JPH0466071B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Technical Field> The present invention relates to an FM modulated signal reproducing device, and more particularly to an FM modulated signal reproducing device having a function of removing noise caused by omission of a reproduced FM modulated signal.

<Description of the Prior Art> When reproducing an FM modulated signal from a recording medium, if there is a lack of signal due to so-called dropout, noise with a large amplitude is generated. If the FM modulated signal has temporal correlation like a video signal, the method for removing the noise is to use the reproduced signal of the correlated part and replace the signal-absent period. The occurrence of such noise can be eliminated. However, in cases where there is no temporal correlation, such as in audio signals, noise has been removed by muting the audio signal during periods when the signal is absent, or by holding the level of the audio signal immediately before dropout.

The present invention is particularly effective when applied to an apparatus for reproducing a signal obtained by FM modulating an information signal having no temporal correlation such as an audio signal. This type of device is a magnetic recording and reproducing device (hereinafter referred to as “magnetic recording and reproducing device”) that uses a rotary head to record and reproduce a signal obtained by frequency multiplexing a video signal for recording with an FM modulated audio signal.
VTR). FIG. 1 is a diagram showing the circuit configuration of a reproduction system conventionally used in VTRs and the like.

In FIG. 1, a terminal to which an FM modulated audio signal reproduced by a reproduction means such as a head is input, 2 is a band pass filter (BPF),
3 is an FM demodulator, 4 is a low pass filter (LPF),
5 is a previous value hold circuit, 6 is a dropout detection circuit, 7 is a monostable multivibrator, 8 is a well-known OR circuit, and 9 is an audio signal output terminal.

FIG. 2 is a waveform diagram showing waveforms at each part a to f in FIG. 1. The operation will be explained below.

In FIG. 1, the reproduced FM modulated audio signal supplied to the input terminal 1 is demodulated by the FM demodulator 3 via the BPF 2, and becomes an audio signal via the LPF 4. When the reproduced FM signal input to the FM demodulator 3 is lost due to dropout, the audio signal obtained at the output of the LPF 4 has a waveform as shown in FIG. 2b. Further, the dropout detection signal obtained by the dropout detection circuit has a waveform as shown in FIG. 2c.

By the way, the reason why the voice noise at the time of dropout as shown in FIG. 2b continues to occur even after the generation period of the dropout detection signal shown in FIG. 2c is as follows.
This is caused by the transient of the LPF 4 connected after the FM demodulator 3.

Therefore, the dropout detection pulse is input to the monostable multivibrator 7 for a period corresponding to the period in which the transient is generated by the LPF 4 (Fig. 2d).
The waveform shown in FIG. 2e is obtained by generating a pulse as shown in FIG. By operating the previous value hold circuit 6 using the output pulse of the OR circuit 8, a reproduced audio signal having the waveform shown in FIG. 2f can be obtained at the audio signal output terminal 9. However, the reproduced audio signal obtained in this way has a problem that the difference from the original recorded audio signal becomes large due to the increase in the previous value hold period accompanying the transient, and the original blue is not faithfully reproduced. .

<Object of the Invention> In view of the above-mentioned drawbacks, the present invention aims to provide an FM modulated signal reproducing device that is faithful to the original signal and can reliably and effectively remove noise caused by dropout. do.

<Description of Examples> The present invention will be described below using Examples. Third
The figure is a diagram showing a reproducing circuit configuration of an apparatus as an embodiment of the present invention. Components similar to those in FIG. 1 are given the same numbers. 10 is a phase comparison circuit;
11 is a voltage controlled oscillator (VCO), 12 is an LPF, 1
3 is a variable time constant circuit. Further, FIG. 4 is a waveform diagram showing waveforms at each part of FIGS. 3a to 3d.

The operation will be explained below.

Playback supplied to input terminal 1 in Figure 3
The FM modulated audio signal is input to the phase comparator circuit 10 via the BPF2. phase comparison circuit 10,
VCO11 and LPF12 are phase locked loops (hereinafter referred to as
The output signal of the VCO 11 is phase-locked with the reproduced FM modulated audio signal input to the phase comparison circuit 10. and
The output of the VCO 11 is input to the FM demodulator 3, and the detected audio signal is output to the audio signal output terminal 9 via the LPF 4.

When the reproduced FM signal input to the phase comparison circuit 10 is lost due to dropout, a dropout detection output (control pulse) as shown in FIG. 4c is obtained at the output of the dropout detection circuit 6.
During the generation period of this control pulse, the time constant variable circuit 1
3 increases the time constant of the LPF 12 constituting the PLL circuit to slow down the response of the PLL circuit. Therefore, when a dropout occurs, the frequency of the output signal of the VCO 11 slowly moves toward the free run frequency of the PLL circuit, depending on the response of the PLL circuit. By setting the free run frequency to the carrier center frequency of the modulated audio FM recording signal, the VCO 11 is input to the FM demodulator 3.
The frequency of the output signal (shown in FIG. 4b) gradually approaches the carrier center frequency. Furthermore, when the dropout period ends, the response of the PLL circuit slows down, and the reproduced FM input to the phase comparator circuit 10
It becomes possible to follow the modulated audio signal quickly enough, and the output signal of the VCO 11 becomes phase-locked to the input signal. A reproduced audio signal waveform as shown in FIG. 4d is obtained by the FM demodulator 3 and LPF 4.

That is, the FM demodulator 3 normally has a reproduced FM
An FM modulated signal that is phase-locked to the modulated audio signal is input, and at dropout, a signal that approaches the free-run frequency (set to the carrier center frequency of the FM modulated audio signal) from the frequency immediately before dropout occurs is input. Therefore, since the input signal is input to the FM demodulator 3 even during the dropout period, the high frequency components in the output signal are reduced, and the noise that occurs when a signal is input from no signal is not generated, and the LPF 4 No transients occur.

<Description of Effects> As explained above using the embodiments, according to the present invention, the FM signal is faithful to the original signal and can reliably remove noise caused by dropout.
A modulated signal reproducing device is obtained.

[Brief explanation of drawings]

Figure 1 is a diagram showing the circuit configuration of an FM modulation signal reproduction system used in conventional VTRs, etc., Figure 2 is a waveform diagram showing waveforms of each part of Figure 1, and Figure 3 is an embodiment of the present invention. Figure 4 showing the regeneration circuit configuration of the device.
The figure is a waveform diagram showing waveforms at various parts in FIG. 3. 1 is a playback signal input terminal from the playback means, 2 is a playback signal input terminal from the playback means;
BPF, 3 is FM demodulator, 4 is LPF, 6 is dropout detection circuit, 10 is phase comparison circuit, 11 is
VCO, 12 is an LPF, and 13 is a variable time constant circuit.

Claims (1)

[Scope of Claims] 1. Reproducing means for reproducing an FM modulated signal from a recording medium; a phase-locked loop circuit for outputting a signal phase-synchronized with the FM modulated signal reproduced by the reproducing means; and the phase-locked loop circuit. an FM demodulator for FM demodulating the output signal of the reproducing means; a pulse generating circuit for detecting a dropout of the FM modulated signal reproduced by the reproducing means and generating a control pulse having a width corresponding to the dropout occurrence period; , a control circuit that reduces the response speed of the phase-locked loop circuit for a period corresponding to a dropout occurrence period.