JPH0754612B2 - FM demodulation circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a frequency demodulation signal demodulation circuit having a noise reduction circuit (noise suppression circuit), that is, an FM demodulation circuit. And an FM demodulation circuit suitable for a video recorder that reproduces a high S / N Hi-Fi audio signal by frequency multiplexing and recording and reproducing.

[Background of the Invention]

FIG. 3 is a block diagram showing a configuration of an FM demodulation circuit in a reproduction FM audio circuit of a conventional video tape recorder (hereinafter abbreviated as "VTR"), where 1 is an input terminal of a reproduced FM audio signal and 2 is an input terminal. FM demodulator, 3 is a low-pass filter, 4 is a pre-value holding circuit, 5 is a noise reduction circuit, 6 is a voltage controlled amplifier (VCA), and 7 rectifier.

In the figure, the previous value holding circuit 4 holds the demodulated output of the demodulator 2 at the level immediately before the dropout occurs during the FM audio signal dropout occurrence period.

The noise reduction circuit 5 controls the gain of the amplifier 6 by the control signal obtained by rectifying the audio signal by the rectifier 7.
The output audio signal is expanded.

Next, the operation of this prior art will be described. The FM audio signal input to the input terminal 1 contains a lot of FM carrier components. For this reason, this voice signal as it is, the previous value holding circuit 4
When the dropout is compensated by supplying the signal to the audio signal, the FM carrier component included in the audio signal may hold the previous value, resulting in an erroneous signal for the audio frequency. Will be held at the level. The fourth
The characteristic curve A in the figure shows the frequency characteristic of the two output signals of the FM demodulator. Therefore, the low-pass filter 3 that removes the FM carrier component and the noise component of the high frequency from the input signal of the previous value holding circuit 4 is provided between the demodulator 2 and the previous value holding circuit 4.

However, if the low-pass filter 3 is provided with a cutoff characteristic that passes only the audible frequency band as shown by the curve B in FIG. 4, the transient response to noise components deteriorates. As shown, the previous value holding width has to be made larger than the width of the actual dropout period, and as a result, the audible sound quality is deteriorated. Therefore, conventionally, in order to avoid mis-holding of the previous value holding circuit 4 as much as possible and to prevent sound quality deterioration due to an increase in holding width, the pass characteristic of the low-pass filter 3 is shown in FIG. By making the characteristic curve C, a dropout-compensated audio signal is obtained from the previous value holding circuit 4 as shown in FIG. 5 (b). This audio signal is supplied to the noise reduction circuit 5 and expanded. However, the noise component input to the noise reduction circuit 5 includes noise D outside the audible frequency band due to FM triangular noise, as shown in FIG. Therefore, the rectifier 7 in the noise reduction circuit 5 malfunctions, and the extension characteristic is
As indicated by the broken line E in FIG. 7, there is a phenomenon that the ratio between the input level Vin and the output level Vout becomes 1: 2, which deviates from the original characteristic shown by the solid line F in FIG.

In the video recorder, the FM audio demodulation circuit that holds the audio signal level immediately before its occurrence in order to compensate for the dropout is the one applied to the video disk recorder. See the bulletin.

[Object of the Invention]

The present invention, except for the above-mentioned drawbacks of the prior art, makes it possible to prevent erroneous dropout compensation operation in the previous value holding circuit and erroneous operation of the noise reduction circuit.
To provide FM demodulation circuit.

[Outline of Invention]

To achieve this object, the present invention provides a first low-pass filter optimized between the FM demodulator and the previous value holding circuit so as to prevent malfunction of the previous value holding circuit, A feature is that a second low-pass filter that removes noise that cannot be removed by the first low-pass filter is provided between the previous value holding circuit and the noise reduction circuit.

Example of Invention

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

FIG. 1 is a block diagram showing an embodiment of an FM demodulation circuit according to the present invention, in which 9 to 11 are resistors, 12 and 13 are capacitors, and 1
Reference numeral 4 is a transistor, 15 is a resistor, 16 is a capacitor, 17 and 18 are low-pass filters, and the same parts as those in FIG.

In FIG. 1, between the FM demodulator 2 and the previous value holding circuit 4,
A first low-pass filter 17 is inserted, and this first low-pass filter is composed of resistors 9 to 11, capacitors 12, 13 and a transistor 14, and its cutoff frequency is resistor 9 and capacitor. 12 and resistor 11 and capacitor 13 determine. First
The cutoff frequency of the low-pass filter 17 is generally determined from the viewpoint of hearing because it has a correlation with the previous value holding width of the previous value holding circuit 4. That is, in the Hi-Fi video tape recorder, since the FM audio signal is recorded and reproduced by the rotary head together with the FM video signal, 60 Hz switching noise occurs in the reproduced audio signal. To
The noise level when the FM demodulation triangular noise starts to be erroneously held is compared with the switching noise level at 1/60 second intervals, which has a wider previous value holding width as the cutoff frequency becomes lower as described above. The cutoff frequency of the low-pass filter 17 and the previous value holding width by the previous value holding circuit 4 are determined so that the noise of (1) becomes the lowest in hearing. This previous value holding width is 10 to 20 μse
This cut-off frequency is about 50 kHz, where c is.

Next, the second low pass filter 18 is inserted between the previous value holding circuit 4 and the noise reduction circuit 5. The second low pass filter 18 is composed of a resistor 15 and a capacitor 16. Considering the upper limit of the audible frequency band of voice as 20 kHz, the cutoff frequency of the low-pass filter 18 is set to be 20 kHz including the influence of the first low-pass filter 17. In addition, when considered as the audio circuit of the video system, the horizontal sync signal (15.75
It is also possible to set the cutoff frequency to 15 kHz in order to prevent malfunction of noise reduction due to (kHz).

In this way, malfunctions of the previous value holding circuit 4 and the noise reduction circuit 5 due to noise outside the audible frequency band can be prevented.

FIG. 2 is a block diagram showing another embodiment of the FM demodulation circuit according to the present invention. The parts corresponding to those in FIG. This embodiment differs from the embodiment shown in FIG. 1 in that the second low pass filter 18 is provided on the input side of the rectifier 7 in the noise reduction circuit 5 and the voltage control in the noise reduction circuit 5 is controlled. Type amplifier VCA (Variable gain amplifier)
6 is that the output signal of the previous value holding circuit 4 is directly supplied.

According to this embodiment, a malfunction due to noise outside the audible frequency band of the rectifier 7 can be prevented, and since no low-pass filter is provided on the signal transmission line, only the first low-pass filter 17 is used. There is an advantage that frequency characteristics in a wide band can be provided.

〔The invention's effect〕

As described above, according to the present invention, the first and second low-pass filters are provided between the FM demodulator and the previous value holding circuit and between the previous value holding circuit and the noise reduction circuit. ,
Since the cutoff frequencies of both low-pass filters were set to the optimum values separately, the malfunction of the pre-value holding circuit due to FM triangular noise was eliminated, and the setting of the pre-value holding width was set to a level at which there was no audible problem. Above, it is possible to prevent the malfunction when the noise reduction is expanded, and so on.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an FM demodulation circuit according to the present invention, FIG. 2 is a block diagram showing another embodiment of an FM demodulation circuit according to the present invention, and FIG. 3 is an example of a conventional FM demodulation circuit. 4 is a block diagram showing the frequency characteristics of the low-pass filter,
5 is a waveform diagram showing the output signal of the previous value holding circuit of FIG. 4, FIG. 6 is a diagram showing the spectrum of the output signal of the previous value holding circuit of FIG. 3, and FIG. 7 is the noise of FIG. It is a characteristic view which shows the expansion characteristic of a reduction circuit. 1 ... FM voice signal input terminal, 2 ... FM demodulator, 4 ... Pre-value holding circuit, 5 ... Noise reduction circuit, 8 ... Output terminal, 17, 18 ... Low-pass filter.

Claims (1)

[Claims] 1. An FM demodulator for inputting an FM voice signal and generating a demodulated signal containing an FM carrier component, and a demodulated signal from the FM demodulator supplied to the FM demodulated signal to generate an FM carrier component contained in the demodulated signal. A first low-pass filter having a cutoff characteristic that is sufficiently attenuated, and an output signal of the first low-pass filter are supplied, and a dropout occurs in the output signal of the first low-pass filter. When it occurs, by holding the previous value of the output signal of the first low-pass filter, a previous value holding circuit for compensating for dropout, and an output signal of the previous value holding circuit are supplied, A second low-pass filter having a cut-off frequency lower than the cut-off frequency of the first low-pass filter and near the upper limit of the required audible frequency, and an output signal of the second low-pass filter are supplied. As well as
A rectifier that rectifies an output signal of the second low-pass filter, and a signal that is rectified by the rectifier as a control signal
2. An FM demodulation circuit comprising a variable amplifier for amplifying a coming-out signal of the low-pass filter, and a noise reduction circuit using the signal amplified by the variable amplifier as an output signal.