JPS6017200B2 - color killer circuit - Google Patents

Description

[Detailed Description of the Invention] In a color television receiver, when receiving a black and white broadcast,
The color signal system is blocked to prevent color noise from occurring.

When performing such a color killer, conventionally, the color burst signal is separated and this burst signal is supplied from the terminal 1 to the comparison circuit 2 as shown in FIG. 1, while the reference voltage Vt from the voltage source 3 is The burst signal is supplied to a comparison circuit 2 to compare the levels thereof, and when the level of the burst signal becomes lower than the reference voltage Vt, a signal is supplied to a switch circuit 4 that cuts off the color signal system to perform color killer. Ta.

In this case, a hysteresis circuit 5 is provided in the voltage source 3 to increase the reference voltage Vt when the color killer is applied and to lower the reference voltage Vt when it is not applied.
This is to prevent unstable operation in border areas, etc.
By the way, the color killer detection circuit described above can be used to
When detecting the level of the strike signal, synchronous detection is used.

On the other hand, in a so-called ACC circuit that adjusts color saturation, a peak value of a color signal is detected, and level control is performed so that the peak value becomes a predetermined level.

Therefore, when the color signal level decreases due to fluctuations in radio wave propagation or antenna mismatching during a strong electric field, the input/output characteristics of the ACC circuit change from a solid line to a broken line as shown in Figure 2, and the output Color killer is applied when the level reaches Vt, that is, when the input level is E2.

This is because when there is a lot of noise in a weak electric field, ACC is multiplied by the peak value of this noise, so the input/output characteristics become like a broken line with respect to the original color signal level.
Color killer is applied when the input level is sufficiently high at E2. Therefore, in areas with a weak electric field, color killer occurs and color image reception becomes impossible even though there is still a color signal level that allows color demodulation.

In view of these points, the present invention is designed to enable normal color image reception and accurate color killer even in areas with weak electric fields.

Hereinafter, one embodiment of the present invention will be described with reference to the drawings.

In FIG. 3, a video signal from a tuner 11 is demodulated through a video intermediate frequency circuit 12 and a video demodulation circuit 13.

This demodulated signal is supplied to the average value detection circuit 14, and this detected signal (AGC signal) is supplied to the tuner 11 and intermediate frequency circuit 12, where it is multiplied by the average value AGC of the RF signal system. Further, a signal from the demodulation circuit 13 is coupled to a matrix circuit 16 through a luminance signal (Y) system amplifier 15. Further, the signal from the demodulation circuit 13 is supplied to the color demodulation circuit 19 through color signal system bandpass amplifiers 17 and 18, and the signal from the bandpass amplifier 17 is supplied to the burst gate circuit 201 to generate a color burst signal. is separated, and this burst signal is supplied to the CW oscillator 21 to generate a continuous wave with the same frequency and the same phase. This continuous wave is then supplied to the color demodulation circuit 19 to demodulate the B-Y and RY color difference signals, and these color difference signals are supplied to the matrix circuit 16 to generate the three primary color signals of R, G, and B. is formed. Further, the burst signal from the burst gate circuit 20 is supplied to the peak value detection circuit 22 for peak value detection, and this detection signal is supplied to the comparator 23, and the reference voltage Vr from the reference voltage source 24 is compared. The comparison output is supplied to the circuit 23 and the bandpass amplifier 17
is supplied to the gain control terminal of the burst signal, and ACC is performed so that the peak value of the burst signal becomes a constant level.

Further, the burst signal from the burst gate circuit 20 is supplied to the synchronous detection circuit 25, and the continuous wave from the oscillator 21 is supplied to the synchronous detection circuit 25, where the burst signal is synchronously detected, and this detected signal is sent to the comparison circuit 26. At the same time, the reference voltage yt from the voltage source 27 is supplied to the comparator circuit 2.
6, the level of the burst signal is set to the reference voltage Vt
A color killer signal is formed when:

This color killer signal is then supplied to the bandpass amplifier 18, and while the color killer signal is being obtained, the gain is reversed and the color signal is cut off. Further, the output voltage Vt of the voltage source 27 is controlled by a hysteresis circuit 28.

A color killer signal from the comparator circuit 26 is supplied to the hysteresis circuit 28, and the value of the reference voltage Vt from the voltage source 27 is increased during the period when this signal is obtained. Further, the AGC signal from the detection circuit 14 is supplied to the hysteresis circuit 28, and when the level of the AGC signal is small, that is, when the received signal is in a weak electric field, the reference voltage V
The value of t is reduced.

Therefore, in this circuit, the AGC signal from the detection circuit 14 changes according to the electric field strength of the received signal, and in response to this change, the value of the reference voltage Vt is reduced in the case of a weak electric field, for example, as shown by Vt' in FIG. The color killer signal is detected at this voltage Vt'.

In this way, a color killer signal is detected and color killer is performed.According to the present invention, by lowering the reference voltage Vt during a weak electric field and lowering the color killer detection level, normality can be achieved even in weak electric field areas. Not only can color image reception be performed, but also accurate color killer can be performed.

Note that the present invention is applicable not only to television receivers but also to VTRs.

[Brief explanation of the drawing]

FIG. 1 is a system diagram of a conventional circuit, FIG. 2 is a characteristic diagram for explaining the present invention, and FIG. 3 is a system diagram of an example of the present invention. 14 is an average value detection circuit, 22 is a peak value detection circuit, 25
is a synchronous detection circuit, 26 is a comparison circuit, 27 is a voltage source, 28
is a hysteresis circuit. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1 AGC that controls the gain of the tuner and intermediate frequency circuit
In a television color killer circuit having a closed-loop ACC circuit that peak-detects the peak value of a burst-gated burst signal and keeps this peak value approximately constant, a detection circuit that detects the burst signal; ,
a comparator that compares the output of the detection circuit with a reference level;
a switch circuit that controls the output of the color difference signal based on this output; a hysteresis circuit that controls the reference level;
Each of the closed loop control circuits is provided to control the hysteresis circuit with the output of the comparator, and the hysteresis circuit is controlled with the output of the AGC circuit, and when the input electric field is weak, the operating point of the switch circuit is controlled. A color killer circuit that does not operate as a killer until the output of the detection circuit drops further.