JPS61139904A - Muting circuit - Google Patents

Abstract

PURPOSE:To eliminate a potential difference between the DC potential and the clamp potential of an output at the time of muting and in a usual video signal period, by fixing a sample and holding circuit to a sample state and controlling an output voltage, at the time of muting. CONSTITUTION:In a state that no mute signal exists, an output of an outputting circuit 6 is compared with a reference voltage from a reference potential source 11 by a comparator 12, and provided to a sample and holding circuit 13. In the circuit 13, by a gate pulse applied to a terminal 15, a comparison voltage is sampled to a capacitor 18 in a gate pulse period, holding is executed in other period, and by controlling a variable DC shifting circuit 10 by this holding voltage, an outputted video signal is clamped. On the other hand, when a mute signal has been impressed to a mute signal input terminal 5, the potential from a DC potential source 8 passes through the circuit and appears in an output terminal 7. In this case, at the time of muting, the circuit 13 is fixed to a holding state, and the potential from the reference potential source 11 is outputted to the terminal 7.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a muting circuit that erases a signal for a predetermined period of time, and is particularly useful when integrating a video signal meeting circuit that involves a clamp operation. This provides a circuit suitable for this purpose.

[Conventional technology]

When recording and reproducing video signals, a muting circuit for forcibly erasing the image using a mute signal is required in order to prevent blank periods or disturbed screens from occurring in the transient state of recording/reproduction switching.

Conventionally, as an image meeting circuit, a circuit that cuts off image input or a circuit that replaces the output with a fixed DC potential has been used. An example of a muting circuit having the configuration shown in FIG. 3 is shown.

In FIG. 3, a video signal applied to an input terminal 1 passes through a clamp circuit 2 and then enters one input of a switching circuit 3. On the other hand, the other input of the switching circuit 3 is supplied with a fixed DC potential source 4 . The switching circuit 3 is switched by the mute signal applied to the mute signal terminal 5.
The input video signal and the fixed DC potential are selectively output to the output. That is, during a period when the mute signal is generated at the mute signal terminal 5, a fixed DC potential is passed through the output circuit 6 and outputted to the output terminal 7 regardless of the input video signal.

[Problem that the invention seeks to solve]

The conventional muting circuit described above has the drawback that a potential difference is likely to occur between the clamped DC potential of the video signal at the output terminal 7 and the DC potential set at the time of muting. This occurs because variations in the clamp DC potential and the potential of the fixed DC potential source 4 and variations in the DC output potential of the switching circuit 3 are multiplied by the gain of the output circuit 6.

As mentioned above, if there is a difference in the DC potential of the output terminal 7 from the set value between mute and normal times, the brightness of the image during mute will deviate from the set value, causing an error on the screen. It feels natural.

In even more severe cases, since the output terminal 7 is normally capacitively coupled, the DC potential cannot respond quickly when switching.
Screen abnormalities may occur.

[Means for solving problems]

The muting circuit of the present invention includes a switching circuit that switches according to an input signal, a predetermined DC potential, and a mute signal applied to a t-mute signal terminal, a DC potential shift circuit whose input is connected to the output of the switching circuit, and a DC potential shift circuit. an output circuit that inputs the output of the circuit; an output terminal that takes out the output from the output circuit; a comparator that compares the output of the output circuit with a first reference voltage; The circuit includes a sample-and-hold circuit that samples the 1-number-and-hold circuit, holds it for another period, and feeds it back to the DC potential shift circuit, and a circuit that fixes the 1-number-and-hold circuit in the sample state during the period when the mute signal is present.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. When there is no mute signal, the input video signal applied to input terminal 1 is applied to one input of switching circuit 9, and variable DC potential shift circuit 10. It passes through the output circuit 6 and is output to the output Hiroko 7. Further, the output of the output circuit 6 is compared with a reference voltage from a first reference potential source 11 by a comparator 12, and is connected to a sample and hold circuit 13. In the sample-and-hold circuit 13, the comparison voltage is sampled into the capacitor 18 during the gate pulse period by the gate pulse applied to the terminal 15, held for another period, and the variable DC potential shift circuit 10t is controlled using this hold voltage. By doing this, the output video signal is clamped.

On the other hand, when the mute signal is applied to the mute signal input terminal 5 and running, the input of the switching circuit 90 is switched to the DC potential source 8 side, and the potential from the DC potential source 8 is changed to the variable DC potential shift circuit 10. It passes through the output circuit 6 and appears at the output terminal 7. Here, at the time of mu+), the sample-and-hold circuit 13 is fixed in a hold state by the OR gate circuit 14 regardless of the presence or absence of a gate pulse, and the output terminal 7 has the same potential as the first reference potential source 11. Output.

In this way, the output potential is controlled during mute by using a feedback loop that determines the clamp potential during the period when the video signal is applied. Therefore, the output DC voltage varies between the mute time and the normal video signal period. There is no potential difference between the clamp potential and the clamp potential.

FIG. 2 shows another embodiment. In FIG. 2, the same parts as in FIG. 1 are given the same numbers.

The difference from FIG. 1 is that a circuit for switching from the first reference DC potential source 11t connected to the comparator 12 to the second reference DC potential source 17 by the mute switching circuit 16 is added.

In a video signal recording/reproducing device, the video signal applied to this circuit normally has a constant video signal amplitude in an automatic gain control circuit, so by adding this circuit, for example, feedback clamping can be performed at the synchronization tip. , it becomes possible to align the output potential during muting to the pedestal level.

〔Effect of the invention〕

As described above, according to the present invention, it is possible to obtain a muting circuit that obtains an output in which the clamp level of the Eirin signal and the muting potential are equal.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a block diagram showing another embodiment of the invention, and FIG. 3 is a block diagram showing a conventional example. 1...Input terminal, 2
... Clamp circuit, 3.9 ... Switching circuit, 4, 8 ... DC potential source, 5 ... Mute signal input terminal, 6 ... ...Output circuit, 7...
...Output terminal, 10...Variable DC potential shift circuit, 11...First reference potential source, 12...
... Comparator, 13 ... Sample and hold circuit, 14 ... OR gate circuit, 15 ...
...Terminal, 16...Switching circuit, 17...
...Second reference potential source.

Claims (2)

[Claims] (1) a switching circuit that switches between an input signal and a predetermined DC potential using a mute signal applied to a mute signal terminal;
A variable DC potential shift circuit to which the output of the switching circuit is connected to an input, an output terminal for taking out the output of the DC potential shift circuit via an output circuit, and a comparison between the output of the output circuit and a first reference voltage. and a sample-and-hold circuit that samples the output of the comparator every predetermined period of the input signal, holds it for another period, and feeds it back to the variable DC potential shift circuit. circuit. (2) Claim (1) further comprising means for switching the first reference voltage connected to the comparator to a second reference voltage during the period when the mute signal exists.
Muting circuit described in section.