JPH01188034A - Noise rejection circuit - Google Patents

Abstract

PURPOSE:To reduce noise by obtaining a recording signal through a low pass filter after signal compression processing at recording, passing a reproducing signal to a low pass filter before signal expansion processing at reproduction and applying peak detection to a signal after the low pass filter at all times. CONSTITUTION:The low pass filter 18 acts on a signal after encoding at recording and acts on a signal before decoding at reproduction. Thus, the noise generated from the low pass filter itself is suppressed lower after decoding. Suppose that a minimum gain of the gain control circuit 16 is -50dB, when the noise level of the low pass filter is the same as that of a conventional circuit, the conversion level at the filter output is advantageous by 50dB. Moreover, the input signal to a peak detection circuit 21 is a signal through the low pass filter 18 even at recording and reproduction. Thus, the error of peak detection and to noise invaded in the input is not more disadvantageous than that of the conventional circuit.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention is directed to the compression and This invention relates to a noise removal circuit that performs decompression.

(Prior Art) For example, in a video tape recorder, a signal processing section for recording and reproducing audio signals is provided with a noise removal circuit as shown in FIG. This circuit has the function of compressing the amplitude of a recorded signal and recording it on a recording medium during recording, and expanding and deriving a reproduced signal during reproduction, and has the effect of reducing noise caused by the characteristics of the recording medium.

The recording input signal is supplied to the terminal 3R of the Swiss-Sochi 3 via the terminal 1, and the reproduction input signal is supplied to the terminal 3P of the switch 3 via the terminal 2. Switch 3 selects the recording signal from terminal 1 during recording, and selects the reproduction signal from terminal 2 during playback. The signal selected by switch 3 is supplied to the non-inverting input terminal of operational amplifier 5 via low-pass filter 4 . The output terminal of the operational amplifier 5 is connected to the recording signal output terminal 9, and is also connected 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 reproduced signal output terminal 10 and also to the terminal 8R of the switch 8.

A terminal 8P of the switch 8 is connected to the output terminal of the operational amplifier 5, and a selection output of the switch 5 is connected to the inverting input terminal of the operational amplifier 5.

In the above noise removal circuit, during recording, the switches 3.8 are switched to the terminals 3R and 8R, respectively, and the recording input signal is supplied to the terminal 1. The input signal is input to an operational amplifier 5 via a low-pass filter 4, subjected to amplitude compression processing, and then output to a recording signal output terminal 9. The above circuit has a 2:1 logarithmic compression/expansion characteristic.

The compression characteristics during recording are set as the encoding characteristics A in FIG. In other words, the gain of the gain control circuit 6 is
It is controlled so that the signal at terminal 9 becomes larger as the level becomes smaller. Since the output of the gain control circuit 6 is supplied to the inverting input terminal of the operational amplifier 5, it is controlled so that the rate of change in the output is small relative to the rate of change in human power.
It will undergo compression processing.

Next, the expansion characteristics during reproduction are set as decoding characteristics B in FIG. Since the switch 8 is switched to the terminal 8P side, the reproduction input signal from terminal 2 to ' is
The operational amplifier 5 performs negative feedback gain control processing. Then, the signal is expanded by the control circuit 6 under the control of the peak detection circuit 7.

In other words, during playback, the rate of change in output is largely controlled relative to the rate of input tabulation. As a result, the overall characteristic of input and output can be expressed as characteristic C.

In the above circuit, the role of the low-pass filter 4 is to remove out-of-band noise. In particular, in video tape recorders, in addition to white noise, there is leakage of 2FLL (FLL: horizontal frequency), 38KHz (audio multiphase signal, FM signal subcarrier), etc., so peak detection does not malfunction due to interference waves. In order to do this, the above-mentioned low-pass filter 4 is necessary.

(Problems to be Solved by the Invention) When the above-mentioned noise removal circuit is integrated into an integrated circuit (IC), noise generated in the low-pass filter 4 itself becomes a problem. The reason for this is that the noise mixed in the encoded signal is suppressed after decoding, but the noise mixed in the pre-encoded signal is encoded and decoded as is. Therefore, the low-pass filter can be used as is.
In order to incorporate it into C, a filter with a very low noise level is required, which is difficult to implement.

[Means for Solving the Problem] This invention obtains a recorded signal through a low-pass filter after signal compression processing during recording, and obtains a recorded signal through a low-pass filter during playback before performing signal expansion processing. The signal is passed through a low-pass filter, and peak detection is always performed on the signal after passing through the low-pass filter.

(Operation) According to the above means, the noise generated by the low-pass filter itself is mixed into the encoded signal, so that it is suppressed to a low level when the signal is subjected to decoding processing during playback. Furthermore, if the transfer characteristics of the low-pass filter are set to be the same as the conventional ones, the transfer characteristics of the entire circuit will not change.

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

Figure 1 is an example of this invention, and it is input! 11 is supplied with one recording signal. The input terminal 11 is connected to a terminal 12R of the switch 12. The output terminal of switch 12 is connected to the non-inverting input terminal of operational amplifier 13. The output terminal of the operational amplifier 13 is connected to the terminal 14P of the switch 14 and the terminal 1 of the switch 15.
5R, and further connected to the gain control circuit 16. The output terminal of the gain control circuit 16 is connected to the terminal 14R of the switch 14 and also to the reproduced signal output terminal 17. The output terminal of switch 14 is connected to the inverting input terminal of operational amplifier 13.

By switching the switch 14 to the inverting input terminal of the operational amplifier 13, the output of the operational amplifier 13 is set to negative during playback.
The output of the gain control circuit 16 can be negatively fed back during recording.

A terminal 15P of the switch 15 is connected to the reproduced signal input terminal 20, and an output terminal is connected to the low-pass filter 18.

The output end of the low-pass filter 18 is connected to a recording signal output terminal 19, a terminal 12p of the switch 12, and further connected to a peak detection circuit 21.

The peak detection circuit 21 detects the peak level of the output from 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 lower the level of the signal at the output terminal 19, the greater the gain.

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

First, during recording, the switches 12, 14.15 are switched to the terminals 12R, 14R, and 15R, respectively. Therefore, operational amplifier 13. A signal compression process is performed in the circuit section formed by the gain control circuit 16, and the signal subjected to the compression process is supplied to the low-pass filter 18 via the switch 15. Here, components outside the desired band are removed and the output terminal 19
is derived as a recording signal.

During playback, the switches 12, 14, and 15 are switched to the terminals 12P, 14P, and 15P, respectively. The input signal for reproduction is introduced into the terminal 20 and input into the low-pass filter 18 via the switch 15.

The output of the low-pass filter 18 is supplied via the switch 12 to the non-inverting input of the operational amplifier 13.

The output of the operational amplifier 13 is expanded by a gain control circuit 16 and output to a reproduction signal output terminal 17.

As described above, in this embodiment, during recording, the recorded signal is obtained through a low-pass filter after signal compression processing, and during playback, the reproduced signal is passed through a low-pass filter before signal expansion processing, and The configuration is such that peak detection is always performed on the signal after passing through the low-pass filter.

VIN is the recording/playback manual power, and VOR is the recording output signal.

The reproduction output signal is vop, the transfer characteristic of the low-pass filter is HLPF, the gain of the gain control circuit 16 is GCCA,
Let it be α times the human power (α: constant fault). Here, the operational amplifier 13
The gain of is assumed to be sufficiently high.

First, let's find the transfer characteristics of the conventional circuit.

During recording H1, Pr' X V IN-V OPV OP-G
CCA
IN)-HLPP during playback X V IN-VOR-(1)VOP=G
CCA

VIN when recording VOP VOP-GCCA X (VOR/HLPP)-
a VORX (VOR/HLPr')VOR=
(1/a)XHLPP x%NNl 2 ・=
(3) HLPP x V IN -V 0R VOP-GCCA x VOR during playback αVORxVOl? VOR-a (HLPt' x V IN) 2- (
4) From the above equations, equation (1), equation (3), equation (2), and (
It can be seen that equation 4) represents the transfer characteristic. The above is the transfer characteristic for the frequency sweep response, but let's consider what happens to the output frequency spectrum for white noise. In this case, the peak detection output is a fixed amount, and therefore the gain GCC^ can be considered to be a constant gain and a 9 value.

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

During recording HLPF X V IN-V 0P VOP-GCCA X VOR Therefore VOR-(HLPP IC, CCΔ)
- (5) On the other hand, in the case of the circuit of the embodiment, the situation is as follows.

When recording VIN-VOP VOP-GCCA X (VOR/HLPP')
Therefore, VOP= (HLPF /GCCA) xV
It can be seen that IN-(6) has the same frequency spectrum as Equations (5) and (6) as described above. Note that during playback, it is clear from the equation for the frequency sweep response that they have the same frequency spectrum.

With the configuration of this embodiment, the low-pass 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 150 dB, the converted level at the final output will be 50 dB more advantageous than the conventional circuit if the noise level of the low-pass filter is the same.

Furthermore, the input signal to 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 reason, there is no disadvantage compared to the conventional method regarding peak detection malfunctions due to noise mixed into the input.

[Effects of the Invention] As explained above, according to the present invention, even if the low-pass filter is configured as a filter with a built-in IC as is commonly done, it is possible to suppress the noise level from greatly affecting the final output. can.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a conventional noise removal circuit, and FIG. 3 is a characteristic diagram shown to explain the operation of the noise removal circuit. 12.14.15... Switch, 13... Operational amplifier, 16... Gain control circuit, 18... Low pass filter, 21... Peak detection circuit. Applicant's agent Patent attorney Takehiko Suzue

Claims (1)

[Claims] an operational amplifier; and a gain control circuit to which the output of the operational amplifier is supplied and whose output terminal is connected to the reproduction signal output terminal; during recording, the output of the gain control circuit is introduced to the inverting input terminal of the operational amplifier; during reproduction, the output of the gain control circuit is connected to the inverting input terminal of the operational amplifier; a first switch means for introducing the output of the operational amplifier directly into the inverting input; a first switch means for introducing the output of the operational amplifier into a low-pass filter during recording, and using the output of the low-pass filter as a recording signal output; and a first switch means for introducing the output of the operational amplifier directly into the inverting input; a second switch means for introducing an input signal into the low-pass filter; a second switch means for introducing a recording input signal into the non-inverting input terminal of the operational amplifier during recording; and a second switching means for introducing the input signal for recording into the non-inverting input terminal of the operational amplifier during reproduction; a third switch means introduced into the inverting input terminal; and a peak detection circuit which detects the peak of the output of the low-pass filter and supplies the output to the gain control terminal of the gain control circuit, and when recording. The input signal is compressed and then passed through the low-pass filter to be output as a recording signal, and during playback, the input signal for reproduction is passed through the low-pass filter and then expanded to be output as a reproduced signal. Noise removal circuit.