JPH05227109A - Stereophonic demodulation circuit - Google Patents

Abstract

PURPOSE:To provide a stereophonic demodulation circuit in which sound quality can be improved, and to which digital processing can be applied, and furthermore, in which high integration can be easily attained. CONSTITUTION:The stereophonic demodulation circuit to perform the stereophonic demodulation of an A/D-converted FM composite signal is equipped with a multi-pass distortion detection circuit 4 consisting of a digital band-pass filter 1 which detects the distortion of a stereophonic pilot signal in the FM composite signal and a level decision device 3, which decides the output level of a digital high-pass filter 2, and which decides the existence of multipass distortion based on the size of distortion, and a data correction circuit 6 which corrects a stereophonic demodulation signal based on linear prediction. When it is decided that the multi-pass distortion exists, the output of the data correction circuit 6 is outputted as the stereophonic demodulation signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM tuner stereo demodulation circuit, and more particularly to a stereo demodulation circuit with reduced multipath distortion.

[0002]

2. Description of the Related Art A conventional circuit of this type utilizes the fact that multipath distortion significantly increases in stereo reception as compared to monaural reception. I was narrowing the band width of the wave stage and muting. Also, in the intermediate frequency stage, the amplitude component due to the interference between the direct wave and the reflected wave is extracted,
There was also a circuit that canceled the delayed wave component with an adaptive filter.

[0003]

However, in the former case of the above-mentioned conventional example, there is a problem that deterioration of sound quality cannot be avoided. Further, in the latter case of the above conventional example, since the amplitude characteristic at the intermediate frequency stage is not completely flat, it is difficult to extract only the amplitude change due to multipath distortion, and conversely an excessive cancel signal is generated. There was a problem that it would end up. In addition, when the tap position of the adaptive filter is selected and the tap coefficient is calculated, the smaller the tap position is, the shorter the calculation time becomes, and the real-time processing becomes possible. On the contrary, the calculation time for calculating the correlation decreases the number of taps used. There was a problem that it increased.

The present invention relates to a stereo pilot signal (hereinafter,
Amplitude distortion and phase distortion (simply referred to as pilot signals) are detected, multipath distortion is determined based on the detected distortion level, and demodulated stereo signals are corrected to improve sound quality. It is an object of the present invention to provide a stereo demodulation circuit that can be performed, can be digitally processed, and can be easily highly integrated.

[0005]

A stereo demodulation circuit of the present invention is a stereo demodulation circuit for stereo demodulating an A / D converted FM composite signal, and a distortion for detecting distortion of a stereo pilot signal in the FM composite signal. Detecting means, multipath distortion determining means for determining the presence or absence of multipath distortion based on the magnitude of distortion detected by the distortion detecting means, and stereo demodulated signal based on linear prediction when it is determined that multipath distortion is present. It is characterized by comprising a correction means for correcting.

Instead of the correction means for correcting on the basis of linear prediction, when it is determined that multipath distortion is present, the stereo demodulated signal in which distortion has occurred is used as an average value of the stereo demodulated signal before and after distortion is not generated. The average value correction means may be used, or the pre-value holding correction means that uses the stereo demodulated signal having distortion when it is determined that multipath distortion is present as the immediately preceding stereo demodulated signal without distortion.

The correcting means may be a correcting means for correcting the stereo demodulated signal based on the distortion correction coefficient correlated with the magnitude of the distortion detected by the distortion detecting means.

When the multipath distortion determining means is a multipath distortion determining means for determining the magnitude of the multipath distortion based on the magnitude of the distortion detected by the distortion detecting means, and the correcting means is determined to have a small multipath distortion. The stereo demodulation signal is corrected to a stereo demodulation signal based on the distortion correction coefficient that correlates to the detected distortion level, and when it is determined that the multipath distortion is large, the stereo demodulation signal is corrected based on the linear prediction. It may be a correction means for outputting as a stereo demodulated signal.

The correcting means may be a correcting means for correcting the FM composite signal based on the distortion correction coefficient correlated with the magnitude of the distortion detected by the distortion detecting means.

[0010]

According to the stereo demodulation circuit of the present invention, the distortion of the stereo pilot signal in the FM composite signal is detected by the distortion detecting means, and the presence or absence of multipath distortion is determined by the magnitude of the distortion detected by the distortion detecting means. When it is determined by the path distortion determination means and it is determined that multipath distortion is present, the stereo demodulated signal is corrected by the correction means based on the linear prediction.

When the average value correcting means is used instead of the correcting means for correcting based on the linear prediction, the stereo demodulated signal which has been distorted when it is determined that multipath distortion is present The correction is made to be the average value of the subsequent stereo demodulated signal. Further, instead of the correction means for correcting based on the linear prediction, when the previous value holding correction means is used, the stereo demodulated signal in which distortion has occurred when it is determined that multipath distortion exists Correction is made to obtain a stereo demodulated signal.

When the correcting means is a correcting means for correcting the stereo demodulated signal based on the distortion correction coefficient correlated with the magnitude of the distortion detected by the distortion detecting means, the magnitude of the distortion in which the stereo demodulated signal is detected is detected. Is corrected based on the distortion correction coefficient correlated with

When the multipath distortion determining means is a multipath distortion determining means for judging the magnitude of the multipath distortion based on the magnitude of the distortion detected by the distortion detecting means, and the correcting means is judged to have a small multipath distortion. The stereo demodulated signal is corrected to a stereo demodulated signal based on the distortion correction coefficient that is correlated with the magnitude of the distortion detected by the distortion detection means, and when it is determined that the multipath distortion is large, the stereo demodulated signal is based on the linear prediction. When it is determined that the multipath distortion is small, the stereo demodulation is performed based on the distortion correction coefficient that correlates with the magnitude of the distortion detected by the distortion detection means. When the signal is corrected and the multipath distortion is determined to be large, the stereo demodulated signal is corrected based on linear prediction. , Corrected stereo demodulated signal is output as a stereo demodulated signal.

Based on a distortion correction coefficient which correlates with the magnitude of the distortion detected by the distortion detecting means, the F
When the correction means for correcting the M composite signal is used,
Based on the distortion correction coefficient that correlates with the magnitude of distortion, the F
The M composite signal is corrected, and the corrected FM composite signal is stereo demodulated.

[0015]

EXAMPLES The present invention will be described below with reference to examples. FIG. 1 is a block diagram showing the configuration of the first embodiment of the present invention, showing an example using a digital processing type PLL circuit.

In FIG. 1, reference numeral 29 is a real public number 62-3.
The stereo demodulation circuit shown in No. 8362 is shown. The stereo demodulation circuit 29 is a digital processing type PLL circuit 20 including a multiplier 16, a filter 17, an arithmetic circuit 18, and a cosine wave arithmetic circuit 19, and sine wave arithmetic circuits 21 and 2.
3, adder 24, multipliers 25 and 26, filters 27 and 2
FM composite signal A that is composed of 8 and is A / D converted
Stereo demodulated, and stereo demodulated signals J ₁ and J ₂ are output from the filters 27 and 28.

The stereo demodulated signals J ₁ and J ₂ are supplied to the data correction circuit 9. The data correction circuit 9 receives the stereo demodulated signals J ₁ and J ₂ and stores the storage circuit 5, the data correction circuit 6 that performs a correction calculation of data from the stored data, the storage circuit 5 and the delay caused by the data correction circuit 6. Stereo demodulation signals J ₁ , J ₂ delayed by receiving the outputs of the delay circuit 8 and the data correction circuit 6 for delaying the stereo demodulation signals J ₁ , J ₂ for compensation and the output of the delay circuit 8 and the stereo demodulation corrected The data switching circuit 7 includes a data switching circuit 7 that selects and outputs _{one of} the signals J ₁ and J _2.
The outputs selected by are output as stereo demodulated signals L and R.

The FM composite signal A is supplied to the multipath distortion detection circuit 4. The multipath distortion detection circuit 4 is F
A digital bandpass filter 1 and a digital highpass filter 2 having the M composite signal A as an input, and a level discriminator 3 that receives the output of the digital bandpass filter 1 and the output of the digital highpass filter 2 and determines their respective levels. Multipath distortion is detected according to the level, and the multipath distortion detection output is output to the data switching circuit 7 as a switching signal.

The operation of the first embodiment constructed as above will be described. The FM composite signal A is the main signal (L +
R), sub-signal (LR) sinωs nT and pilot signal Psin (ωs / 2) nT. Where L is the left channel audio signal, R is the right channel audio signal, P is the level of the pilot signal, ωs is the angular frequency of the audio signal, T is the sampling rate, and n is a positive integer and is the sampling rate. +1 for each.

FIG. 2 shows a pilot signal without distortion and a pilot signal with distortion caused by multipath. 2A shows a pilot signal waveform without distortion, and FIG. 2B shows a pilot signal waveform with distortion caused by multipath. FIG. 3 shows the measurement result of the high frequency spectrum of the pilot signal by the multipath by the simulator. FIG. 3A shows a case where there is no multipath, and FIG.
(B) shows the case where there is multipath.

From the above, it can be seen that many harmonics are generated in the pilot signal when multipath occurs, and the pilot signal exists in the base band of the FM composite signal together with the main signal and the sub signal, and the pilot signal is distorted. The distortion of the stereo demodulated signal can be detected by detecting the.

In the multipath distortion detection circuit 4, 38
The 2nd harmonic component of the pilot signal is extracted by the digital bandpass filter 1 of kHz, and the 3rd harmonic component of the pilot signal is extracted by the digital highpass filter 2. The outputs from the digital bandpass filter 1 and the digital highpass filter 2 are input to the level determiner 3 and compared with preset reference values,
If at least one of them exceeds the reference value, it is determined that multipath distortion has occurred, and a switching signal for switching to the output from the data correction circuit 6 is output.

Next, FIG. 4 shows a specific example of the memory circuit 5 and the data correction circuit 6 in the case of correction by linear prediction. FIG. 4 shows an example of a case where the linear prediction correction circuit 30 including a digital signal processing circuit is used. Input port 10
The stereo demodulated signals J ₁ and J ₂ captured from are stored in the memory circuit 5.
To store. The stereo demodulated signals J ₁ and J ₂ follow the order instructions of the program counter 12 and the program memory 11,
Various operations are performed on the data, and the operation result is output port 1
It is output as corrected data to the data switching circuit 7 through 5. Here, the multiplier shifter 13 sets a multiplicand and a multiplier, and the multiplication ALU accumulator 14 performs data addition / subtraction, a logical operation, and a product-sum operation.

In linear prediction, a prediction coefficient is calculated from a past sampled data string, and the stereo demodulated signals J ₁ and J ₂ determined to have distortion due to multipath distortion are corrected based on this prediction coefficient. .. In the following description, the stereo demodulated signal J ₁ side of _one channel will be described.
Each sample data of the stereo demodulated signal J ₁ is converted into Si (i =
1, 2, 3 ...).

Here, the prediction coefficients a ₁ , a ₂ , a ₃ , a ₄ and past sample data strings are S ₁₀ , S ₉ , S ₈ , S ₇ , S ₆ , S.
₅ , S ₄ , S ₃ , S ₂ , S ₁ , and the corrected data is (mounting S ₀ ).

[0026]

[Formula 1]

[0027]

[Formula 2]

[0028]

[Formula 3]

[0029]

[Formula 4]

[0030]

[Formula 5]

[0031]

[Formula 6]

[0032]

[Formula 7]

[0033]

[Formula 8]

[0034]

[Formula 9]

[0035]

[Formula 10] Is.

The prediction coefficients a ₁ , a ₂ , a ₃ and a ₄ are as follows: a ₁ = D ₁ / D, a ₂ = D ₂ / D, a ₃ = D ₃ / D, a ₄ = D ₄
The corrected data (mounting S ₀ ) obtained from / D is calculated from the amount S ₀ = a ₁ S ₁ + a ₂ S ₂ + a ₃ S ₃ + a ₄ S ₄ .

Therefore, when multipath distortion is detected by the multipath distortion detection circuit 4, a switching signal is output and the data corrected by the data switching circuit 7, that is, (mounting S ₀ ) is selected and output, When the multipath distortion is not detected by the path distortion detection circuit 4, the switching signal is not output, and the stereo demodulated signals J ₁ and J ₂ delayed by the delay circuit 8 are selected and output from the data switching circuit 7. This relationship is illustrated in FIG. The delay circuit 8 delays the stereo demodulated signals J ₁ and J ₂ in order to make the output data continuous in consideration of the calculation time of the (mounting S ₀ ) by the linear prediction correction circuit 30.

Next, as a first modified example of the first embodiment, a case where the correction is performed by the mean value interpolation will be described. Storage circuit 5 for performing a mean value interpolation, the data correction circuit 6 and the delay circuit 8 as shown in FIG. 6, if Shimese for stereo demodulation signal J ₁ side, cascade-connected D flip-delaying sampling data Si P 31, 32, 33, 3
4, an adder 35 for adding the output of the D flip-flop 31 and the output of the D flip-flop 33, and an adder 35
And a divider 36 that halves the output of
The output of the D flip-flop 34 and the output of the divider 36 are supplied to the data switching circuit 7 and one of them is selected and output by the output of the multipath distortion detection circuit 4.

Here, the D flip-flop 34 acts as a delay device for making the output continuous, and the adder 35 and the divider 36 constitute an average value calculation circuit. The stereo demodulated signal J ₂ side is similarly configured.

[0040] In the case of performing the correction by the average value interpolation, sample data sequence ... S _1, S _0, S _-1, in ..., multipath distortion detected multipath distortion detection circuit 4 when the data of the sample data S ₀ When the output is output, the output of the divider 36 is selected, and the averaged output of the sample data S ₁ and the sample data S ₋₁ before and after the sample data S ₀ is used as the correction data and the data correction circuit 7 Is output from. If this corrected data is (mounting S ₀₁ ), then (mounting S ₀₁ ) = (S ₁ + S ₋₁ ) / 2.

Next, a second modification of the first embodiment will be described. The second modified example is a case where the correction is performed by holding the previous value.
Storage circuit 5 for performing the correction before value holding, data correction circuit 6 and the delay circuit 8 as shown in FIG. 7, if Shimese for stereo demodulation signal J ₁ side, the sampling data Si
Cascaded D flip-flop 37 for delaying
38, and the output of the D flip-flop 37 and the output of the D flip-flop 38 are the data switching circuit 7
One of them is selected and output by the multipath distortion detection output of the multipath distortion detection circuit 4. The same applies to the stereo demodulated signal J ₂ side.

[0042] In the case of performing the correction before value holding sample data string ... S _1, S _0, the S _-1 ..., multipath distortion detection output from the multipath distortion detection circuit 4 when the data of the sample data S ₀ Is output, the output of the D flip-flop 38 is selected and the sample data S ₀ is output.
Is replaced with the sample data S ₁ immediately before the sample data S ₀ , and correction is performed by holding the previous value.

Next, the second embodiment will be described. The second embodiment is a case where the distortion correction coefficient corresponding to the magnitude of harmonic distortion is used for correction. FIG. 8 is a block diagram showing the configuration of the second embodiment.

In the second embodiment, the multipath distortion detection circuit 4-A comprises a digital bandpass filter 1 and a digital highpass filter 2, and the FM composite signal A is supplied to the digital bandpass filter 1 and the digital highpass filter 2. The digital high-pass filter 2 obtains the second-order harmonic and the digital high-pass filter 2 obtains the third-order harmonic, which are output as address data of the ROM 50.

A distortion correction coefficient corresponding to the magnitude of high frequency distortion is stored in the ROM 50, and the distortion correction coefficient read out from the ROM 50 is addressed by the output from the digital band pass filter 1 and the digital high pass filter 2. The coefficient and the stereo demodulation output J ₁ are multiplied by the multiplier 48 and output as the stereo demodulation output L, and similarly, the distortion correction coefficient read from the ROM 50 and the stereo demodulation output J ₂ are multiplied by the multiplier 49,
It is configured to output as a stereo demodulation output R.

Therefore, the distortion correction coefficient based on the outputs from the digital band pass filter 1 and the digital high pass filter 2 is multiplied by the stereo demodulation outputs J ₁ and J ₂ , respectively, to be corrected, and the multiplier is corrected to reduce the distortion. The outputs of 48 and 49 are output as stereo demodulation outputs.

Further, by using the distortion correction coefficient stored in the ROM 50 as difference data, the multipliers 48 and 49 can be adders.

Next, a third embodiment will be described. Third
The embodiment is an example in which the linear prediction correction and the correction by the distortion correction coefficient are selected according to the magnitude of the harmonic distortion.
FIG. 9 is a block diagram showing the configuration of the third embodiment.

The delay circuit 8 a stereo demodulated signal J _1, J ₂ via a stereo demodulated signal J ₁ corrected in the linear prediction compensation circuit 30, J _2, ROM 50 reads the stored distortion compensation coefficient to the multiplier 48 supplies a stereo demodulation signal J _1, J _2, which is corrected by multiplying the stereo demodulation signal J _1, J ₂ by 49 to the data switching circuit 7-a, the multipath distortion detection circuit one of them 4- The multipath distortion detection circuit 4-B is configured to select and output based on the multipath distortion detection output from B. The multipath distortion detection circuit 4-B includes a digital bandpass filter 1 and a digital highpass filter 2.
And a level determiner 3-A that compares the output of the digital bandpass filter 1 and the output of the digital highpass filter 2 with a preset reference level and determines multipath in three stages, and switches the determination output. It is supplied as a signal to the data switching circuit 7-A.

In the third embodiment having the above-mentioned structure, when it is determined that the magnitude of the harmonic distortion of the pilot signal is a negligible level, the data switching circuit 7-A sends the stereo demodulated signal J ₁ through the delay circuit 8. , J ₂ , that is, the stereo demodulated signals J ₁ and J ₂ without correction are selected and output.

When it is determined that the magnitude of the harmonic distortion of the pilot signal is smaller than the set reference value, it is corrected by the outputs of the multipliers 48 and 49, that is, the distortion correction coefficient by the data switching circuit 7-A. The stereo demodulated signals J ₁ and J ₂ are selected and output.

When it is determined that the magnitude of the harmonic distortion of the pilot signal is equal to or greater than the set reference value, the data switching circuit 7-A outputs the linear prediction circuit 30, that is, the stereo demodulation signal corrected by the linear prediction. J ₁ and J ₂ are selected and output.

Next, a fourth embodiment will be described. The fourth embodiment is an embodiment in the case where the multipath distortion is corrected in the preceding stage of the stereo demodulation circuit. FIG. 10 is a block diagram showing the configuration of the fourth embodiment.

In the fourth embodiment, the FM composite signal A is supplied to the delay circuit 8-A to be delayed and is also supplied to the multipath distortion detection circuit 4-A, and the FM composite signal A is supplied depending on the magnitude of the harmonic distortion. The output of the multipath detection circuit 4-A is supplied as address data to the ROM 50-A in which the correction coefficient for reducing the multipath distortion of A is stored as the address data, and the FM composite signal A delayed by the delay circuit 8-A and the ROM 50-. The correction coefficient read from A is multiplied by a multiplier 48-A, and the multiplication calculation power is supplied to a stereo demodulator 49 for stereo demodulation.

Here, the delay circuit 8-A is for compensating the processing time by the multipath distortion detection circuit 4-A and the ROM 50-A.

In the fourth embodiment configured as described above, the address of the ROM 50-A is designated by the multipath distortion detection output detected by the multipath distortion detection circuit 4-A and read from the ROM 50-A. The output correction coefficient and the FM composite signal are multiplied to make correction, and stereo demodulated. In this case ROM50-A
Since a correction coefficient determined to reduce the multipath distortion of the FM composite signal A is stored in, the multipath distortion is reduced.

Next, another example of the multipath distortion detection circuit will be described. FIG. 11 is a block diagram showing another example of the multipath distortion detection circuit.

The multipath distortion detection circuit 47 of this example uses sin (ωsnT +) based on the output data D of the arithmetic circuit 18.
D) a sine wave operation circuit 39 for performing the operation, a subtractor 40 for subtracting the stereo demodulation output J ₂ from the stereo demodulation output J ₁ ,
The adder 41 for adding the stereo demodulation output J ₁ and the stereo demodulation output J ₂ , the output of the sine wave operation circuit 39 and the subtractor 40
42, a subtractor 43 that subtracts the output of the multiplier 42 from the FM composite signal A, and a subtracter 4 that subtracts the output of the adder 41 from the output of the subtractor 43.
4. A level difference detector 45 for detecting the level difference between the output of the sine wave calculation circuit 21 and the output of the subtractor 44, and a level determiner 46 for determining the output of the level difference detector 45.

When the digital signal processing type PLL circuit is synchronized, D = 0, and the output K of the sine wave operation circuit 39 is K = sinωsnT.

The output M of the subtractor 40 is M = LR.

Therefore, the output N of the multiplier 42 is N = (LR) sinωsnT, and the sub signal is obtained.

On the other hand, the output O of the adder 41 is O = L + R, and the main signal is obtained from the adder 41.

Therefore, the output of the subtractor 43 is (L + R)
FM of + (LR) sinωsnT + Psin (ωs / 2) nT
It is an output obtained by subtracting the sub-signal (L−R) sin ωsnT from the composite signal, and is (L + R) + Psin (ωs / 2) nT.

Therefore, the output Q of the subtractor 44 becomes the pilot signal Q = Psin (ωs / 2) nT, and the pilot signal is obtained from the subtractor 44.

On the other hand, from the sine wave operation circuit 21, the reference pilot signal F = Psi synchronized with the FM composite signal is obtained.
n (ωs / 2) nT is output.

When the pilot signal now has multipath distortion as shown in FIG. 2, the distorted pilot signal is obtained from the output Q of the subtractor 44. The level difference detector 45 detects the difference between the level of the reference pilot signal F and the level of the output pilot signal Q from the subtractor 44 at each phase. This difference is compared with a predetermined level, that is, a reference value, by the level determiner 46, and the signal when the multipath distortion exceeds the reference value of the predetermined level is the data switching circuit 7,
It is output to 7-A as a switching signal.

[0067]

As described above, according to the stereo demodulation circuit of the present invention, multipath distortion is detected by distortion of the pilot signal, and linear prediction correction, average value correction, previous value holding correction,
Alternatively, the distortion of the stereo demodulated signal is corrected by the correction correlated with the magnitude of the distortion, or the FM composite signal is corrected, so that the sound quality is improved.
Furthermore, there is an effect that all can be performed by digital signal processing, and integration into an integrated circuit is easy.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a first exemplary embodiment of the present invention.

FIG. 2 is a pilot signal waveform diagram with and without multipath distortion.

FIG. 3 is a diagram showing a spectrum of a pilot signal in the presence or absence of multipath distortion.

FIG. 4 is a block diagram showing a configuration of a linear prediction correction circuit according to the first embodiment of the present invention.

FIG. 5 is an explanatory diagram for explaining the operation of the linear prediction correction in the first embodiment of the present invention.

FIG. 6 is a block diagram showing a configuration of a correction circuit in a first modified example of the first embodiment of the present invention.

FIG. 7 is a block diagram showing a configuration of a correction circuit in a second modification of the first embodiment of the present invention.

FIG. 8 is a block diagram showing a configuration of a correction circuit according to a second embodiment of the present invention.

FIG. 9 is a block diagram showing a configuration of a correction circuit according to a third embodiment of the present invention.

FIG. 10 is a block diagram showing a configuration of a correction circuit according to a fourth embodiment of the present invention.

FIG. 11 is a block diagram showing the configuration of another example of the multipath distortion detection circuit in the first to fourth embodiments of the present invention.

[Explanation of symbols]

1 Digital Band Pass Filter 2 Digital High Pass Filter 3 and 46 Level Evaluator 4 and 47 Multipath Distortion Detection Circuit 5 Storage Circuit 6 Data Correction Circuit 7 Data Switching Circuit 8 Delay Circuit 9 Data Correction Circuit 29 Stereo Demodulation Circuit 30 Linear Prediction Correction Circuit 31, 32, 33, 34, 37 and 38 D flip-flops 35 and 41 adder 36 divider 40, 43 and 44 subtractor 42, 48 and 49 multiplier 50 ROM 45 level difference detector

Claims (6)

[Claims] 1. A stereo demodulation circuit for stereo demodulating an A / D converted FM composite signal, wherein:
Distortion detecting means for detecting distortion of the stereo pilot signal in the M composite signal, multipath distortion determining means for determining the presence or absence of multipath distortion based on the magnitude of the distortion detected by the distortion detecting means, and multipath distortion are present. A stereo demodulation circuit comprising: a correction unit that corrects the stereo demodulation signal based on linear prediction when it is determined to be so. 2. A stereo demodulation circuit for stereo demodulating an A / D converted FM composite signal, wherein:
Distortion detecting means for detecting distortion of the stereo pilot signal in the M composite signal, multipath distortion determining means for determining the presence or absence of multipath distortion based on the magnitude of the distortion detected by the distortion detecting means, and multipath distortion are present. A stereo demodulation circuit, comprising: an average value correcting means for making the stereo demodulation signal having distortion when determined to be the average value of the stereo demodulation signals before and after the distortion has not occurred. 3. A stereo demodulation circuit for stereo demodulating an A / D converted FM composite signal, wherein:
Distortion detecting means for detecting distortion of the stereo pilot signal in the M composite signal, multipath distortion determining means for determining the presence or absence of multipath distortion based on the magnitude of the distortion detected by the distortion detecting means, and multipath distortion are present. A stereo demodulation circuit comprising: a pre-value holding / correcting means for making a stereo demodulated signal having distortion when determined to be the immediately preceding stereo demodulated signal having no distortion. 4. A stereo demodulation circuit for stereo demodulating an A / D converted FM composite signal, wherein the F
Distortion detecting means for detecting the distortion of the stereo pilot signal in the M composite signal, and correction means for correcting the stereo demodulated signal based on the distortion correction coefficient correlated with the magnitude of the distortion detected by the distortion detecting means. A stereo demodulation circuit characterized by the above. 5. A stereo demodulation circuit for stereo demodulating an A / D converted FM composite signal, wherein:
Distortion detecting means for detecting distortion of the stereo pilot signal in the M composite signal, multipath distortion determining means for determining the magnitude of multipath distortion based on the magnitude of the distortion detected by the distortion detecting means, and small multipath distortion. When it is determined that the stereo demodulated signal is corrected based on the distortion correction coefficient that correlates to the magnitude of the detected distortion to make a stereo demodulated signal, and when it is determined that the multipath distortion is large, the stereo demodulated signal is based on the linear prediction. A stereo demodulation circuit, comprising: 6. A stereo demodulation circuit for stereo demodulating an A / D converted FM composite signal, wherein:
Distortion detecting means for detecting distortion of the stereo pilot signal in the M composite signal, and correction means for correcting the FM composite signal based on a distortion correction coefficient correlated with the magnitude of the distortion detected by the distortion detecting means. A stereo demodulation circuit characterized by that.