JPH03815B2 - - Google Patents

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 perform compression operations during recording (or transmission) and during playback (or reception).
By making each perform a stretching motion,
It apparently expands the dynamic range of the recording medium (or signal transmission path), and is constructed with a compression circuit on the input side and an expansion circuit on the output side. These compression circuits and expansion circuits include
A variable transfer function circuit and its control circuit are provided to change the transfer function depending on 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 input transiently 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. It becomes impossible to accurately reconstruct the data.

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 an example of the basic configuration of a compression circuit 10 provided with such an amplitude limiting circuit. The compression circuit 10 shown in FIG. 1 is composed of a main signal path 3, a sub signal path 4, and an adder 5, which are arranged between an input terminal 1 and an output terminal 2. The sub-signal path 4 includes a variable cut-off frequency high-pass filter 6;
It is comprised of a control circuit 7 that controls its cutoff frequency, and the above-mentioned amplitude limiting circuit 8.

FIG. 2 is a graph for explaining the frequency response of this compression circuit 10. 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
Almost determined by the characteristics of the high-pass filter 6,
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 the transfer characteristic A and B of these signal paths 3 and 4.
When there is no signal, the characteristic C ₁
Therefore, when the signal level increases, characteristic _C2 is obtained, for example. Through this process, the dynamic range of the input signal is compressed. During playback (or reception), an expansion circuit (not shown) having complementary transfer characteristics to the compression circuit 10
Of course, the expansion operation is performed by , and the original dynamic range is restored.

Next, the amplitude limiting circuit 8 will be explained. 1st
In _the compression circuit 10 shown in the figure, when the amplitude limiting circuit 8 is not provided, if a tone burst signal as shown in _FIG . A signal with a response waveform 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 frequency side as shown in FIG. _2B1 , and immediately after time _t1 , the control circuit Due to the finite rise time constant of 7, the cut-off frequency is not shifted to the high frequency side 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. 3, and an amplitude limiting characteristic is applied to 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 10 of FIG. 1, and corresponding circuit parts are given the same reference numerals. The amplitude limiting circuit 8 in FIG.
PN junction, i.e. two diodes 11,1
Two anodes and two cathodes are connected in parallel in opposite directions. Here, the limiter level of the nonlinear element has a value unique to the element, and the degree of freedom is small. For example silicon
The PN junction has a threshold of about 0.6V, and the limiter level of the amplitude limiting circuit 8 of FIG. 4, which is connected in antiparallel, has a value of about 1.2V _PP . 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, the adding resistors 16 and 17 which become the adder 5 and the operational amplifier 1
5, by appropriately setting the ratio of the adding resistors 16 and 17, the output of the sub signal path 4 amplified by the operational amplifier 13 has a constant ratio to the output of the main signal path 3 (a small addition It is also necessary that the values are added (with a 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 operations to amplify and attenuate the signal level. As a result, accuracy, signal-to-noise ratio, etc. deteriorate. Also,
A distortion current flows through the amplitude limiting circuit 8, and in the configuration shown in FIG. 4, it may affect other circuits 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 limiter level is 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 conventional situation, and eliminates the need for the operational amplifier 13 dedicated to limiter level setting, allows optimum limiter level setting with a simple circuit configuration, improves accuracy, and achieves the above-mentioned functions. The object of the present invention is to provide a noise reduction device that not only can prevent the adverse effects of distortion current on other circuits, but also can easily switch between two or more limiter levels depending on, for example, the characteristics of a recording medium. shall be.

That is, the features of the noise reduction device according to the present invention include a main signal path to which an input signal is supplied;
an operational amplifier comprising a variable transfer function circuit and a sub-signal path to which the input signal is supplied, including a control circuit thereof, and serving as an addition means for the main signal of the main signal path and the sub-signal of the sub-signal path; A means for converting the output of the variable transfer function circuit into a current, a means for switching the output current of the current converting means, and first and second amplitude limiting means, the non-inverting input terminal of the operational amplifier The main signal is applied, a first resistor is connected 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 second resistor is connected to the inverting input terminal of the operational amplifier. The other end of the resistor is connected to one end of the first amplitude limiting means and one switching output terminal of the current converting means output switching means, and the third
One end of the resistor is connected to the second resistor, and the third resistor is connected to the second resistor.
The other end of the resistor hole 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. at least the second
The limiting characteristic of the first amplitude limiting means is set from the value of the resistor, and the limiting characteristic of the second amplitude limiting means is set from at least the value of the third resistor.

Here, prior to describing embodiments 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 prior art to 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, an input terminal 21 and an output terminal 22
A main signal path 23, a sub-signal path 24, and an operational amplifier 25 serving as means for adding the signals of these signal paths 23 and 24 are disposed between the main signal path 23 and the sub-signal path 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. Considering that amplification is required to adjust the level of the limiter, the voltage is converted into a current by a voltage-current converter 33, and is sent to the inverting input terminal of the operational amplifier 25 via a resistor 34. This inverting input terminal and operational amplifier 2
5, that is, the output terminal 22 of this compression circuit 20, a feedback resistor 35 is connected between the output terminal of the voltage-current converter 33 and the operational amplifier 25.
An amplitude limiting circuit 28 is inserted and connected between the output terminal 22 and the output terminal 22 of the amplifier. 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. The output current of the voltage-current converter 33 flows into the output terminal 22 of the operational amplifier 25 via the resistor 34 and the resistor 35. At this time, the voltage drop that occurs across the resistor 35 as the current converted by the voltage-current converter 33 changes 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-to-current converter 33 needs to be negative in order to add the signal on the secondary signal path 24 to the signal on the main signal path 23.

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 has a specific value of approximately 1.2V _PP , but this constant limiter level is generated in the resistors 34 and 35 as the current converted by the voltage-current converter 33 changes. Since it is set for the sum of voltage drops, by appropriately selecting the value of the resistor 34, the effective limiter level in the entire circuit can be freely set independently of the signal level at the output terminal 22. That is, the output signal level is not constrained due to the nominal limiter level of about 1.2V _PP when looking only at the amplitude limiting circuit 28. for example,
When the resistance value of the resistor 34 is set to twice that of the resistor 35, the effective limiter level when viewed from the output terminal 22 of the entire circuit becomes approximately 0.4V _PP . This amplitude limiting operation has no effect on the main signal.

Next, regarding preferred embodiments of the present invention,
This will be explained 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 this Figure 6,
an input terminal 21, an output terminal 22, 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, a control circuit 27, and a voltage
The current converter 33 can be constructed in the same manner as the respective parts shown in FIG. 5 described above 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 40 as means for switching the output current from the voltage-current converter 33, a first amplitude limiting circuit 41, a second amplitude limiting circuit 41, 42 and first, second, and third resistors 51, 52, and 53, the 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 operational amplifier 25. 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 switching switch 40.
(Let this connection point or node be A),
Connecting one end of the resistor 53 to the other end of the resistor 52,
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 (this connection point or node is referred to as B), and the other ends of the amplitude limiting circuits 41 and 42 are connected to each other. Operational amplifier 2
It is connected to the output terminal of 5. Note that as the amplitude limiting circuits 41 and 42, an NP junction structure connected in antiparallel may be used, similar to the amplitude limiting circuit 28 of FIG. 5.

In the compression circuit 50 of the noise reduction device having the above configuration, the addition coefficient of the sub signal path 24 to the main signal path 23 is set by the resistor 51. This addition coefficient is determined by the changeover switch 4.
It does not depend on the 0 state. Now switch 4
0 is switched and connected to the terminal a side, and the sub-signal path current from the voltage-current converter 33 is applied to the connection point (or node) (A), the substantial first limiter level is: It is determined by the sum of voltage drops across resistor 51 and resistor 52. This first limiter level is determined independently of the addition coefficient, that is, the resistor 52
It can be freely set by the resistance value. Next, when the changeover switch 40 is switched to the terminal b side, the sub-signal path current is applied to the connection point (or node) (B), and the substantial second limiter level is set to the resistor 51,
It is determined by the sum of the voltage drops of 52 and 53,
It can be set independently of the addition coefficient and the first limiter level.

That is, the first, second, and third resistors 51,
When the resistance values of 52 and 53 are respectively R ₁ , R ₂ , and R ₃ , the first limiter level is the nominal limiter level, that is, the amplitude limiting circuit 41
or 42 unique limiter level (approximately 1.2V _PP )
R ₁ /R ₁ +R is multiplied by ₂ , and the second limiter level is similarly multiplied by R ₁ /R ₁ +R ₂ +R _{by 3} . By switching the changeover switch 40, one of the first and second limiter levels can be selected and set.

The expansion circuit of the noise reduction device is
Although a circuit symmetrical or complementary to the compression circuit 50 may be constructed separately, in general, in a recording/playback device such as a tape recorder, only one of the recording and playback modes is selected. In view of this, it is preferable to configure the system so that either compression or expansion is selectively performed by switching a switch. , when the expansion operation is selected, the output from the output terminal 22 is inverted and passed through the high-pass filter 26.
Just apply it to .

Next, FIG. 7 shows a more specific example of the circuit configuration of the first embodiment shown in FIG. 6 above. In FIG. 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 has one changeover terminal c connected to the input terminal 21, and the other changeover terminal e connected to the output terminal of an inverting amplifier 30 that inverts the output from the output terminal 22. each connected. The output from this changeover switch 29 is applied to a high-pass filter 26 with a variable cut-off frequency, and the output from this filter 26 is converted into a current by a differential transistor circuit 36 that is part of the voltage-current converter 33. Ru. First and second current switches 43 and 44 corresponding to the changeover switch 40 shown in FIG. 6 are connected to the collectors of the transistors 37 and 38 constituting the differential transistor circuit 36. (Current mirror) circuit 45, 4
6 are connected, and the output currents are added to nodes (A) and (B) above, respectively. By selectively applying a high potential to either one of the switching control terminals 47, 48 of these current switches 43, 44,
The output current of the differential transistor circuit 36 is at the above node (A)
Alternatively, it is selectively sent to either clause (B). Therefore, the first and second limiter levels can be switched by the potentials of the terminals 47 and 48. Furthermore, by switching and connecting the changeover 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.

Next, FIG. 8 shows a second embodiment of the present invention,
The first, second, and third in the first embodiment described above
Resistors 61, 6 corresponding to resistors 51, 52, 53 of
2, 63, and the one end of the third resistor 63 is connected to
It is connected to the above-mentioned one end of the second resistor 62, that is, the connection point between the inverting input terminal of the operational amplifier 25 and the first resistor 61. In this case, the changeover switch 40
The above-mentioned substantial first limiter level when switched and connected to the terminal a side is R ₁ /R ₁ + R ₂ times the above-mentioned nominal limiter level specific to the amplitude limiting circuit 41, whereas when the changeover switch 40 is connected The second limiter level when switched and connected to the terminal b side is R ₁ /R ₁ +R _twice the limiter level specific to the amplitude limiting circuit 42 . Therefore, the resistance value R ₂ of the resistors 62 and 63,
By setting _R3 independently, the first and second limiter levels can be set independently, and these limiter levels can be easily switched and selected by the changeover switch 40. 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. The explanation will be omitted.

As is clear from the above description, according to the noise reduction device according to the present invention, there is no need for a conventional operational amplifier dedicated to limiter level setting, and the operational amplifier 25 as a means for adding the main signal and the sub-signal is not required. By using just one, 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 allows it to function as a voltage follower for the main signal. In order to operate, 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 set two or more limiter levels and select them using a changeover switch. For example, the limiter level of the auxiliary signal path can be changed depending on the type of tape used as the recording medium, the noise reduction method, etc. Switching is easy.

It should be noted that the present invention is not limited to the above-mentioned embodiments; for example, three or more amplitude limiting circuits and four or more resistors may be used to enable switching selection among three or more limiter levels. It can be easily achieved.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the basic configuration of the compression circuit of the noise reduction device, Figure 2 is a graph showing the transfer characteristics of the circuit in Figure 1, and Figure 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. 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;
The figure is a circuit diagram showing a specific circuit configuration of the first embodiment, and shows a configuration in which compression and expansion operations can be switched. FIG. 8 is a circuit diagram showing a noise reduction device capable of switching between compression and expansion operations as a second embodiment of the present invention. 21...Input terminal, 22...Output terminal, 23...
...Main signal path, 24...Sub signal path, 25...Operation amplifier as addition means, 26...High-pass filter with variable cut-off frequency, 27...Control circuit, 28,
41, 42... Amplitude limiting circuit, 33... Voltage-current converter, 40... Changeover switch, 51, 61...
...First resistance, 52, 62...Second resistance, 5
3,63...Third resistance.

Claims (1)

[Claims] 1. Consisting of a main signal path to which an input signal is supplied, and a sub-signal path including a variable transfer function circuit and its control circuit and to which the input signal is supplied, the main signal of the main signal path is and a sub-signal of the sub-signal path; a means for converting the output of the variable transfer function circuit into a current; a means for switching the output current of the current converting means; applying the main signal to the non-inverting input terminal of the operational amplifier; connecting a first resistor between the inverting input terminal and the output terminal of the operational amplifier; One end 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 terminal of the current converting means output switching means. , one end of a third resistor is connected to the second resistor, 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, The other ends of the first and second amplitude limiting means are connected to the output terminal of the operational amplifier, and the limiting characteristic of the first amplitude limiting means is set based on the value of at least the second resistor, and the limiting characteristic of the first amplitude limiting means is set based on the value of at least the second resistor. A noise reduction device characterized in that the limiting characteristic of the second amplitude limiting means is set from the value of the resistance.