JPS62206988A - Muting circuit - Google Patents

Abstract

PURPOSE:To prevent the very hard-to-see color band from being outputted from a color encoder by muting chrominance signals when an input is not supplied to a color decoder circuit. CONSTITUTION:A subcarrier from a subcarrier oscillating circuit 5 is directly supplied to a balanced modulator 4, and supplied through a 90 deg. shifting circuit 6 to a balanced modulator 3. The outputs balanced and modulated by these balanced modulators 3 and 4 are supplied to an adding circuit 7 and the obtained chrominance signals are inputted to a switch 9 for muting. On the other hand, a CP detecting circuit 8 outputs a detecting signal to the switch 9 in accordance with the presence or absence of a CP signal and controls the ON and OFF of the switch 9. As such a result, the switch 9 mutes the chrominance signals in accordance with the detecting signal. Thus, when the input is not supplied to a color encoder circuit, the chrominance signals are muted. Therefore, it can be prevented beforehand that the very hard-to-see color band is outputted from an color encoder.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a muting circuit that mutes color output when a manually applied clamp pulse to a color encoder circuit is absent.

[Conventional technology]

Conventionally, this type of muting circuit has been constructed as shown in FIG. 2, for example. In Fig. 2, clamp circuit 1
The RY signal is input to the clamp circuit 2, and the B-Y signal is input to the clamp circuit 2.
A signal is input, and a clamp signal CP is manually applied to both clamp circuits 1 and 2. Signals clamped by clamp circuits 1 and 2 are input to balanced modulators 3 and 4, respectively. On the other hand, a subcarrier of frequency fsc is input from the subcarrier oscillation circuit 5 directly to the balanced modulator 4 and to the balanced modulator 3 via the 90° shift circuit 6.

The balanced modulated outputs of these balanced modulators 3 and 4 are input to an adder circuit 7, from which a color signal is obtained.

Here, when there is no input, the RY and BY signals are no longer supplied to this circuit, and the CP multiplied signal obtained from the synchronization signal is no longer supplied. As a result, R-Y
, B-Y signal inputs are no longer clamped, so the DC level fluctuates greatly and is input to the balanced modulators 3 and 4. Since these modulators 3 and 4 perform balanced modulation according to the human-powered DC level,
An erroneous output is output to the adder circuit 7, and as a result, an erroneous signal is output as a color signal even though there is no human power. Therefore, if you manually input a video signal created based on this signal to a monitor,
It had the disadvantage of producing very unsightly color bands.

[Problem that the invention seeks to solve]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a muting circuit which eliminates the drawbacks of the prior art as described above and which appropriately mutes the color output when the absence of an input signal is detected.

[Means for solving problems]

To achieve such objects, the present invention provides a color encoder circuit with means for detecting a clamp pulse that clamps the input to the color encoder, and a means for detecting a clamp pulse that clamps the input to the color encoder; and means for muting the color output of the encoder.

[For production]

According to the muting circuit of the present invention, the color signal is muted when input is no longer supplied to the color encoder circuit, so it is possible to prevent a very unsightly color band from being output from the color encoder. can.

〔Example〕

Embodiments of the present invention will be described in detail and specifically below based on the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. In FIG. 1, the same parts as in FIG. 2 are given the same reference numerals.

In FIG. 1, a clamp circuit 1 receives an RY signal, and a clamp circuit 2 receives a BY signal. A clamp signal CP is input to clamp circuits 1 and 2 and a CP detection circuit 8. The signals clamped by clamp circuits 1 and 2 are input to balanced modulators 3 and 4, respectively. The subcarriers from the subcarrier oscillation circuit 5 are directly supplied to the balanced modulator 4 and also supplied to the balanced modulator 3 via the 90° shift circuit 6. The balanced modulated outputs of these balanced modulators 3 and 4 are supplied to an adder circuit 7, and the obtained color signals are input to a muting switch 9.

On the other hand, the CP detection circuit 8 outputs a detection signal to the switch 9 depending on the presence or absence of the CP flaw signal, and controls whether the switch 9 is turned on or off. As a result, the switch 9 mutes the color signal according to the detection signal.

When the human power is exhausted, the CP multiplied signal obtained from the synchronization signal is no longer supplied, and the detection circuit 8 outputs to the switch 9 that there is no CP. Switch 9 is opened accordingly;
Muting the color signal.

Note that the color encoder circuit is not limited to the NTSC system, but may also be of the PAL system or the SECAM system, and it is clear that the muting according to the present invention can be effectively applied to any case.

Furthermore, the detection time of the CP detection circuit 8 can be determined to be as long as possible using a retriggerable mono-multivibrator circuit within a time range in which the color band is not visually noticeable. Since short-term CP absences such as noise and dropouts are not detected every time, there is no fear of malfunctions caused by these noises and dropouts.

〔Effect of the invention〕

As described above, according to the muting circuit of the present invention, the color signal is muted when the input is no longer supplied to the color encoder circuit, so that it is possible to prevent a very unsightly color band from being output from the color encoder. can be prevented.

[Brief explanation of drawings] FIG. 1 is a block diagram showing an embodiment of the present invention; FIG. 2 is a block diagram showing a conventional example. 1...clamp circuit, 2...clamp circuit, 3... Balanced modulator, 4... Balanced modulator, 5... Subcarrier oscillation circuit, 6...90° shift circuit, 7...addition circuit, 8...CP detection circuit, 9... Muting switch. Honto Sun Moon Tsukata Can, the remaining Fukuchi 1. Figure 1

Claims (1)

[Claims] In a color encoder circuit, means for detecting a clamp pulse for clamping an input to the color encoder, and a color output of the color encoder when the detection means detects the absence of the clamp pulse. A muting circuit characterized by comprising: a means for muting; and a muting circuit.