JPH01307971A - Noise eliminating circuit - Google Patents

Abstract

PURPOSE:To completely reproduce a recording signal waveform by constituting a compressing and expanding circuit and a feedback path of an operational amplifier and allowing an output of the operational amplifier to have an enphasis characteristic in a recording state and allowing a reproducing signal to have a de-emphasis characteristic in a reproducing state. CONSTITUTION:An output of an operational amplifier 13 is supplied directly to an FM processing part 19, and also, supplied to a noise reduction (NR) de-emphasis circuit 14 and a peak detecting circuit 17 through an FM de-emphasis circuit 21. Also, a reproducing signal VINP outputted from the FM processing part 19 at the time of reproduction is supplied to a positive input terminal + of the operational amplifier 13 through a recording and reproducing switch 12. That is, at the time of recording, the FM de-emphasis circuit 21 is interposed in a feedback path of the operational amplifier 13, therefore, after a recording signal VINR has been brought to 1/2 logarith mic compression, an FM emphasis processing can be performed, and at the time of reproduction, before the reproducing signal VINP is brought to logarithmic expanding of two times, the FM de-emphasis processing can be executed by the same FM de- emphasis circuit 21 as the time of recording. In such a way, a recording signal waveform can be reproduced completely.

Description

[Detailed Description of the Invention] [Objective of the Invention] (Industrial Application Field) The present invention is directed to the compression and This paper relates to improvements in noise removal circuits that perform decompression.

(Prior Art) As is well known, for example, the high sound quality of VH3 system (HI Fi
) In a video tape recorder (VTR), when recording an audio signal on a recording medium such as a tape or reproducing it from a recording medium, the signal is compressed and expanded to remove noise. FIG. 3 shows such a conventional noise removal circuit. This noise removal circuit has a 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 function of reducing noise caused by the characteristics of the recording medium.

During recording, the recording signal VINR output from the input/output signal processing section 11 of the VTR is supplied to the positive input terminal + of the operational amplifier 13 via the recording/reproducing switch 12 . The output of this operational amplifier 13 is connected to an NR (noise reduction) de-emphasis circuit 14 and a variable gain circuit (hereinafter G
After passing through a feedback path consisting of CA) 15, the signal is fed back to the negative input terminal of the operational amplifier 13 via the recording/reproducing switch 1B. At the same time, the output of the operational amplifier 13 is subjected to amplitude detection by the peak detection circuit 17 and becomes a control voltage for the GCA 15.

Therefore, the operational amplifier 13 outputs a signal whose amplitude is compressed to 1/2 of the recording signal VINR.

This signal is subjected to emphasis processing by the FM emphasis circuit 18 to become an output signal VOR, which is recorded on a tape (not shown) by the head 20 via the FM processing section 19.

NR noise reduction circuit 14. G CA 15.
Peak detection circuit 17 and FM emphasis circuit ■8
The overall transfer function is calculated assuming that the frequency characteristics and its transfer function are as shown in FIG.

First, regarding the feedback path from the output terminal of the operational amplifier 13 to the negative input terminal 1, assuming that the gain of the operational amplifier 13 is infinite and that the negative input terminal 1 is equal to the recording signal VINH, VINR-(GGCA / HNR-)IFM) VOR
...(1). Regarding the path of the peak detection circuit 17, if α is the proportional constant of the gain of the GCA 15 with respect to the peak detection output, GGCA -a (HWH/HFM) VOR・(2
).

When equations (1) and (2) are solved simultaneously, VOR-HF
M [(1/ a) (HNR/HWE) VINR
]”...(3) is obtained. From equation (3), the amplitude of the recording signal VINH is 1/
It is confirmed that the logarithm has been compressed to 2.

During playback, it is read by the bed 20 and sent to the FM processing unit 1.
The reproduced signal VINP outputted from the FM de-emphasis circuit 21 and the recording/reproducing switch 12 is supplied to the positive input terminal of the operational amplifier 13 .

This operational amplifier 13 is connected to the recording/reproducing switch 1 during reproduction.
6 is in a switching position opposite to that shown in the figure, so its output terminal and negative input terminal 1 are connected and used as a voltage follower.

This voltage follower output is connected to the NR de-emphasis circuit 4 and G CA 15, which served as a return path during recording.
The output signal becomes the OP via the input/output signal processing section 11.
is used for audio playback. At the same time, the voltage follower output is amplitude-detected by the peak detection circuit 17, and the G
This is the control voltage for CA15.

In this way, the NR de-emphasis circuit 14 and GCA 15, which were the feedback path of the operational amplifier 18 during recording,
During playback, it will be inserted into the direct signal path. Therefore, in contrast to the 1/2 logarithmic pressure operation during recording, twice the logarithmic expansion operation is performed during reproduction.

Assuming that the frequency characteristics of the FM de-emphasis circuit 21 and its transfer function are as shown in FIG. 3, the overall transfer function during reproduction is calculated in the same manner as during recording. first,
Regarding the signal path from FM de-emphasis circuit 21 to GCA15, VOP-(GGCA/HNR-HF
M") VINP (4). Also, regarding the path of the peak detection circuit 17, 1. As in the case of recording, if α is the proportional constant of the gain of G CA 15 with respect to the peak detection output, then GGCA -a ( HWE/HP)I)
VINP −=C5).

When equations (4) and (5) are solved simultaneously, V 0P-(
1/ HFM″2) [α(RWE/HNR) IVINP
2...(6) is obtained. From equation (6), it is confirmed that the amplitude of the reproduced signal VINP is logarithmically expanded by twice.

Here, when considering the recording and reproducing system, theoretically, V INP - V OR - (7) should be obtained, so equations (3), (6), and (7)
If we combine the equations and solve the relationship between the recording signal VINR and the output signal vOP, we get: VOP-(Hr'M/HI'M-) 2VINR...
(8) becomes. As is clear from equation (8), if the transfer characteristic HFM of the FM emphasis circuit 18 and the transfer characteristic HI'M of the FM de-emphasis path 21 are in a completely opposite relationship, that is, HPM-HPM-. If so, the equation (8) becomes V OP - V INR (9), and the recorded waveform can be perfectly reproduced in the recording/reproducing system.

However, in reality, the FM emphasis circuit 18 and the FM
It is often provided independently of the de-emphasis circuit 21, and the transfer functions of the two do not have completely opposite characteristics, resulting in the problem that the signal waveform cannot be completely reproduced.

Therefore, conventionally, the part surrounded by the dotted line in Figure 3 was
It is being considered to replace it with a configuration as shown in the figure. During recording, the recording/reproducing switch 22 is set to the illustrated switching position, and the FM de-emphasis circuit 23 is switched to the operational amplifier 24.
Emphasis processing is performed on the output of the operational amplifier 13 to obtain the output signal VOR, and at the time of reproduction, the recording/reproducing switch 22 is set to the opposite position to that shown in the figure, and the above-mentioned FM A de-emphasis circuit 22 performs de-emphasis processing.

According to such a configuration, the same FM de-emphasis circuit 23 is used for emphasis processing and de-emphasis processing while the operational amplifier 24 is used to share the time constant, so that the emphasis characteristics and de-emphasis characteristics are can be set so that they have a completely opposite relationship, making it possible to perfectly reproduce the recorded waveform.

However, with the means shown in FIG. 4, since the recording/reproducing switch 22 and the operational amplifier 24 are newly required, there arises a problem that the number of parts is large and the structure becomes large, which is economically disadvantageous.

(Problems to be Solved by the Invention) As described above, in conventional noise removal circuits, it is difficult to provide a completely opposite relationship between emphasis characteristics and de-emphasis characteristics within a practical range due to the configuration. ,
The problem is that the recorded signal waveform cannot be completely reproduced.

Therefore, the present invention has been made in consideration of the above circumstances, and it is an object of the present invention to provide an extremely good noise removal circuit that is capable of perfectly reproducing the recorded signal waveform, has a simple configuration, and is economically advantageous. With the goal.

[Structure of the Invention (Means for Solving the Problem) This invention provides an operational amplifier to which a recording signal and a reproduction signal are selectively supplied to one input terminal in a recording state and a reproduction state, and an operational amplifier in which a recording signal and a reproduction signal are selectively supplied to one input terminal in a recording state and a reproduction state It is interposed between the output end of the operational amplifier and the other input end to form a feedback path, and the operational amplifier generates a signal that is a compressed recording signal. A compression/expansion circuit that generates an expanded signal is interposed between the input terminal of this compression/expansion circuit and the output terminal of the operational amplifier, and in the recording state, together with the compression/expansion circuit, constitutes a return path of the operational amplifier. The device is configured to include a de-emphasis circuit that gives an emphasis characteristic to the output and a de-emphasis characteristic to the reproduced signal outputted via the operational amplifier in the reproduction state.

(Function) According to the above configuration, the same de-emphasis circuit can be used to obtain the emphasis characteristic and the de-emphasis characteristic, so that the recording signal waveform can be perfectly reproduced. Further, there is no need to newly install a recording/reproducing switch, an operational amplifier, etc. as in the conventional case, and the configuration is simple and economically advantageous.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings. In FIG. 1, the same parts as in FIG. 3 are indicated by the same symbols, and only the different parts will be described here. That is, the FM emphasis circuit 18 is eliminated, the output of the operational amplifier 13 is directly supplied to the FM processing section 19, and the output of the operational amplifier 13 is directly supplied to the FM processing section 19, and the N
The signal is supplied to the R de-emphasis circuit 14 and the peak detection circuit 17. Also, during playback, the FM processing section 1
The reproduction signal VINP outputted from 9 is supplied to the positive input terminal of the operational amplifier I3 via the recording and reproduction switch 12.

According to the above configuration, during recording, the operational amplifier 1
Since the FM de-emphasis circuit 21 is interposed in the feedback path of No. 3, the F
M emphasis processing can be applied, and during playback, the same FM de-emphasis circuit 21 as used during recording performs FM de-emphasis processing before logarithmically expanding the playback signal by twice VINP.
De-emphasis processing can be performed.

Therefore, the FM emphasis characteristic and the FM de-emphasis characteristic can be in a completely opposite relationship, and the recorded waveform can be perfectly reproduced.

In addition, in terms of circuitry, the conventional FM emphasis circuit 18 is eliminated, and there is no need to newly install a recording/reproduction switch or an operational amplifier, so the configuration is simple, suitable for miniaturization, and economically advantageous. .

Here, when calculating the transfer characteristic of the circuit of the above example, at the time of recording, VOR=HFM[(1/a)(HNR/HWE
) V INRI”...(10), which matches the above equation (3), and during playback, V 0P-
(1/HPH2) [α(HWE/HNR] VINP
2...(11), which is consistent with the case where DI"M"-HPM is used in the above equation (6).

FIG. 2 shows another embodiment of the invention.

This leads the output of the operational amplifier 13 to the GCAL 5 via the de-emphasis circuit 25, which integrates an NR de-emphasis circuit and an FM de-emphasis circuit and has a transfer characteristic of [HPH/HNRI-1], and The signal is led to the peak detection circuit 21 via the circuit 21, and the transfer characteristics are exactly the same as the circuit of the embodiment shown in FIG.

As shown in FIG. 1, the emphasis characteristic of the VHS7FF type HI FI VTR is such that the transfer characteristic of the NR de-emphasis circuit 14 and the transfer characteristic of the FM de-emphasis circuit 21 are continuous with a time constant of 56 μsec. There is. Therefore, even if the FM de-emphasis is incorporated into the NR de-emphasis, the time constant simply changes, there is no need to make any design changes, and the number of elements does not increase. In addition, the filter characteristics of the peak detection circuit 17 need to be changed as shown in Figure 2 in relation to the time constant of the FM de-emphasis circuit 210, but the circuit scale is almost the same as that in Figure 1. It never changes.

Here, it will be considered that the FM de-emphasis circuit 21 is composed of an active filter with a complete built-in IC (integrated circuit). An active filter with a complete built-in IC requires an adjustment circuit to compensate for the effects of element variations within the IC, but this type of active filter with adjustment function tends to deteriorate S/N and distortion. is common. For this reason, if the FM de-emphasis circuit 21 and the NR de-emphasis circuit 18 are present in the audio signal path, the S/N and distortion of the audio signal will deteriorate if these filters are completely built into an IC. It is something.

However, in the configuration shown in FIG. 2, only the de-emphasis circuit 25 that integrates the NR de-emphasis circuit and the FM de-emphasis circuit is present in the audio signal path. If the conventional external components are configured with elements, there is an advantage that there will be no cause for deterioration of S/N or distortion.

Note that the FM de-emphasis circuit 21 in the preceding stage of the peak detection circuit 15 is interposed in the control path, and is only used to obtain a DC (direct current) voltage after peak-holding with two large-capacity capacitors. Since the FM de-emphasis circuit 21 is used here, the deterioration of S/N and distortion caused by the FM de-emphasis circuit 21 does not cause deterioration of the quality of the audio signal.

Furthermore, the polar time constant of this FM de-emphasis circuit 21 is smaller than 24μSOC, so it is 56μs.
It also has the advantage of requiring less capacity than EC, making it advantageous for incorporating ICs.

It should be noted that the present invention is not limited to the above-mentioned embodiments, and can be implemented with various modifications without departing from the spirit of the invention.

[Effects of the Invention] j As detailed above, according to the present invention, it is possible to provide an extremely good noise removal circuit that can completely reproduce the recorded signal waveform, has a simple configuration, and is economically advantageous. be able to.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a block diagram showing another embodiment of the invention, and FIGS. 3 and 4 are block diagrams each showing a conventional noise removal circuit. FIG. DESCRIPTION OF SYMBOLS 11... Input/output signal processing unit, 12... Recording/reproducing switch, I3... Operational amplifier, 14... NR de-emphasis circuit, 15... GCA, 113... Recording/reproducing switch, 17. ...Peak detection circuit, 18...F
M emphasis circuit, 19...FM processing section, 2a.
...Head, 21...FM de-emphasis circuit, 2
2... Recording/reproduction switch, 23... FM de-emphasis circuit, 24... Operational amplifier, 25... De-emphasis circuit. Applicant's agent Patent attorney Takehiko Suzue

Claims (3)

[Claims] (1) An operational amplifier to which a recording signal and a reproduction signal are selectively supplied to one input terminal in the recording state and the reproduction state, and an operational amplifier interposed between the output terminal and the other input terminal of the operational amplifier in the recording state. a compression/expansion circuit that forms a feedback path and generates a signal obtained by compressing the recorded signal from the operational amplifier, and generates a signal obtained by expanding the reproduced signal by being removed from the feedback path of the operational amplifier in a reproduction state; interposed between the input end of the compression/expansion circuit and the output end of the operational amplifier,
A diode that forms a feedback path of the operational amplifier together with the compression/expansion circuit in a recording state to give an emphasis characteristic to the output of the operational amplifier, and gives a de-emphasis characteristic to the reproduced signal outputted through the operational amplifier in a reproduction state. A noise removal circuit comprising an emphasis circuit. (2) The compression/expansion circuit includes a noise reduction de-emphasis circuit to which the output of the de-emphasis circuit is supplied, a variable gain circuit to which the output of the noise reduction de-emphasis circuit is supplied, and an output of the de-emphasis circuit. 2. The noise removal circuit according to claim 1, further comprising a peak detection circuit that performs peak detection to generate a control voltage for the variable gain circuit. (3) The noise removal circuit according to claim 2, wherein the de-emphasis circuit is installed immediately before the noise reduction de-emphasis circuit and immediately before the peak detection circuit.