JP3363612B2 - Noise reduction circuit of recording / reproducing device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a noise reduction circuit in a recording / reproducing apparatus for recording / reproducing a signal on a magnetic tape or the like.

[0002]

2. Description of the Related Art When an audio signal is recorded on a tape and reproduced, a large-amplitude signal is lowered below the saturation level of the tape, a minute signal is amplified so as to be sufficiently distinguished from noise, and a large signal and a small signal are amplified. It is recorded on the tape with a small relative difference between (compression) and the opposite process (expansion) during playback.
"Dynamic compression / expansion" is known. The compression / expansion of the dynamic range suppresses signal noise, reduces distortion, and expands the dynamic range. The situation is shown in FIG. Figure 2
The dynamic range of the original signal before recording, the dynamic range of the compressed signal, and the dynamic range of the expanded signal are shown.

FIG. 3 shows a noise reduction circuit in a recording / reproducing apparatus for compressing / expanding such a dynamic range. During reproduction, the switch (1) is PB (reproduction).
, The compressed signal (audio signal) from the cassette tape recorder or VTR is applied from the input terminal (2) and applied to the positive input terminal (+) of the differential amplifier (3) to be amplified and then a full-wave rectifier circuit. It is rectified by (4) and smoothed by a smoothing circuit (5).

On the other hand, the output signal of the differential amplifier (3) is CC
It is amplified by A (current control amplifier) (6), and its amplification factor depends on the level detection output of the smoothing circuit (5), and if the level of the input signal increases, CCA
The gain of (6) also increases. Therefore, the input / output characteristics between the input terminal (2) and the output terminal (7) at this time are as shown by the solid line C in FIG. The ordinate and abscissa of FIG. 4 are displayed in logarithm, and a signal having a level of 1 (0 on the axis) is used as a reference, and a signal larger than this is increased in level, and a signal smaller than that. On the other hand, the level has been reduced. The solid line B of FIG. 4 shows the amplification factor in the relationship of 1: 1 input and output, but the solid line C has a steeper curve than the solid line B, and the large signal is larger and the small signal is smaller. Will be done.

On the other hand, when recording, the switch (1) is set to RE.
Since it is on the C side, the negative input terminal (-) of the differential amplifier (3)
Negative feedback is applied from CCA (5) to level control, which is exactly the opposite level of the expansion described above, and a compressed signal is obtained at the output terminal (7). The characteristic at this time is as shown by the solid line A. Therefore, according to the circuit of FIG. 3, compression / expansion of the dynamic range can be performed.

[0006]

FIG. 5 is a characteristic diagram showing the magnitude of gain with respect to frequency at the time of REC of the differential amplifier (3) of FIG. ) Is the characteristic curve when the open loop gain is set, and the dotted line shows the closed loop gain (+
50 dB) and a closed loop gain (-10 when a large signal is input)
dB) is shown. The gain of the differential amplifier (3) in FIG. 3 needs to be changed from +50 dB to −10 dB as shown in FIG. 5, but when the gain of −10 dB is taken, the signal input is large, so that the open loop gain is large. There is a possibility of oscillation due to phase rotation. Therefore, if the open loop gain is reduced as indicated by the chain double-dashed line in FIG. 5, the risk of oscillation is reduced, but this time there is the problem that the level of the high frequency signal is reduced.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and is a noise reduction circuit of a recording / reproducing apparatus for compressing a signal for recording and expanding the compressed signal for reproduction. And a variable gain type differential amplifier to which a recording or reproduction signal is applied to one input terminal, a level detecting means for performing level detection of an output signal of the differential amplifier, and the variable gain type differential amplifier. A variable gain amplifier for amplifying the output signal of the amplifier according to the output signal of the level detection means, and a feedback means for applying the output signal of the variable gain amplifier to the other input terminal of the variable gain type differential amplifier during recording. And adjusting means for changing the gain of the variable gain type differential amplifier according to the output signal of the level detecting means, and the recording signal is obtained from the output terminal of the variable gain type differential amplifier. Characterized by To.

[0008]

According to the present invention, the gain of the differential amplifier is changed by the output signal of the level detecting means for detecting the level of the output signal of the differential amplifier. When the level of the output signal of the differential amplifier is low, the open gain of the differential amplifier is increased. Then, the frequency characteristic of the differential amplifier can be extended to a high range. When the level of the output signal of the differential amplifier is high, the open gain of the differential amplifier is reduced. Then, the amount of feedback to the differential amplifier is reduced (from A to B in FIG. 5) and the risk of oscillation is reduced.

Therefore, a signal can be stably transmitted from a small input to a large input.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a noise reduction circuit of a recording / reproducing apparatus according to the present invention. Is a variable current source whose current value increases when the output signal of the differential amplifier (3) is large.

In FIG. 1, the same circuit blocks as those in FIG. 3 are designated by the same reference numerals and the description thereof will be omitted. Figure 1
The smoothing circuit (5) is originally necessary for controlling the gain of the CCA (6) and converts the magnitude of the output signal of the differential amplifier (3) into a DC voltage.

In FIG. 1, when the output signal of the differential amplifier (3) is small, the current value of the variable current source (8) increases and the gain of the differential amplifier (3) increases. When the gain of the differential amplifier (3) increases, the open loop gain of the differential amplifier (3) increases as shown by the alternate long and short dash line in FIG. 5, and the frequency characteristic of the differential amplifier (3) can be extended to a high frequency range. it can. Therefore, even a high frequency signal can be transmitted.

Next, when the output signal of the differential amplifier (3) is large, the current value of the variable current source (8) decreases, and the gain of the differential amplifier (3) decreases. Differential amplifier (3)
When the gain of the differential amplifier decreases, the open loop gain of the differential amplifier (3) decreases as indicated by the chain double-dashed line in FIG. 5, and the amount of feedback to the differential amplifier (3) decreases (from A to B in FIG. 5). The risk of oscillation is reduced.

Therefore, according to the circuit of FIG. 1, signals can be stably transmitted from a small input to a large input.

[0015]

As described above, according to the present invention, the gain of the differential amplifier is changed by the output signal of the level detecting means for detecting the level of the output signal of the differential amplifier. When the level of the output signal is low, the open gain of the differential amplifier is increased, so that the frequency characteristic of the differential amplifier can be extended to a high frequency band. Further, when the level of the output signal of the differential amplifier is high, the open gain of the differential amplifier is reduced, so that the amount of feedback to the differential amplifier is reduced and the risk of oscillation is reduced. Therefore, a noise-removed signal can be stably transmitted from a small input to a large input.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a noise reduction circuit of a recording / reproducing apparatus of the present invention.

FIG. 2 is a characteristic diagram for explaining a noise reduction circuit of a conventional recording / reproducing apparatus.

FIG. 3 is a circuit diagram showing a noise reduction circuit of a conventional recording / reproducing apparatus.

FIG. 4 is a characteristic diagram for explaining a noise reduction circuit of a conventional recording / reproducing apparatus.

FIG. 5 is a characteristic diagram for explaining a noise reduction circuit of the recording / reproducing apparatus.

[Explanation of symbols]

(3) Differential amplifier (5) Smoothing circuit (6) CCA (8) Variable current source

Claims (2)

(57) [Claims] 1. A noise reduction circuit of a recording / reproducing apparatus for compressing a signal for recording and expanding the compressed signal for reproduction, wherein a gain for applying a recording or reproducing signal to one input terminal. Variable-type differential amplifier, level detection means for detecting the level of the output signal of the differential amplifier, and variable for amplifying the output signal of the variable-gain type differential amplifier according to the output signal of the level detection means. A gain amplifier; feedback means for applying an output signal of the variable gain amplifier to the other input terminal of the variable gain type differential amplifier during recording; and a variable gain type difference according to an output signal of the level detection means. Adjusting means for changing the open gain of the dynamic amplifier,
And a recording signal is obtained from an output end of the variable gain type differential amplifier. 2. The gain of the variable gain differential amplifier is increased when the level of a signal applied to one input terminal of the variable gain differential amplifier is low, and the gain of the variable gain differential amplifier is increased. 7. The adjusting means operates so as to reduce the gain of the variable gain differential amplifier when the level of the signal applied to one input terminal of the dynamic amplifier is high. 2. The noise reduction circuit of the recording / reproducing apparatus according to 1.