JPH05175735A - Fm demodulation circuit - Google Patents

Abstract

PURPOSE:To prevent generation of demodulation noise of an FM modulation signal with a discontinuous phase by detecting a dropout of the FM demodulation circuit by a hold circuit depending on an amplitude of the FM modulation signal and phase information. CONSTITUTION:The PLL demodulation circuit is made up of a phase comparator 4, a voltage controlled oscillator 5 and a low pass filter 6, and a 1st output of a phase shifter 9 divides an output of the voltage controlled oscillator 5 to 1st and 2nd outputs whose phase differs by 90 deg. is fed to the phase comparator 4 to implement PLL demodulation. The 2nd output is fed as a clock signal input to a balanced modulator 10 receiving an FM modulation signal, and an output of the low pass filter 6 is extracted as an output of a sample-and-hold circuit 7 sampling-and-holding in response to the output of a balanced modulator 10.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM demodulation circuit for an FM modulation recording device such as a hi-fi VTR.

[0002]

2. Description of the Related Art A conventional phase-locked loop composed of a voltage controlled oscillator, a phase comparator and a low-pass filter is used for reproducing an FM modulation recording device for audio signals such as a hi-fi VTR.
A (PLL) demodulation circuit is used, and during reproduction, the reproduced FM signal is disturbed due to dropouts on the tape, and a large amount of noise is generated, so that such noise is prevented from being output by means such as hold. There is a need. The disturbance of the reproduced FM signal is generally detected by detecting the input FM signal, and the hold is generally applied when the FM signal is small.

FIG. 2 shows a functional block diagram of such a conventional FM demodulation circuit, in which 1 is an input terminal, 2 is an output terminal, 3 is an input FM modulated signal, and the amplitude is limited. Limiter (LIM) that forms a rectangular wave signal by
4 is a phase comparator (PC), 5 is a voltage controlled oscillator (VCO),
6 is a low pass filter (LPF), 7'is a hold circuit (H
OLD), 8 is a dropout detector (DET),
When the input FM modulation signal is small, the hold circuit 7 '
The hold signal is output to.

This circuit has an FM input from the input terminal 1.
The modulated signal is demodulated by the PLL demodulation circuit including the phase comparator 4, the voltage controlled oscillator 5 and the LPF. However, the tape to be demodulated may have dropout or the like, which may disturb the reproduced FM signal and generate noise. In order to prevent this, the input FM modulation signal is detected, and when the output level is low, it is held by the hold circuit 7 '.

[0005]

In the conventional dropout detection circuit, since the on / off state of the hold circuit 7'is determined only by the magnitude of the input FM signal, the magnitude of the signal is not so small. Also, when the phase is discontinuous and a large noise is generated in the FM modulation signal, or when a large noise is superimposed on the input FM signal and the S / N of the input signal is extremely deteriorated, the hold cannot be performed. However, there was a drawback that a large amount of noise was output.

The present invention eliminates the above-mentioned drawbacks.
An object is to provide an M demodulation circuit.

[0007]

The output of a voltage controlled oscillator is divided into two outputs that are 90 degrees out of phase with each other, and PLL demodulation is performed using one of the outputs and balanced modulation is performed using the other to obtain an input signal. This is solved by an FM demodulation circuit having a sample hold circuit including not only the magnitude but also phase information.

[0008]

The second output of the phase shifter for the clock input of the balanced modulator is 90 degrees out of phase with the first output of the phase shifter for the clock input of the phase comparator for PLL demodulation. In the locked state, the clock input of the phase comparator and the FM modulation signal input are out of phase by 90 degrees.
It is in phase with the modulating signal input, at which time the balanced modulator output is maximum. Now, when the phase of the input FM modulation signal changes discontinuously, the phase of the clock input of the balanced modulator does not respond suddenly, the output of the balanced modulator becomes small, and the phase discontinuity can be detected. This will put the hold on. Even when the size of the input FM modulation signal becomes small, the output of the balanced modulator becomes small, so that dropout can be detected as in the conventional case, and noise can be prevented because the hold is applied.

[0009]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of an FM demodulation circuit according to an embodiment of the present invention. Reference numeral 9 is a phase shifter (PS), 10 is a balanced modulator (BM), and other symbols are the same as those in FIG. To do.

With this configuration, the phase comparator 4, the voltage-controlled oscillator 5, and the low-pass filter 6 constitute a PLL demodulation circuit. In the locked state, the two inputs a and b of the phase comparator 4 have the same frequency. , 90 degrees out of phase. On the other hand, since the two outputs B and C of the phase shifter 9 are out of phase with each other by 90 degrees, the clock input C of the balanced modulator 10 and the FM modulation signal input D are in the same phase. The output of the balanced modulator 10 is maximum. Now, when the phase of the input FM modulation signal changes discontinuously, the phase of the clock input of the balanced modulator 10 does not change abruptly, so a phase difference occurs between the two inputs C and D of the balanced modulator 10, and the output Becomes smaller. Therefore, when the output of the balanced modulator 10 is small, if a hold is applied, noise output can be prevented even when the phase of the input FM signal D is discontinuous. Further, even if the input FM signal becomes small due to dropout or the like, the output of the balanced modulator 10 becomes small, so that the hold can be applied as in the conventional case.

The present invention has been described above. However, since the hold can be applied even when the magnitude of the input FM signal does not change much and only the change in the phase is large, the head of the reproducing apparatus of the FM recorded tape is used. It can also be used as a sample and hold circuit as a countermeasure against noise generation due to switching during switching.

[0013]

As is apparent from the above description, the present invention is capable of effective noise removal not only when the input FM signal is missing but also when the phase is discontinuous. There is an effect that the demodulation circuit can be configured.

[Brief description of drawings]

FIG. 1 is a block diagram of an FM demodulation circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional FM demodulation circuit.

[Explanation of symbols]

3 ... Limiter amplifier, 4 ... Phase comparator, 5 ... Voltage controlled oscillator, 7 ... Sample and hold circuit, 8 ... Dropout detector, 9 ... Phase shifter, 10 ... Balanced modulator.

Claims (1)

[Claims] 1. A voltage-controlled oscillator and its outputs are connected to each other.
Phase shifter for forming first and second outputs having a phase difference of 100 degrees, and a phase comparator for comparing the phases of the first output and the FM modulation signal to be demodulated and extracting a voltage according to the phase difference. And a PLL demodulation circuit whose output is input to the voltage controlled oscillator through a low-pass filter, and a second output of the phase shifter as a clock input, and
A balanced modulator having an M-modulated signal as a signal input, and a sample-hold circuit for sampling and holding the output of the low-pass filter according to the output of the balanced modulator are input from the phase comparator. An FM demodulation circuit, wherein the FM modulated signal is extracted as a demodulation signal from the sample hold circuit.