JPS6131669B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION An object of the present invention is to provide an empty channel discriminating device that can accurately determine whether a television receiver is receiving broadcast waves or is in an empty channel state.

In order to know whether a television receiver is receiving broadcast waves, it is sufficient to detect whether a synchronization signal is present in the composite video signal. However, even when a broadcast wave is not being received on an empty channel, random pulses are present, so a simple synchronization signal determining means cannot determine whether it is an empty channel or not.

The present invention aims to provide an empty channel discrimination device that can accurately discriminate empty channels, and one embodiment of the present invention will be described below with reference to the drawings.

As shown in FIG. 1, a composite video signal is applied to an input terminal, and this is applied to a synchronization separation circuit 2 to separate a horizontal synchronization signal. A first one-shot multivibrator 3 is provided which uses the output pulse of the synchronous separation circuit 2 as a trigger signal. The metastable time of this one-shot multivibrator 3 is set to be slightly longer than the horizontal scanning period, and it is of a type in which it is retriggered every time a trigger pulse is applied.
A second one-shot multivibrator 4 is provided which uses the output of this one-shot multivibrator 3 as a trigger signal. The metastable time of this multivibrator is set to be extremely long, and the level of the output terminal 5 is lowered by being triggered by the fall of the output of the one-shot multivibrator 3.

Next, the operation of this device will be explained. The solid line in FIG. 2a is a horizontal synchronizing signal, and the dotted line is a random pulse generated during an empty channel. In the case of a horizontal synchronization signal, the output of the one-shot multivibrator 3 is as shown in FIG. 2b. That is, since the horizontal synchronizing signal is input before the quasi-stable time of the one-shot multivibrator 3 has elapsed, the output of the one-shot multivibrator 3 is kept at a high level H by being retriggered. However, in the case of an empty channel, the output of the multivibrator 3 becomes as shown in FIG. 2c.
H is the metastable time. Even when the output shown in FIG. 2b is applied to the multivibrator 4, it is not triggered, and since the quasi-stable time is extremely long, it remains at a high level H as shown by the solid line in FIG. 2d. However, when the signal shown in FIG. 2c is applied to the multivibrator 4, the multivibrator 4 is triggered at the falling edge of the signal and becomes low level L as shown by the dotted line in FIG. 2d. Therefore, by applying the output of the multivibrator 4 to, for example, a display, it is possible to clearly indicate that the channel is empty.

As described above, according to the present invention, the first one-shot multivibrator is triggered by the horizontal synchronization pulse that is the output of the synchronization separation circuit, and the falling part of the output of the first one-shot multivibrator is activated. By using a triggered second one-shot multivibrator, it is possible to clearly distinguish between broadcast reception and empty channel, and the output pulse detected by the first one-shot multivibrator Furthermore,
By adding this to the second one-shot multivibrator with an extremely long metastable time, a signal with a constant pulse width can be obtained from the second one-shot multivibrator even if the period of the random pulse generated during an empty channel varies. This makes subsequent circuit design and driving easier. Further, according to the present invention, in a channel where a television signal is present, the horizontal synchronizing signal is output every horizontal scanning period, so that the first one-shot multivibrator is continuously triggered and the second one-shot multivibrator is continuously triggered. is never triggered, whereas in an empty channel there are only noise pulses, and the generation period is random, so there are cases where the interval between occurrences is longer than one horizontal scanning period. In view of this, when pulses are output from the period separation circuit at a generation interval longer than one horizontal period, the output of the first one-shot multivibrator always falls, and at that time the output of the second one-shot multivibrator always falls. By configuring the Yotsuto multivibrator to generate an output when triggered, if such a state with a long occurrence interval, that is, an empty channel state where only noise exists, occurs even once, it is immediately detected as an empty channel. This has the advantage that the detection sensitivity can be extremely high.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an empty channel discriminating device according to an embodiment of the present invention, Fig. 2 a, b, c,
d is a waveform diagram for explaining the device. 1...Composite video signal input terminal, 2...Synchronization separation circuit, 3, 4...One-shot multivibrator, 5...Output terminal.

Claims (1)

[Claims] 1. A synchronization separation circuit that separates a horizontal synchronization signal from a composite video signal;
A one-shot multivibrator is provided, the metastable time of the first one-shot multivibrator is set to be slightly longer than the horizontal scanning period, and the first one-shot multivibrator is triggered at the falling edge of the output of the first one-shot multivibrator. An empty channel discriminating device comprising: a second one-shot multivibrator having an extremely long metastable time; and detecting an empty channel based on the output when the second one-shot multivibrator is triggered. .