JPH0614426B2 - Audio signal monitoring device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an audio signal muting device suitable for use in a video tape recorder or the like.

[Background of the Invention]

Generally, as an audio signal muting device, an analog switch circuit is provided between an audio signal amplifier circuit and a load circuit provided to the output of the amplifier circuit, or an input terminal of the audio signal amplifier circuit is provided. It is known that a switch circuit is provided between the signal source and the switch circuit so that the voice signal is cut off or attenuated during the muting operation (for example, Japanese Patent Laid-Open No. 59-59).
58905).

One of the problems with these conventional muting devices is that the breaking characteristics of the switch circuit must be increased. In other words, since the noise reduction circuit is normally used, the dynamic range of the audio signal is 80 dB or more, and therefore, the cutoff characteristic of the muting circuit also requires a large attenuation of 80 dB or more. Otherwise, the noise cannot be muted sufficiently.
Therefore, the switch circuit configuration becomes complicated, resulting in an increase in the number of parts and an increase in cost.

The second problem is that since a switch circuit is used to sharply cut off and connect the input signal by turning on / off the muting operation, a large level signal is connected rapidly at the end of the muting operation. Then it is very difficult to hear.

The third problem is that the muting operation is turned on because the signal transmission path is directly shut off by the switch circuit.
When it is turned off, the output DC potential is likely to fluctuate, which causes noise.

[Object of the Invention]

An object of the present invention is to solve the above-mentioned problems, to provide an audio signal muting device that can obtain sufficient cutoff characteristics with a simple circuit configuration and that does not generate an abnormal sound when the muting operation is turned on and off. Especially.

[Outline of Invention]

In order to achieve this object, the present invention provides an audio circuit using noise reduction for compressing and expanding the dynamic range of an audio signal, and at the time of muting operation, the gain of the dynamic range expansion circuit that is a component of the noise reduction circuit is reduced. It is characterized in that it is controlled so as to be the minimum section and muted.

Specifically, a muting operation signal is created by detecting a dropout or the like of an audio signal, and the muting operation signal is controlled so that the gain of the variable gain amplifier in the noise reduction circuit is minimized. This variable gain amplifier is normally gain controlled by the output of an audio signal level detector which detects the input signal level when there is no dropout. When a dropout occurs, the input to the audio signal level detector is shut off by the muting operation signal, or the output of this level detector is short-circuited to zero.

Example of Invention

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of an audio signal muting device according to the present invention applied to a magnetic recording / reproducing apparatus for recording / reproducing a frequency-modulated (FM-modulated) audio signal with a rotary magnetic head. Is a magnetic tape, 2 and 3 are rotary magnetic heads, 4 is a switch circuit, 5 is a preamplifier, and 6
Is a BPF (band pass filter), 7 is an FM demodulator, 8 is an LPF (low pass filter), 9 is a hold circuit, 10
Is an input terminal, 11 is an edge detection circuit, 12 is a dropout detection circuit, 13 is a mute detection circuit, 14 is an adder, 15 is a noise reduction circuit, 16 is a voltage follower, and 17 is a CCA (current control type variable gain amplifier). , 18 is a detection circuit, 19 is a capacitor, 20 is a switch, and 21 is an output terminal.

In the figure, the signal reproduced from the magnetic tape 1 by the rotary heads 2 and 3 is converted into a series of reproduced signals by the switch circuit 4 and then amplified by the preamplifier 5. The amplified reproduction signal is input to the BPF 6 and FM
After the audio signal is extracted, it is FM demodulated by the FM demodulator 7. The FM demodulated gospel voice signal is applied to a hold circuit 9 after an unnecessary band component such as carrier leak is removed by the LPF 8, and the hold circuit 9 causes a pulsed nozzle generated at the time of rotary head switching or dropout. At the time of occurrence of, the output level is compensated by holding the previous value at the previous level. Here, the hold circuit 9 detects both the edges of the output signal of the dropout detection circuit 12 for detecting the dropout by detecting the envelope of the reproduced FM audio signal and the rotary head switching signal input from the input terminal 10. It is controlled by the output signal of the adder 14 which receives the output signal of the edge detection circuit 11 to be extracted and the input.

The demodulated audio signal, which has been subjected to the previous value hold compensation by the hold circuit 9, is expanded to the original dynamic range by the noise reduction circuit 15 and output from the output terminal 21.

Here, the noise reduction circuit 15 is composed of a voltage follower 16, a CCA 17 and a detection circuit 18 for detecting the signal level. The output signal 29 of the detection circuit 18 controls the gain of the CCA 17 to expand the signal amplitude. To do. The capacitor 19 is a detection capacitor. An example of the input / output characteristics of the noise reduction circuit 15 is shown in FIG.

As described above, short-term noise such as head switching noise and dropout noise can be sufficiently dealt with by performing front-value hold compensation at the level immediately before the noise occurs in the hold circuit. . However, when the tape on which the FM audio signal carrier is not recorded, such as a blank tape, is reproduced, the dropout detection circuit 12 causes the operation due to noise, and as a result, a large noise is generated. , F
Depending on the presence or absence of M carrier, a mute function for cutting off the audio signal is required.

Therefore, in this embodiment, the dropout detection circuit 12
The mute detection circuit 13 that receives the output signal of
When a dropout or a large noise occurs, the muting operation control signal 27 shown in FIG.
The CCA 17 in the noise reduction circuit 15 is controlled to prevent noise generation.

That is, during the muting period, the switch 20 is closed by the output signal of the first-stage mute detection circuit 13, whereby the gain control signal 29 applied from the detection output of the detector 18 to the CCA 17 becomes zero, and the expansion operation is performed. CC
The gain of A17 is minimized, and thus muting is performed. In this way, at the start of muting, the switch 20 instantly short-circuits the detection capacitor 19, so that the gain control signal 29 of the CCA 17 is reduced.
Becomes instantly zero, as shown in FIG.
The output signal 28 from is muted. On the other hand, at the end of muting, as shown in FIG. 2, the gain control signal 29 proportional to the charge characteristic to the detection capacitor 19 is obtained, so that the output signal 28 is gradually demuted. Therefore, even if a signal with a large amplitude level is added when the muting operation is turned off, the mute is released gradually, and no abnormal sound is generated.

Further, in this embodiment, since the gain control signal for controlling the expansion operation of the noise reduction circuit is switched instead of directly turning on / off the signal transmission path, fluctuations in the output DC potential due to the muting operation may occur. There is no.

FIG. 4 is a block diagram showing another embodiment of the audio signal muting device according to the present invention, in which 22 is a switch, and parts corresponding to those in FIG. To do.

In FIG. 4, the switch 2 is activated during the muting period.
By switching the gain control signal 29 of the CCA 17 according to 2, the gain of the CCA 17 is minimized, thereby performing muting. The function and effect are the same as those of the embodiment shown in FIG.

FIG. 5 is a block diagram showing still another embodiment of the audio signal muting device according to the present invention, in which 23 is a switch, and the parts corresponding to those in FIG. Omit it.

In FIG. 5, the switch 2 is activated during the muting period.
When the input signal to the detection circuit 18 is cut off by 3,
The gain control signal 29 to the CCA 17 is made zero, so that the gain of the CCA 17 is minimized and muting is performed. Also in this embodiment, the CCA1 for performing the expansion operation is not directly turned on / off the signal transmission path.
Detection circuit 18 for generating gain control signal 29
Since the output signal to the circuit is cut off and the input level of the detection circuit 18 is set to the ground level, the output DC potential fluctuation does not occur. Further, as shown in FIG. 6, the gain control signal 29 at the start of muting is a signal proportional to the charge charging characteristic of the detection capacitor 19, so the mute is gradually released. Therefore, the output signal 28 from the CCA 17 is faded out and faded in according to the mute operation on / off, so that no abnormal sound is generated.

FIG. 7 is a block diagram showing still another embodiment of the audio signal muting device according to the present invention, which is 30FMA.
GC (FM automatic gain control circuit), 31 is a phase comparator, 3
Reference numeral 2 is a loop filter, 33 is a VCO (voltage controlled oscillator), 34 is a 90 ° shift circuit, 35 is a synchronous detection circuit, and 36 is a dropout detection circuit. A duplicate description will be omitted.

In FIG. 7, the FM audio signal extracted by the BPF 6 from the reproduction signal is supplied to the FMAGC 30, and the reproduction level fluctuation is absorbed and removed. The output signal of FMAGC30 is
FM demodulation is performed by a PLL type FM demodulation circuit including a phase comparator 31, a loop filter 32, and a voltage controlled oscillator (VCO) 33. The FM demodulated audio signal is output as an output signal of the loop filter 32 and then added to the hold circuit 9, where the rotary head switching noise and the dropout noise are held in the previous value hold compensation. Here, as the dropout detection signal, the output level of the synchronous detection circuit 35 which receives the signal obtained by phase-shifting the phase of the output of the VCO by 90 ° by the 90 ° shift circuit 34 and the output of the FMAG 30 as a dropout detection signal. It is obtained by identification and detection by the circuit 36. The hold circuit 9 is controlled by a signal obtained by adding the output signal of the edge detection circuit 11 which detects both edges of the head switching signal input from the input terminal 10 and the dropout detection signal by the adder 14.

The output signal of the hold circuit 9 is expanded to the original dynamic range by the noise reduction circuit 15,
The signal 28 is output to the output terminal 21.

The noise reduction circuit 15 is provided with a switch 20 for performing a muting operation. Switch 20
The detection capacitor 19 is short-circuited by this, and as a result, the CCA1 which is a component of the noise reduction circuit 15 is
The gain of 7 is minimized to cut off the output signal. The muting operation signal that controls the switch 20 is obtained as the output signal of the mute detection circuit 37 that receives the output signal of the synchronous detection circuit 35 described above. The dropout and mute detection circuit using the synchronous detection shown in this embodiment has an advantage that malfunction due to noise is less likely to occur as compared with the method of detecting the envelope.

The advantage of this embodiment is that, like the embodiment shown in FIG.
The output signal is instantly muted at the start of the mute operation, and faded in at the end of the mute operation.

FIG. 8 is a circuit diagram showing a specific example of the detection circuit 18 of the noise reduction circuit 15 in each of the above embodiments, in which 24 and 25 are input terminals, 26 is an output terminal, and Q1 to Q.
16 is a transistor, R1 to R11 are resistors, and C1 to C3
Is a capacitor.

In the figure, transistors Q1 to Q15 and resistors R1 to
A block composed of R7 and a capacitor C1 constitutes a double-wave rectification circuit, and includes resistors R8 and R9 and a capacitor C.
3 and 3 form a weighting circuit. The audio signal from the voltage follower 16 (FIG. 1) is supplied to the input terminal 24 and weighted by the weighting circuit in accordance with the frequency.
Both wave integers are output and CCA 17 (FIG. 1,
It is output as a gain control signal 29 to FIGS. 4 and 5). The muting operation is performed by the mute control signal 27 added to the input terminal 25. When the mute control signal 27 is input, the transistor Q16 and the resistor R1
The switch composed of 0 and R11 shuts off the current source of the double-wave rectification circuit (composed of power supplies V _B , Q5, Q6, R1 and R2), and as a result, the operation of the double-wave rectification circuit is stopped and the gain is increased. The control signal 29 becomes zero.

Gain control signal 2 by turning on / off the muting operation
The change of 9 is similar to that shown in FIG. 6, and is proportional to the charge / discharge characteristic of the capacitor C1.

FIG. 9 is a circuit diagram showing another specific example of the detection circuit 18 of the noise reduction circuit 15 as well, in which Q17 is a transistor and R12 is a resistor, and the parts corresponding to those in FIG. There is.

Similar to the embodiment shown in FIG. 8, transistors Q1 to Q
15, the resistors R1 to R7 and the capacitor C1 form a double-wave rectification circuit, and the resistors R8 and R9 and the capacitor C3 form a weighting circuit. The muting operation is performed by the mute control signal 27 added to the input terminal 25. When the mute control signal 27 is input,
By the switch composed of the transistor Q17 and the resistor R12, the output transistors Q13 and Q1 of both rectification circuits are
4 is turned off, and as a result, the gain control signal 29 which is the output signal becomes zero, and the muting operation is performed. Also in this embodiment, the gain control signal 29 by turning on / off the muting operation is the same as that shown in FIG.
When the muting operation is turned on / off, the output signal 28 (FIG. 1, FIG. 4, FIG. 5, FIG. 7) of the CCA 17 is faded out and in.

Although the embodiments of the present invention have been described above with reference to the drawings, the gist of the present invention is to effectively utilize the noise reduction circuit,
Muting is performed by using a gain control signal for noise reduction. Therefore, it is possible to use noise reduction circuits of various configurations other than those shown in the embodiments.

〔The invention's effect〕

As described above, according to the present invention, by adding a simple circuit to the noise reduction circuit, the CCA in the noise reduction circuit can be effectively used to generate an abnormal sound such as a pop sound during a muting operation. It is possible to obtain an excellent effect such that the audio signal can be muted very audibly without being emitted.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of an audio signal muting device according to the present invention, FIG. 2 is an operation explanatory diagram thereof, FIG. 3 is a characteristic diagram of a noise reduction circuit in FIG. 1, FIG. 4 and FIG. FIG. 5 is a block diagram showing another embodiment of the audio signal muting device according to the present invention, and FIG.
FIG. 7 is a diagram for explaining the operation of the embodiment shown in FIG. 5, FIG. 7 is a block diagram showing still another embodiment of the audio signal muting device according to the present invention, and FIGS. 8 and 9 show the respective embodiments. It is a circuit diagram which shows the specific example from which the detection circuit in a noise reduction circuit differs. 15: Noise reduction circuit, 17: CCA, 1
8 ... Detection circuit, 20, 22, 23 ... Switch, Q1
6, Q17 ... transistor.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akitoshi Kakuka, 292 Yoshida-cho, Totsuka-ku, Yokohama City, Kanagawa Prefecture, inside Hiritsu Video Engineering Co., Ltd. (56)

Claims (1)

[Claims] 1. A magnetic head for reproducing a frequency-modulated audio signal recorded on a magnetic tape, an amplifier for amplifying the frequency-modulated audio signal reproduced by the magnetic head, and a desired frequency band from an output signal of the amplifier. Of the band-pass filter, the FM demodulation means for frequency demodulating the output signal of the band-pass filter, and the output signal of the band-pass filter are supplied, and the supplied signal is dropped. A dropout period detection circuit for generating a dropout detection pulse having a pulse width of the dropout period detected by detecting the out, and the above-mentioned F during the pulse width period of the dropout detection pulse.
A pre-value holding circuit for holding the frequency-demodulated audio signal output from the M demodulating means, and a muting detection circuit for receiving the output signal of the band pass filter and detecting the mute period of the supplied signal. A level detection circuit provided after the front value holding circuit for detecting the amplitude level of the frequency demodulated audio signal, and a dynamic circuit for expanding the dynamic range of the frequency demodulated audio signal according to the detection signal of the level detection circuit. An audio signal muting device comprising: a range expansion circuit; and control means for controlling the output signal of the level detector to a minimum according to the output signal of the muting detection circuit.