JP2006333074A - Modulation factor detecting circuit, noise processing circuit, and modulation factor detecting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To speedily detect variation from a low modulation factor to a high modulation factor and to make the quantity of variation in modulation factor after the variation to the high modulation factor is detected. <P>SOLUTION: A modulation factor detecting circuit includes an absolute value output circuit which outputs the absolute value of an audio signal, a modulation factor output circuit which outputs a larger value between the absolute value and a comparison value corresponding to a modulation factor which is output immediately before as a modulation factor, a delay circuit which delays the modulation factor output from the modulation factor output circuit by a specified time and outputs it as a second modulation factor, a comparing circuit which outputs a result of comparison between the first modulation factor as the modulation factor which is output from the modulation output circuit immediately before and the second modulation factor output from the delay circuit, and an output circuit which outputs, according to the comparison output from the comparing circuit, the first modulation factor as the comparison value to the modulation factor output circuit when the first modulation factor is larger than the second modulation factor and a value obtained by decreasing the first modulation factor as the comparison value to the modulation factor output circuit when the fist modulation factor is smaller than the second modulation factor. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a modulation degree detection circuit, a noise processing circuit, and a modulation degree detection method.

  In an FM radio receiver, it is common practice to remove multipath noise that occurs when an FM signal transmitted from a broadcasting station is reflected by a mountain or a high-rise building. FIG. 7 is a diagram illustrating an intermediate frequency signal including multipath noise. In the FM radio receiver, multipath noise is detected by the AC component of the envelope of the intermediate frequency signal. When multipath noise is detected, the multipath noise is removed by smoothing the amplitude of the intermediate frequency signal.

  As a process for removing such multipath noise, for example, there is a process of controlling in accordance with the modulation degree indicating the amplitude of the FM signal. FIG. 8 is a diagram showing a spectrum distribution of the intermediate frequency signal according to the modulation degree of the FM signal. As shown in FIG. 8, the higher the degree of modulation, the wider the frequency distribution of the intermediate frequency signal. When band limitation is performed using a bandpass filter or the like in order to remove the influence of adjacent interference or the like, a signal having a frequency outside the band is attenuated when the modulation degree is high.

  FIG. 9 is a diagram illustrating a change in the intermediate frequency signal due to band limitation when the modulation degree is high. As shown in the figure, for example, the high frequency portion outside the band of the intermediate frequency signal is attenuated by the band limitation, and the amplitude becomes small. As described above, some FM radio receivers detect multipath noise by changing the amplitude of an intermediate frequency signal, and such a change in amplitude due to band limitation is misunderstood as multipath noise. May end up.

  FIG. 10 is a diagram illustrating an example of audio signal distortion generated by performing multipath noise removal processing when the degree of modulation is high. In FIG. 10, at a time t1, the modulation degree is switched from a low modulation degree to a high modulation degree. As shown in the part surrounded by the broken line in the figure, when the process of removing multipath noise is performed when the modulation degree is high, signal distortion occurs and sound quality deteriorates.

Therefore, when the modulation degree of the FM signal is detected and the modulation degree is larger than a predetermined reference value, control is performed so as to stop the process of removing multipath noise (for example, Patent Document 1). .) As a method for detecting the modulation degree of the FM signal, a method of taking an average value of signal levels over a predetermined period is generally used (for example, Patent Document 2).
JP 2000-49723 A JP-A-10-135856

  FIG. 11 is a diagram illustrating an example of transition of the detected modulation degree when the modulation degree is detected by the average value. In FIG. 11, at a time t1, the modulation degree is switched from a low modulation degree to a high modulation degree. As shown in the figure, when the modulation degree is detected by the average value, if the average is taken over a long period of time, it cannot react quickly to the change in the modulation degree. For this reason, when the degree of modulation is changed from a low degree of modulation to a high degree of modulation, detection that the degree of modulation is greater than a predetermined reference value is delayed. Therefore, multipath noise removal processing is performed even though the modulation degree is high, and the audio signal is distorted as shown in FIG.

  On the other hand, by averaging the signal level in a short time, a change in the modulation degree can be detected quickly. However, on average over a short period of time, after detecting that the modulation degree has changed to a high modulation degree, the amount of change in the detected modulation degree becomes large, and the multipath noise removal processing is frequently switched on and off. Becomes unstable.

  The present invention has been made in view of the above problems, and the detection of a change from a low modulation degree to a high modulation degree is fast, and after the change to a high modulation degree is detected, the amount of change in the modulation degree detected is small. It is an object of the present invention to provide a small modulation degree detection circuit, a noise processing circuit using the modulation degree detection circuit, and a modulation degree detection method.

  In order to achieve the above object, a modulation degree detection circuit according to the present invention is a modulation degree detection circuit that detects and outputs a modulation degree indicating the amplitude of an FM signal, and the absolute value of the FM signal is output immediately before. When the comparison value is greater than or equal to the modulation degree, a value according to the absolute value is output as the modulation degree of the FM signal, and when the absolute value is less than the comparison value, the FM output immediately before is output. It has a function of reducing the signal modulation and outputting.

  The modulation degree detection circuit outputs an absolute value output circuit that outputs the absolute value of the FM signal, and a modulation degree output circuit that outputs the larger of the absolute value or the comparison value as the modulation degree of the FM signal. And the first modulation degree that is the modulation degree output immediately before from the modulation degree output circuit is the second modulation degree that is the modulation degree output from the modulation degree output circuit immediately before the first modulation degree. If the first modulation degree is greater than the second modulation degree, the first modulation degree is output to the modulation degree output circuit as the comparison value. If the first modulation degree is less than or equal to the second modulation degree, the first modulation degree is output. A comparison value output circuit that outputs a value obtained by decreasing the degree to the modulation degree output circuit as the comparison value.

  The comparison value output circuit includes a delay circuit that outputs the modulation degree output from the modulation degree output circuit with a predetermined time delay as the second modulation degree, and the output that is output from the modulation degree output circuit. A comparison circuit that outputs a comparison result between the first modulation degree and the second modulation degree output from the delay circuit; and the first modulation based on the comparison result output from the comparison circuit. When the degree is larger than the second modulation degree, the first modulation degree is output as the comparison value to the modulation degree output circuit, and the first modulation degree is equal to or less than the second modulation degree. May comprise an output circuit that outputs a value obtained by reducing the first modulation factor as the comparison value to the modulation factor output circuit.

  Further, the output circuit includes a multiplication circuit that outputs a value obtained by multiplying the first modulation factor by a predetermined value less than 1 as a value obtained by reducing the first modulation factor. be able to.

  In addition, the noise processing circuit of the present invention detects a sound signal from the FM signal and outputs the detection signal, and detects a modulation degree indicating the amplitude of the FM signal based on the sound signal output from the detection circuit. When the modulation degree output from the modulation degree detection circuit is less than a predetermined value, noise included in the FM signal is removed and output, and the modulation degree is A noise removal circuit that outputs the noise signal without removing noise included in the FM signal when the value is equal to or greater than a predetermined value, and the modulation degree detection circuit outputs the absolute value of the audio signal immediately before When the comparison value is greater than or equal to the modulation degree, a value according to the absolute value is output as the modulation degree of the FM signal, and when the absolute value is less than the comparison value, the FM output immediately before is output. Reduced signal modulation It is assumed that the output Te.

  The modulation degree detection method of the present invention is a method for detecting and outputting the modulation degree indicating the amplitude of the FM signal, wherein the absolute value of the FM signal is a comparison value corresponding to the modulation degree that was output immediately before. If it is above, a value corresponding to the absolute value is output as the modulation degree of the FM signal, and if the absolute value is less than the comparison value, the modulation degree output immediately before is decreased and output. And

  The detection of a change from a low modulation degree to a high modulation degree is fast, and after detecting a change to a high modulation degree, a modulation degree detection circuit in which the change amount of the detected modulation degree is small, the modulation degree detection circuit is used. A noise processing circuit and a modulation degree detection method can be provided.

== Overall structure ==
FIG. 1 is a diagram illustrating an example of a configuration of an FM radio receiver that includes a modulation degree detection circuit according to an embodiment of the present invention. The FM radio receiver 1 includes an antenna 11, a high frequency amplification circuit 12, a frequency conversion circuit 13, an intermediate frequency filter 14, an intermediate frequency amplification circuit 15, a multipath detection circuit 16, a noise removal circuit 17, a detection circuit 18, and a modulation degree detection circuit. 19, a low frequency amplifier circuit 20, and a speaker 21.

  The antenna 11 receives an FM modulated high frequency signal (FM signal) transmitted from a broadcasting station. The high frequency amplifier circuit 12 amplifies and outputs the high frequency signal received by the antenna 11. The frequency conversion circuit 13 converts the amplified high frequency signal into an intermediate frequency signal and outputs it. The intermediate frequency filter 14 passes the intermediate frequency signal with a predetermined frequency bandwidth and outputs it in order to remove unnecessary components such as adjacent channel waves. The intermediate frequency amplifier circuit 15 amplifies the intermediate frequency signal, converts it into a digital signal by sampling, and outputs it.

  The multipath detection circuit 16 detects the presence or absence of multipath noise based on the change in the amplitude of the intermediate frequency signal. When multipath noise is detected by the multipath noise detection circuit 16, the noise removal circuit 17 performs noise removal by correcting a change in the amplitude of the intermediate frequency signal. The detection circuit 18 extracts and outputs an audio signal from the intermediate frequency signal from which noise has been removed. The modulation degree detection circuit 19 detects and outputs the modulation degree of the FM signal based on the audio signal (detection signal) output from the detection circuit 18. The modulation degree of the FM signal is indicated by the amplitude of the modulated FM signal transmitted from the broadcast station, and the modulation degree of the FM signal increases as the amplitude of the audio signal output from the detection circuit 18 increases. Will be expensive.

  When the modulation degree output from the modulation degree detection circuit 19 is equal to or greater than a predetermined value, the noise removal circuit 17 outputs an intermediate frequency signal without performing noise removal processing. This is because when the modulation degree is high, the frequency outside the band is attenuated by the intermediate frequency filter 14, thereby changing the amplitude of the intermediate frequency signal, and noise removal processing is performed on such a signal. This is because signal distortion occurs and sound quality deteriorates.

  The low frequency amplifier circuit 20 amplifies the low frequency of the audio signal output from the detection circuit 18, converts it to an analog signal, and outputs audio from the speaker 21.

  Note that the multipath detection circuit 16, the noise removal circuit 17, the detection circuit 18, and the modulation degree detection circuit 19 surrounded by a broken line in FIG. 1 correspond to the noise processing circuit of the present invention.

== Modulation degree detection circuit ==
Next, details of the modulation degree detection circuit 19 will be described. FIG. 2 is a diagram illustrating an example of the configuration of the modulation degree detection circuit 19. The modulation degree detection circuit 19 includes an absolute value output circuit 31, a modulation degree output circuit 32, a delay circuit 33, a comparison circuit 34, and an output circuit 35. The delay circuit 33, the comparison circuit 34, and the output circuit 35 constitute the comparison value output circuit of the present invention.

  The absolute value output circuit 31 outputs the absolute value of the detection signal x (n) output from the detection circuit 18. The modulation degree output circuit 32 corresponds to the absolute value | x (n) | of the detection signal output from the absolute value output circuit 31 and the modulation degree y (n−1) output immediately before from the modulation degree output circuit 32. The comparison value y ′ (n) is compared, and the larger value is output as the modulation factor y (n).

  The delay circuit 33 outputs the modulation factor y (n−1) (second modulation factor) output immediately before the modulation factor y (n) (first modulation factor) output by the modulation factor output circuit 32. . That is, the delay circuit 33 is a circuit that delays the modulation degree output from the modulation degree output circuit 32 by a predetermined time and outputs it to the comparison circuit 34, and can be realized by, for example, a register.

  The comparison circuit 34 outputs a comparison result between the modulation factor y (n) output from the modulation factor output circuit 32 and the modulation factor y (n−1) output immediately before. When the modulation degree y (n) is greater than the modulation degree y (n−1) based on the comparison result output from the comparison circuit 34, the output circuit 35 sets the modulation degree y (n) to the next comparison value y. When '(n + 1) is output and the modulation factor y (n) is equal to or less than the modulation factor y (n-1), the value obtained by reducing the modulation factor y (n) is used as the next comparison value y' (n + 1). Output as.

  In the present embodiment, when the value obtained by reducing the modulation factor y (n) is used as the comparison value y ′ (n + 1), the comparison value is obtained by multiplying the modulation factor y (n) by a coefficient less than 1. Let y ′ (n + 1) be obtained. That is, the output circuit 35 includes a multiplication circuit that multiplies the modulation degree y (n) by a coefficient less than 1 and outputs the result. Note that the method of outputting with the modulation degree y (n) decreased is not limited to this, and for example, a predetermined value may be subtracted from the modulation degree y (n) and output.

  The operation of the modulation degree detection circuit 19 will be described with reference to a flowchart. FIG. 3 is a flowchart showing processing of the modulation degree detection circuit 19. First, the detection signal x (n) is input to the absolute value output circuit 31 (S301), and the absolute value output circuit 31 outputs the absolute value | x (n) | (S302). The modulation degree output circuit 32 compares the absolute value | x (n) | output from the absolute value output circuit 31 with the comparison value y ′ (n) output from the output circuit 35 (S303). When the absolute value | x (n) | is equal to or greater than the comparison value y ′ (n) (S303: N), the modulation degree output circuit 32 converts the absolute value | x (n) | to the modulation degree y (n). When the absolute value | x (n) | is less than the comparison value y ′ (n) (S303: Y), the comparison value y ′ (n) is output as the modulation factor y (n). (S305).

  Subsequently, the comparison circuit 34 outputs a comparison result between the modulation factor y (n) output from the modulation factor output circuit 32 and the previous modulation factor y (n−1) output from the delay circuit 33. (S306). When the modulation factor y (n) is greater than the modulation factor y (n−1) (S306: Y), the output circuit 35 outputs the modulation factor y (n) as the next comparison value y ′ (n + 1) ( S307) When the modulation factor y (n) is equal to or less than the modulation factor y (n-1) (S306: N), the value obtained by multiplying the modulation factor y (n) by a coefficient α less than 1 is the next comparison value. It outputs as y '(n + 1) (S308). Then, the same processing (S301 to S308) is repeatedly performed on the next detection signal x (n + 1) (S309).

Next, the transition of the modulation degree output from the modulation degree detection circuit 19 will be described. FIG. 4 is a diagram illustrating a specific example of the transition of the modulation degree output from the modulation degree detection circuit 19 when the modulation degree changes from a low state to a high state.
FIG. 4A shows a waveform of the absolute value | x (n) | of the detection signal x (n). FIG. 4B is a diagram showing the relationship among the absolute value | x (n) |, the comparison value y ′ (n), and the modulation factor y (n). FIG. 4C is a diagram showing the transition of the modulation factor y (n) to be output.

  As shown in FIGS. 4A to 4C, in this example, the modulation factor y (n) output from the modulation factor detection circuit 19 is zero at time T0, which is the initial state. Then, as shown in FIG. 4A, a detection signal having an amplitude of 3 is input to the modulation degree detection circuit 19 from time T0. In this example, the coefficient α for decreasing the modulation factor is 0.9, and the calculation result is rounded off to the second decimal place. Note that the numerical values shown in FIG. 4 are set for the sake of explanation, and do not indicate the numerical values in the actual FM signal.

  The absolute value | x (n) | of the detection signal starts increasing from time T0, and the absolute value | x (n) | of the detection signal at time T1 is 1.5. Since y (n) at time T0 is zero, the comparison value y ′ (n) at time T1 is zero. Therefore, since the absolute value | x (n) | is equal to or greater than the comparison value y ′ (n) at time T1, the absolute value | x (n) | = 1.5 is output as the modulation factor y (n). The Since the modulation factor y (n) is larger than the modulation factor y (n−1) at time T0, the modulation factor y (n) = 1.5 becomes the next comparison value y ′ (n + 1).

  Further, the absolute value | x (n) | of the detection signal increases, and the absolute value | x (n) | at time T2 is 2.5. At this time, since the absolute value | x (n) | is equal to or greater than the comparison value y ′ (n) (y ′ (n + 1) generated at time T1), the absolute value | x (n) | = 2.5 is It is output as the modulation degree y (n). Since the modulation factor y (n) is larger than the modulation factor y (n−1) at time T1, the modulation factor y (n) = 2.5 becomes the next comparison value y ′ (n + 1).

  At time T3, the absolute value | x (n) | of the detection signal is 3 as the peak value. At this time, since the absolute value | x (n) | is equal to or larger than the comparison value y ′ (n) (y ′ (n + 1) generated at time T2), the absolute value | x (n) | = 3 is the modulation factor. Output as y (n). Since the modulation factor y (n) is larger than the modulation factor y (n−1) at time T2, the modulation factor y (n) = 3 becomes the next comparison value y ′ (n + 1).

  Thereafter, the absolute value | x (n) | of the detection signal begins to decrease, and the absolute value | x (n) | of the detection signal at time T4 is 2.5. At this time, since the absolute value | x (n) | is less than the comparison value y ′ (n) (y ′ (n + 1) generated at time T3), the comparison value y ′ (n) = 3 is the modulation factor y. (N) is output. Since the modulation factor y (n) is equal to or less than the modulation factor y (n−1) at time T3, 2.7 obtained by multiplying the modulation factor y (n) by the coefficient α is the next comparison value y ′ ( n + 1).

  Further, the absolute value | x (n) | of the detection signal is decreased, and the absolute value | x (n) | at time T5 is 1.5. At this time, since the absolute value | x (n) | is less than the comparison value y ′ (n) (y ′ (n + 1) generated at time T4), the comparison value y ′ (n) = 2.7 is modulated. It is output as degree y (n). Since the modulation factor y (n) is equal to or less than the modulation factor y (n−1) at time T4, 2.4 obtained by multiplying the modulation factor y (n) by the coefficient α is the next comparison value y ′ ( n + 1).

  At time T6, the absolute value | x (n) | of the detection signal is 0, which is the minimum value. At this time, since the absolute value | x (n) | is less than the comparison value y ′ (n) (y ′ (n + 1) generated at time T5), the comparison value y ′ (n) = 2.4 is modulated. It is output as degree y (n). Since the modulation factor y (n) is equal to or less than the modulation factor y (n−1) at time T5, 2.2 obtained by multiplying the modulation factor y (n) by the coefficient α is the next comparison value y ′ ( n + 1).

  Then, the absolute value | x (n) | of the detection signal begins to increase again from time T6, and the absolute value | x (n) | of the detection signal at time T7 is 1.5. At this time, since the absolute value | x (n) | is less than the comparison value y ′ (n) (y ′ (n + 1) generated at time T6), the comparison value y ′ (n) = 2.2 is modulated. It is output as degree y (n). Since the modulation factor y (n) is equal to or less than the modulation factor y (n−1) at time T6, 2.0 obtained by multiplying the modulation factor y (n) by the coefficient α is the next comparison value y ′ ( n + 1).

  Further, the absolute value | x (n) | of the detection signal increases, and the absolute value | x (n) | at time T8 is 2.5. At this time, since the absolute value | x (n) | is equal to or greater than the comparison value y ′ (n) (y ′ (n + 1) generated at time T7), the absolute value | x (n) | = 2.5 is It is output as the modulation degree y (n). Since the modulation factor y (n) is larger than the modulation factor y (n−1) at time T7, the modulation factor y (n) = 2.5 becomes the next comparison value y ′ (n + 1).

  At time T9, the absolute value | x (n) | of the detection signal is 3 again as the peak value. At this time, since the absolute value | x (n) | is equal to or greater than the comparison value y ′ (n) (y ′ (n + 1) generated at time T8), the absolute value | x (n) | = 3 is the modulation factor. Output as y (n).

  Thus, in the modulation degree detection circuit 19, the absolute value | x (n) | of the detection signal is equal to or greater than the comparison value y ′ (n) corresponding to the modulation degree y (n−1) output immediately before. If the absolute value | x (n) | is less than the comparison value y ′ (n), the absolute value | x (n) | is output as the modulation degree y (n). A value (y ′ (n)) obtained by multiplying (n−1) by a coefficient α is output as the modulation degree y (n). Thereby, as apparent from FIG. 4C, it is possible to quickly detect that the modulation degree has changed from a low state to a high state. Further, when the modulation factor is decreased, a value obtained by multiplying the modulation factor output immediately before by the coefficient α is output as the modulation factor, so that the change amount of the modulation factor can be reduced. In this example, the coefficient α is set to 0.9, but the value of the coefficient α is not limited to this. For example, by making the coefficient α a value closer to 1, it is possible to further reduce the amount of change in the detected modulation degree.

== Simulation result ==
Next, a simulation result in the modulation degree detection circuit 19 is shown. FIG. 5 is a diagram showing the transition of the modulation degree output from the modulation degree detection circuit 19. At time t1, the modulation degree of the audio signal output from the detection circuit 18 is switched from a low modulation degree to a high modulation degree. As is apparent from FIG. 5, when the modulation degree is switched at time t1, the modulation degree detected by the modulation degree detection circuit 19 also quickly changes to a high value, and thereafter, the amount of change in the modulation degree is small.

  FIG. 6 is a diagram showing a waveform of an audio signal output from the detection circuit 18 when such a modulation degree detection circuit 19 is used. As shown in FIG. 6, unlike the waveform of FIG. 10 or FIG. 12 in the conventional example, it can be seen that there is no distortion in the audio signal after switching from a low modulation degree to a high modulation degree at time t1. This is because the modulation degree detection circuit 19 of the present embodiment quickly detects that the modulation degree has been switched to a high degree of modulation, and the control for stopping the multipath noise removal processing in the noise removal circuit 17 is performed quickly.

  The FM radio receiver 1 including the modulation degree detection circuit 19 according to the embodiment of the present invention has been described above. As described above, the modulation degree detection circuit 19 of the present embodiment uses the absolute value of the detection signal as the modulation degree of the FM signal when the absolute value of the detection signal is equal to or larger than the comparison value corresponding to the modulation degree output immediately before. When the absolute value of the detection signal is less than the comparison value, the modulation degree output immediately before is decreased and output. As a result, as is clear from the specific example of FIG. 4 or the simulation result of FIG. 5, it is possible to quickly detect a change from a low modulation degree to a high modulation degree, and to reduce the amount of change in the modulation degree after detection. .

  If the modulation degree detection circuit 19 of this embodiment is applied to a noise processing circuit that stops noise removal processing when the modulation degree is high, effective noise removal processing can be performed. That is, by using the modulation degree detection circuit 19, it is possible to quickly detect a change from a low modulation degree to a high modulation degree, and to quickly stop the multipath noise removal processing in the noise removal circuit 17. Distortion can be suppressed. Further, in the modulation degree detection circuit 19, since the amount of change in the modulation degree after detecting a high modulation degree is small, the multipath noise removal processing in the noise removal circuit 17 is not frequently switched on and off. Can be stabilized.

  As mentioned above, although embodiment of this invention was described, the said embodiment is for making an understanding of this invention easy, and is not for limiting and interpreting this invention. The present invention can be changed and improved without departing from the gist thereof, and the present invention includes equivalents thereof.

  For example, in the present embodiment, the modulation degree output circuit 32 compares the absolute value of the detection signal with the comparison value, and outputs the absolute value as the modulation degree when the absolute value is equal to or greater than the comparison value. The value output as the modulation degree is not limited to the absolute value itself, but may be a value corresponding to the absolute value. For example, a value obtained by multiplying the absolute value by a predetermined coefficient may be output as the modulation degree.

  Further, for example, in the present embodiment, an example in which the modulation degree detection circuit 19 is applied to a noise removal circuit has been shown, but the use of the modulation degree detection circuit 19 is not limited to this. In this embodiment, the modulation degree is detected using a circuit. However, the function realized by the modulation degree detection circuit 19 is realized by software using, for example, a DSP (Digital Signal Processor). Is also possible.

It is a figure which shows an example of a structure of the FM radio receiver comprised including the modulation degree detection circuit which is one Embodiment of this invention. It is a figure which shows an example of a structure of a modulation degree detection circuit. It is a flowchart which shows the process of a modulation degree detection circuit. It is a figure which shows the specific example of transition of the modulation degree output from a modulation degree detection circuit when a modulation degree changes from a low state to a high state. It is a figure which shows transition of the modulation degree output from a modulation degree detection circuit. It is a figure which shows the waveform of the audio | voice signal output from a detection circuit in the case of using a modulation degree detection circuit. It is a figure which shows the intermediate frequency signal containing multipath noise. It is a figure which shows the spectrum distribution of the intermediate frequency signal according to the modulation degree of FM signal. It is a figure which shows the change of the intermediate frequency signal by a band limitation in case a modulation degree is high. It is a figure which shows the example of distortion of the audio | voice signal which generate | occur | produces when a multipath noise removal process is performed when the modulation degree is high. It is a figure which shows the example of transition of the modulation degree detected when detecting a modulation degree by an average value. It is a figure which shows the example of the distortion of the audio | voice signal accompanying the delay of the change to a high modulation degree.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 FM radio receiver 11 Antenna 12 High frequency amplification circuit 13 Frequency conversion circuit 14 Intermediate frequency filter 15 Intermediate frequency amplification circuit 16 Multipath detection circuit 17 Noise removal circuit 18 Detection circuit 19 Modulation degree detection circuit 20 Low frequency amplification circuit 21 Speaker 31 Absolute Value output circuit 32 Modulation degree output circuit 33 Delay circuit 34 Comparison circuit 35 Output circuit

Claims (12)

A modulation degree detection circuit that detects and outputs a modulation degree indicating the amplitude of an FM signal,
If the absolute value of the FM signal is greater than or equal to the comparison value corresponding to the modulation factor output immediately before, the value corresponding to the absolute value is output as the modulation factor of the FM signal, and the absolute value is the comparison value A function of reducing the modulation degree of the FM signal output immediately before if it is less than
A modulation degree detection circuit. The modulation degree detection circuit according to claim 1,
An absolute value output circuit for outputting an absolute value of the FM signal;
A modulation degree output circuit that outputs the larger of the absolute value or the comparison value as the modulation degree of the FM signal;
The first modulation degree that is the modulation degree output immediately before from the modulation degree output circuit is greater than the second modulation degree that is the modulation degree output from the modulation degree output circuit immediately before the first modulation degree. When the first modulation degree is larger, the first modulation degree is output as the comparison value to the modulation degree output circuit. When the first modulation degree is equal to or less than the second modulation degree, the first modulation degree is output. A comparison value output circuit that outputs the reduced value as the comparison value to the modulation degree output circuit;
A modulation degree detection circuit comprising: The modulation degree detection circuit according to claim 2,
The comparison value output circuit includes:
A delay circuit that outputs the modulation factor output from the modulation factor output circuit with a predetermined time delay as the second modulation factor;
A comparison circuit for outputting a comparison result between the first modulation factor output from the modulation factor output circuit and the second modulation factor output from the delay circuit;
Based on the comparison result output from the comparison circuit, when the first modulation degree is larger than the second modulation degree, the first modulation degree is output to the modulation degree output circuit as the comparison value. An output circuit that outputs a value obtained by reducing the first modulation degree to the modulation degree output circuit as the comparison value when the first modulation degree is equal to or less than the second modulation degree;
A modulation degree detection circuit comprising: The modulation degree detection circuit according to claim 3,
The output circuit is
A multiplier circuit configured to output a value obtained by multiplying the first modulation factor by a predetermined value less than 1 as a value obtained by reducing the first modulation factor;
A modulation degree detection circuit. A detection circuit for detecting and outputting an audio signal from the FM signal;
A modulation degree detection circuit that detects and outputs a modulation degree indicating the amplitude of the FM signal based on the audio signal output from the detection circuit;
When the modulation degree output from the modulation degree detection circuit is less than a predetermined value, noise included in the FM signal is removed and output, and when the modulation degree is greater than or equal to the predetermined value, A noise removal circuit that outputs without removing the noise contained in the FM signal;
With
The modulation degree detection circuit includes:
When the absolute value of the audio signal is greater than or equal to the comparison value corresponding to the modulation factor output immediately before, the value corresponding to the absolute value is output as the modulation factor of the FM signal, and the absolute value is the comparison value If it is less than the output, decrease the modulation degree of the FM signal output immediately before,
A noise processing circuit. The noise processing circuit according to claim 5,
The modulation degree detection circuit includes:
An absolute value output circuit for outputting an absolute value of the input audio signal;
A modulation degree output circuit that outputs the larger of the absolute value or the comparison value as the modulation degree of the FM signal;
The first modulation degree that is the modulation degree output immediately before from the modulation degree output circuit is the second modulation degree that is the modulation degree output from the modulation degree output circuit immediately before the first modulation degree. If it is larger, the first modulation factor is output to the modulation factor output circuit as the comparison value, and if the first modulation factor is less than or equal to the second modulation factor, the first modulation factor is output. A comparison value output circuit that outputs a value obtained by reducing the value as the comparison value to the modulation degree output circuit;
A noise processing circuit comprising: The noise processing circuit according to claim 6,
The comparison value output circuit includes:
A delay circuit that outputs the modulation factor output from the modulation factor output circuit with a predetermined time delay as the second modulation factor;
A comparison circuit for outputting a comparison result between the first modulation factor output from the modulation factor output circuit and the second modulation factor output from the delay circuit;
Based on the comparison result output from the comparison circuit, when the first modulation degree is larger than the second modulation degree, the first modulation degree is output to the modulation degree output circuit as the comparison value. An output circuit that outputs a value obtained by reducing the first modulation degree to the modulation degree output circuit as the comparison value when the first modulation degree is equal to or less than the second modulation degree;
A noise processing circuit comprising: The noise processing circuit according to claim 7,
The output circuit is
A multiplier circuit configured to output a value obtained by multiplying the first modulation factor by a predetermined value less than 1 as a value obtained by reducing the first modulation factor;
A noise processing circuit. A method of detecting and outputting a modulation degree indicating the amplitude of an FM signal,
If the absolute value of the FM signal is greater than or equal to the comparison value corresponding to the modulation factor output immediately before, the value corresponding to the absolute value is output as the modulation factor of the FM signal, and the absolute value is the comparison value If it is less than, output the degree of modulation that was output immediately before,
A modulation degree detection method characterized by the above. The modulation degree detection method according to claim 9,
Output the absolute value of the audio signal;
When the first modulation degree that is output immediately before the first modulation degree is greater than the second modulation degree that is output immediately before the first modulation degree, the first modulation degree is When the first modulation factor is equal to or less than the second modulation factor, a value obtained by reducing the first modulation factor is output as a comparison value.
Outputting the absolute value or the comparison value, whichever is greater, as the modulation degree of the audio signal;
A modulation degree detection method characterized by the above. The modulation degree detection method according to claim 10,
When outputting the comparison value,
Outputting the modulation degree to be outputted as the second modulation degree with a predetermined time delay;
Outputting a comparison result between the first modulation factor and the second modulation factor;
Based on the comparison result, when the first modulation degree is larger than the second modulation degree, the first modulation degree is output as the comparison value, and the first modulation degree is the second modulation degree. If the modulation degree is equal to or less than the modulation degree, a value obtained by reducing the first modulation degree is output as the comparison value;
A modulation degree detection method characterized by the above. The modulation degree detection circuit according to claim 11,
When outputting a value obtained by reducing the first modulation degree as the comparison value,
Outputting a value obtained by multiplying the first modulation factor by a predetermined value less than 1,
A modulation degree detection method characterized by the above.

Images