JP2873508B2 - FM demodulation circuit - Google Patents

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 reproduction of a conventional FM modulation recording device for an audio signal such as a Hi-Fi VTR.
A (PLL) demodulation circuit is used. At the time of reproduction, disturbance of the reproduced FM signal due to dropout on the tape occurs, and a large noise is generated. Therefore, the noise is prevented from being output by means such as a hold. There is a need. The disturbance of the reproduced FM signal is generally detected by detecting the input FM signal, and when the FM signal is small, the above hold is generally applied.

FIG. 2 is a functional block diagram of such a conventional FM demodulation circuit, wherein 1 is an input terminal, 2 is an output terminal, and 3 is an amplifier for amplifying an input FM modulation signal and limiting the amplitude. Limiter (LIM) for forming a square wave signal
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 '
Output the hold signal.

This circuit uses an FM signal input from an input terminal 1.
The modulated signal is demodulated by a PLL demodulation circuit including a phase comparator 4, a voltage controlled oscillator 5, and an LPF. The reproduced FM signal may be disturbed due to a dropout or the like in a demodulated tape, and noise may be generated. To prevent this, an input FM modulation signal is detected, and when the output level is low, the hold circuit 7 'holds the signal.

[0005]

In the conventional dropout detection circuit, since the ON / OFF 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. In the case where the phase modulation is discontinuous and large noise appears in the FM modulation signal, or when 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 is a disadvantage that a large noise is output.

SUMMARY OF THE INVENTION The present invention provides an F
It is intended to provide an M demodulation circuit.

[0007]

Means for Solving the Problems The output of a voltage controlled oscillator is divided into two outputs having a phase shift of 90 degrees from each other, and PLL demodulation is performed by using one of the outputs and balanced modulation is performed by using the other to obtain an input signal. The problem is solved by an FM demodulation circuit having a sample and hold circuit including not only the size but also the phase information.

[0008]

The second output of the phase shifter at the clock input of the balanced modulator is 90 degrees out of phase with the first output of the phase shifter at 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 90 degrees out of phase.
The modulating signal input is in phase, at which time the output of the balanced modulator is at a 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, so the output of the balanced modulator becomes small, and the discontinuity of the phase can be detected. This will result in a hold. In addition, even when the magnitude of the input FM modulation signal becomes small, the output of the balanced modulator becomes small, so that the dropout can be detected as in the conventional case, and the hold can be applied, thereby preventing the generation of noise.

[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, wherein 9 is a phase shifter (PS), 10 is a balanced modulator (BM), and other symbols are the same as those in FIG. I do.

In 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 have the same phase. The output of the balanced modulator 10 is at a 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 suddenly. Becomes smaller. Therefore, when the output of the balanced modulator 10 is small, if the hold is applied, the output of noise can be prevented even when the phase of the input FM signal is discontinuous. Further, even when 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 performed as in the related art.

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

[0013]

As described above, the present invention can effectively remove noise not only when the input FM signal is lost but also when the phase is discontinuous. There is an effect that a demodulation circuit can be configured.

[Brief description of the drawings]

FIG. 1 is a block diagram of an FM demodulation circuit according to one 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)

(57) [Claims] 1. A voltage controlled oscillator and its output are connected to each other by 90
A phase shifter formed at first and second outputs having a phase difference of degree, and a phase comparator for comparing the phase of the first output with the FM modulation signal to be demodulated and extracting a voltage corresponding to the phase difference A PLL demodulation circuit having its output input to the voltage controlled oscillator via a low-pass filter, and a second output of the phase shifter as a clock input;
A balanced modulator having an M-modulated signal as a signal input; and a sample-and-hold circuit for performing a sample and hold operation on the output of the low-pass filter in accordance with the output of the balanced modulator. An FM demodulation circuit for extracting the FM modulated signal from the sample hold circuit as a demodulated signal.