JPH04228151A - Audio signal reproducing device - Google Patents

Abstract

PURPOSE:To provide the reproduced signals of audio signals which can remove drop-out noise and head changeover noise in the reproduction and demodulation of the FM audio signals of a VTR. CONSTITUTION:The generation of the drop-out is detected by detecting a change in the amplitude of the reproduced signal outputted from a head and a previous value holding circuit which holds the previous value of demodulation signal in controlled by both signals of the detection signal and the head changeover signal. The operation to hold the previous value is executed both at the time of the generation of the drop-out and the changeover of the head and, therefore, the generation of noise is prevented.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention reproduces an audio signal without pulse noise from a reproduction signal from a recording medium in which a frequency modulated video signal and a frequency modulated audio signal are recorded on the same magnetic tape by a rotating head. The present invention relates to an audio signal reproducing device equipped with a noise removal circuit for eliminating noise.

0002

2. Description of the Related Art Conventionally, when reproducing audio signals from a recording medium recorded by frequency modulating (FM modulating) audio signals, there is a possibility that the reproduction signal is missing due to dropouts, or, for example, a helical scan type magnetic tape. It is inevitable that large amplitude noise will occur due to discontinuities in the reproduced audio FM signal, such as when switching between reproduction tracks when recording is performed using a rotary head of a recording/reproducing apparatus. Therefore, 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 value holding circuit that holds 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. In Figure 1, (a) is the reproduced audio FM signal waveform, (b) is the audio signal that has been FM demodulated and has unnecessary frequency components removed by a low-pass filter (LPF), and (c) is the dropout. The detection signal (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.

[0004] The dropout period △ shown in FIG. 1 (a)
During T, large-amplitude noise as shown in (b) occurs in the reproduced audio signal. On the other hand, the reproduced audio FM signal (a)
By amplitude detecting the dropout detection signal shown in (c), it is possible to obtain a dropout detection signal. therefore,(
c) Maintaining the reproduced audio signal level immediately before the dropout occurs during the dropout period ΔT using the signal shown in
By so-called prior value retention, a substantially noise-free audio signal as shown in (d) can be obtained.

[0005] 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 band-limited by a band pass filter (BPF) that extracts the audio FM signal from the reproduced signal and a low pass filter (LPF) that removes unnecessary frequency components from the demodulated audio signal. The above BP
This is to generate a transient corresponding to the cutoff time constant of F or LPF.

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 wave of the reproduced signal naturally becomes discontinuous at the time of switching the reproduction track. Therefore, large-amplitude noise as shown in FIG. 4(b) occurs in the demodulated audio signal at this point. 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. 4(a). moreover,
The duration of this noise is not the extremely short time required for track switching, but is primarily a period of transients similar to the post-dropout transient.

[0007]

[Problems to be Solved by the Invention] Therefore, the noise removal circuit that operates as shown in FIG. there were.

An object of the present invention was to solve the above-mentioned drawbacks of the prior art, and even when a frequency modulated audio signal is recorded and reproduced by a rotating head using a helical scan type magnetic recording and reproducing device. An object of the present invention is to provide an audio signal reproducing device that can reduce noise to a level that poses no problem in practical use.

[0009]

[Means for Solving the Problems] In order to achieve the above object, a noise removal circuit is provided that prevents noise from occurring both when a dropout occurs and when switching tracks.

0010

[Operation] By using both the detection signal that detects the occurrence of dropout and the track switching signal as control signals that cause the noise removal circuit to perform the noise removal operation, the noise removal operation is performed in any case. .

[0011]

Embodiment FIG. 2 is a block diagram showing an embodiment of the present invention. In the figure, 1 is a playback 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 previous value holding circuit, and 6 is a dropout detection circuit. , 7 is a monostable multivibrator, 8 is an OR circuit, and 9 is a reproduced audio signal output terminal.

FIG. 3 is a signal waveform diagram of each part of the circuit shown in FIG. 2. In FIG. 3, (d) is a monostable multivibrator 7 triggered at the trailing edge of the dropout detection signal (c) detected by the dropout detection circuit 6.
(e) is the output signal waveform of the OR circuit 8,
(F) is the output signal waveform of the previous value holding circuit 5. In addition,
(A), (B), and (C) are the same as the waveforms shown in FIG.

Next, the operation will be explained with reference to FIGS. 2 and 3. B from the playback signal input from playback signal input terminal 1
PF2 extracts only the audio FM signal. As shown in Figure 3 (a), the output signal waveform of BPF2 is FM
During the dropout period of the audio signal demodulated by the demodulator 3 and passed through the LPF 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. 3(c) shows the dropout detection signal detected in this manner. However, as mentioned above, in the output signal of LPF4, BP
Because transients occur even after the dropout period due to the band limitations in F2 and LPF4, noise cannot be completely eliminated by simply holding the previous value directly in the previous value holding circuit 5 using the dropout detection signal (c). Cannot be removed. In order to improve this, the noise is removed by holding the previous value including the period corresponding to the transient period. That is, monostable multivibrator 7
The end of the dropout period is used as a trigger to generate the pulse shown in FIG. 3 (d) corresponding to the transient period, and the OR circuit 8 generates the dropout detection signal pulse shown in FIG. 3 (c) and the pulse shown in (d) above. The pulse shown in FIG. 3(E) is generated by ORing the . By operating the previous value holding circuit 5 using the pulse (e) obtained in this way, noise caused by dropout can be almost completely removed.

[0015] Next, noise removal at the time of switching reproduction tracks when using a helical scan type magnetic recording/reproduction device will be explained. The noise in this case is shown in Figure 4(a).
As shown in FIG. 4, this noise is caused by the discontinuity of the FM signal at the time tc of switching the reproduction track, so the timing tc at which this noise occurs is clear in advance, as shown in FIG. Using the playback track switching signal itself that instructs switching of the playback track shown in the figure (d).
) can create an instruction signal to hold the previous value. It goes without saying that the pulse width of this instruction signal may be approximately the same width as the pulse shown in FIG. 3(d), which has a width corresponding to the transient period. Therefore, by applying the instruction signal shown in FIG. 4(d) to the previous value holding circuit 5 of FIG. 2, impulsive noise can be removed as shown in FIG. 4(e).

Furthermore, in a helical scan type magnetic tape recording/playback device, etc., it is possible to use the same circuit to remove noise when switching playback tracks and remove noise during the dropout period. Needless to say.

That is, as shown in FIG. 2, for example, 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. 4(d) is sent to the terminal 10. It suffices to apply the voltage via the .

Note that the period for extending the previous value holding period at the end of the dropout period and the period for holding the previous value at the time of switching the playback track should include the above-mentioned transient period, and should not be excessively long than necessary. It is clear that this is not preferable because the waveform distortion due to holding the previous value increases.

[0019]

As explained above, by using the present invention, large-amplitude noise caused by dropouts in the reproduced signal of an FM-modulated audio signal or discontinuity of the FM signal at the time of switching the reproduction track can be eliminated. can be almost completely eliminated with a simple circuit configuration, and the effect is great.

[Brief explanation of the drawing]

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;

[Figure 3]
Signal waveform diagram of each part of the circuit shown in Fig. 2,

FIG. 4 is a signal waveform diagram illustrating noise removal at the time of switching playback tracks.

[Explanation of symbols]

2...BPF 3...FM demodulator 4...LPF 5...Previous value holding circuit 6...Dropout detection circuit 7... Monostable multivibrator 8...OR circuit

Claims (2)

[Claims] 1. An audio signal reproducing device that reproduces and demodulates a frequency modulated audio signal recorded by a helical scan recording method on tracks inclined at a constant angle with respect to the longitudinal direction of a magnetic tape by sequentially switching the tracks. 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; It consists of a low-pass filter that removes , and a noise removal circuit that removes noise in the demodulated audio signal from the low-pass filter, and this noise removal circuit:
An audio signal reproducing device characterized in that a noise removal operation is performed in response to a dropout detection signal indicating a dropout of the reproduced signal and a reproduction track switching signal instructing switching of a track to be reproduced. 2. The audio signal reproducing apparatus according to claim 1, wherein the noise removal circuit comprises a previous value holding circuit that holds the demodulated audio signal for a predetermined period according to the dropout detection signal and the reproduced track signal. .