JPH0474886B2 - - Google Patents

Description

[Detailed Description of the Invention] "Field of Industrial Application" This invention is applicable to analog tape recorders, PCM
The present invention relates to a noise reduction processing circuit applied to tape recorders and the like.

"Background Art and Its Problems" In a tape recorder, a known method for reducing noise generated during recording and playback is to compress the input signal during recording, and expand and restore it during playback. In addition, in order to remove noise modulation accompanying this companding operation, a pre-emphasis characteristic is provided, and in particular, high-frequency signals are boosted for the purpose of suppressing high-frequency noise that is easily audible. FIG. 1 shows an outline of the recording system of a tape recorder, in which 1 is an audio signal input terminal, 2 is a limiter, 3 is a noise reduction circuit (performs compression operation), 4 is an audio signal processing circuit,
5 is an output terminal for recording signals. This recording signal is recorded on the magnetic tape.

This noise reduction circuit 3 has not only a compression characteristic but also a pre-emphasis characteristic 6 that boosts a high frequency signal of (5 to 7) KHz by up to 15 dB, as shown in FIG. have. This noise reduction circuit 3 has a third
In the figure, it has a dynamic range indicated by DR ₁ . Further, the input analog signal has a dynamic range indicated by DR ₂ , and after compression has a dynamic range indicated by DR ₃ . These dynamic ranges DR ₁ , DR ₂ , DR ₃ relate to the positive side of the signal. If the input analog signal is supplied as is to the noise reduction circuit 3, it will exceed the clip level indicated by 7, so the limiter 2
The amplitude is suppressed uniformly by Furthermore, the noise reduction circuit 3 compresses the dynamic range of the output signal of the limiter 2 to _DR3 .

Here, since the noise reduction circuit 3 has a pre-emphasis characteristic, the high frequency clip level 7 is lowered, and therefore the limiter 2
Peak level 8 as shown by the broken line in Figure 3
It is necessary to limit the amplitude of the input signal to 1st
In the case of pre-emphasis characteristic 6 shown in the figure, it is necessary to suppress the amplitude of the input signal to a point 15 dB lower than the clip level in the low and middle ranges. For this reason, it is necessary to increase the amount of amplitude restriction for input signals in the low and middle ranges. In the limiter 2, the distortion generally increases as the amount of amplitude restriction increases, so as described above, it is not preferable to increase the amount of amplitude restriction. In other words, the circuit design of the limiter 2 becomes difficult, and the dynamic range DR ₁ of the noise reduction circuit 3 cannot be effectively utilized.

Furthermore, when analog audio signals are digitized and recorded using PCM modulation, signal clips appear more clearly than in the analog case, so it is necessary to pay close attention to the clip level. Therefore, the amount of amplitude limitation by the limiter 2 must be precisely matched to the clip level 7.

``Object of the Invention'' This invention reduces the amount of amplitude restriction for the low-range and mid-range components of the input signal to reduce the generation of distortion when limiting the amplitude of the input signal within the dynamic range of a noise reduction circuit. The purpose of this is to realize a noise reduction processing circuit that can prevent such noise. Also,
The present invention provides a noise reduction processing circuit capable of adjusting the level of an input signal, that is, the recording level, by adjusting the level of a control signal.

"Summary of the Invention" The present invention provides a noise reduction circuit whose gain is controlled according to at least the level of an input signal and which performs an operation of compressing the dynamic range of this input signal and a pre-emphasis operation, and a stage preceding the noise reduction circuit. and a gain control circuit provided in the
A signal related to the control signal of the noise reduction circuit is supplied as a control signal to the gain control circuit. The present invention also provides a gain control circuit in which a switch circuit switches between a signal related to the control signal of the noise reduction circuit and a signal formed by a manual level adjuster and supplies the signal as a control signal. It is.

Embodiment FIG. 4 shows the basic configuration of an embodiment of the present invention. In this example, the present invention is applied when recording a stereo audio signal, and includes a configuration from an input terminal 1R for a right channel audio signal to an output terminal 5R for a recording signal, and an input terminal 1L for a left channel audio signal. The configuration from the to the recording signal output terminal 5L is the same. Therefore, corresponding circuit blocks of these signal paths are designated with the same reference numerals.

A gain control amplifier 6 is connected upstream of the noise reduction circuit 3 having compression characteristics and pre-emphasis characteristics for compressing the dynamic range of the input signal. The gain of the gain control amplifier 6 is controlled according to the level of a DC voltage control signal. The control signal for this gain control amplifier 6 is:
It is passed through a switch circuit 7. The switch circuit 7 switches between a signal related to the control signal of the noise reduction circuit 3 and a control signal generated by the manual level adjuster 8.

A more detailed configuration of one channel in an embodiment of the present invention is shown in FIG. The noise reduction circuit 3 is a subtraction circuit 9 using a differential amplifier.
and a high-pass filter 10 that defines transient characteristics.
, a detection circuit 11, a gain control amplifier 12 whose gain is controlled by the output of the detection circuit 11, and a low-pass filter 13 that defines pre-emphasis characteristics. The detection circuit 11 generates a detection output proportional to the level of the output signal of the high-pass filter 10, and the gain change characteristic with respect to the detection output of the gain control amplifier 12 has a compression characteristic. Further, by supplying the signal passed through the low-pass filter 13 to the subtraction circuit 9, the high frequency signal is boosted.

Gain control amplifier 12 of the above-mentioned noise reduction circuit 3
A control voltage for the gain control amplifier 6 is supplied to the control terminal of the gain control amplifier 6 via the switch circuit 7. Further, as shown in FIG. 6, the output signal of the high-pass filter 10 is supplied to a separate detection circuit 14, and the control voltage formed by this detection circuit 14 is sent to the gain control amplifier 6 via the switch circuit 7. It may also be supplied. Although not shown, the input side of the high-pass filter 10, that is, the output signal of the noise reduction circuit 3 may be detected to form the control voltage for the gain control amplifier 6. The gain control amplifier 6 operates to reduce the gain only in response to a control voltage equal to or higher than a predetermined level. In other words,
This suppresses the amplitude of a signal that may exceed clip level 7 (see FIG. 3) of the noise reduction circuit 3.

In this way, by using the control voltage of the noise reduction circuit 3 or a signal related thereto as the control voltage for the gain control amplifier 6, the gain control amplifier 6 can have a frequency characteristic opposite to that of the noise reduction circuit 3. Can be done. Since the control voltage fed back from the noise reduction circuit 3 is the detected output of the pre-emphasized signal, the gain control amplifier 6 can substantially provide de-emphasis characteristics. To explain using FIG. 3, the gain control amplifier 6 only responds to high-level signals that may exceed the clip level 7 of the noise reduction circuit 3.
responds, and the amplitude of the signal can be compressed to a level slightly lower than this clip level 7, as shown by a dashed line 15 in FIG.

Therefore, for high-frequency components of large-level signals, the combined frequency characteristics of the gain control amplifier 6 and the noise reduction circuit 3 become flat, and the pre-emphasis characteristics are weakened. However, the noise reduction circuit 3 provides pre-emphasis characteristics to low- and mid-range signals or signals at a level to which the gain control amplifier 6 does not respond. Further, the gain control amplifier 6 adapts the dynamic range of the input signal to the dynamic range of the noise reduction circuit 3, and the compression operation of the noise reduction circuit 3 is as follows.
It is not affected by anything. The noise reduction circuit 3 originally prevents low-level signals from being affected by tape noise, and there is no particular problem even if the pre-emphasis characteristics of high-level signals are weakened.

Further, when the switch circuit 7 is switched and the control voltage formed by the manual level regulator 8 is supplied, the dynamic range of the input signal can be arbitrarily varied by the level regulator 8. . This adjustment is the recording level adjustment.
It can be adjusted by the user according to the input signal. As the level adjuster 8, in addition to a variable resistor, an electronic level adjuster that generates an arbitrary voltage using a counter and a D/A converter may be used.

In addition to manual switching, the switch circuit 7 can also be operated by switching the adjustment knob 16 of the level adjuster 8 provided on the tape recorder in conjunction with the opening and closing of the cover 17, as shown in FIG. It is also possible to try to improve the quality. Although not shown, the switch circuit 7 is connected by the rotating end of the lid 17.
The switch circuit 7 is constituted by an optical switch, and the switch is operated by opening and closing the lid 17.

"Application example" Noise reduction circuits whose companding characteristics change depending on both the input level and input frequency,
There are known devices that compress the dynamic range of the input signal by 1/2 in decibels during recording, and expand it to 2x during playback. The present invention can also be applied when using such a noise reduction circuit.

"Effects of the Invention" According to the present invention, when suppressing the amplitude of an input signal within the dynamic range of a noise reduction circuit, unlike limiting the amplitude with a simple limiter,
The amount of amplitude restriction can be reduced. Therefore, not only does the noise reduction circuit not clip the signal waveform, but also the distortion caused by amplitude limitation can be reduced, and the gain control circuit that performs amplitude limitation can be easily designed. In other words,
According to the present invention, the dynamic range of the noise reduction circuit can be effectively used, and the noise reduction operation can be performed satisfactorily.

Further, in the present invention, the recording level can be adjusted by applying a control voltage to the gain control circuit using a level adjuster. Conventional recording level adjusters are inserted into a signal path parallel to the signal path containing the AGC circuit that automatically limits the recording level, and require a switch circuit to switch this signal path. However, in the present invention, it is only necessary to switch the control voltage, and there is no need for two signal paths and a switch to switch between them as in the conventional system, and the configuration can be simplified. It is possible to avoid the generation of noise caused by the switching circuit.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of a conventional noise reduction processing circuit, Fig. 2 is a graph showing an example of pre-emphasis characteristics, Fig. 3 is a schematic diagram used to explain the conventional noise reduction processing circuit, and Fig. 4 is a block diagram showing the configuration of an embodiment of the present invention, FIGS. 5 and 6 are block diagrams showing a more detailed configuration of the embodiment of the invention, and a partially modified configuration, respectively. FIG. 2 is a perspective view used to explain a configuration for switching a switch circuit in an embodiment of the present invention. 1, 1R, 1L... Audio signal input terminal, 3... Noise reduction circuit, 5, 5R, 5L...
Recording signal output terminal, 6...Gain control amplifier, 7
...Switch circuit, 8...Level adjuster.

Claims (1)

[Claims] 1. A calculation circuit that calculates an input signal, a detection circuit that detects the output of the calculation circuit, a first gain control circuit whose gain is controlled according to the output of the detection circuit, a low-pass filter to which the output of the first gain control circuit is supplied; by supplying the output of the low-pass filter to the arithmetic circuit, the gain is controlled according to at least the level of the input signal; A noise reduction circuit that performs an operation of compressing the range and a pre-emphasis operation, and a second gain control circuit provided in the preceding stage of this noise reduction circuit, the signal related to the control signal of the noise reduction circuit is A noise reduction processing circuit that is supplied as a control signal to a gain control circuit. 2. An arithmetic circuit that calculates an input signal, a detection circuit that detects the output of the arithmetic circuit, a first gain control circuit whose gain is controlled according to the output of the detection circuit, and the first gain control circuit. and a low-pass filter to which the output of the low-pass filter is supplied, and by supplying the output of the low-pass filter to the arithmetic circuit, the gain is controlled according to at least the level of the input signal, and the dynamic range of the input signal is compressed. A noise reduction circuit that performs a pre-emphasis operation, a second gain control circuit provided before the noise reduction circuit, and an output signal of the detection circuit in the noise reduction circuit and a manual level adjuster. and a switch circuit for selecting one of the control signals and supplying the selected control signal to the second gain control circuit. 3. The noise according to claim 2, characterized in that a lid that can be opened and closed is provided to cover the adjustment knob of the manual level adjuster, and the switch circuit is switched in conjunction with the opening and closing of the lid. Reduction processing circuit.