JPH0535480Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to a muting circuit for a video tape recorder (VTR), etc.

(b) Prior Art Fixed head audio circuits of tape recorders, VTRs, etc. are usually provided with a muting circuit. This muting circuit, for example,
As shown in No. 118518, it is equipped with a muting transistor that grounds the output of the recording amplifier when the muting switch is closed.

Further, in a VTR or the like, for example, a changeover switch is provided for input selection. Recently, analog switch ICs (such as the TK15021) have been used in this changeover switch for the purpose of miniaturization. This analog switch IC is usually provided with changeover switches for a plurality of circuits. Therefore, IC for analog switch
When used in a VTR, etc., some of the analog switches in the IC may not be used.

Therefore, it is conceivable to use the remaining analog switches to construct a muting circuit instead of the muting transistor. FIG. 2 shows this configuration.

The output of recording amplifier 1 is connected to one input terminal 2 of 2-input-1-output analog switch (TK15021) 2.
connected to a. The other input terminal 2b is in an oven state. A control signal from analog switch 2 is input to control terminal 3.

At times other than mute, the input terminal 2a is selected and the audio signal is supplied to the recording/reproducing head 4.
When the mute control signal goes to L level,
Analog switch 2 selects the other input terminal 2b. This performs a muting operation.

However, in the above configuration, each input terminal 2
The difference in DC offset between a and 2b appears on the output side (because there is a difference even in the same IC). This becomes a noise component, and noise as shown in FIG. 3a is recorded and reproduced.

(c) Problems to be solved by the invention As mentioned above, when a muting circuit is constructed using an analog switch, there is a drawback that noise is generated during the muting operation and when it is released.

(d) Means for solving the problem In this invention, the input terminal to which a signal is applied in the analog switch and the input terminal selected during the muting operation are connected via a DC impedance element. The selected input terminal is AC grounded.

(e) Effect Therefore, since the two input terminals are connected with a certain DC impedance, the difference in DC offset becomes small. Since the input terminal selected during the muting operation is grounded in terms of AC, the input signal is muted.

(F) Example An example of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit block diagram of an embodiment. The embodiment is used in a fixed head audio circuit in a so-called HiFi VTR. In the figure, 6 is an analog switch IC (TK15021), 7 is a recording amplifier,
8 is an oscillation circuit for AC bias, 9 is a recording head,
Reference numeral 10 designates an input terminal for an audio signal which is the output of a VTR built-in tuner, 11 an input terminal related to an external audio signal, 12 an input terminal for a simulcast control signal, and 13 an input terminal for a mute control signal. Also, 14 is a changeover switch, and 15 is a changeover switch.
It is a HiFi audio circuit.

IC (TK15021) 2 is provided with two analog switches with 2 inputs and 1 output. One analog switch 16 selects an audio signal to be recorded with a fixed head. Another analog switch 17 is used for muting operations. In other words, the leftover analog switch is being used for muting. An audio signal selected by the analog switch 16 is applied to one input terminal 17a of the muting analog switch 17. A resistor R (impedance element) for reducing DC offset is connected to this input terminal 17a and the other input terminal 17b, and they are connected in a DC manner. The input terminal 17b is also grounded through a capacitor C1 (about 100 .mu.F) having sufficiently low impedance for audio signals.

The resistor R reduces the DC offset between the input terminals 17a and 17b, and serves as a load impedance when viewed from the output terminal 16c of the other analog switch 16. And this IC
A value of 10KΩ was selected based on the relationship between load impedance and distortion factor in (TK15021). In addition, capacitors C2, C3, C4, C5, C
6 is for coupling and has a capacitance of about 10 μF.

The VTR of the embodiment is configured to be capable of so-called simulcast recording.
In addition to recording audio signals on the audio track in the longitudinal direction of the tape using a fixed head 9, the VTR is also capable of recording FM audio signals (together with video signals) using a rotating head. During normal recording, the analog switch 16 selects the input terminal 16a.
From the external audio signal selected by 4
The audio signal from the VTR's built-in tuner is recorded by the rotary head and the fixed head. During simulcast recording, the analog switch 16 selects the input terminal 16b. The fixed head 9 records the audio signal related to the video signal being recorded, while the rotary head records the external audio signal.

The muting operation is performed at the start of recording, at the start of a pause, at the time of canceling the pause, etc. This is because the tape runs at a different speed than the normal speed at the start of a pause, and if the tape were recorded without muting, an abnormal sound would be output during playback.

Therefore, the muting control signal is generated as a signal at L level for a predetermined period in synchronization with the timing of the mode change. When the muting control signal becomes L level, the muting analog switch 17 selects the input terminal 17b side. Since the input terminals 17a and 17b are connected through the resistor R, the difference in DC offset is small, and no noise is generated as described above. and input terminal 1
7b is grounded with a sufficiently low impedance in terms of alternating current, so no audio signal is output and a muting operation is performed.

Although the above embodiment is a muting circuit compatible with a fixed head recording circuit, it is also possible to apply it to a muting circuit for other purposes.

(g) Effects of the invention As described above, according to the invention, it is possible to prevent unnecessary noise from occurring when starting and canceling the muting operation.

[Brief explanation of the drawing]

FIG. 1 is a circuit block diagram of an embodiment of the present invention.
FIG. 2 is a circuit block diagram of a muting circuit using an analog switch, and FIG. 3 is a waveform diagram showing noise generation. 17... Analog switch, R... Resistor (impedance element), C1... Capacitor.

Claims (1)

[Scope of utility model registration request] an analog switch that selects and outputs one of two input terminals in response to a control signal, means for supplying a signal to one input terminal, and an impedance element that connects the two input terminals in a DC manner;
A muting circuit consisting of means for AC grounding the other input terminal to which no signal is supplied.