JPH0262913B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention provides a method for reproducing an audio signal without pulse noise from a reproduction signal from a recording medium in which a frequency modulated signal is recorded on a magnetic tape by a rotating head. The present invention relates to an audio signal reproducing device equipped with a noise removal circuit.

[Background of the invention]

Conventionally, when reproducing an audio signal from a recording medium recorded by frequency modulating (FM modulating) an audio signal, there is a possibility that the reproduction signal may be missing due to dropouts, or the rotary head of a helical scan type magnetic tape recording/reproducing device, for example. Playback audio as at the time of switching the playback track when recorded with
It was inevitable that large amplitude noise would occur due to discontinuities in the FM signal. For this reason, noise has been removed by, for example, reducing the output signal level of the audio signal amplifier during the dropout period, or by using a previous hold circuit that maintains the signal level immediately before the dropout period during the dropout period.

FIG. 1 is a reproduced audio signal waveform diagram showing the operation of a conventionally used noise removal circuit. 1st
In the figure, A is the reproduced audio FM signal waveform,
(b) is an audio signal that has been FM demodulated and has unnecessary frequency components removed by a low-pass filter (LPF), (c) is a dropout detection signal, and (d) is an audio signal waveform from which noise has been removed based on the dropout detection signal (c). . Further, ΔT indicates a dropout period.

During the dropout period ΔT shown in FIG. 1A, large-amplitude noise as shown in B occurs in the reproduced audio signal. On the other hand, by amplitude-detecting the reproduced audio FM signal A, a dropout detection signal as shown in C can be obtained. Therefore, by using the signal shown in C to maintain the reproduced audio signal level immediately before the dropout occurs during the dropout period ΔT, so-called previous value holding, it is possible to obtain an audio signal from which noise has been substantially removed as shown in D. .

However, noise caused by dropouts in the audio signal after FM demodulation is not limited to just the dropout period, but also accompanies transients after the dropout. This is a bandpass filter (BPF) that extracts the audio FM signal from the playback signal.
The band is limited by a low-pass filter (LPF) for removing unnecessary frequency components from the demodulated audio signal, so a transient corresponding to the cut-off time constant of the BPF or LPF is generated. It's for a reason.

Furthermore, when a frequency modulated audio signal is recorded and reproduced using a rotary head using a helical scan type magnetic tape recording and reproducing apparatus, the phase of the carrier degree of the reproduced signal naturally becomes discontinuous at the time of switching the reproduction track. Therefore, at this point, there is a fourth signal in the audio signal after demodulation.
Large-amplitude noise as shown in Figure b is generated. The occurrence of noise at the time of track switching cannot be detected by the dropout detection circuit because, unlike dropout, there is no large amplitude variation in the carrier wave, as shown in FIG. 4a. Moreover, the duration of this noise is not the extremely short time required for track switching, but is dominated by a period similar to the transient after dropout.

Therefore, the noise removal circuit operating as shown in FIG. 1 not only fails to remove the transient portion, but also has difficulty in removing noise at the time of switching the reproduction track.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art, and to eliminate the noise that occurs when switching the reproduction track when reproducing a frequency modulated audio signal recorded on a magnetic tape using the helical scan method, without causing any practical problems. An object of the present invention is to provide an audio signal reproducing device that can reduce the amount of noise to a certain degree.

[Summary of the invention]

The present invention includes a bandpass filter for extracting a frequency modulated audio signal from a reproduced signal reproduced from a diagonal track of a magnetic tape, a frequency demodulation circuit for frequency demodulating the output signal of the bandpass filter, and a frequency demodulation circuit for frequency demodulating the output signal of the bandpass filter. A low-pass filter removes unnecessary frequency components from the output signal of the frequency demodulation circuit, and noise in the demodulated audio signal from the low-pass filter is removed for a predetermined period from the time when the track to be played back is switched. The predetermined period is determined to be at least a period corresponding to the cut-off time constant of the narrower of the middle bands of the two filters, thereby reducing the impulse at the time of switching the reproduction track. It is possible to prevent the occurrence of sexual noise.

[Embodiments of the invention]

FIG. 2 is a block diagram showing one embodiment of the present invention. In the figure, 1 is a reproduction signal input terminal;
2 is a BPF for audio FM signal extraction, 3 is an FM demodulator,
4 is an LPF for removing unnecessary frequency components; 5 is a front-row holding circuit; 6 is a dropout detection circuit; 7 is a monostable multivibrator; 8 is an OR circuit; 9 is a reproduced audio signal output terminal; 10 is an output track switching signal (fourth This is an input terminal for the front direct hold instruction signal (FIG. 4 d) obtained based on FIG. 4 c).

FIG. 3 is a signal waveform diagram of each part of the circuit shown in FIG. 2. In FIG. 3, D is the output signal waveform of the monostable multivibrator 7 triggered at the trailing edge of the dropout detection signal detected by the dropout detection circuit 6, E is the output signal waveform of the OR circuit 8, and H is the output signal waveform of the OR circuit 8. is the reproduction signal waveform of the immediately preceding holding circuit 5. Note that A, B, and C are the same waveforms as shown in FIG.

Next, the operation will be explained with reference to FIGS. 2 and 3. The BPF 2 extracts only the audio FM signal from the reproduced signal input from the reproduced signal input terminal 1.
As shown in Figure 3A, the output signal waveform of BPF 2 loses its signal during the dropout period and becomes only a noise component, so it is demodulated by FM demodulator 3 and passed through LPF.
During the dropout period of the audio signal that has passed through the signal line 4, large-amplitude noise as shown in FIG. 3(b) occurs.

On the other hand, such a dropout period of the reproduced signal can be detected by a dropout detection circuit 6 that detects the amplitude of the reproduced signal. FIG. 3C shows the dropout detection signal detected in this manner. However, as mentioned above, the output signal of LPF 4 is affected by the band limitation in BPF 2 and LPF 4, and transients occur even after the dropout period. Noise cannot be completely removed just by holding it. In order to improve this, the noise is removed by holding the previous value including the period corresponding to the transient period. That is, the monostable multivibrator 7 generates the pulse shown in FIG. 3D corresponding to the transient period using the end of the dropout period as a trigger, and the OR circuit 8 generates the dropout detection signal pulse shown in FIG. By ORing the pulses shown in (d), the pulses shown in FIG. 3 (e) are generated. By operating the previous value holding circuit 5 using the pulse E thus obtained, noise caused by dropout can be almost completely removed.

Next, noise removal at the time of switching the reproduction track when a helical scan type magnetic recording/reproduction device is used will be explained. In this case, the noise occurs at the playback track switching time tc, as shown in Figure 4a.
Since this noise is caused by discontinuity in the FM signal, the timing at which this noise occurs is as shown in Figure b.
Since tc is known in advance, it is possible to easily create a command signal for holding the previous value as shown in FIG. 4d using the reproducing track switching signal itself, which instructs switching of the reproducing track shown in FIG. 4c. It goes without saying that the pulse width of this instruction signal should be approximately the same width as the pulse shown in FIG. 3D, which has a width corresponding to the transient period. Therefore, by applying the instruction signal shown in FIG. 4d to the previous value holding circuit 5 of FIG. 2, impulsive noise can be removed as shown in FIG. 4e.

It goes without saying that in a helical scan type magnetic tape recording and reproducing device, the same circuit can be used to remove noise during playback track switching and to remove noise during the dropout period. .

That is, for example, as shown in FIG. 2, in addition to the pulse signal that causes the previous value holding circuit 5 to perform the previous value holding operation based on dropout detection, the instruction signal shown in FIG. 4d is applied via the terminal 10. All you have to do is do it.

Note that the period for extending the noise removal period at the end of the dropout period and the period for noise removal at the time of switching the playback track should be set to include the above-mentioned transient period, and if they are made too long than necessary, the waveform due to noise removal Obviously, this is undesirable because it increases distortion.

As explained above, if the present invention is used, FM
Dropout in the playback signal of a modulated audio signal or FM at the time of switching the playback track
Large-amplitude noise caused by signal discontinuity can be almost completely removed with a simple circuit configuration, and the effect is great.

[Brief explanation of drawings]

FIG. 1 is a reproduced audio signal waveform diagram showing the operation of a conventional noise removal circuit, FIG. 2 is a block diagram showing an embodiment of the present invention, and FIG. 3 is a diagram of the circuit shown in FIG. 2. FIG. 4 is a signal waveform diagram illustrating noise removal at the time of switching the reproduction track. Code explanation, 2... BPF, 3... FM demodulator, 4
...LPF, 5 ... Previous value holding circuit, 6 ... Dropout detection circuit, 7 ... Monostable multivibrator, 8 ... OR circuit.

Claims (1)

[Scope of Claims] 1. An audio signal reproducing device that reproduces and demodulates a frequency modulated audio signal recorded as a track inclined at a certain angle with respect to the longitudinal direction of a magnetic tape by sequentially switching tracks, the audio signal reproducing device reproducing and demodulating the frequency modulated audio signal recorded as a track inclined at a constant angle with respect to the longitudinal direction of a magnetic tape, a bandpass filter for extracting the frequency modulated audio signal from the reproduced signal; a frequency demodulation circuit for frequency demodulating the output signal of the bandpass filter; and a frequency demodulation circuit for frequency demodulating the output signal of the frequency demodulation circuit. a noise removal circuit that removes noise in the demodulated audio signal from the low-pass filter for a predetermined period from the time of switching the track to be played; An audio signal reproducing device characterized in that the period corresponds to at least a cut-off time constant of the narrower of the middle bands of the filters. 2. The noise removal circuit determines the level of the demodulated audio signal from the low-pass filter at the time of switching the track to be reproduced.
The audio signal reproducing device according to claim 1, comprising a previous value holding circuit that holds the previous value for the predetermined period.