JPH0419883Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] This invention relates to a level adjustment circuit for video signals, and in particular to a video synchronization signal expander that expands a synchronization signal included in a video signal to a predetermined level. It is related to.

[Background technology and its problems]

Video signals reproduced from magnetic recording and reproducing devices (VTRs), etc., are subject to distortion in the recording/reproducing system, nonlinear characteristics of demodulation circuits, and dynamic range characteristics of amplifiers, clamp circuits, etc. There is a "sink blockage" where the signal level shrinks.

When such a sink jam occurs, for example, the first
As shown in the figure, it is composed of an amplifier 1, a clamp circuit 2, and a clip circuit 3.
A sink that automatically controls the video signal level using the synchronization signal level clipped by
When the video signal [A] causing the sink jam is input to the AGC circuit, the gain of amplifier 1 will increase, so the white level of the output video signal [B] will be higher than the predetermined level. A phenomenon occurs.

Then, when such a video signal [B] is modulated by the FM modulation circuit 4 and recorded again, the frequency deviation becomes excessive, causing a deterioration of the S/N ratio and the occurrence of a moiré phenomenon.

Furthermore, even if the image is clipped at the white level, there is a problem in that the image becomes smeared and waveform characteristics such as Kpd deteriorate.

[Purpose of invention]

This invention was made in view of the above-mentioned circumstances, and is intended to provide a video synchronization signal expander for eliminating synchronization clogging of synchronization signals included in video signals.

[Summary of the idea]

In this invention, a negative feedback circuit is configured by a buffer amplifier provided with a high-impedance emitter resistor for an amplifier that amplifies a video signal with a gain of A, and the buffer amplifier has a feedback rate of β during the video signal period. The negative feedback amplifier is configured such that the feedback rate decreases to β-x during the synchronization signal period.

In the video synchronization signal expander of this invention, the input video signal intentionally causes a sink blockage in the negative feedback circuit, so this sink blockage becomes the amount of feedback to increase the gain of the negative feedback amplifier. The synchronizing signal portion of the video signal that has been processed is expanded.

Also, when the video signal expanded by this synchronization signal is input to the sink AGC circuit, a constant V/S ratio (V
A standard video signal is obtained in which S is a picture level and S is a sync level.

〔Example〕

FIG. 2 shows a block diagram of a video synchronizing signal expander showing an embodiment of this invention, in which 10 is an input terminal, 11 is an adder circuit, 12 is an amplifier, 13 is a sink blockage circuit, and 14 is a negative feedback circuit. signal generation circuit,
15 indicates an output terminal.

In this circuit, when the gain of the amplifier 12 is A, and the feedback rate of the feedback path consisting of the sink blockage circuit 13 and the negative feedback signal generation circuit 14 is β, the gain G when it normally functions as a negative feedback amplifier is G=A/ 1-Aβ
become.

By the way, in this invention, a sink blockage is intentionally generated in the video signal component fed back by the sink blockage circuit 13 inserted in the feedback path.

That is, as shown in FIG. 3, the amplified waveform of the input video signal V _io is converted in the sink blockage circuit 13 into a signal in which only the synchronizing signal part is compressed, as seen in the feedback voltage V _K , and this signal is Please leave negative feedback. Therefore, the feedback rate is, for example, β during the video signal period T _p , but changes to β−x during the synchronization signal period T _H. x changes depending on the synchronization signal level of the input video signal.

Therefore, the video synchronization signal expander of this invention operates so that the gain G becomes G=A/1-A(β-x) during the synchronization signal period T _H.

FIG _. 4 shows a simple negative feedback amplifier circuit showing an embodiment of _the video synchronization signal expander _of this invention. 1
2 is formed. The signal at output terminal 15 is a resistance
The average video signal level (APL) is fed back by r ₁ , r ₂ , r ₃ , r ₄ and capacitor C ₁ .

Further, a feedback path for the AC signal is formed by the emitter follower-connected transistor Q ₄ , resistor r, and capacitor C ₂ .

However, in this feedback path, the emitter resistance R _E of transistor Q ₄ has a high impedance, and the capacitor C _L is connected to the base of transistor Q _2.
is added. Therefore, when a part of the synchronization signal that suddenly becomes a negative level is input, the potential of the capacitor C _L does not change as it follows this negative level, and during this period, the transistor Q ₄ is almost in a cut-off operation. The amount of feedback decreases.

Therefore, a sync jam of the video signal occurs in this feedback path, and the gain of the amplification system becomes high during the synchronization signal period _TH .

Note that S represents a constant current source, and R _L represents a load resistance.

Nonlinear active elements may be used in the circuit that causes the sink blockage.

[Effect of idea]

As explained above, the video signal expander of this invention has a direct current feedback circuit that feeds back the average video signal level to the feedback path of the negative feedback amplifier, and a synchronization signal part where the signal level suddenly drops to the negative. An AC feedback circuit is provided, and this AC feedback circuit causes the sink to become clogged and returns the video signal so that the signal is expanded during the synchronization signal period, which is particularly useful when playing back a VTR. So, APL
It is possible to effectively extend the sync jam of the video signal, which tends to occur almost in proportion to the height of the video signal. Therefore, when combined with a sync AGC circuit, the video level becomes constant, which has the advantage of preventing deterioration of waveform characteristics in the VTR recording/playback system.

Furthermore, when combined with a synchronizing signal separation circuit, stable operation is achieved even in the face of threshold level fluctuations.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram for explaining the operation of the sink AGC circuit, Fig. 2 is a block diagram of the video synchronization signal expander of this invention, Fig. 3 is a waveform diagram explaining the sink jam waveform and feedback rate, and Fig. 4 The figure is a circuit diagram showing an embodiment of this invention. In the figure, 10 is an input terminal, 11 is an adder circuit, 12
is an amplifier, 13 is a sink blockage circuit (element), 1
4 indicates a negative feedback signal generation circuit.

Claims (1)

[Claims for Utility Model Registration] A video synchronization signal expander comprising an amplifier for amplifying the level of an input video signal and a negative feedback circuit connected between the output of the amplifier and the input end, The circuit includes a DC feedback circuit that feeds back the average video signal level of the input video signal, and an AC feedback circuit that is configured to reduce the amount of feedback when the signal level at the output terminal of the amplifier suddenly changes to a negative level. a feedback circuit, wherein the feedback rate of the negative feedback circuit is configured to be lower in a synchronization signal period of the input video signal than in a video signal period.