JPS58116819A - Noise reduction device - Google Patents

Abstract

PURPOSE:To select different limiter levels by providing amplitude limiting circuits to a circuit which does not require any operational amplifier used exclusively for limiter level setting. CONSTITUTION:A changeover switch 40 for switching the output of a voltage- current converter 33, amplitude limiting circuits 41 and 42, and resistance 51-53 are provided. Then, a resistance 51 is connected between the inverted input terminal of an operational amplifier 25 and an output terminal 22; and one terminal of a resistance 52 is connected to the inverted input terminal of the amplifier 25 and the other terminal is connected to one terminal of the limiting circut 41 and a changeover terminal (a) of the switch 40. One terminal of a resistance 53 is connected to the other terminal of the resistance 52 and the other terminal is connected to one terminal of the limiting circuit 42 and a changeover terminal (b) of the switch 40. Further, the other-side terminals of the limiting circuits 41 and 42 are connected to the output terminal 22. Thus, different limiter levels are set selectively through the switching operation of the switch 40.

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 apparently expand 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 high-level signal is temporarily input while in a high gain state, and a signal with an extremely large peak, so-called overshoot, 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.
The main signal path 3 and the auxiliary signal path arranged between the main signal path 3 and the sub signal path are comprised of 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 1o. In FIG. 2, the transfer characteristic A of the main signal path 3 is 1. For example, it has 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 B2, for example. The transfer characteristic C between the input and output terminals 1 and 2 of the compression circuit 10 is a combination of the transfer characteristics A and B of these signal paths 3 and 4, and becomes the characteristic C□ when there is no signal, and the signal level is In the increased state, for example, characteristic C2 is obtained.

Through this process, the dynamic range of the input signal is compressed. Of course, during playback (or reception), an expansion operation is performed by an expansion circuit (not shown) having transfer characteristics complementary to the compression circuit 10, and the original dynamic range is restored.

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 1 from time t1.
If the signal is input up to 2, a signal with a response waveform as shown in FIG. 3B, for example, is output. That is, since there is almost no signal before time t1, the transfer characteristic of the high-pass filter 6 has a cutoff frequency on the low side as shown in FIG. Because of the rise time constant of , the cutoff frequency is not shifted to the higher frequency range instantly, and an overshoot 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. ing.

Such amplitude limiting circuit 8 is constructed using nonlinear elements, but a PN junction is most commonly used. FIG. 4 shows a more specific configuration of the compression circuit bar shown in FIG. 1, and corresponding circuit parts are given the same reference numerals. The amplitude limiting circuit 8' in FIG. 4 uses a PN junction connected in antiparallel, that is, the anode and cathode of two diodes 11 and 12 are connected in parallel in opposite directions. There is. Here, the IJ level of the nonlinear element has a value unique to the element, and the degree of freedom is small. For example, a silicon PN junction has a threshold of about 0.6cm, and the limiter level of the amplitude limiting circuit 8 shown in FIG.
The value is 2Vpp. This value is large compared to the signal level that is 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 high-pass filter 6 It is necessary to amplify the output of the limiter 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, the adder 5
By appropriately setting the ratio of the summing resistor 16.17 and the operational amplifier 15, the output of the sub signal path 4 amplified by the operational amplifier 13 can be compared to the output of the main signal path 3. It is also necessary that the values are added 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 width signal path 4, which complicates the circuit configuration and requires extra steps to amplify and attenuate the signal level. Accuracy, SN ratio, etc. deteriorate due to operation. 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.

Furthermore, when applying such a noise reduction device to a recording/playback device such as a tape recorder, when the types of recording media such as tapes are different, their recording characteristics are also different, and the above limiter level may be set to a value of 2 or more. It is desirable to switch between the two. In addition, when switching the noise reduction method itself, it is necessary to switch the limiter level.

The present invention has been made in view of the above-mentioned conventional situation, and is directed to the operational amplifier 13 dedicated to limiter level setting.
This eliminates the need for an optimum limiter level setting with a simple circuit configuration, improves accuracy, and prevents the above-mentioned distortion current from adversely affecting other circuits. An object of the present invention is to provide a noise reduction device that allows easy selection of switching between two or more limiter levels.

That is, the characteristics of the noise reduction device according to the present invention are that the main signal path, the variable transfer function circuit, and its control circuit.
an operational amplifier serving as a means for adding the main signal and the sub-signal, a means for converting the output of the variable transfer function circuit into a current, a means for switching the current output, and at least a first . and second amplitude limiting means,
A main signal is applied to the non-inverting input terminal of the operational amplifier, a first resistor is connected between the inverting input terminal and the output terminal, and one end of the second resistor is connected to the inverting input terminal of the operational amplifier. The other end is connected to one end of the first amplitude limiting means and one output terminal of the current converting means output switching means, one end of the third resistor is connected to the second resistor, and the other end is connected to one end of the first amplitude limiting means and one output terminal of the current converting means output switching means. one end of the second amplitude limiting means is connected to one end of the second amplitude limiting means and the other output terminal of the current converting means output switching means, and the other ends of the first and second amplitude limiting means are connected to the output terminal of the operational amplifier. It is about being connected.

Here, prior to a detailed explanation of the present invention, the basic configuration and operating principle of a noise reduction device that has been proposed by the inventor and is also a prior art of the present invention will be explained with reference to the drawings.

That is, FIG. 5 shows a noise reduction device as a prior art of the present invention, and shows an example of a circuit configuration for performing a compression operation. In the compression circuit 20 of FIG. 5, a main signal path 23, a sub-signal path 24, and an operational amplifier 25 serving as means for adding signals of these signal paths 23 and 24 are provided between an input terminal 21 and an output terminal 22. is installed. 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. It is converted into a current by the resistor 3@, and the resistor 3
4 to the inverting input terminal of the operational amplifier 35. A feedback resistor 35 is connected between this inverting input terminal and the output terminal of the operational amplifier 25, that is, the output terminal 22 of the compression circuit 20. An amplitude limiting circuit 28 is inserted and connected between the two. Further, a main signal path 23 is connected to a non-inverting input terminal of the operational amplifier 25.

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 set by the conversion coefficient of the voltage-current converter 33 and the resistor 35. The conversion coefficient of the voltage-current converter 33 is determined by the sub-signal path 24.
In order to add the signal to the signal on the main signal path 23, it is necessary to make it negative.

On the other hand, the amplitude limiting circuit 28 is generally constructed by connecting PN junction elements in antiparallel. The limiter level of this amplitude limiting circuit 28 is approximately 1.
It has a specific value of 2Vp-p, but this constant limiter level is set for the sum of the voltage drops across resistors 34 and 35, so by appropriately selecting the value of resistor 34, the output The signal level at the terminal 22 can be set independently. That is, the output signal level is not limited by the nominal limiter level of about 1.2 Vp-p when focusing only on the amplitude limiting circuit 28. For example, if the resistor 34 is set to twice the resistance value of the resistor 35, the effective limiter level when viewed from the output terminal 22 of the entire circuit will be approximately 0.4 Vpp. This amplitude limiting operation has no effect on the main signal.

Next, preferred embodiments of the present invention will be described with reference to FIGS. 6 to 8.

FIG. 6 shows a first embodiment of a noise reduction device according to the present invention, and shows an example of a circuit configuration for performing a compression operation. In FIG. 6, an input terminal 21, an output terminal 22.7, a main signal path 23, a sub signal path 24, an operational amplifier 25 as an adding means, a high-pass filter 26 with a variable cut-off frequency as a variable transfer function circuit, and a control The circuit 2γ and the voltage-current converter 33 can be configured in the same manner as the above-described tenth parts and operate in the same manner, so corresponding parts will be given the same reference numerals and a description thereof will be omitted.

Here, the characteristic configuration of the first embodiment of the present invention includes a changeover switch 4o as means for switching the output current of the voltage-current converter 33, and first and second amplitude limiting circuits 41. 42 and 1st. second and third resistors 51,5
2, 53, a resistor 51 is connected between the inverting input terminal and the output terminal of the operational amplifier 25, and one end of the resistor 52 is connected to the connection point between the inverting input terminal of the operational amplifier 25 and the resistor 51. , the other end of the resistor 52 is connected to one end of the amplitude limiting circuit 41 and one switching output terminal a of the changeover switch 40, and this connection point or node is designated as A. ), one end of the resistor 53 is connected to the other end of the resistor 52, and the other end of the resistor 53 is connected to one end of the amplitude limiting circuit 42 and the other switching output terminal b of the changeover switch 40 (if the two are connected, this connection point or node is connected). (B)
, the other ends of the amplitude limiting circuits 41 and 42 may be connected in antiparallel in the same way as the operational amplifier 2528, using a PN junction configuration.

In the compression circuit 50 of the noise reduction device having the above configuration, the sub signal path 2 for the main signal path 23 is
The addition coefficient of 4 is set by the resistor 51. This addition coefficient does not depend on the state of the changeover switch 40. Now, the changeover switch 40 is connected to the terminal a side, and the sub-signal path current from the voltage-current converter 33 is connected to the connection point (
Alternatively, the substantial first limiter level when added to clause (A) is determined by the sum of the voltage drops across resistor 51 and resistor 52. This first limiter level can be freely set independently of the addition coefficient, that is, by the resistance value of the resistor 52. Next (Y), when the selector switch '40 is switched to the terminal side, the sub-signal path current flows to the connection point (
or clause) (B), and the substantial second limiter level is resistors 51, 52 . and 53 voltage drops, and can be set independently of the addition coefficient and the first limiter level.

That is, the above 1. Second. Third resistance 51°52.5
The first limiter level is the nominal limiter level, that is, the inherent limiter level of the amplitude limiting circuits 41 and 42 (approximately 1.2 V p -p) wo naru. and,
By switching the changeover switch 40, these first
．． Either one of the second riveter levels can be selected and set.

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 50, but generally, in a recording/playback device such as a tape recorder, either recording or playback is performed. Considering that only one mode is selected, compression,
It is preferable to configure such that one of the expansion operations is selectively performed. In this case, for example, a selector switch may be inserted and connected to the input terminal of the high-pass filter 26 to select the expansion operation. In some cases, the output from the output terminal 22 may be inverted and applied to the high-pass filter 26.

Next, FIG. 7 shows a more specific example of the circuit configuration of the first embodiment shown in FIG. 6 above. In this Figure 7,
56 and 57 are positive and negative power supply terminals, respectively. The changeover switch 29 is for switching between the compression and expansion operations described above, and one of the changeover terminals C is connected to the input terminal 21, and the other changeover terminal e is connected to the inverting amplifier 3 that inverts the output from the output terminal 22.
0 output terminal, respectively. The output of this changeover switch 29 is applied to a high-pass filter 26 with a variable cut-off frequency, and the output of this filter 26 is
The current is converted by a differential transistor circuit 36 that is part of the voltage-current converter 33. First and second current switches 43 and 44 corresponding to the changeover switch 40 shown in FIG. (Current mirror) circuits 45 and 46 are connected, and their respective output currents are the above-mentioned nodes (A) and (
B). And these current switches 43
．． By selectively applying a high potential to one of the switching control terminals 47 and 48 of 44, the output current of the differential transistor circuit 36 is selectively applied to either node (A) or node (B). sent to. Therefore, terminal 47
．． 48, the above-mentioned 1. It becomes possible to switch the second limiter level. Also, set the changeover switch 29 to terminal C.
By switching the connection to the terminal e side, a compression operation is performed, and by switching the connection to the terminal e side, an expansion operation is performed, and these compression and expansion operations are complementary to each other.

Next, FIG. 8 shows a second embodiment of the present invention. Second. Third resistor 51,5
Resistors 61, 62 and 63 corresponding to resistors 2 and 53 are provided, and the third
The above-mentioned one end of the resistor 63 is connected to the above-mentioned end of the second resistor 62,
That is, the inverting input terminal of the operational amplifier 25 and the first resistor 6
It is connected to the connection point with 1. In this case, when the changeover switch 40 is connected to the terminal a side, the above-mentioned substantial first limiter level is unique to the amplitude limiting circuit 41, whereas when the changeover switch 40 is connected to the terminal a side, the substantial first limiter level is unique to the amplitude limiting circuit 41. The second limiter level limits the amplitude. Therefore, by independently setting the resistance values R, , R3 of the resistors 62 and 63, the first and second limiter levels can be set independently, and these limiter levels can be set by the changeover switch 40. Switching can be easily selected.

In the second embodiment shown in FIG. 8, other configurations and operations are similar to those of the first embodiment described above, so parts corresponding to those in FIGS. 6 and 7 have the same reference numerals. will be added and the 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 follower, 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. Furthermore, it is easy to maintain two or more limiter levels at a constant level and select them using a changeover switch. can be easily switched.

It should be noted that the present invention is not limited to the above-mentioned embodiments; for example, it is possible to switch between three or more limiter levels by using three or more amplitude limiting circuits and four or more resistors. can also be easily achieved.

[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 a compression circuit of a noise reduction device as a prior art of the present invention. FIG. 6 is a circuit diagram showing a compression circuit of a noise reduction device according to a first embodiment of the present invention, and FIG. 7 is a circuit diagram showing a specific circuit configuration of the first embodiment. FIG. 3 is a circuit diagram showing a configuration that allows switching of expansion operations. FIG. 8 shows compression as a second embodiment of the present invention. FIG. 2 is a circuit diagram showing a noise reduction device capable of switching expansion operations. 21...Input terminal 22...Output terminal 23...Main signal path 24...Sub signal path 25...Operation amplifier 26 as addition means...High-pass filter 27 with variable cut-off frequency ... Control circuit 28, 41, 42 ... Amplitude limiting circuit 33 ... Voltage-current converter 40 ... Changeover switch 51.61 ... First resistor 52.62 ... Second resistor 53.63...Third resistance % applicant Sony Corporation agent Patent attorney Koike Mima 1) Sakae Mura - 11th 2nd - 3rd m 14 vs Kaya 5! 11 z'/ 6th am Tsuta 7 Figure 2 105 Figure 8?

Claims (1)

[Claims] an operational amplifier comprising a main signal path and a sub-signal path including a variable transfer function means and a control circuit thereof, and which is a means for adding the main signal of the main signal path and the sub-signal of the sub-signal path; means for converting the output of the variable transfer function circuit into a current;
output current switching means of the current converting means; and at least a first. applying the main signal to the non-inverting input terminal of the operational amplifier, and connecting a first resistor between the inverting input terminal and the output terminal of the operational amplifier; One end of a second resistor is connected to the inverting input terminal of the operational amplifier, and the other end of the second resistor is connected to one end of the first amplitude limiting means and one switching output of the current converting means output switching means. terminal, one end of a third resistor is connected to the second resistor, and the other end of the third resistor is connected to one end of the second amplitude limiting means and the other switching output terminal of the switching means. and the other ends of the first and second amplitude limiting means are connected to the output terminal of the operational amplifier.