JPH03231568A - Synchronizing separator circuit - Google Patents

Abstract

PURPOSE:To attain stable synchronizing separation by extracting a current when a synchronizing signal crosses a threshold level of a 1st comparator and the synchronizing signal does not cross a threshold level of a 2nd comparator to prevent a level of a synchronizing signal tip of an inputted video signal from being boosted. CONSTITUTION:Suppose that a brightness of an input video signal is decreased suddenly and the amplitude is decreased, then the level of the clamped SYNC tip is boosted and the level enters between a threshold level of a comparator 2 and a threshold level of a comparator 3, then the two comparators output a high level simultaneously and an AND circuit 4 outputs a high level. Thus, a switch S is closed and a current source extracts the current to decrease the level at a terminal (a) thereby decreasing the clamp level of the SYNC tip. Thus, sure synchronization separation is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a synchronization separation circuit for extracting a synchronization signal from a video signal.

[Prior art 1] Figure 3 is a circuit diagram of a conventional synchronous separation circuit.
1 is a sync tip clamp circuit, and 2 is a comparator. The video signal is input to terminal a of the sync tip clamp circuit 1. At terminal a, the sink chip of this video signal is connected from the potential of constant voltage source Vl to transistor Ql.
The voltage is clamped to a potential lowered by the base-emitter voltage Vbe. This clamped video signal is passed through emitter follower Q2 to transistor Q3 of comparator 2.
input to the base of When the base voltage of the transistor Q3 is lower than the base voltage of the transistor Q4, the comparator 2 outputs the blocking voltage to the terminal. Therefore,
If the potential of the constant voltage source v2 is set so that the threshold value crosses the synchronization signal of the video signal input to the comparator 2, as shown in FIG.
) is output.

[Problem to be solved by the invention]

Conventional synchronization separation circuits are configured as described above, so that when the brightness of the input video signal decreases and the amplitude becomes small, or when the video signal enters the vertical blanking period, the terminal aK There is a problem in that the potential of the sync chip in the comparator 2 rises, and the synchronization signal in the comparator 2 no longer crosses the threshold, making it impossible to perform synchronization separation.

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to provide a synchronous separation circuit that can perform stable synchronous separation.

[Failure to solve the problem]

The sync separation circuit according to this 8AK has a sync tip clamp circuit that clamps the video signal to a certain reference voltage, a threshold between the sync tip level of the clamped video signal, and the base level, and has a sync tip clamp circuit that clamps the video signal to a certain reference voltage. a first comparator to detect; a second comparator having a threshold voltage between the sync tip level of the clamped video signal and the threshold of the first comparator; By increasing the current value of the current source connected to the input terminal of the sync tip clamp circuit when the sync signal crosses the threshold and the sync signal does not cross the threshold of the second comparator. The device is equipped with a circuit that works to lower the clamp potential.

[Function] The synchronization separation circuit according to the present invention prevents the potential of the input video signal sync chip from rising and enables stable synchronization separation.

[Example] Hereinafter, an example of the present invention will be explained with reference to the drawings.
In the figure, 1 is a sync tip clamp circuit, 2 and 3 are comparators, and 4 is an AND circuit. The video signal is input to the terminal a of the sync chip clamp circuit 1. At the terminal a, the sync chip of this video signal is clamped to a potential lower than the potential of the constant voltage source v1 by the pace emitter voltage Vbe of the transistor Ql. . The clamped video signal is input to the base of the transistor Q3 of the comparator 2 through the emitter follower Q, 2t-.

When the base voltage of the transistor Q3 is lower than the base voltage of the transistor Q4, the comparator 2 outputs a high voltage from two terminals. Therefore, as shown in FIG. 2(a), if the potential of the constant voltage source v2 is set so that the threshold value crosses the synchronization signal of the video signal input to the comparator 2, the terminal A synchronizing signal as shown in FIG. 2(b) is output.

The clamped video ff1-f is also input to the base of the transistor Q7 of the comparator 3.

This comparator 3 is active when the base voltage of transistor Q7 is higher than the base voltage of transistor QB.
ii pressure is output, and the polarity is opposite to that of comparator 2. Further, the potential of the constant voltage source v3 is set so that the threshold potential of the comparator 3 is higher than the potential of the clamped sync chip and lower than the threshold potential of the comparator 2. With this setting, the comparator 3 outputs a synchronizing signal as shown in FIG. 2(c) K. Therefore, normally, the outputs of the two compensators do not go high at the same time, and the AND circuit 4 to which these two outputs are connected goes low.
ow i[Outputting pressure. The switch S connected to the output of the AσD circuit 4 indicates that the output of the AND circuit is High.
Set it to turn on when the voltage is reached. As mentioned above, when the output of the AND circuit 4 is at a low voltage, the switch S is otrr, so the current source does not work and this synchronous separation circuit operates in the same way as a conventional synchronous separation circuit.

Assume here that the brightness of the input video signal suddenly drops and the amplitude becomes small. Assume that the potential of the clamped sync chip rises and the sync chip enters between the threshold values of comparator 2 and comparator 3 as shown in FIG. 2(d). At this time, the two comparators simultaneously output High [pressure], and the AND circuit 4 outputs High [pressure]. Therefore, switch S is 0NK
1, the current source pulls a current, which lowers the potential at terminal a, and lowers the clamp potential of the sink chip. When the potential of the sync chip becomes lower than the threshold of the comparator 3 again, the switch S is turned off, so that the potential of the terminal a does not drop too much.

〔Effect of the invention〕

As described above, according to the present invention, the sync tip of the video signal always works to prevent it from lifting up, and synchronization separation is reliably performed.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a synchronous separation circuit showing an embodiment of the present invention, FIG. 2 is a waveform diagram explaining the operation of the circuit of FIG. 1, FIG. 3 is a circuit diagram of a conventional synchronous separation circuit, and FIG. FIG. 4 is a waveform diagram illustrating the operation of the circuit of FIG. 3. In the figure, 1 is a sync chip clamp circuit, 2 and 3 are comparators, 4 is an AND circuit, a%b is a terminal, Q, 1
%=Q 10 is a transistor, ■1%v3 is a constant voltage source, and S is a switch. In addition, in the figures, the same reference numerals indicate the same or equivalent parts. Figure 1: 'l'y'! +tsul"7inpu'oi2.3°funparator 4: ANDI f each Qt~QIO nido -71 register Vt-v3: constant voltage 5 source S: switch 1 day Figure 3 Figure 4 (α) Chair Shi J

Claims (1)

[Claims] a sync tip clamp circuit that clamps the video signal to a certain reference voltage; a first comparator that has a threshold between the sync tip level and the pedestal level of the clamped video signal and detects a synchronization signal; a second comparator having a threshold potential between the sync tip level of the video signal and the threshold of the first comparator; 1. A sync separation circuit comprising a circuit that lowers the potential of a sync chip by pulling a current when a sync signal does not cross a threshold of a comparator.