JPH07320205A - Recording/reproducing switching circuit - Google Patents

Abstract

PURPOSE:To generate an AC bias current with a simple and inexpensive oscillator by providing an adjustment means varying the output signal level of an AC bias generation circuit according to the output signal of a level detection circuit. CONSTITUTION:An audio signal current superimposed on the AC bias current flows through a magnetic head coil 13, and signal is recorded. The superimposed current flows through a resistor 23 used as the measurement resistor of a recording current to be converted into a voltage and to be impressed to an HPF 17. Only an AC bias signal is extracted from the HPF 17, and it is level detected by a level detection circuit 18. The detected signal is level-compared with a reference voltage by the circuit 18, and when a difference component exists, a value of a variable source 20 being an adjustment means is varied so that the difference is canceled. Then, the amplitude of the AC bias signal generated from the AC bias generation circuit 3 is varied, and the recording current flowing through the resistor 23 is varied. Then, finally, the detection level of the level detection circuit 18 becomes equal to the incorporated reference voltage, and a stable state is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording / reproducing switching circuit for recording / reproducing a signal by using a magnetic head for a recording medium such as a magnetic tape, and more particularly to a recording / reproducing switching circuit for preventing generation of noise. Regarding

[0002]

2. Description of the Related Art In a VTR in which a single magnetic head is used for both recording and reproduction, a tape head switching switch for switching the magnetic head is provided, and the switch is switched according to recording or reproducing operation. Need to take control. The magnetic head coil is also used in the recording mode and the reproducing mode. For example, in the recording mode, one end is grounded and the recording signal is applied from the other end. In the reproduction mode, the other end of the magnetic head coil is grounded and the reproduction signal is obtained from the one end. In that case, especially the side grounded in the reproduction mode must be completely grounded. This is because the gain of the reproduction amplifier that amplifies the signal from the magnetic head coil is as high as about 60 dB, and if even a small amount of noise leaks from the recording amplifier side, the original reproduction signal will be greatly disturbed.

Such a recording / reproducing circuit is disclosed in JP-A-62-1.
No. 65490.

[0004]

As described above, a transistor switch is required to be grounded in the reproduction mode. However, since an AC bias voltage of 100 V _PP is applied to the transistor during recording, the transistor switch is used. When the switch is integrated into an IC, the IC must be formed by a high breakdown voltage process. However, an element such as a transistor in an IC manufactured by a high breakdown voltage process is apt to generate noise, which is a problem.

[0005]

The present invention has been made in view of the above points, and a magnetic head coil for recording and reproducing a signal on a recording medium, and a recording amplifier connected to one end of the magnetic head coil. A reproduction amplifier connected to one end of the magnetic head coil, an oscillation bias generation circuit composed of a Colpitts oscillation circuit connected to the other end of the magnetic head coil, and the oscillation bias circuit operated during recording for reproduction. A switch for stopping the operation of the oscillation bias circuit and a control unit for controlling the amplitude of the AC output signal of the oscillation bias circuit are provided.

[0006]

According to the present invention, the amplitude of the AC output signal of the oscillation bias circuit is adjusted by the oscillation bias circuit itself and then output. Therefore, the amplitude is adjusted between the oscillation bias circuit and the magnetic head coil. No means is required, and both can be directly connected. Therefore, a transistor switch for grounding the other end of the magnetic head coil during reproduction is not required. Therefore, when the recording / reproducing switching circuit is integrated into an IC, it is not necessary to use a high breakdown voltage process, and it is possible to use a low nozzle and fine process, thereby reducing the chip area.

[0007]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.
(13) is a magnetic head coil having one end connected to the output side of the recording amplifier (5) and the other end connected to the AC bias generation circuit (3), (15) a switch closed during recording, and (16) a switch closed during reproduction. , (17) is an HPF that extracts only the AC bias current from the AC bias current and the signal current flowing through the magnetic head coil (13), and (18) is the HPF.
This is a level detection circuit that performs level detection of the output signal of (17) and changes the amplitude of the AC bias generation circuit (3) according to the detection output.

In FIG. 1, the same circuit elements as those in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted. First, at the time of recording, the switch (15) is closed and the switch (16) is opened. Further, when the switch (19) is closed, the AC bias generation circuit (3) starts operating according to the voltage from the variable power source (20). Then, the resonance coil (2
In 1), an AC signal having an AC bias of 70 KHz to 80 KHz is generated.

On the other hand, the audio signal from the input terminal (4) is
It is amplified by the recording amplifier (5) and applied to the magnetic head coil (13) through the resistor (23) and the capacitor (24). At this time, the resistor (23), the capacitor (24) and the magnetic head coil (13) form a series resonance circuit, and impart an equalizer characteristic to the audio signal. At this time, the impedance when the resonance coil (21) is viewed from the magnetic head coil (13) is about 20 to 30 [Ω], and is almost grounded. Therefore, the recording signal can be supplied to the magnetic head coil (13).

Therefore, an audio signal current superimposed on the AC bias current flows through the magnetic head coil (13) to record the signal. This superimposed current flows through the resistor (23) used as a recording current measuring resistor, is converted into a voltage, and is applied to the HPF (17). Then H
Only the AC bias signal is extracted from the PF (17), and its level detection is performed by the level detection circuit (18).
In the level detection circuit (18), the detected signal and the reference voltage are compared in level, and if there is a difference component, the value of the variable power supply (20) is changed so as to eliminate the difference. Then, the amplitude of the AC bias signal generated from the AC bias generating circuit (3) changes, and the recording current flowing through the resistor (23) changes. In this way, finally, the detection level of the level detection circuit (18) becomes equal to the built-in reference voltage, and a stable state is achieved.

Therefore, a desired recording current can be passed through the magnetic head coil (13) according to the reference voltage. During reproduction, the switch (15) is opened, the switch (16) is closed, and the switch (19) is opened. Then, the AC bias generation circuit (3) stops oscillation.
Then, the resonance coil (21) also has no AC bias signal, and one end of the magnetic head coil (13) is grounded. Since the output impedance of the recording amplifier (5) is low, the reproduction signal current from the magnetic head coil (13) is
It flows to the output end of the recording amplifier (5). Also in this case, the magnetic head coil (13), the capacitor (24) and the resistor (23) form a parallel resonance circuit, and give the reproduced signal a frequency characteristic. The playback signal with frequency characteristics is
It is amplified by the regenerative amplifier (10) via the capacitor (9) and the switch (16) and obtained at the output terminal (11).

Therefore, according to the circuit of FIG. 1, it is possible to record and reproduce a signal without connecting a switch to both ends of the magnetic head coil (1). The AC bias generation circuit (3) of FIG. 1 is a kind of Colpitts oscillation circuit called a clapper oscillation circuit, and the oscillation frequency is substantially determined by the resonance coil (21) and the resonance capacitor (22). The output terminal of the clapper oscillator circuit is normally led from the emitter of the transistor (23). Transistor (2
A large output as a current can be obtained from the emitter of 3), but a large output as a voltage cannot be obtained. For that reason,
If the emitter of the transistor (23) is directly connected to the magnetic head coil (13), the magnetic head coil (13)
Due to its high impedance (60 KΩ), there is a problem that a sufficient current cannot be supplied to the magnetic head coil (13). Therefore, in the present invention, a resonance coil (2
The oscillation output signal is obtained from one end of 1).

FIG. 2 shows another embodiment of the AC bias generating circuit (3) shown in FIG.
The point 3) is different from that of FIG. 1 in that the collector power source is connected to the variable power source 20 of FIG. As compared with that of FIG. 1, the oscillator circuit of FIG. 2 has a narrower variable range with respect to the control amount because the collector potential of the transistor (23) moves according to the control voltage.

[0014]

As described above, according to the present invention, it is possible to provide a recording / reproducing switching circuit capable of automatically adjusting the recording current. Further, according to the present invention, since the competes oscillating circuit is used, the AC bias current can be generated by an inexpensive oscillator having a simple structure as compared with the conventional oscillator which requires two coils.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a recording / reproducing switching circuit of the present invention.

FIG. 2 is a circuit diagram showing another embodiment of the AC bias generating circuit shown in FIG.

[Explanation of symbols]

 (3) AC bias generation circuit (5) Recording amplifier (10) Reproduction amplifier (13) Magnetic head coil (17) HPF (18) Level detection circuit

Claims (2)

[Claims] 1. A magnetic head coil for recording and reproducing a signal on a recording medium, a recording amplifier connected to the magnetic head coil, a reproduction amplifier connected to the magnetic head coil, and a connection to the magnetic head coil. An AC bias generation circuit including a Colpitts oscillator circuit, an HPF for detecting an AC bias current flowing through the magnetic head coil, a level detection circuit for level comparison between an output signal of the HPF and a reference voltage, and the level detection circuit. Adjusting means for changing the output signal level of the AC bias generating circuit in accordance with the output signal of the recording / reproducing switching circuit. 2. The recording / reproducing switching circuit according to claim 1, wherein the AC bias generating circuit includes a resonance coil, and the resonance coil is connected to the magnetic head coil.