flip 180°. Further, at this time, the subcarriers are continuous.

Further, FIG. 5 shows the phase of a burst signal when the same line and frame are repeated in digital processing using a line memory, frame memory, etc. As shown in the figure, when the same line is repeated, the first line is connected to the first line, and when the same frame is repeated, the 525th line is connected to the first line. . In this way, if the same line and the same frame are repeated, the phase of the burst signal will be discontinuous by 180 degrees.

FIG. 6 is a block system diagram showing the color synchronization circuit 3 in FIG. 3 in detail. This shows the configuration of an APC (automatic phase control) type color synchronization circuit.

In the figure, a burst signal is extracted from the input composite video signal by a burst gate 11, and this burst signal is supplied to a phase detector 12. Then, the phase detector 12 detects the phase of the burst signal. That is, the phase detector 12 detects the input burst signal and the voltage controlled oscillator (VCO) 1.
A voltage (phase error signal) corresponding to the phase difference with the output of VCO 14 is generated, and after being smoothed by an integrating circuit 13, this voltage is supplied to a frequency control terminal of a VCO 14. The oscillation frequency of the VCO 14 is controlled by a DC voltage input to a frequency control terminal, and the generated voltage (signal) is supplied to the phase detector 12 while being output as a reference subcarrier.

The circuit shown in Figure 6 is a so-called phase-locked circuit, and the time it takes for the phases to become synchronized depends on the integrator circuit, and if the time constant of the integrator circuit is made small in order to shorten the pull-in time, noise etc. There are some problems that make it easier to be influenced.

Therefore, in such conventional APC circuits, it is not possible to instantaneously track and synchronize with discontinuities in the phase of burst signals due to repetition of the same line or frame, etc. There is a problem in that normal color demodulation cannot be performed until the phases are synchronized, resulting in uneven hue.

Furthermore, in the conventional circuit as described above,
Since a crystal oscillator is used in the APC circuit, the cost increases accordingly.

Therefore, the present invention solves the above-mentioned problems of the conventional technology, and provides a color demodulation system that can perform color demodulation without hue unevenness even if subcarrier discontinuity occurs due to repetition of the same line or frame. The purpose is to provide a demodulation circuit.

(Means for Solving the Problems) In order to achieve the above object, the present invention, as shown in FIG. 1, inserts a carrier color signal subjected to double balance modulation into a composite video signal. In a color demodulation circuit that performs synchronous detection and demodulation using a reference subcarrier synchronized with a burst signal, a voltage controlled oscillator 25 that generates a signal with a frequency N times that of the burst signal (N is a positive integer); A control means 23 that generates a phase error signal corresponding to the phase difference between the burst signal and the signal obtained by dividing the output of ,24,2
6, a sampling means 27 for sampling the composite video signal using the clock signal b, using the output of the voltage controlled oscillator 25 as a clock signal b for determining the sampling point of the composite video signal; A phase detection means 39 detects the phases of the burst signals a, g, h, and i of the composite video signal, and a signal j which divides the output of the voltage controlled oscillator 25 to 1/N and is detected by the phase detection means 39. The output of this frequency divider 36 is used as reference subcarriers c, d, e, f, and is synchronously detected and demodulated using the reference subcarriers c, d, e, f. The present invention provides a color demodulation circuit characterized by the following features.

(Function) In the color demodulation circuit configured as described above, the output of the voltage controlled oscillator 25 is used as the clock signal b to sample the composite video signal, and the voltage controlled oscillator 25 is used as the clock signal b to sample the composite video signal.
The reference subcarrier c, which is reset by the signal j obtained by dividing the output of 5 by 1/N and detecting the sampled burst signals a, g, h, and i of the composite video signal.
It performs synchronous detection and demodulation using d, e, and f.

(Example) Regarding an example of the color demodulation circuit according to the present invention,
This will be explained below along with the drawings.

FIG. 1 is a block system diagram showing an embodiment of the color demodulation circuit according to the present invention. Note that this is to sample a clock signal with a frequency that is N times (N is a positive integer) the burst signal of the input composite video signal. As an example, N = 4.
The case will be explained below.

In FIG. 1, a burst signal is extracted from a composite video signal inputted through an input terminal 21 at a burst gate 22.
A clock signal is generated in a phase locked circuit composed of a phase detector 23, an integrating circuit 24, a VCO 25, and a 1/N (1/4) frequency division counter 26.
A clock signal is obtained from the output of VCO25.
As a result, the clock signal is synchronized in frequency and phase with a signal four times as large as the input burst signal.

Therefore, as shown in FIG.
The sampling points for one period are 4 points,
Each sampling point has a fixed phase, and each
They have a phase difference of 90 degrees, and the clock signal b is synchronized with a signal four times as large as the input burst signal a.

On the other hand, in FIG. 1, a composite video signal input through an input terminal 21 is digitized by an A/D converter 27, and digitally processed by a line memory, a frame memory 28, etc. Then, the data in the line memory and frame memory 28 is read out, and the same line and frame data is digitally processed and visualized, thereby performing image processing such as repetition of the same line and frame. .

Furthermore, the digitally processed composite video signal is
The Y/C separation circuit 29 separates the luminance signal Y and the carrier color signal C.
The luminance signal Y and the carrier color signal C
are converted into analogs by D/A converters 30a and 30b, respectively. Note that the A/D converter 27, line memory, frame memory 28, Y/C separation circuit 29
A clock signal synchronized with the burst signal described above is supplied to the D/A converters 30a and 30b, respectively, and digital processing is performed.

The luminance signal Y converted into an analog signal by the D/A converter 30a is output to the output terminal 31. Also, D/A
The carrier color signal C converted into analog by the converter 30b is converted to B-Y by the B-Y demodulator 32 and R-Y demodulator 33.
The signals are demodulated into a Y signal and an RY signal, respectively, and output to output terminals 34 and 35, respectively. At this time, the reference subcarrier used for demodulation is one obtained by dividing the clock signal output from the VCO 25 by 1/N (1/4) by a 1/N (1/4) frequency division counter 36.
That is, while supplying the output of the 1/N (1/4) frequency dividing counter 36 to the BY demodulator 32,
4) The output of the frequency dividing counter 36 is phase-advanced by 90 degrees using a 90 degrees phase shifter 37 and then supplied to the RY demodulator 33 .

The frequency of the clock signal is divided by 1/ by the frequency division counter 36.
If the frequency is divided into 4, 4 of c, d, e, f in Fig. 2
The signal will be one of the two types of phase signals, and the frequency will be the same but the phase will be shifted.

Therefore, in order to make the reference subcarrier coincide with the phase of the burst signal, the burst signal digitally processed by the burst gate 38 is extracted from the signal immediately before the D/A converter 30b, and the phase of this burst signal is detected by the phase detection circuit. By detecting the phase at step 39 and supplying this detected signal (for example, the pulse j in FIG. 2) to the frequency division counter 36 as a reset pulse, the reference subcarrier whose frequency is divided to 1/4 by the frequency division counter 36 is generated. The phase of the burst signal is made to match the phase of the burst signal.

Here, the reset pulse by the phase detection circuit 39 is obtained from the most significant digit (MSB) of the digital signal data (binary word) of the burst signal immediately before D/A conversion. Further, since the carrier color signal C is an offset binary or two's complement code, the MSB of the burst signal extracted from the carrier color signal C is a pulse having the period of the burst signal. for example,
If this is waveform c in Fig. 2, then this waveform c and the waveform d delayed by one clock period.
The waveform (pulse) j is obtained by taking NAND (inversion of logical product). This pulse j becomes a reset pulse, which can be obtained with an extremely simple circuit (NAND gate).

Line memory and frame memory 28 in Figure 1
The composite video signal digitally processed by
Since this is a signal with a phase sampled by the A/D converter 27, the phase is shifted only to the phase fixed to one of the waveforms a, g, h, and i in FIG.

On the other hand, since the clock signal is synchronized with the input burst signal, a signal obtained by dividing the clock signal into 1/4 has the phase of any one of the waveforms a, g, h, and i in FIG.

Here, as described above, the digitally processed burst signal (output of the burst gate 38)
The phase is detected by the phase detection circuit 39, and the pulse j in Fig. 2 is generated, which resets the frequency division counter 36 in Fig. 1, so the clock signal (output of the VCO 25) is reduced to 1/4. The reference subcarrier obtained by frequency division matches the phase of the burst signal D/A converted by the D/A converter 30b,
For the phases of the burst signals a, g, h, and i in FIG. 2, the waveforms c, d, e, and f serve as reference subcarriers, respectively.

As a result, even if the phase of the burst signal is discontinuous in units of one line (horizontal period), the reference subcarrier has a predetermined phase in units of the burst signal period.

In addition, in the above embodiment, the case where N=4 was explained as an example, but the case where N=4 is a positive integer other than 4 can also be explained in the same way.

(Effects of the Invention) As described above, according to the color demodulation circuit of the present invention, when a composite video signal is digitally processed, when subcarrier discontinuity occurs due to repetition of the same line or frame, etc. Even if a phase shift occurs in the carrier color signal in units of clock signals as shown in FIG. In the APC circuit, a crystal oscillator was used, but in the present invention, the frequency-divided clock signal is used as the reference subcarrier, so the crystal oscillator can be omitted, and the cost can be reduced accordingly. have

[Brief explanation of the drawing]

Fig. 1 is a block system diagram showing an embodiment of the color demodulation circuit according to the present invention, Fig. 2 is a diagram showing waveforms of each part of the block system diagram of the invention shown in Fig. 1, and Fig. 3 is a diagram showing a conventional color demodulation circuit. A block system diagram showing the circuit. Figure 4 shows the phase of the burst signal for each line. Figure 5 shows the burst signal when the same line and frame are repeated by line memory, frame memory, etc. 6 are block diagrams showing the configuration of a conventional APC color synchronization circuit. 21... Input terminal, 22, 38... Burst gate, 23... Phase detector, 24... Integrating circuit, 25...
Voltage controlled oscillator (VCO), 26, 36...1/N frequency division counter, 27...A/D converter, 28...line memory, frame memory, 29...Y/C separation circuit, 30a, 30b...D/A conversion vessel, 31,3
4, 35... Output terminal, 32... B-Y demodulator, 33
...RY demodulator, 37...90° phase shifter, 39...phase detector.

Claims (1)

[Scope of Claims] 1. A color demodulation circuit that synchronously detects and demodulates a carrier color signal subjected to double balance modulation using a reference subcarrier synchronized with a burst signal inserted into a composite video signal, comprising the steps of: a voltage controlled oscillator that generates a signal with a frequency N times (N is a positive integer) the voltage controlled oscillator, and a phase error corresponding to the phase difference between the signal obtained by dividing the output of the voltage controlled oscillator by 1/N and the burst signal. control means for generating a signal and controlling the frequency of the signal generated by the voltage controlled oscillator using the phase error signal; sampling means for sampling the composite video signal using a signal; phase detection means for detecting the phase of the burst signal of the composite video signal sampled by the sampling means; Divide the frequency into
and a frequency divider that is reset by the signal detected by the phase detection means, the output of this frequency divider is used as the reference subcarrier, and the reference subcarrier is used for synchronous detection and demodulation. Characteristic color demodulation circuit.