JPS58116818A - Noise reduction device - Google Patents

Abstract

PURPOSE:To eliminate the need for an operational amplifier used exclusively for limiter level setting and to set an optimum limit level by inserting and connecting an amplitude limiting circuit to a prescribed position. CONSTITUTION:The output of a high-pass filter 26 of variable cutoff frequency is sent to the inverted input terminal of the operational amplifier 25 as an adding means through a voltage-current converter 33 and a resistance 34. A feedback resistance 35 is connected between the inverted input terminal and the output terminal 22 of the amplifier 25, and the amplitude limiting circuit 28 is inserted between the output terminal of the converter 33 and the output terminal 22. The uninverted input terminal of the amplifier 25 connects with a main signal line 23. Thus, the circuit is formed to eliminate the need for an operational amplifier used exclusively for limiter level setting. Further, resistance values of resistances 34 and 35 are selected to set an optimum limit level.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a noise reduction device that apparently expands the dynamic range of a transmission system or recording/playback system through a signal dynamic range compression/expansion process.

In general, noise reduction devices disguise the dynamic range of the recording medium (or signal transmission path) by performing compression operations during recording (or transmission) and expansion operations during playback (or reception). It is configured with a compression circuit on the input side and an expansion circuit on the output side. These compression circuits and expansion circuits are provided with a variable transfer function circuit and its control circuit, and change the transfer function according to the signal level and frequency. These variable transfer function circuits and control circuits have response time constants, which may cause problems in transient response when the signal level or the like suddenly changes.

For example, in the compression circuit described above, the gain is generally controlled to be large when the signal level is low, and the gain is controlled to be small when the signal level is large. Due to the time constant, a thick pel signal is input while in a transient high gain state, and a signal with an extremely large peak, so-called oversheet, is output. Recording media, etc., have a maximum permissible level such as a saturation level, and even if a signal with a level exceeding this level is supplied, normal recording/playback or transmission cannot be performed, and the decompression circuit returns the original signal. Accurate restoration of data will no longer be possible.

Therefore, it is necessary to provide an amplitude limiting circuit using a diode or the like in the compression circuit to prevent the above-mentioned overshoot from occurring.

FIG. 1 shows a compression circuit 1 equipped with such an amplitude limiting circuit.
This shows an example of the basic configuration of 0.

The compression circuit 10 in FIG. 1 has an input terminal 1 and an output terminal 2.
It consists of a main signal path 3, a sub signal path 4, and an adder 5, which are disposed between the main signal path 3 and the adder 5. The sub-signal path 4 includes a high-pass filter 6 with a variable cut-off frequency, a control circuit 7 for controlling the cut-off frequency, and the above-mentioned amplitude limiting circuit 8.

FIG. 2 is a graph for explaining the frequency response of this compression circuit 100. In FIG. 2, the transfer characteristic A of the main signal path 3 has, for example, a gain of 1 and a flat frequency characteristic.

On the other hand, the transfer characteristic B of the sub-signal path 4 is largely determined by the characteristics of the high-pass filter 6, and the cutoff frequency changes depending on the signal level. That is, when there is no signal, the cutoff frequency becomes characteristic B1, which is the lowest, and as the signal level increases, the cutoff frequency increases, and becomes characteristic B, for example. The transfer characteristic C between the input and output terminals 1 and 2 of the compression circuit 100 is a combination of the transfer characteristics A and B of these signal paths 3 and 4, and becomes the characteristic C3 when there is no signal, and the signal level increases. For example, the state becomes characteristic C2. Through this process, the dynamic range of the input signal is compressed. During playback (or reception), an expansion circuit (
Of course, the original dynamic range is restored by an expansion operation (not shown).

Next, the amplitude limiting circuit 8 will be explained. In the compression circuit 10 of FIG. 1, if the amplitude limiting circuit 8 is not provided, the third
The tone burst signal as shown in Figure A is transmitted from time t to time t.
, a signal having a response waveform as shown in FIG. 3B, for example, is output. In other words, since there is almost no signal before time t□, the transfer characteristic of the high-pass filter 6 has a cutoff frequency on the low side as shown in Fig. 2B, and immediately after time t, the control circuit 7 Due to the finite rise time constant of , the cutoff frequency is not shifted to the higher frequency side instantly, and overshoot 4 occurs in the output waveform.

Transmission lines and recording media have their own clipping levels or maximum allowable levels, and there is a possibility that the overshoot that occurs will exceed the clipping level. In order to avoid such a situation, a limiter level L within the above-mentioned clipping level is set by the amplitude limiting circuit 8 as shown in FIG. 3, and an amplitude limiting characteristic is provided for overshoot exceeding this limiter level L. There is.

Such amplitude limiting circuit 8 is constructed using nonlinear elements, and a PN junction is most commonly used. FIG. 4 shows a more specific configuration of the compression circuit shown in FIG. 1, and corresponding circuit parts are given the same reference numerals. The amplitude limiting circuit 8 in FIG. 4 includes a PN junction connected in antiparallel, that is, two diodes 11.
．． Twelve anodes and cathodes are connected in parallel in opposite directions. Here, the limiter level of the nonlinear element is a value specific to the element. The limiter level of the amplitude limiting circuit 8 in FIG. 4 is approximately 1.2 V P
−P value. This value is quite large compared to the signal level normally set, and cannot be used as is. Therefore, an operational amplifier 13 is provided before the amplitude limiting circuit 8 (on the input side), and the output from the high-pass filter 6 is It is necessary to amplify the output and apply it to the amplitude limiting circuit 8 so that the limiter level is substantially the optimum value for the signal. In this case, in the adder 5, the adding resistors 16 and 17, and the operational amplifier 15,
By appropriately setting the ratio of addition resistors 16 and 17,
It is also necessary that the output of the auxiliary signal path 4 amplified by the operational amplifier 13 is added to the output of the main signal path 3 at a constant ratio (with a small addition coefficient).

However, in the configuration shown in FIG. 4, the operational amplifier 13 for setting the limiter level is required in the sub-signal path 4, which complicates the circuit configuration and requires extra steps to amplify and attenuate the signal level. Depending on the operation, accuracy, 8N ratio, etc. will deteriorate. Further, a distortion current flows through the amplitude limiting circuit 8,
In the configuration shown in FIG. 4, there is a possibility that other circuits are affected through the common impedance of the ground circuit.

The present invention has been made in order to eliminate these conventional drawbacks, and eliminates the need for an operational amplifier that only serves to match the limiter level and signal level. The limiter level can be set to the optimum value for the signal, improving characteristics such as accuracy and S/N ratio, and the distortion current flowing through the amplitude limiting circuit does not flow through the grounding circuit, affecting other circuits. The purpose of the present invention is to provide a noise reduction device equipped with a compression circuit and an expansion circuit that do not cause noise.

That is, the noise reduction device according to the present invention is characterized by comprising a main signal path and a sub-signal path including a variable transfer function circuit and its control circuit, and an operational amplifier serving as a means for adding the main signal and the sub-signal; It comprises means for current converting the output of the variable transfer function circuit, and amplitude limiting means, a main signal is applied to the non-inverting input terminal of the operational amplifier, and one end of the first resistor is applied to the output of the current converting means. One end of the amplitude limiting means is connected, the other end of the first resistor and one end of the second resistor are connected to the inverting input terminal of the operational amplifier, and each of the second resistor and the amplitude limiting means is connected. The end thereof is connected to the output terminal of the operational amplifier.

Preferred embodiments of the present invention will be described below with reference to the drawings.

FIG. 5 shows a compression circuit 20 of a noise reduction device as an embodiment of the present invention. In FIG. 5, there is a main signal path 2 between the input terminal 21 and the output terminal 22.
An operational amplifier 25 is provided as means for summing the three sub-signal paths 24 and the output signals from these signal paths 23,24.

The sub-signal path 24 includes a high-pass filter 26 with a variable cut-off frequency, which is a variable transfer function circuit, and a control circuit 27 that controls the cut-off frequency. The current is converted into a current by the amplifier 38 and sent to the inverting input terminal of the operational amplifier 35 via the resistor 34. Between this inverting input terminal and the output terminal of the operational amplifier 25, that is, the output terminal 22 of this compression circuit 20,
A feedback resistor 35 is connected, and an amplitude limiting circuit 28 is inserted and connected between the output terminal of the voltage-current converter 33 and the output terminal 22 of the operational amplifier 25. In addition, the operational amplifier 25
A main signal path 23 is connected to the non-inverting input terminal of.

In the compression circuit 20 of the noise reduction device having the above configuration, the operational amplifier 25 operates as a voltage follower for the main signal path 23 and as an inverting amplifier for the sub signal path 24. Voltage-current converter 3
3 flows into the output terminal 22 of the operational amplifier 25 via the resistor 34 and the resistor 35. At this time, resistance 35
The resulting voltage drop contributes as an output signal. Therefore, the addition coefficient for the sub-signal path 24 can be designed using the conversion coefficient of the voltage-to-current converter 33 and the resistor 35. The conversion coefficient of the voltage-to-current converter 33 is such that in order to add the signal of the secondary signal path 24 to the signal of the main signal path 23,
Must be negative.

On the other hand, the amplitude limiting circuit 28 is generally constructed by connecting PN junction elements in antiparallel, and in this embodiment, two silicon diodes 31 and 32 are connected in parallel so that their anode-cathode directions are opposite to each other. I am using the one that I made. The limiter level of this amplitude limiting circuit 28 is approximately 1.2
It has a specific value of V P, +, but since this constant limiter level is set for the sum of the voltage drops across resistors 34 and 35, by appropriately selecting the value of resistor 34, Independent of the signal level at the output terminal 22, the substantial limiter level for the entire circuit can be freely set.

- Here, the resistance values of resistors 34 and 35 are respectively l(, +
R3, the above-mentioned substantial limiter level is the nominal limiter level specific to the amplitude limiting element 28. For example, if the resistance value R3 of the resistor 34 is set to twice the resistance value R1 of the resistor 35, the actual limiter level of the output terminal 22 is The actual limiter level is approximately 0
．． 4 V pp. This amplitude limiting operation has no effect on the main signal.

Here, although various specific circuit configurations of the voltage-current converter 33 can be considered, it is preferable to configure it as shown in FIG. 6, for example. In FIG. 6, an input terminal 41 is connected to the output terminal of the variable cut-off frequency high-pass filter 26, and an output terminal 42 is connected to the connection point between the first resistor 34 and the amplitude limiting circuit 28. . Terminals 43 and 44 are
These are positive and negative power supply terminals, respectively. The output of the high-pass filter 26 is converted into a current by a differential transistor circuit 46. Current inversion (current mirror) circuits 53 and 54 are connected to the collectors of the transistors 47 and 48 constituting the differential transistor circuit 46, respectively, and the output of the current inversion circuit 53 is connected to the output terminal 42, and the current inversion circuit 5
The outputs of No. 4 are current-inverted by a current inverting circuit 55 and supplied to the output terminals 42, respectively.

Note that the decompression circuit of the noise reduction device may be configured as a separate circuit that is symmetrical or complementary to the compression circuit 20, but generally speaking, in a recording/playback device such as a tape recorder, recording and playback are In view of the fact that only one of the modes is selected, the configuration may be such that either the compression or expansion operation is selected by switching the changeover switch 29 as shown in FIG.

That is, in FIG. 7, one switching terminal C of the changeover switch 29 is connected to the input terminal 21, and the other switching terminal e is connected to the output terminal of an inverting amplifier 30 that inverts the output from the output terminal 22. There is. By switching and connecting the selector switch 29 to the terminal C side, a compression operation is performed, and by switching and connecting the changeover switch 29 to the terminal e side, an expansion operation is performed, and these compression and expansion operations are complementary to each other. The other configurations are the same as those shown in FIG. 5, so their explanation will be omitted.

As is clear from the above description, the noise reduction device according to the present invention eliminates the need for a conventional operational amplifier dedicated to limiter level setting, and eliminates the need for an operational amplifier as a means for adding the main signal and sub-signal. By using only one 25, the optimum practical limiter level can be set. Therefore, the circuit configuration is simplified, and the extra operation of raising the signal level, setting the limiter level, and then attenuating it as in the conventional method is omitted, which not only improves accuracy, but also increases the voltage Since it operates as a 7-lower, the gain does not depend on the resistance ratio as in the conventional case, and the accuracy is high. Further, the distortion current flowing through the amplitude limiting circuit can be prevented from flowing through the ground circuit as in the conventional case, and the influence on other circuits can be prevented.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the basic configuration of the compression circuit of the noise reduction device, FIG. 2 is a graph showing the transfer characteristics of the circuit in FIG. 1, and FIG. 3 is a time chart showing the input/output response of the tone burst signal. FIG. 4 is a circuit diagram showing a conventional example of the circuit shown in FIG. 1. FIG. 5 is a circuit diagram showing an embodiment of the compression circuit of the noise reduction device according to the present invention, and FIG. 6 is a circuit diagram showing an example of a specific circuit configuration of the voltage-current converter in the circuit of FIG. 7
This figure is a circuit diagram showing an example of a configuration in which the circuit of FIG. 5 can be switched between compression and expansion operations. 21...Input terminal 22...Output terminal 23...Main signal path 24...Sub signal path 25......Operation amplifier 2 as addition means
6......High-pass filter with variable cut-off frequency 27...Control circuit 28...Amplitude limiting circuit 33... Voltage-current converter 34...
・・・・・・First resistance 35・・・・・・・・・Second resistance Utility model registration applicant Sony Corporation representative Patent attorney Koike Mimi 1) Sakae Mura - 1st m 211 3rd g 4s 6th M Sparrow! '/ Figure 6 3

Claims (1)

[Claims] It is composed of a main signal path and a sub-signal path including a variable transfer function circuit and its control circuit, and includes an operational amplifier which is an addition means for the main signal of the main signal path and the sub-signal of the sub-signal path, and the variable transfer function circuit. It comprises means for converting the output of the transfer function circuit into a current and amplitude limiting means, the main signal is applied to the non-inverting input terminal of the operational amplifier, and one end of the first resistor is applied to the output of the current converting means. and one end of the amplitude limiting means,
The other end of the first resistor and one end of the second resistor are connected to the inverting input terminal of the operational amplifier, and the other ends of the second resistor and the amplitude limiting means are connected to the output terminal of the operational amplifier. A noise reduction device characterized by being connected to each other.