JPS59140782A - Fm signal demodulating circuit - Google Patents

Abstract

PURPOSE:To perform excellent FM demodulation free of inversion by discriminating the carrier frequency of an FM signal and controlling the limit of a demodulation frequency on the basis of the discrimination output. CONSTITUTION:An FM luminance signal from a head 1 is led out through a regenerative amplifier 2 and supplied to an FM demodulator 4 through a limiter 3. This FM luminance signal is led to a band-pass filter 5 to limit its band on the upper and lower sides and then supplied to an FM demodulator 7 through a limiter 6. The demodulation frequency limit of the FM demodulator 4 is controlled according to the signal from this FM demodulator 7. Further, the output of the demodulator 4 is supplied to a variable gain amplifier 8 and the signal from the demodulator 7 is supplied to the control terminal of an amplifier 8, so that a gain-controlled luminance signal appears at an output terminal 9.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an FM demodulation circuit used, for example, in a reproduction system of a video tape recorder (VTR).

Background Art and its Problems For example, in a VTR, a luminance signal is recorded as an FM signal with a low frequency on the black side and a high frequency on the white side.

In such a VTR reproduction system, the luminance signal is demodulated in an FM demodulation circuit provided with a predetermined demodulation frequency limit.

In that case, for example, as shown in Figure 1A, the low frequency (black)
When a signal that suddenly changes from high frequency to high frequency (white) is supplied, FM demodulation is performed using a signal such as that shown in FIG. 1B, which is obtained by limiting the amplitude of this signal. At this time, if the demodulation frequency limit of the FM demodulation circuit is set low, the no. (black). i.e. 1
When there is a sudden change from black to white in a horizontal scan line, a blacker signal is generated at the point of change, and this signal invades the white side, causing black spots.

In response to this, it is conceivable to set the demodulation frequency limit high. However, in the above-mentioned VTR, the F on the tape
The M signal is recorded in saturation, and a large amount of odd-order harmonics are generated due to distortion. In that case, for example, if the third harmonic shown by the broken line d is superimposed on a low frequency signal as shown in Figure 1C, the amplitude-limited output will become as shown in Figure 1, where the demodulation frequency limit is If it is set high, the signal of e is detected, and the signal that should originally be demodulated at the wavelength of f is demodulated at the wavelength of g, resulting in a high frequency (white) signal.

In other words, white spots appear on a black screen.

Therefore, in the past, the demodulation frequency limit was set at a position where the above-mentioned black and white spots (inversion phenomenon') occur with the same probability, but this is not always good, and it is also possible to increase the modulation degree. could not.

OBJECTS OF THE INVENTION In view of these points, the present invention is intended to enable good FM demodulation without causing an inversion phenomenon.

Summary of the Invention The present invention is an FM signal demodulation circuit that discriminates the carrier frequency of an FM signal and controls the demodulation frequency limit of an FM demodulator based on the discrimination output. It is possible to perform good FM demodulation without this occurring.

Embodiment In FIG. 2, an FM brightness signal from a head (1) is extracted through a reproducing amplifier f(2), and this FM brightness signal is supplied to an FM demodulator (4) through a limiter (3). In addition, the FM luminance signal from the reproducing amplifier (2) is supplied to the Nonondo T4 filter (5) to limit the upper and lower bands, and this band-limited signal is sent to the FM demodulator (7) through the limiter (6). Supplied. The signal from this demodulator (7) is supplied to the control terminal of the FM demodulator (4), and the demodulation frequency limit of the demodulator (4) is controlled in accordance with the signal from the demodulator (7). Further, the output of the demodulator (4) is supplied to a variable gain amplifier (8), the signal from the demodulator (7) is supplied to the control terminal of amplifier (8), and the gain-controlled luminance signal is output to the output terminal. It is taken out at (9).

In this circuit, the FM demodulator (7) demodulates the FM signal within a predetermined range from the center frequency, and outputs a differential carrier wave frequency from which lower frequencies and higher frequencies than the predetermined frequencies due to the above-mentioned inversion phenomenon have been removed. Served. By supplying this discrimination output to the demodulator (4), control is performed to normally make the demodulation frequency limit sufficiently high, and to lower the demodulation frequency limit when the carrier frequency of the FM signal is low. be exposed. Furthermore, by controlling the demodulation frequency limit, the demodulation sensitivity of the demodulator (7) is also changed, so the discrimination output is supplied to the variable gain amplifier 7' (s), and the output signal level is changed due to the change in the demodulation sensitivity. Gain control is performed to compensate for the changes.

Therefore, in this circuit, the demodulation frequency limit is usually set high enough to avoid the inversion phenomenon caused by neglecting the i4 pulse of a, and the demodulation frequency limit is lowered when the carrier frequency of the FM signal is low. It is possible to prevent the reversal phenomenon caused by the nozzle 4 from occurring.

Since the inversion phenomenon is improved in this way, it becomes possible to increase the degree of modulation of the FM signal to be recorded.

Furthermore, FIG. 3 shows another example, in which a separate circuit for frequency discrimination is not provided, and control is performed by feedback of the output signal.

That is, in the figure, the output signal of the variable gain amplifier f(8) is passed through the low bus filter α1. It is taken out to the output terminal (9) through the series circuit of α, and the low/4th filter α
A signal at the midpoint of the connection between Q and α is supplied to the control terminal of the FM demodulator (4) and variable gain amplifier (8).

In this circuit, the loaf 4-sphere filter 91° (
In 11), the desired characteristics can be obtained by combining the characteristics of both, and low) J? The filter Q0 is a low-order filter that does not cause carrier wave leakage or time delay.

Therefore, in this circuit, the carrier frequency discrimination output is extracted from the output of the low-pass filter (IQ), and the same control as described above is performed.

Although some time delay occurs in the low-pass filter α, no problem occurs because the FM luminance signal originally reproduced is band-limited and there are no steep steps.

Further, in the above examples, control is performed in real time, but a time constant may be provided to perform control using an average value.

Effects of the Invention According to the present invention, it has become possible to perform good FM demodulation without producing inversion noise.

[Brief explanation of drawings]

It is. (4) and (7) are FM demodulators, (5) is a bandpass filter, and (8) is a variable gain amplifier. Figure 1

Claims (1)

[Claims] An FM signal demodulation circuit that discriminates the carrier frequency of an FM signal and controls the demodulation frequency limit of an FM demodulator based on the discrimination output.