JPH0466073B2 - - 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 discontinuities in a reproduced FM modulated signal.

<Description of Prior Art> When reproducing an FM modulated signal from a recording medium, if signal discontinuity occurs due to, for example, dropout or switching of reproduction tracks, large amplitude noise will be generated after demodulation. When the FM modulated signal is an information signal with temporal correlation, such as a video signal, the noise removal method is to use the reproduced signal of the correlated part and include the discontinuous period of the signal. By replacing the parts, the occurrence of such noise can be eliminated. However, in the case of information signals that have no temporal correlation, such as audio signals, it is possible to mute the audio signal during discontinuous periods of the signal, or hold the level of the audio signal immediately before the discontinuous period for a predetermined period. method to remove noise.

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. An example of this type of device is a magnetic recording and reproducing device (hereinafter referred to as a VTR) which records and reproduces a signal obtained by frequency multiplexing a recording video signal and an FM modulated audio signal using two rotating heads. FIG. 1 is a diagram showing the circuit configuration of a reproduction system conventionally used in VTRs and the like.

In Fig. 1, 1 is a terminal to which an FM modulated audio signal reproduced by a reproducing means such as a head is input, 2 is a band pass filter (BPF), 3 is an FM demodulator, and 4 is a low pass filter ( 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 noise generated during dropout as shown in FIG. 2b continues to occur even after the dropout period shown in FIG. 2c is because the FM demodulator 3
This is caused by the transient of LPF4 connected after.

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. Of course, it is well known that similar processing is performed when switching the reproduction track, that is, when switching the reproduction head. However, the reproduced audio signal obtained in this manner has a problem in 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 signal is not faithfully reproduced.

<Object of the Invention> The present invention has been made in view of the above-mentioned drawbacks, and provides an FM modulation signal that is faithful to the original signal and can reliably remove noise caused by discontinuities in the reproduced FM modulation signal. The purpose is to provide a playback device.

<Description of Examples> The present invention will be described below using Examples. Figure 3 shows the FM of a VTR as an embodiment of the present invention.
FIG. 3 is a diagram showing a modulated audio signal reproduction system. Fourth
The figure is a timing chart showing the waveforms of each part of FIGS. 3a to 3g.

Reference numeral 1 denotes a terminal to which a reproduced FM modulated audio signal is input, and the amplitude component of the input FM modulated signal is removed by a limiter 12 to form a signal corresponding to FM modulation. This signal is fed to a phase comparator 24 of an APC oscillator 26 which includes a phase locked loop (PLL). The error signal that is the output of the phase comparator 24 is sent to the B side terminal of the switch 28, LPF3.
0 and the B-side terminal of the switch 32 to a voltage controlled oscillator (VCO) 34. The phase comparator 24 compares the phase of the output of the VCO 34 and the signal corresponding to the above-mentioned FM modulation, and forms a so-called PLL. Further, the oscillation output of the VCO 34 is supplied to an FM demodulator 36, and a reproduced audio signal is obtained from the demodulator 36 and outputted from an output terminal 38.

For example, if dust or the like adheres to the recording medium and a dropout occurs in the reproduced FM modulated audio signal, the output of the limiter 12 will be as shown in Figure 4a.
The waveform becomes as shown in the figure below, and if left as is, large-amplitude noise will occur in the reproduced audio signal. 1
4 is a dropout detection circuit, which generates a pulse as shown in FIG. 4b when dropout occurs. This pulse changes the connection of the switch 28 to the A side terminal. In this way, during the dropout occurrence period, the error voltage output from the phase comparator 24 is held at the previous value by the error hold circuit 20. Therefore, the VCO 34 maintains the oscillation frequency immediately before dropout.

Furthermore, the mono multivibrator 16 is turned on for a certain period in synchronization with the fall of the dropout detection pulse b. (shown in FIG. 4c) This signal and the dropout detection pulse are connected to the OR gate 18.
The OR gate 18 outputs a switching pulse as shown in FIG. 4d.

In other words, during the dropout period, VCO34
remains at the frequency of the FM modulated audio signal just before dropout. Also, while the mono multi 16 is outputting a high-level output signal, the above-mentioned process is used to match the phase between the output of the VCO 34, which is held at a constant frequency immediately after dropout, and the reproduced FM modulated audio signal that appears again. PLL
This is a period to compensate for internal disturbances. During this period, abnormal oscillation can be prevented by connecting the LPF 22, which has a lower time constant than the LPF 30, to the error hold circuit 20.

FIG. 4e shows the output error voltage of the phase comparator 24. When there is no dropout, this voltage becomes a signal waveform centered around a predetermined reference voltage, and is held at a constant voltage during the dropout occurrence period, and reappears immediately after the dropout ends as a reproduced FM modulated audio signal.
A voltage is generated according to the phase difference between the outputs of the VCO 34. The disturbance at this point has a larger time constant.
It is gradually compensated by the LPF 22 and returns to the state before the dropout occurs when the monomulti 16 falls.

According to the playback device configured as described above, since the output of the phase comparator 24 in the PLL is held at a constant level during the dropout occurrence period, the output of the phase comparator 24 in the PLL is held at a constant level during the dropout occurrence period. Abnormal oscillation of VCO34 in PLL, that is, error voltage of phase comparator 24 is not as large as on the left.
The period during which transient noise occurs due to PLL can be shortened. Furthermore, as in this embodiment, by increasing the time constant of the filter for the period including immediately after the end of dropout and slowing down the response speed of the PLL, it is possible to obtain a reproduced signal that is relatively close to the original signal.

Figure 5 shows a VTR as another embodiment of the present invention.
1 is a diagram showing an FM modulated audio signal reproduction system of FIG. Components similar to those in FIG. 3 are denoted by the same reference numerals, and explanations thereof will be omitted. The configuration shown in FIG. 5 is the output of the APC oscillator 26 (output of the VCO 34) only during the period when the output of the OR gate 14 is at a high level.
In this configuration, the output of the limiter 12 is directly demodulated by the FM demodulator 36 during a period not related to dropout. This switching is performed by controlling the switch 42 with the output of the OR gate 14. 40 is a delay circuit;
This compensates for the time delay in the output signal of the APC oscillator 26 due to the LPF 22.

With the configuration shown in FIG. 5, the third
It goes without saying that the same effect as the configuration shown in the figure is exhibited.

In the embodiment shown in FIGS. 3 and 5,
The output of the VCO 34 is used as the output of the APC oscillator 26. However, the output of switch 32,
In other words, since the input signal to the VCO 34 is considered to be approximately the same as the demodulated audio signal,
Output of switch 32, i.e. LPF22 or LPF
It is also possible to use the output of No. 30 as it is as a demodulated audio signal.

<Description of Effects> As explained above using the embodiments, according to the present invention, by shortening the period during which transient noise occurs, it is possible to reproduce faithfully and reliably the original signal.
An object of the present invention is to obtain an FM modulated signal reproducing device that can remove noise generated due to discontinuity of an FM modulated signal.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the circuit configuration of an FM modulated audio signal reproduction system conventionally used in VTRs, etc. Fig. 2 is a timing chart showing waveforms of various parts of Fig. 1, and Fig. 3 is an embodiment of the present invention. Figure 4 showing
FIG. 3 is a timing chart showing waveforms of various parts, and FIG. 5 is a diagram showing another embodiment of the present invention. 14 is a dropout detection circuit as a discontinuity detection circuit, 16 is a mono multivibrator, 18
is an OR gate, 20 is an error hold circuit as a previous value hold circuit, and 22 is a large time constant.
LPF, 24 is a phase comparator, 26 is an APC oscillator, 28 is a switch, 30 is an LPF, 32 is a switch, 34 is a VCO as a controlled oscillator, 36 is a
It is an FM demodulator.

Claims (1)

[Claims] 1. Reproducing means for reproducing an FM modulated signal from a recording medium; a controlled oscillator whose oscillation frequency is controlled according to a control input; a phase comparator that compares the phases of the FM modulated signal and outputs an error signal having a level corresponding to the phase difference; and a detection circuit that detects discontinuity in the FM modulated signal reproduced by the reproducing means. , a previous value hold circuit that operates according to the output of the detection circuit and holds the error signal at a previous value; an output signal of the previous value hold circuit is input, and the output is connected to the control input of the controlled oscillator. An FM modulated signal reproducing device, comprising: a low-pass filter; and an FM demodulator that performs FM demodulation on the output of the controlled oscillator. 2. In the device according to claim 1,
An FM modulated signal reproducing device characterized in that the time constant of the low-pass filter is varied according to the output of the detection circuit.