JP3382126B2 - Low carrier frequency modulation circuit for color video signal - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low carrier frequency modulation circuit for a color video signal. 2. Description of the Related Art Heretofore, there has been known a color television doorphone which can monitor an image transmitted from a camera of an entrance terminal as a color image in a parent device installed indoors. In such a TV door phone, a video signal captured by a camera is scanned, and a luminance signal is obtained from red, blue, and green primary color video signals extracted from the video signal.
Further, a red difference signal and a blue difference signal are created from the red and blue primary color signals, respectively, and added to the luminance signal to create a video signal, thereby providing compatibility with a monochrome television. Furthermore, in order to enable the video signal to be transmitted by one carrier, the red difference signal and the blue difference signal are respectively modulated by modulating the color subcarriers having a predetermined phase difference, and are combined with the luminance signal. The signal synthesized in this way is a horizontal synchronization signal,
A vertical synchronization signal is added to create a color video signal. The color video signal generated in this manner is transmitted to the monitor side. In order to demodulate the color video signal in the monitor, a horizontal synchronization signal is detected from the color video signal in order to synchronously detect the color video signal. Oscillation signals having a predetermined phase difference are generated at the same frequency, and a red difference signal and a blue difference signal are extracted from the color video signal by using the oscillation signals. For this, a voltage frequency conversion circuit is used. When an oscillation signal oscillated from the oscillation circuit is input and the frequency of the oscillation signal is maintained at a predetermined frequency, the phase difference between the horizontal synchronizing signal and the oscillation signal becomes a predetermined level. The color difference signal is extracted from the color video signal by controlling the voltage so that the phase difference is obtained. As shown in FIG. 3, a voltage frequency conversion circuit VCO101 has a terminal IN for inputting a color video signal LN101 transmitted from a video signal source SG101 and a terminal connected between the video signal source SG101 and a reference potential point. ENABLE and color video signal LN10
Frequency modulated signal LN102 whose frequency is modulated according to the voltage of 1
During the period when the terminal ENABLE is at a low level, that is, when a low-voltage horizontal synchronization signal is detected,
The color video signal LN101 input to the terminal IN is subjected to voltage frequency modulation, and the frequency modulation signal LN102 is output from the terminal OUT. [0006] However, in such a voltage frequency conversion circuit, the detection timing may be different because the pulse width of the horizontal synchronizing signal of the color video signal is wide. If the detection timing of the horizontal synchronization signal is different, the synchronization timing will be different for each horizontal scanning section, and the amplitude and phase of the extracted color difference signal will change, so that the color of the monitored image will change. Or moire may occur. SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks, and the change in the amplitude and phase of the demodulated color difference signal can be reduced without the timing of the modulation of the video color signal being different for each horizontal scanning section. It is another object of the present invention to provide a low carrier frequency modulation circuit for a color video signal which does not cause a color change of a monitored video. In order to achieve the above object, a low carrier frequency modulation circuit for a color video signal according to the present invention separates a horizontal synchronizing signal from a color video signal of a video signal source.
Separated sync separation circuit and separated horizontal sync signal are input
And the input of the differentiated differential signal
And the impulse signal obtained from the comparator.
Signal that oscillates in synchronization with the signal and frequency-modulates the color video signal.
And a pressure frequency conversion circuit . In this low carrier frequency modulation circuit, a horizontal synchronizing signal is separated from a color video signal transmitted from a video signal source, the separated horizontal synchronizing signal is differentiated, and an impulse signal is obtained from the differentiated differentiated signal. The width of the impulse signal is narrow, and the voltage frequency conversion of the color video signal is performed in synchronization with the impulse signal.
The phase and amplitude of the color video signal can be set to the same timing in each horizontal scanning section, so that the monitored video does not change in color. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a preferred embodiment of a low carrier frequency modulation circuit for a color video signal according to the present invention will be described with reference to the drawings. As shown in FIG. 1, a low carrier frequency modulation circuit for a color video signal is a voltage frequency conversion circuit VCO to which a color video signal LN1 transmitted from a video signal source SG1 is input.
1 and a synchronization separation circuit B1, a differentiation circuit B2 connected to the synchronization separation circuit B1, and a comparator B3 connected to the differentiation circuit B2.
And are provided. A video signal source SG1 scans a video signal picked up by a camera, obtains a luminance signal from red, blue, and green primary color video signals extracted from the video signal, and further obtains a luminance signal from the red and blue primary color signals, respectively. The created red difference signal and blue difference signal are each subjected to balanced modulation of a color subcarrier having a predetermined phase difference and synthesized with a luminance signal to provide a composite signal, which is compatible with a black-and-white television and has one It is possible to transmit on a low frequency carrier. Furthermore, the composite signal is
The horizontal synchronizing signal and the vertical synchronizing signal are added and output as a color video signal LN1. As shown in FIG. 2, the synchronization separation circuit B1 to which the color video signal LN1 is input separates the horizontal synchronization signal LN2 from the color video signal LN1 and outputs the same. The synchronization separation circuit B1 separates using the difference between the amplitudes of the color video signals LN1 and cut-off characteristics of the transistors. The differentiating circuit B2, to which the horizontal synchronizing signal LN2 output from the synchronizing separation circuit B1 is input, differentiates the horizontal synchronizing signal LN2 and outputs a differentiated signal LN3. Any known circuit can be applied to the differentiating circuit B2. Further, the differential signal output from the differentiating circuit B2
The comparator B3 to which LN3 is input converts the differential signal LN3 into an impulse signal LN4. The impulse signal LN4 is synchronized with the horizontal synchronizing signal LN1, and has a pulse width much smaller than that of the horizontal synchronizing signal LN1. The impulse signal LN4 thus formed is
Are input to the voltage frequency conversion circuit VCO1, the terminal ENABLE to which the impulse signal LN4 is input, the terminal In to which the color video signal LN1 is input from the video signal source SG1, and the output terminal OUT
Are provided. The voltage frequency conversion circuit VCO1 is connected to the terminal ENAB
Reset by the impulse signal LN4 input from LE. Then, the color video signal LN input from the terminal IN
From 1, the frequency modulation signal LN5 corresponding to the color difference signal obtained by modulating the frequency of the oscillation signal according to the voltage is output from the terminal OUT. For this reason, from the color video signal LN1 input to the terminal IN to the impulse signal LN4, the frequency modulation signal LN2
When the impulse signal LN4 that is a narrow pulse is input to the terminal ENABLE of the voltage frequency conversion circuit VCO1 at the timing of the rising position of each horizontal synchronization signal of
Since the frequency modulation signal LN5 is transmitted from the output terminal OUT in synchronization with the rise of each horizontal synchronization signal, the monitored color video can be output in a normal color. The above description is an explanation of one embodiment of the present invention. The present invention is not limited to this, and the voltage / frequency modulation of the color video signal by the horizontal synchronizing signal separated by the synchronizing separation circuit is performed. It is possible to modulate the timings of the respective horizontal scanning sections so as to coincide with each other, so that the captured color video can be monitored as it is without color shift of the video. As is clear from the above description, according to the low carrier frequency modulation circuit for a color video signal of the present invention, the timing of the modulation of the video color signal does not differ for each horizontal scanning section. In addition, it is possible to reduce a change in the amplitude and phase of the demodulated color difference signal and monitor an image in which no color change occurs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of an embodiment of a color video signal low carrier frequency modulation circuit according to the present invention. FIG. 2 is a diagram showing signals obtained by the embodiment shown in FIG. 1; FIG. 3 is a block diagram of a conventional example. [Explanation of Signs] SG1 ... Video signal source B1 ... Sync separation circuit B2 ... Differentiation circuit B3 ... Comparator VCO1 ... Voltage Frequency conversion circuit LN1 Color video signal LN2 Horizontal synchronization signal LN3 Differentiation signal LN4 Impulse signal

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04N 11/00-11/22 H04N 9/00-9/78

Claims (1)

(57) [Claims] [Claim 1] A color video signal (LN) of a video signal source (SG1)
Sync separation circuit (B1) that separates the horizontal sync signal from 1)
And the differential that receives the separated horizontal synchronization signal (LN2)
The circuit (B2) and the differentiated signal (LN3) are input.
(B3) and the comparator
Oscillates in synchronization with the impulse signal (LN4)
-A voltage-frequency converter (VCO) that modulates the frequency of video signals
1) Low transport of color video signals characterized by having
Transmission frequency modulation circuit.