JPS6161293B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to frequency selective circuits with adjustable characteristics, and in particular to adaptive filter circuits for incorporation into audio recording and playback schemes where it is desirable to reduce noise effects. .

When recording and reproducing acoustic signals, noise components generated at the frequencies of high-level desired signal components cannot be heard by the ears due to auditory masking characteristics. On the other hand, noise components having frequencies that do not contain the desired signal components are audible and thus distort the program information. In order to remove such unmasked noise components and pass desired signal components, it is common to change a certain band of the frequency spectrum of the audio recording/reproducing system.

For example, Dutch Patent Application No. 710869 (1972)
shows a noise reduction device having a phase shifter, the input of which is the output of the device. This phase shifter has two outputs which respectively connect the inputs of the adder directly and indirectly via an additional signal processing channel. This signal processing channel includes in series a high-pass filter that passes signal components with frequencies above 4.5 KHz, a peak limiter, and an automatic gain control.

This device dynamically waves signals. That is, at frequencies higher than 4.5 KHz, the transfer coefficient depends on the level of the input signal components at this frequency and decreases as the level decreases. The transfer factor is changed by subtracting the components of the input signal at frequencies higher than 4.5 KHz from the broadband signal. Since this subtraction is maximal only at a single frequency where a 180° phase shift is possible, it provides only a partial reduction in high frequency noise.

This device therefore provides only a low noise reduction of 3-5 dB for various frequencies above 4.5 KHz.

The dynamic noise reduction filter disclosed in U.S. Pat. No. 3,678,416 has means for controlling the cutoff frequency. This means includes a controllable reduction and high-pass filter with an adjustable cut-off frequency and a control circuit. Each of these control circuits consists of a filter circuit having a band spanning the adjustment range of the cut-off frequency of the respective controllable filter, an amplifier, and an amplitude detector. To adjust the cut-off frequencies of the controllable low-pass and high-pass filters, their control circuits are subjected to the action of the signals entering from the inputs of the above means via the high-pass and low-pass filters, respectively. . As a result, the above-mentioned cut-off frequency is adjusted by the level of the signal component passing through the control circuit via the high-pass and low-pass filters, respectively. The greater the input to this means, the narrower the controllable filter band and the lower the output noise level. Also, the smaller the input into the means, the wider the bandwidth, but the higher the noise level becomes unacceptably large.

In the filter described above, the cutoff frequency of the controllable filter is inaccurately adjusted, so that it is not possible to reduce the modulation noise during the adjustment. This is due to the fact that the cutoff frequency is determined by the level of the component of the input signal within the band of the uncontrolled filter circuit of the control circuit. It is known that the envelope of the average statistical frequency spectrum of an acoustic signal program tends to fall in the region below 400 Hz and in the region above 1500 Hz. Assuming that the cutoff frequencies of the filter circuit in the control circuit are fixed values of 300 and 1000 Hz, the cutoff frequency of the controllable filter should be such that as the level of the signal component near these values increases, the cutoff frequency reaches the limit of the operating frequency range. Adjustments occur. As a result, noise at frequencies that occur near these limits in the absence of mask signal components becomes audible. As the level of the intermediate frequency components of the signal changes and the low level of these components, if any, are at the limits of the operating frequency range, variations in the noise level, or modulation noise, become audible.

The present invention therefore contemplates an adaptive filter circuit capable of providing bandwidth adjustment of audio recording and playback schemes as determined by the frequency limits of the desired components of the input audio signal.

The main object of the present invention is to provide an adaptive filter circuit that can provide a higher noise reduction effect in audio recording and playback systems compared to conventional circuits.

Another object of the invention is to provide an adaptive filter circuit capable of providing reduced audible modulation noise.

Yet another object is to provide an adaptive filter circuit with adjustable bandwidth having a highly controllable cutoff frequency determined by the frequency limits of the desired component of the input acoustic signal.

Another object is to provide an adaptive filter circuit whose bandwidth can be varied depending on the sensitivity of the applied signal.

Yet another object is to provide an adaptive filter circuit with an adjustable noise reduction threshold.

It has a controllable filter with an adjustable cut-off frequency, the input and output of this filter form an input and an output, respectively, the input and output of said filter are connected to the input of an amplitude detector, and the input and output of said filter are connected to the input of an amplitude detector. In an adaptive filter circuit whose output is now connected to the control input of the filter, an algebraic adder generates a control signal, a weighting filter transforms the spectrum of the control signal, and a noise reduction threshold level. a threshold limiter that sets
a control signal frequency modifier connected in series, one input of the adder being connected to the input of the controllable filter, and a second input of the adder being connected to the output of the filter. An adaptive filter circuit is disclosed herein, characterized in that the output of the modifier is connected to the input of the detector.

This embodiment allows adjustment of the cut-off frequency of the controllable filter as determined by the frequency limits of the desired component of the input signal. Therefore, the bandwidth of this adaptive filter circuit becomes wider the higher the frequency limit of the component of the input signal that exceeds the threshold level.
On the other hand, the bandwidth of this circuit is narrower when there are no high frequency components of the signal present for both low and high levels of intermediate frequency components.

As a result, the cutoff frequency can be adjusted with greater precision, so that the adaptive filter circuit provides significant noise reduction and less perceptibility of modulation noise.

Preferably, the adaptive filter circuit comprises an uncontrolled filter having a width equal to the maximum bandwidth of the controllable filter, the input and output of the uncontrolled filter being respectively connected to the input and threshold limiter of the adaptive filter circuit. Connect to the input.

This embodiment eliminates the occurrence of components of the input signal outside the operating frequency range at the control input of the controllable filter.

The adaptive filter circuit may include an uncontrolled filter having an input connected to an input of the controllable filter and an output connected to one of the inputs of the algebraic adder.

In this embodiment, components of the input signal occurring within the bandwidth of the controllable filter are suppressed as much as possible at the output of this adder. This is achieved if the amplitude-frequency and phase-frequency characteristics of the controllable and non-controllable filters exactly match each other with the cutoff frequency of the controllable filter adjusted to the frequency limit of the operating frequency range.

Advantageously, the adaptive filter circuit has an uncontrolled filter having an input connected to the output of the algebraic adder and an output connected to the input of the weighting filter.

This embodiment provides an uncontrolled filter with an amplitude-frequency response characteristic that has a steeper slope outside the bandwidth, thereby preventing the occurrence of signal components beyond the operating frequency range at the control input of the controllable filter. The result is a significant exclusion. Under these conditions, the uncontrolled filter random phase-frequency response characteristics can be maintained within this bandwidth.

Preferably, this adaptive filter circuit consists of a control signal frequency modifier constructed as an integrator and a controllable filter constructed as a high-pass filter for the operating frequency range.

This embodiment eliminates unwanted noise occurring within the acoustic frequency range.
Enables reduction of unmasked low frequency components.

The adaptive filter circuit may consist of a controlled frequency modifier constructed as a differentiator and a controllable filter constructed as a low-pass filter.

This embodiment provides a reduction of unwanted, unmasked high frequency components within the acoustic frequency range.

The adaptive filter circuit of FIG. 1 has a controllable filter 1 with an adjustable cut-off frequency, which is used to change its bandwidth. Input 2 of filter 1 is connected to input 3 of the adaptive filter circuit. Output 4 of filter 1 is connected to input 5 of the adaptive filter circuit. Input 2 of filter 1 is also connected to input 6 of algebraic adder 7, whose input 8 is connected to output 4 of filter 1.

Adder 7 generates a control signal with a band equal to the difference between the frequency bands of the input and output signals of filter 1. This is done by subtracting components that are within the band of filter 1 from the input signal content.

The output of the adder 7 is connected to the input of a weighting filter 9 which forms a different control signal when the hearing changes with respect to the sound signal frequency.

The filter 9 may be constructed in the form of a parallel bandpass filter with an adjustable transmission coefficient so that the amplitude-frequency characteristic can be adjusted.

The output of the filter 9 is connected to the input of a threshold limiter 10, the level of which determines the noise reduction threshold of the adaptive filter circuit.

The output of the threshold limiter 10 is connected to the input of a control signal frequency modifier 11, which modifier changes its control input 1 when the frequency of the controllable filter 1 changes.
It operates to change the level of the second control signal.

The output of the corrector 11 is connected to an amplitude detector 13 having an output connected to the control input 12 of the controllable filter 1.
Connect to the input of Detector 13 rectifies the control signal entering its input from the output of modifier 11.

FIG. 2 is a block diagram of one embodiment of an adaptive filter circuit that allows maximum suppression of input signal components at the output of algebraic adder 7. Here, the input of the uncontrolled filter 14 is connected to the input 2 of the controllable filter 1, and the output of the filter 14 is connected to the adder 7.
Connect to input 6 of

FIG. 3 shows another embodiment of an adaptive filter circuit designed to prevent the generation of false control signals in the case of random phase-frequency response characteristics within its bandwidth of uncontrolled filter 14. Here the input of the uncontrolled filter 14 is connected to the output of the adder 7, the output of which is connected to the input of the weighting filter 9.

FIG. 4 is a circuit diagram of another embodiment of an adaptive filter circuit designed to suppress high frequency noise components.

This adaptive filter circuit consists of resistors 15,
16 and capacitors 17 and 18 as passive elements, the controllable low-pass filter 1 is configured as a secondary active filter. An operational amplifier 19 is used as an active element. Resistors 15 and 16 are connected in series between input 3 of the adaptive filter circuit and the non-inverting input of operational amplifier 19. Resistance 15,1
A capacitor 17 is connected between the connection point of 6 and the output of the amplifier 19 to form a positive feedback circuit. A capacitor 1 is connected between the non-inverting input of the amplifier 19 and the ground point.
8 is connected. A resistor 20 is connected between the inverting input of the amplifier 19 and a ground point, and a resistor 21 is connected between the inverting input and its output. The resistance ratio of the resistors 21 and 20 forming the negative feedback circuit determines the transfer coefficient of the operational amplifier 19. A resistor 22 is connected between the input of the adaptive filter circuit and ground. The series resistors 22, 15, 16 are responsive to DC conditions at the non-inverting input of the amplifier 19.

The electric field transistors 23 and 24 are connected to the resistors 15 and 16.
Connect in parallel. The sources of these transistors are both connected to the connection point of resistors 15 and 16. The drain of transistor 23 is connected through a blocking capacitor 25 to the input 3 of the adaptive filter circuit, and the drain of transistor 24 is connected through a blocking capacitor 26 to the non-inverting input of amplifier 19. Transistors 23 and 24 act as variable resistors. Blocking capacitors 25, 26 prevent direct current from passing through the channels of transistors 23, 24, eliminating the inherent noise of the control process.

The output of the operational amplifier 19 is connected to the output 5 of the adaptive filter circuit.

Adder 7 is basically constructed as an operational amplifier 27 with an inverting input connected to input 3 of the adaptive filter circuit through resistor 28 and a non-inverting input connected to the junction of resistors 15 and 16 through resistor 29. . The connection point above is controllable filter 1
is used as the output of the first part. The output of operational amplifier 27 is connected through resistor 30 to its inverting input, and its non-inverting input is connected through resistor 31 to ground. The values of resistors 28 and 30 are resistors 29 and 31, respectively.
be equivalent to.

The transfer coefficient of adder 7 is determined by the ratio of resistors 30 and 28. In this configuration, the first
The incoming out-of-phase signals occurring at the input and output of the section are subtracted from each other. The adder 7 is connected to this first part since the phase distortion caused by the controllable filter 1 is smaller overall than that caused by a two-part controllable filter. This significantly attenuates the signal components within the band of the controllable filter 1 at the output of the adder 7 so that they do not significantly affect the adjustment of the cutoff frequency of the filter 1.

The input of weighting filter 9 is connected to the output of adder 7.

Filter 9 is a bandpass filter with two conventional RC circuits. Resistor 32 and capacitor 33
The first RC circuit consisting of a variable resistor 35 and a capacitor 34 is a low-pass filter.
The RC circuit is a high-pass filter. The cutoff frequencies of these two filters are chosen such that the transmission coefficient of filter 9 has its maximum value in the audio intermediate frequency band and decreases on either side of this maximum value.

The threshold limiter 10 is connected at its input to a cursor of a variable resistor 35 which controls the noise reduction threshold. The limiter 10 consists of diodes 36 and 37 connected in parallel in opposite directions.

The output of the limiter 10 is connected to the input of a control signal frequency modifier 11 (differentiator) consisting of an operational amplifier 38. A capacitor 39 is connected between the output of the limiter 10 and the inverting input of the amplifier 38, and a negative feedback resistor 40 is connected between the inverting input and its output.

An amplitude detector 13 consisting of diodes 41 and 42, resistors 43 and 44, and a capacitor 45 is connected to the output of this differentiator.

A resistor divider consisting of resistors 47-50 is connected in parallel between the output of the detector 13 and the output of the amplifier 19 through a capacitor 46. Resistors 48 and 47
The gate of the field transistor 24 is connected to the connection point of the resistors 49 and 50, and the gate of the transistor 23 is connected to the connection point of the resistors 49 and 50. Transistor 23,2
The gate of No. 4 receives the signal from the output of the amplifier 19 through the divider, and compensates for nonlinear distortion.

In the initial state, transistors 23 and 24 connect to amplifier 19 through resistors 43, 44, 48, and 50.
are cut off by the DC voltage that enters their gates from the outputs of the gates. This voltage is set at the non-inverting input of the amplifier 38 by a diode 51 and a resistor 52 connected in series to the reference voltage +E.

The diode 51 is the field transistor 23, 24
It acts as a temperature compensating element to stabilize the channel resistance. As the ambient temperature increases, the channel resistance of transistors 23 and 24 decreases. At the same time, the voltage across the diode 51 also decreases, but the voltage across the resistor 52 increases, causing the transistors 23 and 24 to cease conducting, thereby increasing their channel resistance.

A diode 41 connected to the output terminal of the operational amplifier 38 via a resistor 43 is connected to the transistors 23 and 2.
This is to prevent the gate-source junction of No. 4 from being reversely energized in dynamic mode.

Since sound recording and playback is mostly affected by the interference of intermediate and high frequency noise components, a preferred embodiment of the present invention provides an adapter with a controllable low-pass filter and a differentiator that acts as a control signal frequency modifier. It is a filter circuit.

The adaptive filter circuit having the above structure operates as follows.

When no signal enters the input 3, the output 4 of the low frequency controllable filter 1, that is, the output 5 of the adder 7
and input 8 have only narrow-band noise components within the initial band of controllable filter 1, and input 6 of adder 7
has wide area noise from input 3.

By subtracting the narrow band noise from the wide band noise, the adder 7
Only the high frequency noise component remains in the output of , which enters the limiter 10 through the weighting filter 9 . In this case, the limit level of the limiter 10, which determines the noise reduction threshold, is higher than the noise component level at the input of the limiter 10 and no signal is generated at its output;
As a result, the cutoff frequency of filter 1 does not change.
Thus, in the absence of a signal, the frequency band of this circuit is limited to its upper limit, so that the noise components occurring above the reference cutoff frequency of the filter 1 are reduced and become imperceptible.

When the signal enters input 3, the output of filter 1 is
Therefore, at the input 9 of the adder 7 there is a low frequency component of the input signal which lies within the initial band of the filter 1.

At the output 6 of the adder 7 are all input signal components in the operating frequency band. As a result of the subtraction of the low frequency components of the signal, only the high frequency components of this signal are present at the output of the adder 7. These components are applied to the input of limiter 10 through filter 9. If the level of the high frequency component at the input of limiter 10 exceeds a predetermined threshold, the output of limiter 10 is modified by control signal frequency modifier 11, which according to this embodiment of the invention is a differentiator. A signal is generated. From the output of the corrector 11 this signal is applied to the input of the detector 13 and, once rectified there, is applied to the control input of the controllable filter 1 with a cutoff frequency occurring in the high frequency limit.

High frequency components of the input signal that exceed a predetermined noise reduction threshold pass unattenuated to the output 5 of the adaptive filter circuit. The high frequency components of the signal that are in the input signal and exceed a predetermined noise reduction threshold are sent unattenuated to this output. At the same time, the adder 7 subtracts the signal components within the expanded band of the filter 1 from the broadband signal. As a result, stronger components adjacent to the cutoff frequency of the filter 1 are removed from the control signal high frequency components and do not significantly influence further adjustment of the cutoff frequency.

As the input signal spectrum widens to include high frequency regions with component levels that exceed the intended noise reduction threshold, the cutoff frequency of filter 1 increases until it reaches the upper limit of its operating frequency band and all signal components are attenuated. It is adjusted so that it outputs output 5 without any problem.

Despite the widening of the frequency band of this circuit, the high frequency noise generated in this signal can be ignored due to the mask characteristics of the ear. As the input signal spectrum narrows and the level of high frequency components decreases, the circuit operates such that the cutoff frequency is reduced and high frequency noise at its output is attenuated.

An adaptive filter circuit with a controllable high-pass filter operates similarly and modifier 11 is an integrator in this case.

In specialized radio devices, a controllable low-pass filter can be used in combination with a controllable high-pass filter, resulting in a bandpass filter whose upper and lower cutoff frequencies can be adjusted independently.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an adaptive filter circuit according to the present invention, and FIG. 2 is a block diagram of an adaptive filter circuit according to the present invention designed to suppress as much as possible the components of the input signal at the output of an algebraic adder. FIG. 3 is a block diagram of another embodiment of the adaptive filter circuit that prevents the generation of an erroneous control signal in the random phase-frequency response characteristic of a non-controlled filter; FIG. The figure is a circuit diagram of another embodiment that suppresses high frequency noise components. DESCRIPTION OF SYMBOLS 1... Controllable filter, 7... Adder, 9... Weighting filter, 10... Threshold limiter, 11... Control signal frequency modifier, 13... Amplitude detector, 14... Uncontrolled filter.

Claims (1)

[Claims] 1. An adaptive filter circuit comprising a controllable filter, an algebraic adder, a weighting filter, a threshold limit, a control signal frequency modifier, and an amplitude detector. The controllable filter has an adjustable cutoff frequency and changes the passband of the adaptive filter circuit, and the controllable filter has an adjustable cutoff frequency and a pass band of the adaptive filter circuit. said algebraic adder is for generating a control signal and has one input, another input and one output, said algebraic adder having an input, an output and a control input respectively connected to said one one input is connected to the input of the controllable filter, the other input is connected to the output of the controllable filter, and the weighting filter is configured to control the control signal by changing the transmission coefficient of the control signal spectral element. for transforming the control signal spectrum by varying the load sensitivity with respect to frequency, having an input and an output, said input being connected to said output of said algebraic adder, said threshold limiter being connected to said adapter; the control signal frequency modifier is for setting the noise reduction threshold level of the weighting filter and has an input and an output, said input being connected to said output of said weighting filter; It is for changing the control signal level at the control input of the filter according to frequency, and has an input and an output, the input is connected to the output of the threshold limiter, and the amplitude detector is connected to the output of the threshold limiter. for shaping the control signal applied to said control input of said controllable filter, having an input and an output, said input being connected to said output of said frequency modifier, and said output being connected to said output of said controllable filter. An adaptive filter circuit connected to the control input, characterized in that: 2. An adaptive filter circuit comprising a controllable filter, an algebraic adder, a weighting filter, a threshold limiter, a control signal frequency modifier, an amplitude detector, and an uncontrolled filter, The controllable filter has an adjustable cutoff frequency and changes the passband of the adaptive filter circuit, and the controllable filter has an adjustable cutoff frequency and a pass band of the adaptive filter circuit. each having an input, an output and a control input connected thereto, said algebraic adder being for generating a control signal and having one input, another input and one output, said one an input is connected to the input of the controllable filter, the other input is connected to the output of the controllable filter, and the weighting filter adjusts the frequency by changing the transfer coefficient of the control signal spectral elements. for transforming the control signal spectrum by varying the load sensitivity of the adapter, having an input and an output, the input being connected to the output of the algebraic adder, and the threshold limiter being connected to the output of the algebraic adder; the control signal frequency modifier is for setting the noise reduction threshold level of the controllable filter, and has an input and an output, the input being connected to the output of the weighting filter; The amplitude detector is for changing the control signal level at the control input of the converter according to the frequency, and has an input and an output, the input is connected to the output of the threshold limiter, and the amplitude detector is connected to the output of the threshold limiter. for shaping the control signal applied to said control input of said control filter, having an input and an output, said input being connected to said output of said frequency modifier, said output being connected to said output of said controllable filter; connected to a control input, said uncontrolled filter having a band equal to the frequency width of the desired signal and preventing the occurrence of input signal components exceeding the operating frequency width and spurious control signals at the control input of said controllable filter. An adaptive filter circuit configured to prevent the formation of an adaptive filter, wherein the uncontrolled filter is connected between an input of the controllable filter and one input of the algebraic adder. 3. The controllable filter is configured as a low-pass type, and the control signal frequency modifier is configured as a differentiator, the differentiator being configured to adjust the control signal level at the control input of the controllable filter and the control signal 2. The adaptive filter circuit according to claim 1, wherein the adaptive filter circuit is configured to provide a proportional relationship with frequency. 4. The controllable filter is constituted by a bypass filter, and the control signal frequency modifier is constituted as an integrator, and the integrator has a voltage between the control signal level at the input of the controllable filter and the control signal frequency. 2. The adaptive filter circuit according to claim 1, wherein the adaptive filter circuit is configured to provide an inversely proportional relationship. 5. An adaptive filter circuit comprising a controllable filter, an algebraic adder, a weighting filter, a threshold limiter, a control signal frequency modifier, an amplitude detector, and an uncontrolled filter. The controllable filter has an adjustable cutoff frequency and changes the passband of the adaptive filter circuit, and the controllable filter has an adjustable cutoff frequency and a pass band of the adaptive filter circuit. each having an input, an output and a control input connected thereto, said algebraic adder being for generating a control signal and having one input, another input and one output, said one an input is connected to the input of the controllable filter, the other input is connected to the output of the controllable filter, and the weighting filter adjusts the frequency by changing the transfer coefficient of the control signal spectral elements. for transforming the control signal spectrum by varying the load sensitivity of the adapter, having an input and an output, the input being connected to the output of the algebraic adder, and the threshold limiter being connected to the output of the algebraic adder; the control signal frequency modifier is for setting the noise reduction threshold level of the controllable filter, and has an input and an output, the input being connected to the output of the weighting filter; The amplitude detector is for changing the control signal level at the control input of the converter according to the frequency, and has an input and an output, the input is connected to the output of the threshold limiter, and the amplitude detector is connected to the output of the threshold limiter. for shaping the control signal applied to said control input of said control filter, having an input and an output, said input being connected to said output of said frequency modifier, said output being connected to said output of said controllable filter; connected to a control input, said uncontrolled filter having a band equal to the frequency width of the desired signal and preventing the occurrence of input signal components exceeding the operating frequency width and spurious control signals at the control input of said controllable filter. An adaptive filter circuit configured to prevent the formation of an adaptive filter, wherein the uncontrolled filter is connected between the output of the algebraic adder and the input of a weighting filter. 6. The controllable filter is constituted by a controllable low-pass filter, and the control signal frequency modifier is constituted by a differentiator, the differentiator having a control signal level at the control input of the controllable low-pass filter. , the adaptive filter circuit according to claim 2, wherein the adaptive filter circuit is configured to provide a proportional relationship between the control signal frequency and the control signal frequency. 7. The controllable filter consists of a bypass filter, the control signal frequency modifier consists of an integrator,
The integrator has a voltage between the control signal level at the control input of the controllable filter and the control signal frequency.
3. The adaptive filter circuit according to claim 2, wherein the adaptive filter circuit is configured to provide an inversely proportional relationship.