JP2804036B2 - Noise removal circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention] (Industrial application field) The present invention relates to a method for compressing a signal when recording or reproducing an audio signal on a recording medium such as a magnetic tape. The present invention relates to a noise removing circuit for performing expansion.

(Prior Art) For example, in a video tape recorder, a signal processing unit for recording and reproducing an audio signal is provided with a noise removing circuit as shown in FIG. This circuit has a function of compressing the amplitude of a recording signal during recording and recording the signal on a recording medium, and extending and deriving a reproduction signal during reproduction, and has a function of reducing noise generated due to characteristics of the recording medium.

The input signal for recording is supplied to the terminal of the switch 3 through the terminal 1.
The input signal for reproduction is supplied to the terminal 3 P of the switch 3 via the terminal 2. The switch 3 selects a recording signal from the terminal 1 during recording, and selects a reproduced signal from the terminal 2 during reproduction. The signal selected by the switch 3 is supplied to the non-inverting input terminal of the operational amplifier 5 via the reduction filter 4. The output terminal of the operational amplifier 5 is connected to the recording signal output terminal 9 and to the input terminals of the gain control circuit 6 and the peak detection circuit 7. The peak detection circuit 7 controls the gain of the gain control circuit 6 in proportion to the signal amplitude on the input side of the gain control circuit 6.

The output terminal of the gain control circuit 6 is connected to the reproduction signal output terminal 10 and to the terminal 8R of the switch 8. The terminal 8P of the switch 8 is connected to the output terminal of the operational amplifier 5,
The selected output of the switch 5 is connected to the inverting input terminal of the operational amplifier 5.

In the above-described noise elimination circuit, at the time of recording, the switches 3 and 8 are switched to the terminals 3R and 8R, respectively, and the input signal for recording is supplied to the terminal 1. The input signal is input to the operational amplifier 5 via the low-pass filter 4, undergoes amplitude compression processing, and is led out to the recording signal output terminal 9. The above circuit has a 2: 1 logarithmic compression / expansion characteristic.

The compression characteristic at the time of recording is set like the encoding characteristic A in FIG. That is, the gain of the gain control circuit 6 is controlled so as to decrease as the signal at the terminal 9 decreases. The output of the gain control circuit 6 is
Are controlled so that the rate of change of the output is smaller than the rate of change of the input, and undergoes compression processing.

Next, the decompression characteristic at the time of reproduction is set like the decoding characteristic B in FIG. The input signal for reproduction from the terminal 2 is generated because the switch 8 is switched to the terminal 8P side.
The signal is input to the gain control circuit 6 via the operational amplifier 5 in a voltage follower connection. Then, in the gain control circuit 6,
An extension process is performed under the control of the peak detection circuit 7. That is, at the time of reproduction, the rate of change of the output is controlled to be greater than the rate of change of the input. As a result, the total characteristic of the input and output can be expressed as a characteristic C.

In the above circuit, the role of the low-pass filter 4 is to remove noise outside the band. Especially in video tape recorders, in addition to white noise, 2fH (fH: horizontal frequency), 38
Since there is leakage of Khz (subcarrier of an audio multiplex signal or FM signal) or the like, the low-pass filter 4 is necessary to prevent the peak detection from malfunctioning due to an interference wave.

(Problems to be Solved by the Invention) When the above-described noise elimination circuit is formed into an integrated circuit (IC), noise generated by the low-pass filter 4 itself becomes a problem. The reason is that the noise mixed in the signal after encoding is kept low after decoding, but the noise mixed in the signal before encoding is encoded and decoded as it is. Therefore, the low-pass filter can be
In order to incorporate such a filter, a filter having a very low noise level is required, which is difficult to realize.

[Constitution of the Invention] (Means for Solving the Problems) According to the present invention, a recording signal is obtained through a low-pass filter after a signal compression signal at the time of recording, and the reproduction signal is reduced before performing signal expansion processing at the time of reproduction. The signal is passed through a band-pass filter, and the peak detection is always performed on the signal after passing through the low-pass filter.

(Operation) According to the above-described means, since the noise generated by the low-pass filter itself is mixed into the signal after the encoding, the noise is suppressed to a low level when the decoding process is performed during reproduction. If the transfer characteristics of the low-pass filter are the same as those of the related art, the transfer characteristics of the entire circuit do not change.

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

FIG. 1 shows an embodiment of the present invention. A recording signal is supplied to an input terminal 11. The input terminal 11 is connected to the terminal 12R of the switch 12. The output terminal of the switch 12 is connected to the non-inverting input terminal of the operational amplifier 13. Operational amplifier 13
The output terminal of the switch 14 is connected to the terminal 14P of the switch 14, is connected to the terminal 15R of the switch 15, and is further connected to the gain control circuit 16. An output terminal of the gain control circuit 16 is connected to a terminal 14R of the switch 14 and to a reproduction signal output terminal 17. The output of switch 14 is an operational amplifier
Connected to 13 inverting inputs.

By switching the switch 14 to the inverting input terminal of the operational amplifier 13, the output of the operational amplifier 13 can be negatively fed back during reproduction, and the output of the gain control circuit 16 can be negatively fed back during recording.

The terminal 15P of the switch 15 is connected to the reproduction signal input terminal 20, and the output terminal is connected to the low-pass filter 18. The output end of the low-pass filter 18 is connected to the recording signal output terminal 19, is connected to the terminal 12P of the switch 12, and is further connected to the peak detection circuit 21.

The peak detection circuit 21 detects the peak level of the output of the output terminal 19, and supplies the output to the gain control terminal of the gain control circuit 16. The gain control circuit 16 is controlled so that the smaller the signal at the output terminal 19 is, the lower the gain is.

This embodiment is configured as described above, and the operation will be described below.

First, at the time of recording, the switches 12, 14, and 15
Switch to R, 14R, 15R side. Therefore, in the circuit section formed by the operational amplifier 13 and the gain control circuit 16, signal compression processing is performed, and the signal subjected to the compression processing is supplied to the low-pass filter 18 via the switch 15. Here, components outside the desired band are removed, and the resulting signal is output to the output terminal 19 as a recording signal.

During playback, switches 12, 14, and 15 are connected to terminals 12P and 14 respectively.
Switch to P, 15P side. The input signal for reproduction is introduced into the terminal 20 and inputted to the low-pass filter 18 via the switch 15. The output of the low-pass filter 18 is supplied to the non-inverting input terminal of the operational amplifier 13 via the switch 12. The output of the operational amplifier 13 is expanded by the gain control circuit 16 and
Derived to 17.

As described above, in the present embodiment, a recording signal is obtained through a low-pass filter after signal compression processing during recording, a reproduced signal is passed through a low-pass filter before signal expansion processing is performed during reproduction, and The peak detection is configured to always be performed on the signal after passing through the low-pass filter.

The recording / reproducing input is VIN, the recording output signal is VOR, the reproducing output signal is VOP, the transfer characteristic of the low-pass filter is HLPF, the gain of the gain control circuit 16 is GCCA, and α times the input (α: constant). . Here, it is assumed that the gain of the operational amplifier 13 is sufficiently high.

 First, the transfer characteristics of the conventional circuit will be obtained.

During recording HLPF × VIN = VOP VOP = GCCA × VOR = αVOR × VOR Therefore, VOR = {(1 / α) × HLPF × VIN} ^1/2 (1) During reproduction HLPF × VIN = VOP VOP = GCCA × VOR = ΑVOR × VOR Therefore, VOP = α (HLPF × VIN) ² (2) On the other hand, the circuit of this embodiment is as follows.

During recording VIN = VOP VOP = GCCA × (VOR / HLPF) = αVOR × (VOR / HLPF) VOR = {(1 / α) × HLPF × VIN} ^1/2 (3) During playback HLPF × VIN = VOR VOP = GCCA × VOR = αVOR × VOR VOP = α (HLPF × VIN) ² (4) From the above equations, the same transfer characteristics are obtained by the equations (1) and (3), and the equations (2) and (4). It shows that it shows. The transfer characteristics for the frequency sweep response have been described above, but what happens to the output frequency spectrum for white noise will be considered. In this case, the peak detection output is a fixed amount, and therefore, it can be considered that the gain GCCA is equivalent to a constant gain.

 In this case, the transfer characteristics of the conventional circuit are as follows.

At the time of recording HLPF × VIN = VOP VOP = GCCA × VOR Therefore, VOR = (HLPF / GCCA) × VIN (5) On the other hand, in the case of the circuit of the embodiment, the following is obtained.

At the time of recording VIN = VOP VOP = GCCA × (VOR / HLPF) Therefore, VOR = (HLPF / GCCA) × VIN (6) As described above, the equations (5) and (6) may have the same frequency spectrum. Recognize. At the time of reproduction, it is apparent from the equation for the frequency sweep response that the frequency spectrum has the same frequency spectrum.

With the configuration as in this embodiment, the reduction filter 18 acts on the signal after encoding during recording, and acts on the signal before decoding during reproduction. Therefore, the noise generated by the low-pass filter itself is kept low after decoding. Assuming that the minimum gain of the gain control circuit 16 is -50 dB, if the noise level of the low-pass filter is the same as that of the conventional circuit, the conversion level at the final output becomes as advantageous as 50 dB.

The input signal of the peak detection circuit 21 is a signal that has passed through the low-pass filter 18 both during recording and during reproduction. For this,
Malfunction of peak detection due to noise or the like mixed into the input is not disadvantageous as compared with the related art.

[Effects of the Invention] As described above, according to the present invention, even if a low-pass filter is configured as an IC built-in filter so as to be generally performed, it is possible to suppress the noise level from greatly affecting the final output. it can.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a conventional noise elimination circuit, and FIG. 3 is a characteristic diagram shown to explain the operation of the noise elimination circuit. 12, 14, 15 ... switch, 13 ... operational amplifier, 16 ... gain control circuit, 18 ... low-pass filter, 21 ... peak detection circuit.

Claims (4)

(57) [Claims] 1. An apparatus for recording on a recording medium via signal compression means for compressing a signal to be recorded by increasing the gain of the signal at a lower level during recording, wherein a low-frequency component of the compressed signal to be recorded is reduced. , Using a first low-pass filter to extract and output a recording signal,
Detecting the amplitude of the output signal of the first low-pass filter;
A noise elimination circuit, which is used as an amplitude control signal for signal compression in the signal compression means based on the detection signal. 2. The signal compression means has an input connected to a non-inverting input of an operational amplifier, an output connected to an output of the operational amplifier, and an inverting input of the operational amplifier via a first gain control circuit. 2. The noise elimination circuit according to claim 1, wherein the amplitude control signal controls the gain of said first gain control circuit. 3. The recording signal is taken out as a reproduction signal from a recording medium which has been recorded through a signal compression means for compressing a signal to be recorded by increasing the gain of a signal having a smaller level in advance during recording. An apparatus that outputs the reproduced signal through signal expansion means that expands the reproduced signal by reducing the gain as the signal has a smaller level, wherein a low-frequency component of the reproduced signal is extracted using a second low-pass filter. The signal is expanded through the signal expansion unit, and the amplitude of the output signal of the second low-pass filter is detected, and the detected signal is used as an amplitude control signal for signal expansion in the signal expansion unit. A noise elimination circuit characterized by the above-mentioned. 4. The signal decompressing means comprises a second gain control circuit, and the gain of the second gain control circuit is controlled by the amplitude control signal. Noise removal circuit.