JPS592226B2 - Video signal processing circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video signal processing circuit for stabilizing the black level of a video signal and reducing low frequency conversion noise in a television broadcast.

Conventionally, a circuit as shown in FIG. 1 has been used as means for fixing the black level of a video signal from an image pickup tube. In this circuit, first, a video signal from an image pickup tube (not shown) is input to an input terminal 1 of a preamplifier 1 having a feedback level setting circuit 2.
1 with DC coupling.

This video signal is amplified by the preamplifier 1, and then the horizontal retrace period is blanked by the blanking gate circuit 6 to remove noise during this period. Next, the video signal output of the blanking gate circuit 6 is input to a video signal black level automatic fixing circuit consisting of a clamp circuit □, a sample hold circuit 8, a high gain amplifier 9, and a blanking gate circuit 10, and the black level is adjusted. Stabilize. but,
In the circuit shown in Figure 1, the DC level of the video signal at the output terminal 12 of the preamplifier 1 fluctuates greatly, making it difficult to stabilize the black level of the video signal using the automatic black level fixing circuit □-10. There was a drawback that black level fluctuations were large.

The present invention is characterized in that it is configured by combining a preamplifier that stabilizes the video signal DC level by a double feedback type and an automatic black level fixing circuit, and its purpose is to stabilize the black level of the video signal, Reduce conversion noise and eliminate it. FIG. 2 shows an embodiment of the present invention, in which 1 is a preamplifier, 2 is a feedback level setting circuit, 3 is a sample and hold circuit, 4 is a high gain comparison amplifier, and 5 is a feedback level setting circuit. The circuit No. 5 constitutes a preamplifier with double DC feedback.

6 is a blanking gate circuit that blanks the horizontal retrace period of the video, □ is a clamp circuit, 8 is a sample hold circuit, 9 is a high gain comparison amplifier, and 10 is a blanking gate circuit. ″ constitutes an automatic fixing circuit for the black level of the video signal.

In FIG. 2, first, a video signal (shown in FIG. 3 (1)) from an image pickup tube (not shown) is inputted to the human power terminal 11 of the preamplifier 1 by direct current coupling.

Here, the J sample and hold circuit 3 is open during the horizontal scanning period of the video signal, and the feedback level setting circuit 2 is the only feedback circuit for the preamplifier 1. On the other hand, the sample hold circuit 3 is closed during the horizontal synchronization period of the horizontal retrace period, and the video signal during this period is sampled and held. Next, this sampled and held voltage is compared with the reduced voltage Es in the high gain comparison amplifier 4, and the error voltage is negatively fed back to the preamplifier 1 through the feedback level setting circuit 5. As a result, fluctuations in the DC level occurring in the preamplifier 1 are reduced, and a video signal with a stabilized DC level is obtained at the output terminal 12.

Next, the horizontal retrace period of this stabilized video signal at the DC level is blanked by the blanking gate circuit 6, and noise (including low-frequency conversion noise caused by the integration operation of the previous stage circuit) during this period is ) is removed, and Figure 3 (2
) is a video signal from which noise during the horizontal retrace period has been removed.

The video signal output of the blanking gate circuit 6 is supplied to video signal automatic black level fixing circuits 7 to 10.

In this circuit, a video signal is clamped to a certain clamp potential by a clamp circuit 7, and a sample/hold circuit 8
A portion of the clamped video signal corresponding to the video signal period of the vertical blanking period is sampled and held, and this sampled and held signal is compared with the reference voltage Es in the high gain comparison amplifier 9, and the error is calculated as the clamped video signal. The potential during the horizontal retrace period of the video signal is clamped to this sampled and held potential. As a result, the DC voltage fluctuation of the video signal at the output terminal 14 of the clamp circuit 7 is further reduced, and the black level of the video signal is always fixed at a constant potential Es as shown in FIG. 3(3). Next, the blanking gate circuit 10 blanks the horizontal retrace period of this video signal using the ramp potential, that is, the reference voltage Es, thereby completely eliminating fluctuations that have occurred from the preamplifier 1 to the clamp circuit 7. A video signal with a stabilized black level as shown in FIG. 3(4) is obtained. That is, DC level fluctuations are reduced in the preamplifier 1 to feedback level setting circuit 5, and noise in the horizontal retrace period of the video signal input is removed by the blanking gate circuit 6, so that the automatic black level fixing circuits 7 to 10 This clamp operation eliminates the generation of low-frequency conversion noise and provides a video signal with a stabilized black level.

As described above, according to the present invention, the stability of the black level of a video signal in a television camera device can be improved, and low frequency conversion noise of the video signal can be completely removed.

[Brief explanation of the drawing]

FIG. 1 shows a conventional black level fixing circuit for a video signal, FIG. 2 shows an embodiment of the circuit of the present invention, and FIG. 3 shows operating waveforms of the circuit of the present invention. 1 is a preamplifier, 2 is a retrace level setting circuit, 3 is a sample and hold circuit, 4 is a high gain comparison amplifier, and 5 is a feedback level setting circuit. 6 is a blanking gate circuit, 7 is a clamp circuit, 8 is a sample and hold circuit, 9 is a high gain comparison amplifier, and 10 is a blanking gate circuit.

Claims (1)

[Claims] 1. The amplifier stabilizes the DC level of the video signal by double DC feedback, the blanking gate circuit removes noise during the horizontal blanking period of the video signal, and the blanking gate circuit eliminates noise during the vertical blanking period of the video signal. A video signal processing circuit comprising a circuit that samples a signal corresponding to a video signal period, clamps the voltage of the horizontal retrace period of the video signal to the sampling signal voltage, and automatically fixes the black level.