JP2005286788A - Noise eliminating circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a noise eliminating circuit in which a sense of discontinuity due to the size of an output level or the difference in frequency characteristics does not occur at the time of switching from DSB reception to SSB reception. <P>SOLUTION: If a detector does not detect reception interference; an extractor extracts a carrier wave, a first intermediate frequency signal, and a second intermediate frequency signal, and a switch outputs an output of the detecting section. If the detector detects reception interference; the extractor extracts any one of a combination of the carrier wave and the first intermediate frequency signal, and a combination of the carrier wave and the second intermediate frequency signal, and the switch outputs an output of a sound quality corrector. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a noise removal circuit.

  Amplitude modulation (AM) is a method of changing the amplitude of a carrier wave with a modulation signal such as an audio signal. At this time, if the carrier wave is amplitude-modulated with the modulated signal, side waves corresponding to the frequency components of the received signal are collected on both sides of the carrier wave. This collection of side waves is generated because various frequency components are included in the modulation signal, and this collection of side waves is referred to as a side band. A sideband having a frequency component higher than the carrier wave is referred to as an upper sideband (hereinafter referred to as USB), and a sideband having a frequency component lower than the carrier wave is referred to as a lower sideband (hereinafter referred to as LSB). ).

A method of receiving both sideband waves generated during amplitude modulation is called double sideband (hereinafter referred to as DSB) reception.
In the USB and LSB, a signal based on the frequency component of the modulation signal is formed symmetrically with respect to the carrier wave. Therefore, it is possible to perform reception using either USB or LSB. A reception method using one side of USB or LSB is called single side band (hereinafter referred to as SSB) reception.

  In this way, in amplitude modulation, there are sidebands on both sides of the carrier wave, so if one sideband overlaps the other sideband with the adjacent broadcast station, the signal level of one sideband increases. Therefore, adjacent interference occurs due to the influence of sidebands of adjacent stations. However, in the occurrence of this adjacent interference, it is possible to effectively prevent adjacent interference by receiving SSB of sidebands that are not subjected to adjacent interference.

  Therefore, the noise removal circuit detects the presence / absence of adjacent interference based on the comparison of the signal levels of both sidebands, and when adjacent interference is detected in one sideband, DSB reception from the side not receiving the adjacent interference is received. (For example, refer to Patent Document 1).

FIG. 6 is a diagram for explaining DSB reception and SSB reception.
When the carrier wave having the frequency fc (for example, 450 kHz) is amplitude-modulated with the modulation signal, a USB is generated according to the frequency of the modulation signal to a frequency higher than the carrier of fc as shown in FIG. An LSB is generated according to the frequency of the modulation signal. At this time, if the low frequency (for example, 500 Hz) of the modulation signal is fa and the frequency (for example, 1 kHz) higher than fa is fb, USBs corresponding to fc + fa and fc + fb are generated for frequencies higher than fc, LSBs corresponding to fc-fa and fc-fb are generated at frequencies lower than fc.

When performing DSB reception, for example, the USB, LSB, and carrier wave are extracted with the filter characteristic indicated by the solid line a, and the extracted signal is detected.
Further, when the signal level of one sideband increases and adjacent interference is detected, reception with less influence of adjacent interference can be performed by extracting the carrier wave and the other sideband and performing SSB reception.
JP-A-6-276027

  When performing SSB reception, if all the carrier waves are not extracted, the AM detection characteristics deteriorate. Therefore, for extracting SSB reception, USB and fc carrier waves are extracted with a filter characteristic as shown by a dotted line b in FIG. At this time, even if a filter having a steep filter characteristic is used, a component (for example, fc-fa) of an LSB band that is not intended to be used as shown in FIG. 6 is also extracted. Therefore, the extracted IF signal includes many signals of frequency components on the low frequency side of the modulation signal.

  When detection is performed using the IF signal extracted by such filter characteristics, an audio signal having a large low-frequency component is output.

  Moreover, since only one side of the sideband wave is extracted in SSB reception, the detection output level is halved compared to DSB reception.

  FIG. 7 is a diagram illustrating an example of output level characteristics of DSB reception and SSB reception. The horizontal axis indicates the frequency, and the vertical axis indicates the output level. In the SSB reception indicated by the dotted line, the output level is reduced and the output level is increased at a low frequency compared to the DSB reception indicated by the solid line.

  As described above, in the conventional noise removal circuit, the output level characteristics of the DSB and the SSB are different, and thus a discontinuity in output occurs when switching from the DSB reception to the SSB reception.

  An object of the present invention is to provide a noise removal circuit that can eliminate the discontinuity of output due to the difference in output level and frequency characteristics when switching from DSB reception to SSB reception.

  A main invention according to the present invention is that an intermediate frequency signal obtained from a received signal includes a first intermediate frequency signal in a frequency band higher than a carrier frequency and a second intermediate frequency signal in a frequency band lower than the carrier frequency. A detection unit for detecting reception interference based on a difference in detection level; the carrier and the first intermediate frequency signal; the carrier and the second intermediate frequency signal; the carrier, the first intermediate frequency signal and the second intermediate An extraction unit that extracts one of the frequency signals according to the output of the detection unit, a detection unit that detects the output of the extraction unit, and a sound quality correction unit that corrects the output of the detection unit to a predetermined characteristic; A switching unit that switches and outputs the output of the detection unit and the output of the sound quality correction unit according to the output of the detection unit, and when the detection unit does not detect the reception interference, the extraction unit Said transport And the first intermediate frequency signal and the second intermediate frequency signal are extracted, and the switching unit outputs an output of the detection unit, and when the detection unit detects the reception interference, the extraction unit One of the carrier wave and the first intermediate frequency signal, the carrier wave and the second intermediate frequency signal is extracted, and the switching unit outputs the output of the sound quality correction unit.

  Other features of the present invention will become apparent from the accompanying drawings and the description of this specification.

  According to the present invention, when reception interference is detected, the extraction band of the intermediate frequency signal is changed and the output of the detector is corrected to a predetermined characteristic, so that it is possible to eliminate the discontinuity of the output.

  In the following embodiments of the present invention, a case where the noise removal circuit of the present invention is applied to an AM receiver will be described.

=== AM receiver configuration ===
FIG. 1 is a block diagram showing an example of the configuration of an AM receiver using the noise removal circuit 100 of the present invention. The AM receiver shown in FIG. 1 includes a front end (hereinafter referred to as FE) unit 10, an IF unit 12, a noise removal circuit 100, and a low frequency amplification circuit 22.

  The FE unit 10 amplifies the reception signal received by the antenna 1 so that the signal has a level required by the IF unit 12 at the next stage. At this time, amplification is limited to a target reception signal and a frequency band including the reception signal so as not to amplify a signal other than the target and noise such as noise.

  The IF unit 12 has a function of converting a carrier frequency, and a local oscillation circuit (not shown) that outputs a local oscillation signal for frequency conversion of a reception signal frequency, and a reception signal and the local oscillation signal are mixed. And a mixing circuit (not shown). Then, the IF unit 12 converts the received signal into a predetermined intermediate frequency (for example, 450 kHz). Further, only a desired signal is extracted by a band-pass filter (BPF: not shown) having the intermediate frequency as a center frequency, and then amplified by an amplifier circuit (not shown) and output as an IF signal. Here, although the intermediate frequency is set to 450 kHz, when a part of the circuit is shared with the FM tuner, the IF signal of the FM tuner and the intermediate frequency may be aligned by temporarily up-converting to 10 or 7 MHz.

The noise removal circuit 100 detects the presence / absence of adjacent interference from the IF signal output from the IF unit 12, and when detecting adjacent interference (“reception interference”), performs detection by switching from DBS reception to SSB reception. The resulting audio signal is corrected. The noise removal circuit 100 includes an IF filter 14, a detection unit 16, a sound quality correction unit 18, a switching unit 20, an adjacent disturbance detection unit 24, and a filter coefficient setting unit 26.
The IF filter 14 includes, among IF signals, a carrier wave and USB (“first intermediate frequency signal”), a carrier wave and LSB (“second intermediate frequency signal”), and a carrier wave and USB and LSB. Extract according to 24 outputs.
The detector 16 removes the carrier wave component from the IF signal and outputs an audio signal that is the original modulation signal. For example, synchronous detection using a PLL (phase locked loop) can be used as the detection unit 16.
The sound quality correction unit 18 corrects the output of the detection unit 16 to a predetermined characteristic.
The switching unit 20 switches and outputs the audio signal output from the detection unit 16 and the audio signal corrected by the sound quality correction unit 18 according to the detection output of the adjacent disturbance detection unit 24.
The adjacent interference detection unit 24 (“detection unit”) detects adjacent interference based on the difference in detection level between the USB and the LSB in the IF signal.
The filter coefficient setting unit 26 (“coefficient setting unit”) sets a multiplication coefficient of the IF filter 14 described later in accordance with the output of the adjacent interference detection unit 24. In combination with the filter coefficient setting unit 26 and the IF filter 14 (“extraction unit”), according to the output of the adjacent interference detection unit 24, the carrier wave and USB, the carrier wave and LSB, the carrier wave and USB and LSB, Any of these can be extracted.
The low frequency amplifier circuit 22 amplifies the output of the noise removal circuit 100 and supplies necessary power to the speaker 3.

In the AM receiver having the above-described configuration, the reception signal received by the antenna 1 is amplified in the high frequency range by the front end unit 10 and is subjected to intermediate frequency conversion by the IF unit 12 to be an IF signal. The IF signal is input to the adjacent interference detection unit 24, and adjacent interference is detected based on the difference between the detection levels of the USB and LSB.
When the adjacent interference detection unit 24 does not detect adjacent interference, the filter coefficient setting unit 26 sets coefficients for extracting the carrier wave, USB, and LSB from the IF signal in the IF filter 14. The detector 16 detects the IF signal extracted by the IF filter 14 and outputs an audio signal. The audio signal is output from the switching unit 20, amplified by the low frequency amplifier circuit 22, and output from the speaker 3.
On the other hand, when the adjacent interference detection unit 24 detects the adjacent interference, the filter coefficient setting unit 26 sets a coefficient for extracting any one of the carrier wave, the USB, and the carrier wave and the LSB from the IF signal in the IF filter 14. The IF signal extracted by the IF filter 14 is detected by the detection unit 16 to be an audio signal, and is corrected to a predetermined characteristic by the sound quality correction unit 18. The audio signal corrected by the sound quality correction unit 18 is output from the switching unit 20, amplified by the low frequency amplifier circuit 22, and output from the speaker 3.

=== Configuration of Adjacent Interference Detection Unit 24 ===
FIG. 2 is a block diagram showing an example of the configuration of the adjacent disturbance detection unit 24 of the noise removal circuit 100 of the present invention.
The adjacent disturbance detection unit 24 includes bandpass filters (hereinafter referred to as BPF) 30 and 34, level detection units 32 and 36, and a comparison unit 38.
The BPF 30 (“first band filter”) extracts a USB having a frequency higher than the carrier frequency from the IF signal.
The level detection unit 32 (“first level detection unit”) performs level detection of the output of the BPF 30.
The BPF 34 (“second band filter”) extracts an LSB having a frequency lower than the carrier frequency from the IF signal.
The level detection unit 36 (“second level detection unit”) performs level detection of the output of the BPF 34.
The comparison unit 38 compares the output difference between the level detection unit 32 and the level detection unit 36 with a predetermined threshold value, and detects adjacent interference.

Next, the operation of the adjacent disturbance detection unit 24 will be described.
The comparison unit 38 compares the USB level that is the output of the level detection unit 32 with the LSB that is the output of the level detection unit 36 and compares the difference with a predetermined threshold value.
If the difference is smaller than the threshold value as a result of the comparison, it is determined that no adjacent interference has occurred. At this time, the switching unit 20 outputs an audio signal that is the output of the detection unit 16, and the filter coefficient setting unit 26 sets a multiplication coefficient for extracting the carrier wave, USB, and LSB from the IF signal in the IF filter 14.
On the other hand, as a result of the comparison, when it is detected that the difference is greater than the threshold and the output of the level detection unit 36 is greater than the output of the level detection 32, the comparison unit 38 outputs a detection signal indicating adjacent interference detection. Based on this detection signal, the switching unit 20 switches to the output of the sound quality correction unit 20, and the filter coefficient setting unit 26 sets a multiplication coefficient for extracting the carrier wave and the USB from the IF signal in the IF filter 14.
As a result of the comparison, when it is detected that the difference is greater than the threshold and the output of the level detection unit 32 is greater than the output of the level detection 36, the comparison unit 38 outputs a detection signal indicating adjacent interference detection. Based on this detection signal, the switching unit 20 switches to the output of the sound quality correction unit 20, and the filter coefficient setting unit 26 sets a multiplication coefficient for extracting the carrier wave and the LSB from the IF signal in the IF filter 14.

=== Configuration of IF Filter 14 ===
FIG. 5 is a block diagram illustrating an example of the configuration of the IF filter 14. For example, an IIR (Infinite Impulse Response) filter shown in FIG. 5 can be used as the IF filter 14.

  The IF filter 14 delays the input signal for each sampling cycle unit (“predetermined period”) and outputs the delayed signal D1 to D6, and any multiplication corresponding to the input signal and the output of the delay circuit D1 to D4. Multipliers A1 to A6 having coefficients a1 to a6 and multiplying the input signal by a multiplication coefficient and outputting, and arbitrary multiplication coefficients b2, b3, b5 and b6 corresponding to the outputs of the delay circuits D3 to D6 The multiplication circuit B2, B3, B5, B6 for multiplying the input signal by the multiplication coefficient and outputting it, and the addition circuit for adding and outputting the outputs of the multiplication circuits A1 to A3 and the outputs of the multiplication circuits B2, B3 And an adder circuit 52 that adds and outputs the outputs of the multiplier circuits A4 to A6 and the outputs of the multiplier circuits B5 and B6.

The input signal Xn (n is the number of samples) is delayed by one sample period in each of the delay circuits D1 and D2, and is multiplied by multiplication coefficients a1 to a3 in the corresponding multiplication circuits A1 to A3.
The outputs of the multiplier circuits A1 to A3 are added together with the outputs of the multiplier circuits B2 and B3 by the adder circuit 50 and output as the output signal Yn of the adder circuit 50.
The output signal Yn is delayed by one sample period in each of the delay circuits D3 and D4, and multiplied by the multiplication coefficients b2 and b3 in the corresponding multiplication circuits B2 and B3, respectively.
The outputs of the multiplying circuits B2 and B3 are added together with the outputs of the multiplying circuits A1 to A3 by the adding circuit 50 and output as the output signal Yn of the adding circuit 50.
Since the multiplication circuits A4 to A6, B5, B6 and the delay circuits D3 to D6 on the adder circuit 52 side perform the same operation, the description is omitted for convenience.

As described above, the IIR filter used as the IF filter 14 has feedback in which the signal output from the adder circuit 50 returns to the adder circuit 50 again after passing through the delay circuit and the multiplier circuit. Such an IIR filter can realize desired characteristics with a small order. Further, the frequency characteristics of the filter can be arbitrarily changed by setting the multiplication coefficients a1 to a6, b2, b3, b5, and b6 of the IF filter 14.
In the filter coefficient setting unit 26, the IF filter 14 uses multiplication coefficients a1 to a6, b2, b3, b5 for extracting the carrier wave and USB, the carrier wave and LSB, and the carrier wave and USB and LSB from the IF signal. b6 is preset.
The IF filter 14 has multiplication coefficients a1 to a6, b2, b3, b5, and b6 set by the filter coefficient setting unit 26. From the IF signal, any of carrier wave and USB, carrier wave and LSB, carrier wave and USB and LSB is selected. I do some extraction.
In the embodiment of the present invention, the IF filter 14 is an IIR filter, but another filter, for example, an FIR filter may be used. Also, an IIR filter other than the configuration of FIG. 5 may be used.

=== Configuration of Sound Quality Correction Unit 18 ===
FIG. 3 is a block diagram showing an example of the configuration of the sound quality correction unit 18 in the noise removal circuit 100 of the present invention.
The sound quality correction unit 18 amplifies an audio signal output from the detection unit 16 with an amplification factor a, and a high-pass filter (HPF) 42 (amplifies the low frequency component of the output of the amplification unit 40. Low-pass suppression filter ”).
The amplification factor a of the amplifying unit 40 is set to, for example, 2 as a value for adjusting the magnitude of the SSB detection output to the magnitude of the DSB detection output.
In SSB reception, the audio signal that is the output of the detection unit 16 input to the sound quality correction unit 18 is amplified, for example, twice by the amplification unit 40, and then the low frequency component is attenuated by the HPF 42 and output. Is done.

  FIG. 4 is a diagram showing the characteristics of the output level of the audio signal corrected by the sound quality correction unit 18 when SSB reception is performed by the noise removal circuit 100 of the present invention. In the noise removal circuit 100 of the present invention, there is no increase in low frequency components generated due to the characteristics of SSB reception in FIG. 7, and the output level is equivalent to that in DSB reception in FIG. Therefore, even when switching from the audio signal output from the detection unit 16 at the time of DSB reception to the audio signal corrected by the sound quality correction unit 18 at the time of SSB reception, there is no discontinuity in output at the time of switching.

As described above, the noise removal circuit 100 of the present invention can eliminate the discontinuity in output by correcting the output characteristics of SSB reception when switching from DSB reception to SSB reception.
This correction can be realized by a simple configuration in which the amplification unit 40 of the sound quality correction unit 18 amplifies the signal size of the SSB reception so as to be almost equal to that of the DSB reception, and suppresses the low frequency boost by the HPF 42. it can.
Further, the adjacent interference detection unit 24 can detect the occurrence of adjacent interference by comparing the difference between the USB level detection and the LSB level detection with the threshold value.

Furthermore, the frequency characteristic of the IF filter 14 can be changed by changing the setting of the multiplication coefficient of the IF filter 14, and according to the output of the filter coefficient setting unit 26, the carrier wave and the USB, the carrier wave and the LSB are obtained from the IF signal. Any one of carrier wave, USB, and LSB can be extracted.
And when adjacent interference occurs, it is possible to receive without being affected by adjacent interference by selecting the sideband side where interference has not occurred and receiving SSB. DSB reception can be performed.

  As described above, the present embodiment has been specifically described based on the embodiment. However, the present embodiment is not limited to this, and various modifications can be made without departing from the scope of the present embodiment.

It is a block diagram of AM receiver using the noise removal circuit of this invention. It is a block diagram which shows the structure of the adjacent disturbance detection part of the noise removal circuit of this invention. It is a block diagram which shows the structure of the sound quality correction | amendment part of the noise removal circuit of this invention. It is a figure which shows the output level characteristic of the SSB reception of the noise removal circuit of this invention. It is a block diagram which shows the structure of IF filter. It is a figure for demonstrating DSB reception and SSB reception. It is a figure which shows the output level characteristic of DSB reception and SSB reception.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 FE part 12 IF part 14 IF filter 16 Detection part 18 Sound quality correction part 20 Switching part 22 Low frequency amplification part 24 Adjacent disturbance detection part 26 Filter coefficient setting part 30, 34 BPF
32, 36 Level detection unit 38 Comparison unit 40 Amplification unit 42 HPF
50, 52 Adder circuit 100 Noise removal circuit

Claims (6)

Received based on a difference in detection level between a first intermediate frequency signal in a frequency band higher than a carrier frequency and a second intermediate frequency signal in a frequency band lower than the carrier frequency among intermediate frequency signals obtained from the reception signal. A detection unit for detecting interference;
The carrier wave and the first intermediate frequency signal, the carrier wave and the second intermediate frequency signal, the carrier wave, the first intermediate frequency signal and the second intermediate frequency signal according to the output of the detection unit. An extractor for extracting;
A detection unit for detecting the output of the extraction unit;
A sound quality correction unit that corrects the output of the detection unit to a predetermined characteristic;
A switching unit that switches and outputs the output of the detection unit and the output of the sound quality correction unit according to the output of the detection unit;
With
When the detection unit does not detect the reception interference,
The extraction unit extracts the carrier wave, the first intermediate frequency signal, and the second intermediate frequency signal, and the switching unit outputs an output of the detection unit,
When the detection unit detects the reception interference,
The extraction unit extracts one of the carrier wave and the first intermediate frequency signal, the carrier wave and the second intermediate frequency signal, and the switching unit outputs an output of the sound quality correction unit.
A noise elimination circuit characterized by that. The sound quality correction unit
An amplifier for amplifying the output of the detector;
A low-pass suppression filter for attenuating the low-frequency component of the output of the amplification unit;
The noise removal circuit according to claim 1, further comprising: correcting and outputting an audio signal that is an output of the detection unit. The detector is
A first bandpass filter for extracting the first intermediate frequency signal;
A first level detector for detecting an output level of the first bandpass filter;
A second bandpass filter for extracting the second intermediate frequency signal;
A second level detector for detecting the output level of the second bandpass filter;
A comparison unit that compares the difference between the output of the first level detection unit and the output of the second level detection unit and a predetermined threshold;
With
The comparison unit includes:
3. The noise removal circuit according to claim 1, wherein when the difference is larger than the threshold value, a detection signal indicating the reception interference is output. The extraction unit includes:
A plurality of delay circuits for outputting an input signal after a predetermined period of time;
A plurality of multiplication circuits for multiplying outputs of the plurality of delay circuits by an arbitrary multiplication coefficient;
At least one addition circuit for adding the outputs of the multiplication circuits;
A coefficient setting unit that sets the multiplication coefficient of the multiplication circuit according to the output of the detection unit;
With
The carrier and the first intermediate frequency signal, the carrier and the second intermediate frequency signal, the carrier, the first intermediate frequency signal, and the second are changed by changing the multiplication coefficient according to the output of the detection unit. 4. The noise removal circuit according to claim 1, wherein any one of the intermediate frequency signals is extracted. The extraction unit includes:
When the output of the detection unit indicates that the difference is greater than the threshold value and the first intermediate frequency signal is greater than the second intermediate frequency signal, the carrier and the second intermediate frequency signal are extracted,
When the output of the detection unit indicates that the difference is greater than the threshold and the second intermediate frequency signal is greater than the first intermediate frequency signal, the carrier and the first intermediate frequency signal are extracted.
The noise removal circuit according to claim 3 or 4, wherein The extraction unit includes:
6. The carrier wave, the first intermediate frequency signal, and the second intermediate frequency signal are extracted when the output of the detection unit indicates that the difference is smaller than the threshold value. The noise removal circuit of crab.

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