JPH01291589A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To improve a state where a convergence time is increased due to excessive dynamic range and to prevent malfunction from being generated by detecting the abnormal operation of a PLL, and limiting the amplification factor of a variable amplifier in an AGC circuit at constant width. CONSTITUTION:The frequency of an oscillator in a PLL circuit which generates a demodulation processing signal in a demodulation circuit is discriminated by a frequency detector, and when the abnormal operation is performed, the amplification factor of the variable amplifier or a gain control circuit is limited at the constant width at a gain limiting circuit, or the amplification factor of the variable amplifier is set at a constant level. It is practicable that a gain change rate limiting circuit 11 by the frequency detector 10 functions directly on the variable amplifier 1, or the gain change rate limiting circuit 11 functions on the gain control circuit 5. Also, the gain change rate limiting circuit can be constituted in the same circuit as that of the gain control circuit 5. Gain limitation in the gain change rate limiting circuit 11 can be performed by providing certain limiting width or fixing it at a constant gain.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an automatic gain control (AGC) circuit during reproduction in a magnetic recording and reproducing apparatus, such as a VTR.

2. Description of the Related Art Conventionally, a color signal reproducing circuit for a VTR has had a configuration as shown in FIG. In FIG. 2, the low-frequency converted signal is regenerated, inputted to the variable amplifier 1 from the terminal A, and returned to the carrier frequency of the television signal, for example, 3.579545M Ilz in the NTSC system, by the demodulation circuit 2. In order to remove unnecessary frequency components generated at the same time during demodulation, the demodulated signal is passed through a bandpass filter that passes the carrier frequency as a frequency selector 3, and level detection for AGC is performed by a detector 4 using the demodulated signal appearing at the terminal C. As a result, the gain adjustment circuit 5
The variable amplifier 1 operates with the value determined by .

In order to create a signal to be used in the demodulation circuit 2, the demodulated signal at the terminal C is compared with the carrier wave reference frequency input from the terminal B at the phase comparator 6, and as a result, the frequency of the oscillator 7 is adjusted and the low frequency Demodulation frequency generation circuit 8 according to the conversion method
A signal is created by Figure 3 shows each state (a), (
It is a symbol schematic diagram in b), (e), and (d).

These operations demodulate the color signal, stabilize the amplitude in the demodulation, and absorb variations due to components.

Problems to be Solved by the Invention In such a conventional configuration, the PLL circuit 9 for demodulation
If the RLL circuit includes a temporally unstable period, such as when the VTR is not operating normally or during special playback where the playback speed is changed, the output amplitude of the frequency selector 3 may change. As a result, the AGC loop operates to increase the amplification factor of the variable amplifier 1. This is shown in Figure 3 (b). However, if the amount of deviation is too large, the amplification factor of the variable amplifier 1 increases. As shown in (C), there is a problem in that the dynamic range of a variable amplifier, etc. is exceeded, the amplitude is limited, and the waveform is distorted.When the dynamic range is exceeded in this way, waveform abnormalities occur, and the PLL loop This can cause a long time to recover, or a stable state of malfunction may occur that cannot be recovered.This is a serious problem, especially when signal processing involves A/D conversion and digital signal processing. .

The present invention solves these problems by detecting abnormalities in PLL operation, limiting the amplification factor of the variable amplifier of the AGC circuit to a certain range, and preventing outputs exceeding the dynamic range of each component. The purpose is to prevent this from occurring in each component.

Means for Solving the Problem In order to solve this problem, the present invention uses a frequency detector to determine the frequency of the oscillator of the PLL circuit that generates the demodulated signal in the demodulation circuit. , the amplification factor of the variable amplifier or the gain adjustment circuit is set to a certain limit, or the amplification factor of the variable amplifier is made constant.

Effect: With this configuration, even if the amount of deviation in the PLL circuit is large, the amplification factor in the AGC circuit is limited, thereby preventing waveform distortion or malfunction from occurring in each component.

Embodiment FIG. 1 is an AGC circuit diagram of a color signal processing circuit of a VTR according to an embodiment of the present invention. In FIG. Alternatively, the gain change rate limiting circuit 11 may act on the gain adjustment circuit 5. Further, the gain change rate limiting circuit can also be configured in the same circuit as the gain adjustment circuit 5. The gain limit in the gain change rate limiting circuit 11 can have a certain limit width or can be fixed to a constant gain.

Effects of the Invention As described above, the present invention is useful for improving the time required for convergence due to dynamic range overflow in an AGC circuit, and for preventing malfunctions. Particularly when digitally processing a video signal, it is useful to prevent malfunctions, increase the margin of bit number of the A/D converter, and effectively utilize the dynamic range.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an AGC circuit according to an embodiment of the present invention, FIG. 2 is a block diagram showing a conventional AGC circuit,
FIG. 3 is a schematic diagram of signals in each state (a) to (d). 1... variable amplifier, 2... demodulation circuit,
3... Frequency selector, 4... Level detector, 5... Gain adjuster, 6... Phase comparator, 7... - Oscillator, 8... Demodulation frequency generator, 9... PLL circuit, 10...
...Frequency detector, 11...Gain change rate limiting circuit, A...Reproduction signal input terminal, B...
Reference frequency input terminal, C...Reproduction output terminal. Name of agent Patent attorney Toshio Nakao One idiot Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] Variable amplifier, demodulation circuit, frequency selector, phase comparator,
a PLL circuit having an oscillator and a demodulation frequency generator;
a frequency detection circuit that detects the signal of the oscillator, a gain change width limiting circuit that is controlled by this detection signal, a level detection circuit that detects the signal of the frequency selector, and a gain that is controlled by the signal of this level detection circuit. An automatic gain control circuit comprising: an adjustment circuit, the gain change width limiting circuit and the gain adjustment circuit being coupled to the variable amplifier.