JPS5845848B2 - electronic circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electronic circuit useful for use in a noise reduction system, and more particularly to an amplifier whose gain and frequency characteristics are controlled by control signals.

Noise and
When applying a reduction system, if the S/N improvement of the system is not very good, the modulation of the noise level will not be noticeable.

However, noise reduction with good S/N improvement
Modulation of the noise level cannot be ignored in the system.

Particularly when applied to a transmission system with poor S/H, modulation of the noise level becomes noticeable and may cause practical problems depending on the sound source.

This is shown in Fig. 1 by a noise filter with excellent S/N improvement.
The reduction system will be explained with reference to FIG. 2 as an example.

The system shown in FIG. 1 has an encoder 2 used during recording as a means for compressing the dynamic range of an analog information signal on the input side of a recording medium such as a tape recorder or a transmission medium 1, and a compressed signal on the output side. This system reduces noise by disposing a decoder 3 used during playback as a means of expanding the original dynamic range.

The VCA (voltage control amplifier) 21 of the concoder 2 and the VCA 31 of the decoder 3 function as a kind of multiplier, and provide a DC level E to the input signal ei.

Here, the exponent of E becomes a negative sign during encoding and a positive sign during decoding.

Furthermore, the level sensor 22 of the encoder 2 and the level sensor 32 of the decoder 3 detect signal levels, and send out a DC level E corresponding to the level of the input signal e.

Therefore, when the signal ei1 is applied to the encoder 2, it becomes its output signal (an input signal to the recording medium 1), and upon decoding, the level change of the signal is expanded by a factor of two on a log scale.

FIG. 2 is an operational characteristic diagram of the noise reduction system.

The compression operation of encoder 2 that operates during recording follows straight line A.

Therefore, for example, an input signal of +20 dB is suppressed to +10 dB and recorded, thereby improving the peak margin.

Also, for example, an input signal of -60 dB becomes -30 dB, so the dynamic range of the input signal as a whole is compressed in half and recorded.

The expansion operation of the decoder 3 that operates during playback follows straight line B.

Therefore, the signal recorded at +10 dB on recording medium 1 is restored to +20 dB.

Also, the signal that was recorded at -30dB is now -60dB again.
However, at the same time, the noise is also reduced by 30 dB, so that for the -60 dB signal input to the encoder 2, the S/N is improved by 30 dB at the output of the encoder.

Therefore, the S/N of this noise reduction system is
The degree of improvement can generally be expressed as 1/2X (input signal level [dB]).

However, in this system, for example, in the case of a -100 dB input signal, the S/N is as high as 50 dB.
While the degree of improvement is obtained and the noise is almost inaudible, Od
In the case of the input signal B, the S/N improvement degree is OdB, so that the noise is modulated according to changes in the volume level.

With many sound sources, the noise becomes inaudible due to the masking effect when the volume is high, so the noise modulation described above is not a practical problem, but in the case of a sound source with a relatively simple harmonic structure, such as a piano solo, the noise is masked. I can hear it without being heard.

Moreover, in this case, the level of the noise changes with the rhythm of the sound source, so if the degree of change is large, the auditory sense is more sensitive than if the same level of noise is constantly heard. This appears as a drawback in that it stimulates the conscious level of people to a large extent.

The configuration shown in FIG. 3 is known as one method for making this defect less noticeable.

This is a de-emphasis circuit 3 with the characteristics shown in Figure 4.
3 lowers the gain of the band in which a lot of high-frequency noise is included in the recording medium 1, thereby making the noise itself difficult to hear.

By inserting this de-emphasis circuit, the high range of the musical tone is also attenuated, so when encoding, the pre-emphasis circuit 2 with the characteristics shown in Figure 4a, which has an inverse function relationship with this,
It is necessary to insert 3.

In this way, increasing the high frequency range during encoding reduces the peak margin relative to the saturation level of the recording medium.
Since the possibility of distortion in the high frequency range increases, a weighting circuit 24, 34 having the characteristics shown in FIG. 4C is used, which increases the sensitivity of the level sensor in the high frequency range for greater compression.

However, even with these measures, unlike small cassette tape recorders, the recording level is high and the saturation level is low.
When used for recording media with a narrow band, not only will noise modulation be noticeable if the sound source has a simple spectrum structure, but if the level is high for a sound source whose spectrum spans the entire band, the high range will be saturated due to high frequency reproduction. However, you can only get a playback sound that is not crisp and has little spread.

FIG. 5 shows an example of the recording/playback frequency characteristics of a general cassette tape recorder.

It can be seen that the higher the level is, the worse the high frequency characteristics become.

That is, even if a musical tone is not encoded, if it is recorded at a high level, the high frequency range will not be reproduced, resulting in a deterioration in sound quality, but when an encoded signal with an emphasis on the high frequency range is recorded, this deterioration is even more significant.

If a large amount of emphasis is used to make the modulation of the noise level inaudible, high-frequency saturation becomes a problem.

However, noise level modulation becomes a problem when the signal level is relatively small, and high frequency saturation becomes a problem when the signal level is large, so it is necessary to use variable emphasis so that the amount of emphasis is large for small levels and small for large levels. It can make up for the shortcomings of tobacco.

FIG. 6 shows the main signal circuit of an encoder that realizes this characteristic.

In this figure, 65 is an operational amplifier. 64 is a voltage controlled current amplifier in which a current Ae times the current flowing through the resistor Ri flows into the output.

FIG. 7 shows one circuit side of this voltage-controlled current amplifier 64, which includes an operational amplifier 71 and four transistors Q1 to Q4.
Connect as shown in the figure.

The relationship between the input and output currents of this circuit is It is expressed as

Now, the transfer function in FIG. 6 is expressed by the equation below using each constant in the diagram.

The characteristics in parentheses on the left side of the right side of the above equation are expressed using G as a parameter, as shown in FIG.

The equation on the right side is static high frequency pre-emphasis.

Desired characteristics can be obtained by controlling the amplification factor Ae of the voltage-controlled current amplifier 64 so that it increases when the signal level is low and decreases when the signal level is high. FIG. 9 also shows an example of the circuit diagram of the decoder. It shows,
In the figure, 91 is a voltage-controlled current amplifier such that a current Ad times the current input thereto flows into its output, and 92 is an operational amplifier.

The inverse transfer function of this circuit is obtained as in the case of FIG. 6, which is completely opposite to the transfer function of the encoder shown in the figure.

The characteristics in parentheses can be expressed as shown in Figure 10 using G as a parameter.

Now, the electronic circuit according to the present invention includes the encoder shown in FIG.
This is designed to switch the decoder circuit shown in Figure 9.
An example thereof is shown in FIG.

That is, this includes a first resistor R1 connected between the first node n1 and the second node, and a second resistor R1 connected between the second node n2 and the third node n3. A control voltage V is applied to the current value connected between the resistor R2, the third node n, and the fourth node n4, and flows from the third node n3.

a current amplifier 113 that causes a current value multiplied by a current amplification degree controlled by to flow from the fourth node n4;
an operational amplifier 114 whose inverting input terminal is connected to the fourth node n4 and whose output terminal is connected to the fifth node n5; A first terminal a is connected to the third resistor R3, a fourth resistor R4 connected between the sixth node n6 and the fourth node n4, and the first node n1. a first switch device 110 having a second terminal connected to the node n3 and a common terminal connected to one end of a first series circuit including a fifth resistor R5 and a first capacitor C1; A second switch device 111 has a first terminal a connected to a node n4, a second terminal connected to a fifth node n, and a common terminal connected to the other end of the first series circuit; 1 terminal a to the 2nd terminal
A second series circuit is connected to the sixth node n6 through a second series circuit including a capacitor C2 and a sixth resistor R6, and the second terminal is connected to a third series circuit including a third capacitor C3 and a seventh resistor R7. a third switch device 112 connected to the second node n2 through a series circuit with a common terminal set to ground potential;
Therefore, the first node n1 is connected to the signal input side, and the fifth node n1 is connected to the signal input side.
- n5 is the signal output side, the gain and frequency characteristics are controlled simultaneously by the control voltage, and the switching between the a contact and the b contact of each of the switches and the polarity and bias value of the control signal are simultaneously performed. By doing this, it is an electronic circuit that can obtain two transfer characteristics having a relationship of substantially inverse functions to each other.

When the switch device 110゜111.112 is set to the first terminal a side, it is an encoder, and at this time, R1+
R2=Ri, R5=1%.

C1-Cr, C22C6+ R6= Rer R5=
If R4 near is satisfied, it becomes completely equivalent to FIG. 6.

Furthermore, if the switch devices 110, 111, and 112 are placed on the second terminal side and C32Ce, R7=Re, and R1-R2- are satisfied, the circuit becomes completely equivalent to that shown in FIG.

In other words, to summarize, R1"R2"R3"R4"
” 2 + R5° Horse FC1=Cr, C22C3=Ce
, R6=R7=Re (at this time, Ri=R4=R), and "・=shoulder@'/fat:t,,h 5
R: :] When switching at the time of 71-D -yv@JfJ*, two transfer characteristics having substantially inverse functions to each other can be obtained.

Also, if we allow the product of the encoder transfer function and the inverse function of the decoder transfer function to become a certain constant, R6
It is also possible to obtain the same characteristics using relationships between other constants between paths that satisfy C2=R7C3.

As described above, if the electronic circuit of the present invention is used, the encoder and decoder of the highly effective noise reduction system can be switched with a relatively simple circuit, so stable operation can be expected.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the basic structure of the noise reduction system, Figure 2 is a diagram showing the input/output level characteristics of the system, and Figure 3 is a diagram showing the configuration of a conventional improved noise reduction system. , FIG. 4 is a diagram showing the frequency response characteristics of each part in FIG. 3, FIG. 5 is a diagram showing an example of the recording and playback frequency characteristics of a general cassette tape recorder,
FIG. 6 is a circuit diagram of the main part of the encoder of the improved noise reduction system, and FIG. 7 is a circuit diagram of the voltage-controlled current amplifier used in the noise reduction system f11. Fig. 8 is a characteristic diagram of the circuit shown in Fig. 6 when the resistance Re is set to infinity, Fig. 9 is a circuit diagram of the main part of the decoder of the improved noise reduction system, and Fig. 10 is the circuit diagram of the main part of the decoder of the improved noise reduction system. The characteristic diagram when the resistance Re is set to infinity in the circuit shown in Fig. 9, and the characteristic diagram shown in Fig. 11 is the electronic circuit according to the present invention.
This is a circuit that switches between the encoder in FIG. 6 and the decoder in FIG. 9. 110-112...Switch, n1-n6...
...Node, n1 to n7...Resistor, C1
~C3... Capacitor, 113... Current amplifier, 114... Operational amplifier.

Claims (1)

[Claims] 1 a first resistor connected between a first node and a second node, a second resistor connected between the second node and a third node, and a second resistor connected between the second node and the third node; 3 nodes and the 4th node
a current amplifier connected between the fourth node and the fourth node so that a current value obtained by multiplying the current value flowing from the third node by a current amplification degree controlled by the control signal flows from the fourth node; an operational amplifier having an input end connected to the fifth node and an output signal circuit connected to the fifth node; a third resistor connected between the fifth node and the sixth node;
a fourth resistor connected between the sixth node and the fourth node; a first terminal connected to the first node; and a second terminal connected to the third node. a first switch device having a common terminal connected to one end of a first series circuit including a fifth resistor and a first capacitor; a first terminal connected to the fourth node; and a first switch device connected to the fourth node; a second switch device having two terminals connected to each other and a common terminal connected to the other end of the first series circuit; and a second series circuit including a first terminal connected to a second capacitor and a sixth resistor. A second terminal is connected to the second node via a third series circuit including a third capacitor and a seventh resistor, and a common terminal is connected to the ground potential. and a third switch, wherein the first node is on the signal input side and the fifth node is on the signal output side, and the switch device provides two transfer characteristics having substantially inverse functions to each other.