JPS586613A - Noise reduction common-use circuit - Google Patents

Abstract

PURPOSE:To prevent the generation of a click noise during recording and reproduction switching by extracting an AC component from an input signal to a compressing and expanding circuit, and then feeding it back to a preceding amplifying circuit. CONSTITUTION:A recording input signal after being amplified by an AC amplifier 13 is inputted through an amplifier 15 to a characteristic varying circuit 6, where it is compressed. A playback input signal, on the other hand, is amplified by an AC amplifier 14 and then inputted through the amplifier 15 to the characteristic varying circuit 6, where it is expanded. The output of the amplifier 15 is fed negatively back to amplifiers 13 and 14 through the AC feedback circuit consisting of a resistance and a capacitor. During recording and reproduction switching, only the amplifier 13 or 14 is activated. At this time, the DC levels of both the amplifiers are nearly equal even if there is variance in offset, and an AC feedback circuit is used in common, reducing click noises that a tape deck generate as much as possible.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise reduction system for a tape deck, and more particularly to a common compression and expansion circuit of such a noise reduction system.

In tape decks that perform analog recording using magnetic tape, noise reduction systems such as the Dolby noise reduction system are widely used to expand the dynamic range of recording and playback characteristics.

Such a noise reduction system includes a compression circuit and an expansion circuit that compress an input signal to be recorded to a certain level, record it on a tape, and expand and restore the playback output obtained from the tape.

The compression circuit and decompression circuit have exactly opposite characteristics to each other, and most parts can be shared, so usually,
It is configured as a single shared circuit, and depending on external specifications,
It is designed to act as a compression circuit or as an expansion circuit.

A conventional example of such a shared circuit is shown in FIG. In the circuit shown in Figure 1, the input signal to be recorded is supplied to input terminal 1, and the playback signal obtained from the tape deck is supplied to input terminal 2.
connected to be supplied to the The recording input signal passing through terminal 1 is supplied to one input terminal of the first DC amplifier circuit (hereinafter referred to as amplifier) 3, and the playback input signal passing through terminal 2 is supplied to the second DC amplification circuit (hereinafter referred to as amplifier) 3.
It is supplied to one input terminal of the amplifier 4. 1st and 2nd
Feedback resistors R1 and R2 are connected between the other input terminal and output terminal of the DC amplifiers 3 and 4, respectively, and the other input terminals are both connected to a reference potential v7 via resistors R3 and R4. . The output terminals of the first and second DC amplifiers 3゜4 are the analog changeover switch 50, two input terminals 5cL,
5b. The input terminal 5α or 5b is selectively connected to the output terminal 5G according to the analog cut-off playback (PB) designation control signal. The output terminal 5c is connected to the characteristic variable circuit 6. The characteristic variable circuit 6 becomes a compression characteristic circuit or an expansion characteristic circuit depending on the recording control designation control signal or the playback designation control signal. The characteristic variable circuit 6 outputs a level-compressed signal to be supplied to a deck (not shown) at an output terminal 6a.
and a reproduction processing circuit (not shown) including speakers, etc.
It compresses or expands the level-expanded signal to be supplied to the output signal depending on the output signal, but when switching from the recording designation control signal to the playback designation control signal, a large level click is generated from the characteristic variable circuit 6. Noise occurs.・This is an amplifier in which DC amplifiers 3 and 4 perform DC amplification.
This is because the output DC potentials are significantly different from each other due to variations in the offset voltages of the internal active elements.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems in the conventional example and to provide a noise reduction common circuit that does not generate click noise even when switching between recording and playback.

The noise reduction common circuit according to the present invention includes a first low-gain differential amplifier activated by a recording designation control signal and a low-gain second differential amplifier activated by a playback designation control signal.
a high-gain third amplifier that amplifies the output signal of the differential amplifier, and feeds back only the AC component of the output signal of the third amplifier to the input terminals of the first and second differential amplifiers; The first and second differential amplifiers are characterized in that they include switching circuits configured to operate as AC amplifiers.

Hereinafter, embodiments of the present invention will be described with reference to FIG.

In the noise reduction common circuit according to the present invention shown in FIG. 2, input terminals 11 and 12 are supplied with a recording input signal and a reproduction input signal, respectively. The first and second differential amplifiers 13 and 14 are each a low-gain differential amplifier consisting of, for example, a pair of transistors, and one input terminal of the differential amplifiers 13 and 14 is connected to the DC cut capacitors C1 and C2. input terminals 11 and 12 K are connected and resistor R5
, R6 to the reference potential v7. Also,
The remaining terminals of the differential amplifiers 13 and 14 are grounded through a series circuit of a resistor R8 and a capacitor C3. Furthermore, the differential amplifiers 13 and 14 are activated by a recording designation control signal and a reproduction designation control signal. For example, it is possible to configure the constant current supply to the differential pair of the differential amplifiers 13 and 14 to be turned on and off. The output terminals of the differential amplifiers 13 and 14 are connected to the input terminal of the third amplifier 15. The amplifier 15 is, for example, a multi-stage amplifier and has a high gain. The terminal of the amplifier 15 is connected to the input terminal of the variable characteristic circuit 6, and the function of the variable characteristic circuit 60 and the connection relationship between the output terminals 6a and 6b are the same as in the conventional example. A feedback resistor R7 is connected between the output terminal of the amplifier 15 and the common connection input terminal of the differential amplifiers 13 and 14, and a capacitor C3 is connected to the resistors R7 and R8.
An AC feedback circuit is formed.

In the noise reduction shared circuit according to the present invention configured as described above, when the recording designation and the playback designation are switched, the active states of the differential amplifiers 13 and 14 are switched. By configuring them in the same amplifier, the DC levels of both are almost equal to the reference potential vr and have a low gain, and the gain is 1 in terms of DC, so even if there is some variation in the offset of the internal active elements, the output potential will not fluctuate. is extremely small, and no click noise occurs. In addition, the resistor R8 and the capacitor Ca, ;l”
This grounding circuit is shared by the differential amplifiers 13 and 14, and when this noise reduction shared circuit is integrated into an IC, it is said that each of the differential amplifiers 13 and 14 is not required and contributes to a reduction in the number of components. Only one external pin for the capacitor is required between the resistor R8 and the capacitor C3, which also contributes to reducing the number of bins. Furthermore, by making the C1R5 and C2R60 time constants the same, click noise becomes extremely small even when recording/playback is switched in an unstable state when the power is turned on.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional example, and FIG. 2 is a block diagram showing an embodiment of the present invention. Explanation of symbols of main parts 11, 12... Input terminals 13, 14...
Differential amplifier 15...Third amplifier

Claims (1)

[Scope of Claims] a high-gain third amplifier circuit having an input terminal connected to both output terminals of the amplifier circuit; and a high-gain third amplifier circuit having an input terminal connected to both output terminals of the amplifier circuit; an AC input circuit that supplies an AC input signal to the other input terminal of each of the first and second differential amplifier circuits; and an AC input circuit that supplies the output signal of the third differential amplifier circuit to the first or second differential amplifier circuit; A noise reduction shared circuit comprising a characteristic variable circuit that compresses or expands according to a control signal. (2) The feedback circuit is connected at one end to the output terminal of the third amplifier, and a resistor circuit connected at the other end to the one input terminal of the first and second differential amplifiers; and a series circuit of a resistor and a capacitor connected between the other end of the resistor circuit and a reference potential point. Noise recovery S ratio according to claim 1, characterized in that
(3) The noise reduction FE [IBJ according to claim 2, characterized in that the capacitor of the series circuit is connected to the reference potential point side.