JPH0423345B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic gain control (hereinafter abbreviated as AGC) circuit used in a magnetic recording and reproducing device (hereinafter abbreviated as VTR) for recording and reproducing video signals.

Conventional home VTRs are equipped with an AGC circuit to obtain a constant amplitude video signal from an input video signal whose amplitude is fluctuating, and can monitor the input video signal as it is on the TV, as well as recording good frequency modulation of the luminance signal. is possible.

FIG. 1 shows a circuit diagram of a conventional VTR equipped with an AGC circuit, and FIGS. 2 and 3 show waveform diagrams for explaining the operation of FIG. 1. In FIG. 1, the operation during recording will first be explained.

The input video signal 1 passes through the AGC amplifier circuit 2, and its amplitude is controlled to a constant level. Next, it passes through a low-pass filter (hereinafter abbreviated as LPF) 3 for color signal removal, and the luminance signal 5 shown in FIG. 2a is obtained at the output end of the buffer amplifier 4. The luminance signal 5 is supplied to a synchronizing signal separating circuit 7 via a switch circuit 6 which is switched depending on playback and recording, and is sent to the output terminal of the synchronizing signal separating circuit 7 as shown in FIG.
A synchronizing signal 8 as shown in FIG.

On the other hand, the luminance signal 5 is supplied to a clamp circuit 9. The clamp circuit 9 operates using the synchronization signal 8 as a key pulse so that the tip of the synchronization part of the luminance signal 5 is fixed at a predetermined DC voltage 10, as shown in FIG. 2c. In this way, the output signal 13 of the clamp circuit 9 is supplied to the frequency modulation circuit 11 and the test wave signal generation circuit 12, which are comprised of a voltage-to-frequency converter and which require a luminance signal with a specified DC level. The frequency modulation circuit 11 and the test wave signal generation circuit 12 operate normally.

Here, the delayed pulse generator 14 generates a delayed pulse 16 delayed by a predetermined time 15 from the trailing edge of the input synchronizing signal 8, as shown in FIG. 2d. In addition, in the test wave signal generation circuit 12, the input luminance signal 13 is amplified to a predetermined times (140/40 times in the case of an NTSC signal) the level difference with the tip of the synchronization part only during the period when the delayed pulse 16 is input. It works like this. By the way, since the delayed pulse 16 is in the back porch period, the synchronization part level 17 of the input luminance signal 13 is amplified 140/40 times. This 140/40 times amplified synchronization section level is
For NTSC signals, the level is 100% white. Therefore, from the test wave signal generation circuit 12,
A test wave signal 1 as shown in FIG.
8 is output.

Note that the above-mentioned test wave signal generation circuit 12 is well-known, and includes, for example, Hitachi linear IC, HA11725,
HA11745 etc. can be used.

Next, the test wave signal 18 is transmitted to the AGC detection circuit 1
9, and the AGC control signal 20 is detected.
The signal is input to the amplifier circuit 2.

In the conventional AGC circuit as described above, as the amplitude of the input video signal 1 increases, the voltage of the additional pulse increases, the AGC control signal 20 increases, and the gain of the AGC amplifier circuit 2 decreases. Negative feedback control. Conversely, when the amplitude of the input video signal 1 becomes smaller, the voltage of the additional pulse becomes lower, and the AGC control signal 20 becomes lower.
The gain of the AGC amplifier circuit 2 is controlled to be large. Thus, the synchronization part level of the video signal is always controlled to have a constant amplitude, or when the synchronization part level is smaller than a predetermined ratio, the amplitude of the video signal is controlled to be constant.

By performing such an AGC operation, the frequency modulation circuit 11 can generate frequencies corresponding to each level of the video signal within a predetermined band, making it possible to record good brightness signal frequency modulation. There is.

Further, the output 21 of the AGC amplifier circuit 2 is supplied to an external television via a switch circuit 22 that switches between recording and reproduction, and an RF converter 23, and is monitored on the television.

On the other hand, during reproduction, the supplied reproduction luminance signal 2
4 and the reproduced color signal 25 are superimposed by the mixer circuit 26 to form the reproduced video signal 27, which is then output to the switch circuit 2.
2. It is observed on a television via the RF converter 23. The reproduced luminance signal 24 is also supplied to the synchronization signal separation circuit 7 via the switch circuit 6 to obtain a synchronization signal. Note that during playback, the switch circuits 6 and 22 are connected to the upper contacts in FIG.

By the way, in such conventional technology, when switching from playback to recording, the RF converter 2
The disadvantages were that the video signal input to 3 had a large amplitude, causing the entire screen to turn white and causing overmodulation on the television, producing buzzing noise.

The reason why such a defect occurs will be explained in detail below with reference to FIG. In Figure 3, time
Time from t ₀ to t ₁ represents the time of reproduction, and time after time t ₁ represents the time of recording. The reproduced video signal 27 is generated only during reproduction, as shown in FIG. 3a. By the way, in order to prevent the recording video signal from leaking into the reproduced image and as a measure to reduce power consumption, circuits that are not used during reproduction, such as the AGC detection circuit 19 and the detected wave generation circuit 12, are rendered inactive. Therefore, the AGC control signal 200 is
As shown in FIG. 3b, it does not occur during reproduction, but increases slowly in accordance with the time constant of the AGC detection circuit when switching from reproduction to recording. Therefore, the amplitude of the monitor video signal 21 that is the output of the AGC amplifier circuit 2 changes in accordance with the AGC control signal 20, as shown in FIG. 3c. Then, the input signal of the RF converter 23 undergoes a significant amplitude change at the time of switching, as shown in FIG. 3d.

Note that even if an attempt is made to eliminate the above drawback without making the AGC detection circuit 19 etc. inoperative during playback, the synchronization signal 8 corresponding to the input video signal 1 cannot be obtained from the synchronization separation circuit 7 during playback, and the AGC operation cannot be performed normally. Therefore, good screen switching cannot be achieved.

An object of the present invention is to eliminate the above-mentioned drawbacks of the prior art and provide an AGC circuit for a magnetic recording and reproducing device that does not generate buzzing noise and provides a good screen even when switching from a reproducing state to a recording state. There is a particular thing.

A feature of the present invention is that the output of the AGC detection circuit is supplied as the control signal for the AGC amplifier circuit during recording.
In addition to performing AGC operation, a signal of a certain constant voltage is supplied during reproduction, thereby significantly reducing the fluctuation in the control signal when switching from reproduction to recording.

The present invention will be explained below using examples. Fourth
The figure shows a block diagram of one embodiment of the present invention.
This figure is a signal waveform diagram of the main part of the block diagram for explaining the operation of the embodiment shown in FIG. 4. In FIG. 4, numeral 28 is a switch circuit that is switched for recording/reproduction, 29 is a reference voltage source, and other symbols are the same as or equivalent to those in FIG. 1.

Next, the operation of this embodiment will be explained. During recording, the output 20 of the AGC detection circuit 19 is selected by the switch circuit 28, and as in the conventional case,
The signal is supplied to the AGC amplifier circuit 2 and performs normal AGC operation.

On the other hand, during reproduction, the reference voltage source 29 is selected by the switch circuit 28 and supplied to the AGC amplifier circuit 2. The voltage of the reference voltage source 29 is
The approximate average value of the voltage output from the AGC detection circuit is given. Therefore, even when switching from the playback state to the recording state, the AGC control voltage fluctuation is much smaller than before, so the RF converter 23
The change in amplitude of the video signal input to the switch also becomes smaller at the time of switching.

The operation of this embodiment will be further explained in detail using the waveform diagram shown in FIG. FIG. 5 shows a situation where the AGC control voltage signal 20 and the RF converter input 30 are switched from the playback state to the record state. Fifth
Figures a, b, and c show cases in which the amplitude of the input video signal 1 at the switching point is smaller than the average, average, and larger than the average, respectively.

The output of the AGC detection circuit 19 is the reference voltage source 2
9, the gain of the AGC amplifier circuit 2 at the time of switching does not change, as shown in FIG. 5b.
Therefore, the screen changes smoothly.

When the input video signal is smaller or larger than the average, some gain changes occur during switching, as shown in FIGS. 5a and 5c. However, the changes in AGC control voltage 20 are much smaller than in the past, and the level changes appearing at the RF converter input 30 are significantly reduced than in the past.

FIG. 6 shows a specific example of the AGC detection circuit 19, switch circuit 28, and reference voltage source 29 suitable for integration. 31 indicates an integration range. 32-4
3 is a transistor, 44 to 50 are resistors, and 51 is an external capacitor. 32 to 52 constitute an AGC detection circuit 19, a reference voltage source 29, and a switch circuit 28. Note that 52 is a power supply voltage line for driving the circuit, and 53 is a low level signal during reproduction and a high level signal during recording.

In this circuit, during reproduction, the signal 53 is at a low level, so the transistor 39 is turned off. Therefore, the transistor 33 becomes conductive and the test wave signal 18 output from the test wave signal generation circuit 12 is short-circuited to ground. Further, the transistors 40 and 42 are turned on and the transistor 41 is turned off, and a predetermined voltage determined according to the resistors 45 and 46 is input to the AGC amplifier circuit 2. That is,
A reference voltage is input to the AGC amplifier circuit 2.

On the other hand, during recording, the signal 53 is at a high level, so the transistor 39 is turned on. Therefore, the transistors 33, 40, and 42 are turned off, and the transistors 32 and 41 are turned on, so that the test wave signal 1
8 is smoothed by capacitor 51, and as AGC control voltage,
The signal is supplied to the AGC amplifier circuit 2.

As shown in FIG. 6, this specific example has a configuration suitable for integration, and can realize small size, light weight, and low cost.

As described above, according to the present invention, it is possible to significantly reduce the change in the AGC control signal when switching from the playback state to the recording state, so it is possible to significantly reduce the amplitude fluctuation of the video signal that occurs at the time of switching, and the generation of buzzing noise and screen It has the effect of eliminating the deterioration of

[Brief explanation of drawings]

Figure 1 is a conventional VTR AGC circuit, Figures 2 and 3 are waveform diagrams for explaining the operation of the conventional circuit in Figure 1, and Figure 4 is a VTR showing an embodiment of the present invention.
5 is a waveform diagram explaining the operation of the embodiment of the present invention shown in FIG. 4, and FIG. 6 is an AGC circuit diagram of the present invention.
FIG. 2 is a circuit diagram showing a specific example of an AGC detection circuit, a switch circuit, and a reference voltage source. 1...Input video signal, 2...AGC amplifier circuit, 3...
LPF, 4... Buffer amplifier, 7... Synchronous signal separation circuit, 9... Clamp circuit, 11... FM modulation circuit, 12... Test signal generation circuit, 14... Delay pulse generator, 19... AGC detection circuit, 6, 22 ,28
...Switch circuit, 29...Reference voltage source, 23...RF
Converter, 24...playback video signal.

Claims (1)

[Claims] 1. A variable gain amplifier 2 that amplifies a video signal to be recorded or reproduced; and a test wave signal generation circuit 12 that performs waveform processing on the video signal amplified by the variable gain amplifier to generate a test wave signal. a first transistor 41 having a base to which the signal to be detected is supplied and an emitter to which a peak detection capacitor is connected; a collector-emitter path connected in parallel to the peak detection capacitor; a second transistor 42 forming a discharge path for the capacitance, and a collector-emitter path connected in parallel with the collector-emitter path of the first transistor;
a third transistor 40 whose base is directly connected to the base of the second transistor; a control means 39 which is connected to the bases of the second and third transistors and controls the base potential to be low during recording and high during reproduction; means 33 for lowering the base potential of the first transistor as the base potential decreases; during recording, the second and third transistors are turned off and the first transistor performs a peak detection operation, and for reproduction. A magnetic recording and reproducing device characterized in that the second and third transistors are sometimes turned on to fix the emitter potential of the first transistor, and gain control means controls the gain of the variable gain amplifier using the emitter potential. Automatic gain control circuit of the device. 2. The base potentials of the second and third transistors during reproduction are determined such that the emitter potential of the first transistor, which is fixed during reproduction, is approximately the average value of the emitter potential of the first transistor during recording. An automatic gain control circuit for a magnetic recording/reproducing apparatus according to claim 1.