JPS6387085A - Agc circuit - Google Patents

Abstract

PURPOSE:To obtain a stable AGC characteristic in any condition by widening the width of a drop-out detecting pulse and holding a peak AGC detecting output at the time of a variable speed reproducing mode of a cue, review and so forth. CONSTITUTION:A reproducing video signal (RF signal) supplied to an input terminal 1 is supplied through a head amplifier to an HPF3 and a drop-out detecting circuit 4 to separate a Y signal component. The Y signal separated by the HPF3 is demodulated by an FM demodulator 5 and sent through a de-emphasis circuit 6 and a comb.line filter 7 to a voltage control type amplifier VCA 11. At a VCA 11, an AGC loop is provided, a drop out detecting pulse is inputted, and then, an AGC voltage is held only for the section. Here, at the time of the variable speed reproduction, the drop-out detecting circuit 4 makes the output pulse width wider than the time of the usual reproduction and secures a stable AGC action.

Description

DETAILED DESCRIPTION OF THE INVENTION A. Industrial Application Field The present invention relates to an AGC circuit for video signals, and in particular, performs peak AGC operation on a reproduced video signal obtained by variable speed reproduction by a video tape recorder or the like. This invention relates to an AGC circuit that performs.

B0 Summary of the Invention The present invention holds the AGC voltage of the video signal using a dropout detection signal or a dropout compensation signal of the reproduced video signal in a peak AGC circuit that performs peak level detection and applies AGC to the reproduced video signal. In addition, during variable speed playback such as so-called cue operation or revier operation, the pulse width of the dropout detection signal is widened and the AGC voltage is held only during this pulse width to prevent malfunction of the AGC circuit due to noise bars. It is.

C0 Conventional technology - In IG, so-called peak A is applied to a video signal obtained from a video signal reproducing device such as a VTR (video tape recorder).
In the AGC circuit for applying GC, the AGC circuit detects a dropout, a so-called dropout, in the reproduced video signal from a VTR, etc., and holds the AGC voltage at a constant value during the period where the dropout occurs. This stabilizes the gain of the variable gain amplifier, etc.

D1 Problems to be Solved by the Invention Incidentally, in such a peak AGC circuit, when the above-mentioned VTR or the like performs so-called cue operation, review operation, or variable speed playback such as quick motion playback or reverse playback, the following occurs. Problems like this arise.

That is, first, a peak detection operation occurs due to the noise bar during variable speed playback, and peak ACC is applied by this noise bar, and the gain of the variable gain amplifier etc. in the AGC circuit decreases, causing the level of the actual playback video signal to decrease. decreases.

Next, even in this noise bar section, if the AGC voltage is held according to the dropout detection signal, the time difference between the dropout pulse and the video signal at the point where actual AGC detection is performed will cause , peak AGC operation is performed to some extent, and complete hold cannot be performed.

In order to compensate for the above-mentioned time lag, if the dropout detection pulse generation timing is advanced and the rise is made steeper, the AGC hold will easily be applied due to noise etc. even during normal operation, and the screen will not be displayed. have a negative impact.

The present invention has been made in view of the above circumstances, and provides stable peak AGC with a simple circuit configuration even for video signals obtained by variable speed playback such as during so-called cue and review. The purpose is to provide AC and C circuits that enable operation.

Means for Solving the E0 Problem In order to solve the above-mentioned problems, the AGC generation circuit according to the present invention provides an AGC generating circuit that performs an AGC operation on a reproduced video signal from a video signal reproducing device by peak level detection.
The circuit includes means for holding the AGC voltage in response to the dropout detection signal of the reproduced video signal, and switching means for widening the pulse width of the dropout detection signal in variable speed reproduction mode than in other modes. It is characterized by consisting of

The dropout pulse generation operation is switched depending on whether the F0 effect video signal is one obtained by variable speed reproduction or another normal video signal. Stable A by widening the width
GC operation is ensured.

G. Example Hereinafter, regarding an AGC circuit as an example of the present invention,
This will be explained with reference to the drawings.

FIG. 1 shows a so-called 81VTR (
1 is a block circuit diagram schematically showing the basic configuration of an AGC circuit and its neighboring circuits used in a video signal reproducing device such as an 8 mm video tape recorder. The video signal reproducing device in this case is capable of reproduction in variable speed reproduction mode (so-called jog mode) such as so-called cue, revie, slow motion run reproduction, quick motion reproduction, or reverse reproduction. A reproduced video signal from a video head or the like is supplied to an input terminal 1 shown in FIG. 1, and a variable speed reproduction mode identification signal indicating whether or not the variable speed reproduction mode is selected is supplied to an input terminal 2.

The reproduced video signal (so-called reproduced RF signal) supplied to the input terminal 1 passes through a head amplifier and is passed through an HPF (bypass filter) 3 for separating the Y signal (luminance signal) component.
and is supplied to the dropout detection circuit 4. HPF3
The Y signal component separated in type amplifier) 11. This VCA II constitutes an AGC loop together with a clamp circuit 12 and a peak AGC detection circuit 13. Here, regarding the AGC-14 pressure of this AGC loop, when the dropout detection pulse is input,
The AGC voltage is held only during that pulse period. This means that if the above peak detection is performed even when the noise bar when playing in the variable speed playback mode (so-called jog mode) or the noise in the normal dropout section occurs, the AGC will be activated at the level of the noise. In consideration of the fact that the level of the video signal decreases due to the peak detection, the above-mentioned adverse effect is prevented by holding the AGCt pressure and sending it to the VCA II instead of the above-mentioned peak detection.

In the example shown in FIG. 1, a changeover switch 14 is provided at the gain control terminal of VCAII, and the dropout detection circuit 4
The configuration is such that the output from the AGC detection circuit 13 and the output from the voltage source 15 are switched depending on the output from the AGC detection circuit 13 and sent to the control terminal of the VCA II. That is, when a dropout detection pulse is input from the dropout detection circuit 4, the changeover switch 14 is switched to the voltage source 15 side, and the AGC voltage is held at a constant DC level.

In addition, the AGC detection circuit 13 itself may be provided with a DC level hold function to stop the peak detection operation and hold the Acct pressure when a dropout detection pulse is input. The output is taken out from the output terminal 8.

Next, the dropout detection circuit 4 generates a pulse in the noise section of the reproduced video signal (reproduced RF signal), and an example of a specific circuit configuration thereof is shown in FIG. is configured to switch the output pulse width between normal playback and variable speed playback, and in order to switch the pulse width, for example, according to the variable speed playback mode identification signal supplied to input terminal 2. The capacitor of the time constant circuit 21, the voltage source of the reference level generating circuit 22, etc. are respectively switched by changeover switches 23 and 24. Here, the variable speed reproduction mode identification signal is assumed to be "H" (high level) during the variable speed reproduction mode.

Specifically, as shown in FIG. 2, for example, the transistor 41 is connected between a transistor 41 whose base is connected to the input terminal 4A of the dropout detection circuit 4 and a differential transistor pair 42 for level comparison. In the time constant circuit 21, a series connection circuit of a capacitor C2 and a switching transistor 23° is connected in parallel to the capacitor C1, and the variable speed reproduction mode identification signal (“H” in variable speed reproduction mode) from the input terminal 2 ) is inverted by an inverter 25 (“L” in variable speed reproduction mode) and supplied to the base of the switching transistor 23'. Therefore,
In the normal playback mode, the transistor 23'' is on and the time constant is determined by the parallel connection capacitance of the capacitors CI and 02, whereas in the variable speed playback mode, the switching transistor 23'' is turned off and the capacitor CI is disconnected. As a result, only the capacitor C3 becomes effective, reducing the time constant and producing an effect of making the rise of the pulse steeper.Also, the comparison target is connected to the other input side of the differential transistor pair 42 via the transistor 43. Reference level generation circuit 2 for supplying the reference level
2 consists of resistors R1 and R2 for voltage division, and a series circuit of a resistor R1 and a switching transistor 24' connected in parallel to the resistors R, and the variable speed reproduction mode identification signal is connected to the base of the switching transistor 24'. supplying. Therefore, in the normal playback mode, the transistor 24' is off and the reference level to be compared is determined only by the voltage dividing resistors R, Rg, whereas in the variable speed playback mode, the transistor 24' is on and the resistance By connecting R2 and resistor R1 in parallel, the reference level is lowered and the fall timing of the output pulse is delayed.

As a result of the switching operation described above, the pulse width of the dropout detection pulse obtained at the output terminal 4B of the dropout detection circuit 4 is, as shown in FIGS. 3 and 4, more normal in the variable speed playback mode. wider than in playback mode. That is, FIG. 3 shows the signal waveform in the normal playback mode, and FIG. 4 shows the signal waveform in the variable speed playback mode, both of which are waveform A.
indicates the video signal supplied to the VCA II of the AGC loop, and waveform B indicates the dropout detection pulse from the output terminal 4B of the dropout detection circuit 4, respectively. As is clear from these FIGS. 3 and 4, the dropout sections T and P that occur in the playback video signal (third doujinshi) during normal playback are short, whereas in the variable speed playback mode, As shown in FIG. 4A, a continuous wide noise bar section T□ occurs. In this case, if the pulse width T of the dropout detection pulse is set to the same level as in the normal case above, the time delay of the reproduced video signal up to the VCA II shown in Figure 1, the transient part of the pulse switching, etc. Since the pressure may not be able to be held in Aacl, a phenomenon occurs in which the video signal level after passing through AGCill is reduced to some extent. Considering this point, in the variable speed playback mode, the dropout detection circuit 4 shown in FIGS.
By switching the time constant of the time constant circuit 21 to a shorter value to advance the timing of the leading edge of the pulse, and by switching the reference level for comparison in the circuit 22 to a lower side to delay the timing of the trailing edge of the pulse, the noise bar section TN3 is A dropout detection pulse with a pulse width T as shown in FIG. 4B, which is wider than that shown in FIG. 4B, is generated.

By the way, even in cases other than the above-mentioned variable-speed playback mode, such as during normal forwarding, the same as in the above-mentioned variable-speed playback mode,
It is conceivable to shorten the time constant of the time constant circuit 21 and lower the reference level for comparison.

However, in general, the dropout detection level setting etc. has many delicate parts depending on circuit conditions, reproduced signal conditions, etc., and the time constant of the time constant circuit 21 and the reference level of the reference level generation circuit 22 during normal operation. etc. are pre-adjusted values that maintain a certain degree of accuracy, so if the same time constants and reference levels as in the variable speed playback mode are used during normal playback, the dropout detection operation settings themselves will change. For example, a dropout detection pulse may easily occur due to simple noise, and a normal dropout detection operation may not be performed. From this, only in the variable speed playback mode mentioned above,
The above-mentioned time constant, reference level, etc. are controlled by switching.

Note that the present invention is not limited to the above-mentioned embodiments, and for example,
Instead of holding at a constant DC level, the GC4 pressure may be held by holding the previous peak level detection output as it is, or by changing the width of the dropout detection pulse in variable speed playback mode. The configuration for expanding is not limited to the illustrated example, and it goes without saying that various configurations are possible without departing from the gist of the present invention.

H0 Effects of the Invention According to the AGC circuit according to the present invention, dropout detection during normal dropout detection operation and peak AGC40 wave output hold operation is not affected in any way, and dropout is detected during variable speed playback modes such as cue and review. By widening the width of the out detection pulse and holding the CC detection output to the peak, stable AGC characteristics can be obtained under any conditions.

[Brief explanation of the drawing]

FIG. 1 is a block circuit diagram showing the basic configuration of an embodiment of the AGq circuit according to the present invention, FIG. 2 is a circuit diagram showing a specific configuration example of the dropout detection circuit of the embodiment, and FIGS. FIG. 4 is a time chart for explaining the operation of this embodiment. 1... Reproduction RF signal input terminal 2... Variable speed reproduction mode identification signal input terminal 4... Dropout detection circuit 11... (7) VCA for AGC 13... AGC detection circuit 14.23.24 ...Selector switch 15...Hold voltage source 21...Time constant circuit 22...Reference level generation circuit

Claims (1)

[Claims] In an AGC circuit that performs an AGC operation on a reproduced video signal from a video signal reproducing device by peak level detection, A
An AGC circuit comprising: means for holding a GC voltage; and switching means for widening the pulse width of the dropout detection signal in variable speed reproduction mode than in other modes.