JP2004242221A - Fm receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an FM receiver capable of suppressing a distortion that may generate due to overmodulation when performing demodulation by digital processing. <P>SOLUTION: When an intermediate frequency signal is outputted from an intermediate frequency filter 14, it is converted into digital data by an A/D conversion unit 22, and intermediate frequency data converted into a frequency in the baseband area are outputted by a digital down-converter 24. An FM demodulation unit 26 performs FM demodulation processing to the intermediate frequency data to output audio data. A peak maintenance determination unit 32 detects overmodulation when both the level of the intermediate frequency data outputted from an IF level detection unit 28 and the level of the audio data outputted from an audio level detection unit 30 drop to maintain and output earlier audio data. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an FM receiver that receives and demodulates a broadcast wave or the like subjected to FM modulation.
[0002]
[Prior art]
Recently, there has been known an FM receiver that performs various kinds of processing such as demodulation by performing signal processing using a DSP (Digital Signal Processor) on an intermediate frequency signal (for example, see Patent Document 1). ). In this FM receiver, A / D (analog-digital) conversion is performed on a signal converted into an intermediate frequency signal by a front end, and FM demodulation processing and stereo demodulation processing are performed using a DSP. By using the DSP, various processes such as sound quality adjustment can be easily performed. In addition, when the DSP is used in another unit such as a CD player, the parts are used in the FM receiver and other units. There is also an advantage such that it becomes possible to share
[0003]
[Patent Document 1]
JP 2001-156667A (page 3, FIG. 1)
[0004]
[Problems to be solved by the invention]
By the way, in the FM receiver disclosed in Patent Document 1 described above, when performing A / D conversion on an intermediate frequency signal, an anti-aliasing filter is usually provided at an input stage in order to prevent generation of aliasing noise. Is provided to remove components other than the necessary band components. For example, the specification of an anti-aliasing filter and a sampling frequency in an A / D converter are determined so that sufficient signal processing can be performed on an FM modulation signal having a modulation factor of 100%.
[0005]
However, in practice, an FM-modulated signal exceeding 100% of the modulation degree may be received, and the level of the intermediate frequency signal after the A / D conversion is significantly reduced, and the signal after the FM demodulation is over-modulated. There is a problem that large distortion occurs.
FIG. 5 is an explanatory diagram of the cause of the overmodulation distortion. The horizontal axis of FIG. 5 indicates the frequency of the modulated wave signal. The overmodulated FM signal contains components B and C exceeding a predetermined bandwidth A (FIG. 5A), and has a predetermined pass bandwidth (FIG. 5B). By passing through an anti-aliasing filter, the high-frequency side C and the low-frequency side B are removed, and only a signal component included in the bandwidth A is obtained (FIG. 5C).
[0006]
FIG. 6 is a diagram showing the relationship between the intermediate frequency signal after A / D conversion and the audio signal after FM modulation. The horizontal axis in FIG. 6 corresponds to the passage of time. Since normal A / D conversion is performed on the FM modulation signal in which overmodulation does not occur, an intermediate frequency signal in which the frequency shift of the FM modulation signal is reproduced as it is can be obtained (FIG. 6A). In the audio signal obtained corresponding to the intermediate frequency signal, the largest part of the frequency shift of the intermediate frequency signal becomes the peak of the amplitude (FIG. 6B).
[0007]
However, for an overmodulated FM signal, the overmodulated frequency component is removed by an anti-aliasing filter and A / D converted, so that the intermediate frequency signal after the A / D conversion is applied to the overmodulated portion. The waveform is such that the corresponding component is missing (FIG. 6C). Therefore, when the FM demodulation process is performed using such an intermediate frequency signal, an audio signal partially missing the signal component is obtained (FIG. 6D). As described above, when the intermediate frequency signal is A / D converted and the subsequent processing is performed, a large distortion of the demodulated signal occurs due to overmodulation.
[0008]
The present invention has been made in view of such a point, and an object of the present invention is to provide an FM receiver that can suppress occurrence of distortion due to overmodulation when demodulating by digital processing. .
[0009]
[Means for Solving the Problems]
In order to solve the above-described problems, an FM receiver according to the present invention includes: an analog-to-digital conversion unit that converts an analog intermediate frequency signal into digital intermediate frequency data; and performing FM demodulation processing using the intermediate frequency data. , Demodulation processing means for generating audio data, first level detection means for detecting the level of intermediate frequency data, second level detection means for detecting the level of audio data, and first and second level detection An overmodulation determining means for detecting the presence or absence of overmodulation of the FM modulation signal based on the result, and holding and outputting audio data before the overmodulation has occurred when overmodulation has occurred. When overmodulation occurs, the level of the intermediate-frequency data after digital-analog conversion and the level of audio data generated using this intermediate-frequency data fluctuate greatly. Therefore, the occurrence of overmodulation is accurately detected based on these levels. can do. Also, when overmodulation is detected, the audio data before that is held and output, so that the level of the audio data can be prevented from greatly dropping, and distortion due to overmodulation can be prevented when demodulating by digital processing. Generation can be suppressed.
[0010]
Further, the above-described overmodulation determining means determines that the current level of the intermediate frequency data is lower than the immediately preceding level by a predetermined reference value or more, and the current level of the audio data is a predetermined level lower than the immediately preceding level. It is desirable to detect the occurrence of overmodulation when the voltage drops below the reference value. By detecting the occurrence of overmodulation by comparing the reduction in the level of these two data with a predetermined reference, it becomes possible to know the occurrence of overmodulation reliably with simple processing.
[0011]
In addition, the above-described overmodulation determining means may be configured such that the current level of the intermediate frequency data is higher than the immediately preceding level by a predetermined reference value and the current level of the audio data is a predetermined level higher than the immediately preceding level. It is desirable to release the holding operation of the audio data when it rises above the reference value. In this way, by retaining the audio data before the occurrence of overmodulation until the overmodulation state is released, it is possible to reliably suppress the occurrence of distortion due to overmodulation.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an FM receiver according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram illustrating a configuration of an FM receiver according to an embodiment. The FM receiver shown in FIG. 1 includes a high-frequency amplifier 10, a mixing circuit 12, an intermediate frequency filter 14, a local oscillation circuit (LO) 16, an A / D converter 22, a digital down converter (DDC) 24, an FM demodulator 26, It includes an IF (intermediate frequency) level detection unit 28, an audio level detection unit 30, and a peak holding determination unit 32. Among them, the A / D converter 22, the digital down converter (DDC) 24, the FM demodulator 26, the IF level detector 28, the audio level detector 30, and the peak holding determination unit 32 are realized by the DSP 20.
[0013]
The high-frequency amplifier 10 performs high-frequency amplification on a broadcast wave received by the antenna 90. The mixing circuit 12 mixes the local oscillation signal output from the local oscillation circuit 16 and the FM modulation signal output from the high-frequency amplifier 10 and outputs a difference component or a sum component of the signals. The intermediate frequency filter 14 selectively passes a predetermined frequency band component from signals output from the mixing circuit 12.
[0014]
The A / D converter 22 samples a signal output from the intermediate frequency filter 14 at a predetermined sampling frequency and converts the signal into digital data. The digital down converter 24 finally converts the output data of the A / D converter 22 into intermediate frequency data having a frequency in the baseband region by thinning out the data at a predetermined interval. The FM demodulation unit 26 performs FM demodulation processing on the intermediate frequency data output from the digital down converter 24 to generate data (audio data) corresponding to an audio signal.
[0015]
The IF level detection unit 28 detects the signal level of the intermediate frequency data output from the digital down converter 24. The audio level detector 30 detects a signal level of audio data output from the FM demodulator 26. The peak holding determination unit 32 holds or outputs the audio data output from the FM demodulation unit 26 without holding, based on the detection results of the IF level detection unit 28 and the audio level detection unit 30.
[0016]
The above-mentioned A / D converter 22 is analog-to-digital converter, FM demodulator 26 is demodulator, IF level detector 28 is first level detector, and audio level detector 30 is second level detector. The peak holding determination unit 32 corresponds to the detection unit, and corresponds to the overmodulation determination unit.
[0017]
The FM receiver of the present embodiment has such a configuration, and its operation will be described next.
In the FM receiver of the present embodiment, when overmodulation has not occurred, the audio data output from the FM demodulation unit 26 is not held in the peak holding determination unit 32, but is output as it is. As a result, the same FM demodulation processing as in the related art using the DSP is performed.
[0018]
When overmodulation occurs, the peak holding determination unit 32 detects this overmodulation state, and holds the previous audio data value. Hereinafter, an operation of detecting the overmodulation and starting the holding of the audio data, and conversely, an operation of detecting the end of the overmodulation state and starting the holding of the audio data will be described.
[0019]
FIG. 2 is a flowchart showing an operation procedure for detecting overmodulation and starting holding audio data, and shows an operation procedure by the peak holding determination unit 32.
First, the peak holding determination unit 32 determines the level of the intermediate frequency data immediately before the current level of the intermediate frequency data (current output of the IF level detection unit 28) (the immediately preceding output of the IF level detection unit 28). Is subtracted (step 100), and it is determined whether or not the level decrease is equal to or more than a predetermined reference value (step 101). If the lowering of the level is smaller than the reference value, a negative determination is made, and the series of processing ends.
[0020]
If the decrease in the level is equal to or more than the predetermined reference value, an affirmative determination is made in the determination of step 101, and then the peak holding determination unit 32 determines whether the current audio data (the current audio data of the audio level detection unit 30) The previous audio data (the previous output of the audio level detection unit 30) is subtracted from the (output) (step 102), and it is determined whether or not the decrease in the level is equal to or more than a predetermined reference value (step 102). 103). If the level decrease is smaller than the predetermined reference value, a negative determination is made, and the series of processing ends.
[0021]
If the decrease in the level is equal to or more than the predetermined reference value, an affirmative determination is made in the determination of step 103, and then the peak holding determination unit 32 determines that the level of the immediately preceding audio data is the predetermined reference value. It is determined whether or not the above is true (step 104). If it is smaller than the reference value, a negative determination is made, and the series of processing ends. On the other hand, if the level of the previous audio data is equal to or higher than the predetermined reference value, an affirmative determination is made in the determination of step 104, and the peak holding determination unit 32 next determines the value of the previous audio data. And outputs it as the current audio data (step 105). Thereafter, the output of the held audio data is continued until the held state is released.
[0022]
FIG. 3 is a flowchart showing an operation procedure for starting to hold audio data, and shows an operation procedure by the peak holding determination unit 32.
First, the peak holding determination unit 32 subtracts the level of the immediately preceding intermediate frequency data from the current level of the intermediate frequency data (step 200), and determines whether the level increase is equal to or greater than a predetermined reference value. (Step 201). If the level increase is smaller than the reference value, a negative determination is made, and the series of processing ends.
[0023]
If the level increase is equal to or more than the predetermined reference value, an affirmative determination is made in the determination of step 201, and then the peak holding determination unit 32 subtracts the immediately preceding audio data from the current audio data. Then, it is determined whether or not the level rise is equal to or higher than a predetermined reference value (step 203). If the level increase is smaller than the predetermined reference value, a negative determination is made, and the series of processing ends.
[0024]
If the increase in the level is equal to or higher than the predetermined reference value, an affirmative determination is made in the determination of step 203, and then the peak holding determination unit 32 determines that the current audio data level is equal to or higher than the predetermined reference value. It is determined whether or not there is (step 204). If it is smaller than the reference value, a negative determination is made, and the series of processing ends. On the other hand, if the current level of the audio data is equal to or higher than the predetermined reference value, an affirmative determination is made in the determination of step 204, and then the peak holding determination unit 32 determines that the audio data held so far has been held. Release (step 205). Thereafter, until the holding of the audio data is started, the audio data output each time from the FM demodulation unit 26 is output from the peak holding determination unit 32 as it is.
[0025]
FIG. 4 is a diagram illustrating a relationship between the intermediate frequency data after A / D conversion and the audio data after FM modulation according to the present embodiment. The horizontal axis in FIG. 4 corresponds to the passage of time. When the overmodulated FM signal is received and the corresponding intermediate frequency signal is output from the intermediate frequency filter 14, the digital downconverter 24 outputs intermediate frequency data whose level is greatly reduced corresponding to the overmodulation portion. (FIG. 4A). In addition, since the FM demodulation process is performed using the intermediate frequency data, the FM demodulation unit 26 outputs audio data whose level is greatly reduced corresponding to the overmodulation part (FIG. 4B).
[0026]
When the level of the intermediate frequency data and the level of the audio data both drop at the same time, the peak holding determination unit 32 detects the overmodulation state, and holds the level of the previous audio data and outputs the same. (FIG. 4C). Therefore, it is possible to prevent the level of audio data from being significantly reduced due to overmodulation.
[0027]
As described above, in the FM receiver according to the present embodiment, when performing various processes on the intermediate frequency signal by digital processing by the DSP 20 and outputting audio data, the level of the intermediate frequency data and the level of the audio data are simultaneously reduced. Then, the occurrence of overmodulation is detected, and the operation of maintaining the level of the audio data before that is performed. As a result, it is possible to prevent the level of audio data from being significantly reduced due to overmodulation, and to suppress the occurrence of distortion due to overmodulation when demodulating by digital processing.
[0028]
In particular, by detecting the occurrence of overmodulation by comparing the decrease in the level of each of the intermediate frequency data and the audio data with a predetermined reference, it is possible to know the occurrence of overmodulation reliably with simple processing. . Also, by retaining the audio data before the occurrence of overmodulation until the overmodulation state is released, it is possible to reliably suppress the occurrence of distortion due to overmodulation.
[0029]
Note that the present invention is not limited to the above embodiment, and various modifications can be made within the scope of the present invention. For example, in the above-described embodiment, the A / D converter 22 is included in the DSP 20. However, an A / D converter may be provided before the DSP 20. Further, since it is sufficient that digital processing can be performed on the intermediate frequency data, a processor other than the DSP may be used.
[0030]
【The invention's effect】
As described above, according to the present invention, the occurrence of overmodulation can be accurately detected based on the respective levels of the intermediate frequency data and the audio data. By outputting the audio data, it is possible to prevent the level of the audio data from being greatly reduced, and to suppress the occurrence of distortion due to overmodulation when demodulating by digital processing.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of an FM receiver according to an embodiment.
FIG. 2 is a flowchart showing an operation procedure of detecting overmodulation and starting holding audio data.
FIG. 3 is a flowchart showing an operation procedure for starting holding audio data.
FIG. 4 is a diagram illustrating a relationship between intermediate frequency data after A / D conversion and audio data after FM modulation according to the embodiment;
FIG. 5 is an explanatory diagram of a cause of occurrence of overmodulation distortion.
FIG. 6 is a diagram showing a relationship between an intermediate frequency signal after A / D conversion and an audio signal after FM modulation in a conventional FM receiver.
[Explanation of symbols]
Reference Signs List 10 High frequency amplifier 12 Mixing circuit 14 Intermediate frequency filter 16 Local oscillation circuit (LO)
20 DSP
22 A / D converter 24 Digital down converter (DDC)
26 FM demodulation unit 28 IF level detection unit 30 Audio level detection unit 32 Peak holding determination unit

Claims (3)

Analog-digital conversion means for converting an analog intermediate frequency signal into digital intermediate frequency data,
Demodulation processing means for generating audio data by performing FM demodulation processing using the intermediate frequency data;
First level detection means for detecting the level of the intermediate frequency data;
Second level detecting means for detecting the level of the audio data;
Overmodulation determining means for detecting the presence or absence of overmodulation of the FM modulation signal based on the first and second level detection results, and holding and outputting the audio data before that when overmodulation occurs When,
An FM receiver comprising: In claim 1,
The overmodulation judging means is configured so that the current level of the intermediate frequency data is lower than the immediately preceding level by a predetermined reference value and the current level of the audio data is a predetermined level lower than the immediately preceding level. An FM receiver characterized by detecting occurrence of overmodulation when the value decreases by more than a value. In claim 1 or 2,
The overmodulation judging means is configured such that the current level of the intermediate frequency data rises by a predetermined reference value or more from the immediately preceding level, and the current level of the audio data is a predetermined reference level higher than the immediately preceding level. The FM receiver cancels the holding operation of the audio data when the value increases by not less than a value.

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