JPS5946451B2 - Display method for the amount of interference distortion caused by multipath interference of FM radio waves - Google Patents

Description

[Detailed Description of the Invention] Interference distortion that occurs in an FM demodulated signal due to interference between FM signals is caused by the fact that the received radio waves at the FM broadcast receiver are transmitted through multiple propagation paths (multipaths). In the case of a synthesized wave, so-called multipath distortion appears as a very harsh and unpleasant sound in the reproduced sound.

In other words, in addition to the direct waves of FM broadcast waves, the FM receiver receives
Indirect waves such as reflected waves and diffracted waves may also be input.
Therefore, interference distortion occurs in the FM demodulated signal based on interference between direct waves and indirect waves, but multipath disturbance due to the generation of interference distortion described above is becoming more and more noticeable as urbanization progresses and the number of high-rise buildings increases. It has become a serious problem in modern times.
Now, the carrier wave frequency ωC of the FM broadcast wave is the modulated signal f (), the direct wave is CD (), the reflected wave is cR (), the direct wave CD (), and the reflected wave CR (). The composite wave is C', and the delay time due to the path difference between the direct wave and the reflected wave is τ.
Then, the above-mentioned direct wave CD ( ), reflected wave CR ( ), composite wave c' ( ), etc. are each expressed in the following (1)
~ (3) It is shown by formula.

CD (wo) ■ Acos (ωc + f (wo)) ...
(1) CR (a)■ Acos(ωct+f(a-τ)
-ωcτ)...(2)c'(a)■CD(a)f-C
R (on) - on) cos (ωc + f (on) + φ (
)) ・・・・・・(3) (However, A=1) In the above equation (3), Env() is the direct wave CD(
) and the reflected wave CR(), the composite wave C'(
) is a term that indicates the amplitude fluctuation that occurred in ), and also φ( )
is a term indicating phase fluctuation, and if the interference ratio between the direct wave and the reflected wave is, then the following (4) and (
5) It is shown as Eq.

2+2K cos(f(tHf(−τ)
)+ωcτ)・・・・・・・・・(4)−4■Ksin
(f(tHf(wo-τ), cτ)φ(wo)-tan・
...(5) 1+Kcos(f(←f(−τ)→, c
τ), (where CD()>CR()) and the interference ratio is a coefficient determined by loss due to reflecting objects and attenuation due to the tendency characteristics of the receiving antenna, so the above value is Generally, <1, and therefore, the above equation (4) may be expressed as the following equation (4a).

When the composite wave C'(t) having the amplitude fluctuation envelope Env(t) as described above is FM demodulated by the FM demodulator, the FM demodulated signal E as shown in the following equation (6) is obtained.
.

(t) occurs. (6) In the formula, the first molecule of the right-hand side
f'(t) shown in the section is the undistorted demodulated signal,
All other signal components are interference distortion components.

Interference distortion that occurs in the demodulated signal due to interference between FM wave signals is caused by the frequency and modulation degree of the modulated signal f(t), the delay time τ caused by the path difference between the direct wave and the reflected wave, and the direct wave It has the property that the content of distortion, especially the frequency of distortion, changes depending on the signal level difference between the wave and the reflected wave, that is, it has the property of dynamic distortion.
Furthermore, since this distortion is extremely unpleasant to the sense of hearing, the presence of interference distortion occurring in the demodulated signal poses a major hindrance to high-fidelity reception of FM broadcast waves.

To this end, in order to eliminate or reduce multipath interference, antennas that exhibit sharp directivity characteristics have been used as antennas for receiving FM waves, or interference distortion has been eliminated or reduced using electrical circuits. Attempts have been made to eliminate or reduce the interference caused by interference between FM wave signals, and the applicant company has so far attempted to eliminate or reduce the interference caused in the demodulated signal by interference between FM wave signals. Distortion removal method (
(e.g., Japanese Patent Publication No. 56)
-51702, Special Publication No. 56-51703, Special Publication No. 51703
7-16536, JP 57-16537, JP 57-16538, JP 54-12511, JP 54-152415, JP 55-6972, JP 55-6973 (Please refer to the official bulletin).

As mentioned above, the interference distortion that occurs in the FM demodulated signal due to interference between FM wave signals significantly degrades the quality of reproduced sound. If we could quantitatively display whether or not
This is extremely useful when attempting to perform FM reception with high fidelity. FIGS. 1 and 2 are block diagrams of a conventional example of an interference distortion amount display device that displays the presence or absence of interference distortion and the amount of distortion on an indicating instrument as described above. In each figure, 1 is a receiving antenna, 2 is a front end of an FM tuner, 3 is an intermediate frequency amplification stage, 4 is an FM demodulator,
5 is a multiflex circuit, 6 and 7 are output terminals for left and right signals in a stereo signal, 10 is an indicator, and in FIG. 1, 8 is an envelope detection circuit, 9 is a signal conversion circuit, and In FIG. 2, 11 is a high-pass filter, and 12 is a signal conversion circuit.

The FM radio waves received by the receiving antenna 1 are subjected to high frequency amplification and frequency conversion in the front end 2, thereby being converted into an intermediate frequency signal and provided to the intermediate frequency amplification stage 3.

The intermediate frequency signal amplified by the intermediate frequency amplification stage 3 is
Demodulated by FM demodulator 4 and sent to multiflex circuit 5
However, when the FM radio waves are a stereo broadcast program, the composite signal from the stereo broadcast from the FM demodulator 4 is converted into left and right signals by signal processing in the multiflex circuit 5, and these are output. It is sent to terminal 6.7. Output terminal 6 from the front end 2 mentioned above
, 7 to 7 are similar to those of a normal FM tuner. Now, the conventional interference distortion amount display device shown in FIG. Variation {This is the amplitude variation of the FM radio waves received by receiving antenna 1, that is, En shown in equations (3) and (4)
v(t)}, and the output signal from the envelope detection circuit 8 is rectified and filtered in a signal conversion circuit 9 and is then provided to an indicator 10.

That is, in the conventional device shown in FIG.
) and the reflected wave CR(t), the composite wave c'(t) is (3)
．． Amplitude fluctuation Env(t
), and focusing on the fact that the amplitude variation Env(t) has a property that the amount changes depending on the interference ratio K, we can find the amplitude corresponding to the amplitude variation of the composite wave C5(t). The envelope detection circuit 8 extracts the amplitude variation of the intermediate frequency signal having the variation and displays it on the indicator 10 as the amount of interference distortion (multipath distortion).

In this manner, in the conventional device shown in FIG. 1, the amount that changes depending on the interference ratio K between the direct wave and the reflected wave is displayed on the indicator 10 as the amount of interference distortion. Since the actual amount of interference distortion does not always correspond correctly to the value of the interference ratio K between the direct wave and the reflected wave,
The conventional device shown in FIG. It is difficult to know the amount of distortion.

In other words, the amount of interference distortion has the property of varying according to changes in the modulation signal frequency and modulation degree, and even if the interference ratio K is the same, the modulation signal frequency i and modulation degree change. In this case, the amount of interference distortion naturally changes, but in the conventional device having the configuration shown in Figure 1, the indicated value of the indicator 10 does not change as long as the interference ratio K remains the same. It is. The above-mentioned problems in the conventional device shown in FIG. In the configuration shown in FIG. 2 as an example device, the output signal of the FM demodulator 4 is applied to a high-pass filter 11 whose cut-off frequency is set to a frequency value of 60 KHz or more. The extracted signal components are applied to the indicator 10 via the signal conversion circuit 12, and the amount of interference distortion is displayed on the indicator 10. However, in the conventional device shown in the second diagram, a signal component having a frequency component higher than the highest frequency value (53 KHz) of the sub-channel signal band during stereo broadcasting is extracted as an interference distortion component. There is a problem in that the perceived distortion does not correspond to the value displayed on the indicator 10.

The above problem will be explained in more detail with reference to FIGS. 3 to 5 as follows.

First, Fig. 3 is a frequency allocation diagram of a stereo composite signal based on the carrier suppression AM-FM method.
The solid line in the figure is a frequency allocation diagram in an ideal state, and the dotted line is an actual frequency allocation diagram, where the (L+R) signal is a baseband signal that occupies a frequency band up to 15KHz, and 23KHz to 53KHz. In the frequency band, both side bands of the carrier-suppressed AM signal by the (L-R) signal exist as sub-channel signals, and furthermore, a pilot signal with a frequency value of 19 KHz exists in the gap between the two frequency bands. ing. Furthermore, FIGS. 4 and 5 are characteristic diagrams for explaining the nature of interference distortion, with FIG. 4 showing the relationship between the modulation signal frequency and the amount of interference distortion, and FIG. 5 showing the relationship between the modulation signal frequency and the amount of interference distortion. FIG. 3 is a characteristic diagram showing the relationship between frequency, modulation degree, time difference (delay time) between a direct wave and a reflected wave, and frequency of interference distortion.

As shown in FIGS. 4 and 5 above, the amount of interference distortion increases as the modulation signal frequency increases,
Furthermore, as the modulation signal frequency increases, the frequency of interference distortion increases, and similarly, as the degree of modulation and delay time increase, the frequency of interference distortion increases. Among the interference distortion components extracted through the high-pass filter 11, those generated based on the baseband signal are those when a large change in modulation degree occurs in the high-frequency signal of the baseband signal. However, most of the interference distortion components that occur when the modulation signal has a moderate degree of modulation in the frequency range of several hundred hertz to several kilohertz are distributed within the frequency band of 15 KHz or less, 60 extracted by high-pass filter 11
Since there are very few frequency components of KHz or higher, the problem is that the perceived distortion does not correspond to the displayed value of the amount of distortion on the indicator 10.

It is no exaggeration to say that it is almost impossible to solve the above-mentioned problems in the conventional device shown in FIG. 2 when considered from the viewpoint of its construction principle. In other words, the actual stereo composite signal is
As mentioned above, the signal energy is distributed up to the frequency range shown by the dotted line in Figure 3, so in order to extract the interference distortion component, it is necessary to change the energy distribution of the stereo composite signal. As no frequency band,
This is because a frequency band of 60 KHz or higher must be selected (if a signal in a frequency band of 60 KHz or lower is extracted, the extracted signal is a mixture of stereo composite signal components and interference distortion components). (The amount of interference distortion cannot be determined from this.)

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for displaying the amount of interference distortion caused by multipath interference of FM radio waves, which satisfactorily eliminates the problems in the conventional devices shown in FIGS. 1 and 2. The specific contents will be clarified below with reference to the attached drawings.

FIG. 6 is a block diagram of one embodiment of the method for displaying the amount of interference distortion caused by multipath interference of FM radio waves according to the present invention, and the parts numbered 1 to 7 in the figure have already been described. These are the same structural parts as the block diagrams shown in the first and second figures.

In FIG. 6, the FM demodulated signal from the FM demodulator 4 is supplied to a first bandpass filter BPF, whose passband center frequency is 19 KHz and whose passband is narrow.

Therefore, in the first bandpass filter BPFi, F
Only the signal component around 19 KHz in the FM demodulated signal from the M demodulator 4 is extracted 2 and fed to the high gain amplitude limiting amplifier HAL which functions as both a high gain amplifier and a limiter. It is also given as its subtraction signal to circuit SUB. In the three states in which the FM receiver is receiving a stereo broadcast program, the signal output from the first bandpass filter BPFl has a frequency value of 19KH.
z pilot signal and the interference distortion generated based on the signal in the baseband signal band, which is a frequency component around 19KHz. In this case, the interference distortion of 319K
When a frequency component around Hz is present, the pilot signal, which originally has a constant amplitude, becomes as if it were amplitude modulated.

In addition, while the FM receiver is receiving a monaural broadcast program, the signal output from the first bandpass filter BPFl4l has a frequency of 19 KHz within the interference distortion generated based on the signal in the frequency band of the modulated signal. Only nearby frequency components. The above-mentioned high gain amplitude limiting amplifier HAL performs a good limiter operation when the input signal applied thereto includes a pilot signal, that is, when the FM receiver is receiving a stereo broadcast program. The limiter operates so as to output a square wave of constant amplitude on the output side, and when the input signal applied to it does not include a pilot signal, that is, when the FM receiver is receiving a monaural broadcast program. Its operating point is set so that it can operate as a simple amplifier without performing any

The output signal from the high gain amplitude limited amplifier HAL has a pass band center frequency value of 19 KHz and a narrow band second
The above-mentioned arithmetic circuit SU
The arithmetic circuit SUB performs an operation of subtracting the output signal of the first bandpass filter BPFl from the output signal of the second bandpass filter BPF2. The resulting signal is applied to a signal conversion circuit SCC, and the pointer of the indicator M is swung by the signal made suitable for driving the indicator M by being rectified and filtered in the signal conversion circuit SCC. In this way, the amount of interference distortion is displayed by the deflection of the pointer of the indicator M.

FIGS. 7A to 7E are waveform example diagrams for explaining the operation of the method of displaying the amount of interference distortion shown in the block diagram of FIG. ．． Figure a2 is a waveform example diagram of output signals Sal and sa2 from the FM demodulator 4 {Drawing numbers Al, a2 and Ve2 in Figure 7
The subscript numeral 1 in drawing symbols Sal, sa2...Se2, etc. in FIG. Shows waveform diagrams and signals of each part when affected by interference distortion}.

The stereo composite signal Sal when not affected by interference distortion is transferred from the FM demodulator 4 to the first bandpass filter B.
When supplied to PF1, the output signal from the first bandpass filter BPF becomes a signal with a constant amplitude like the signal Sbl shown in Fig. 7b1, and when it is affected by interference distortion, When the stereo composite signal Sa2 is supplied from the FM demodulator 4 to the first bandpass filter BPFl, the output signal from the first bandpass filter BPFl has an amplitude like the signal Sb2 shown in FIG. 7B2. The amplitude fluctuation is caused by adding and subtracting the 19 KHz distortion component of the interference distortion caused by the signal in the baseband signal band to the 19 KHz pilot signal. -ing

The output signal Sbl or Sb2 from the first bandpass filter BPFl is converted into a constant amplitude square wave signal Sc as shown in FIG. By being given to BPF2,
A sine wave signal Sd is output from the second bandpass filter BPF2.

When the output signals from the first and second bandpass filters BPFl and BPF2 are subjected to a calculation such as {(sine wave signal Sd) - (signal Sbl or Sb2)} in the calculation circuit SUB, the stereo composite signal interferes. The output from the arithmetic circuit SUB when it is not affected by distortion becomes zero as shown in the signal Se shown in FIG. 7 e1, and
The output from the arithmetic circuit SUB when the stereo composite signal is affected by interference distortion is shown in Fig. 7 E2.
A signal Se2 resulting from the interference distortion component as shown in the figure is obtained.

The output signal Se2 from the arithmetic circuit SUB is rectified and filtered in the signal conversion circuit SCC and then given to the indicator M, and the indicator M displays the amount of interference distortion. This is the output signal S from the arithmetic circuit SUB when the stereo composite signal is not affected by interference distortion.
This can be easily achieved by adjusting the phase of the passing signal in the high gain amplitude limiting amplifier HAL so that el becomes zero.

When an FM receiver is receiving a monaural broadcast program, the output signal from the first bandpass filter BPFl is only a 19KHz component of interference distortion, but the output signal is sufficiently high-gain amplitude-limiting amplifier HAL. After being amplified, the signal is subjected to some attenuation by the second bandpass filter BPF2, and then passed through the arithmetic circuit SUB as it is to be applied to the signal converter SCC.

The display of the amount of interference distortion by the indicator M based on the output signal from the signal converter SCC is the same as when receiving the stereo broadcast program described above. Furthermore, although there is a difference in the amount of interference distortion when receiving a monaural broadcast program and when receiving a stereo broadcast program, as shown in Figure 6, the high gain amplitude limiting amplifier HAL is If the indicator M is configured to operate as a gain amplifier and display the interference distortion component in an amplified state, the indicator M will be displayed when receiving a monaural broadcast program and when receiving a stereo broadcast program. A special means for switching the sensitivity of the meter M can be omitted. As is clear from the above explanation, the method of displaying the amount of interference distortion of the present invention extracts only the signal component around 19 KHz in the output signal of the FM demodulator, and calculates the amount of variation in the extracted signal component. Since the amount of interference distortion (multipulse distortion) is displayed on the indicator M, the FM broadcast wave being received is a monaural broadcast program or a stereo broadcast program. Even interference distortion that occurs when a signal in the frequency band of several hundred hertz to several kilohertz in a modulated signal has a relatively small modulation degree can be effectively extracted from the signal component around 19 kHz, so that the distortion does not appear to the auditory sense. The amount of distortion can be displayed on the indicator M using the corresponding indicated amount.

In addition, since the method of the present invention extracts signal components around 19 KHz as described above, when receiving stereo broadcasting, the presence or absence of amplitude fluctuations of the 19 KHz pilot signal and the magnitude of the amplitude fluctuations are determined by interference distortion. It is used to determine the presence or absence of the signal and to display the magnitude of the amount of distortion, and it is easy to extract signal components around 19 KHz that are free of baseband signal components and sub-channel signal components. Therefore, malfunctions due to the mixing of baseband signal components and sub-channel signal components do not occur, and among the interference distortion components that occur following changes in the modulation signal frequency and modulation degree, it is particularly effective for auditory sense. The perceived distortion component can be dynamically displayed by the indicator M in a state that matches the sense of hearing, and the method of the present invention satisfactorily eliminates all the problems in the conventional device described above. be.

By adopting the method of the present invention, it is possible to easily set the direction of the receiving antenna and adjust the interference distortion removal circuit so that a high-fidelity reproduction state can be obtained when receiving FM broadcasting.

[Brief explanation of drawings]

1 and 2 are block diagrams of a conventional device, FIG. 3 is a frequency allocation diagram of a stereo composite signal, FIGS. 4 and 5 are characteristic diagrams for explaining the nature of interference distortion, and FIG. FIG. 6 is a block diagram of one embodiment of the system of the present invention, and FIGS. 7A1 to E2 are signal waveform diagrams for explaining the operation of the system of the present invention. 1...Reception antenna, 2...Front end, 3.
...Intermediate frequency amplification stage, 4...FM demodulator, 5...Multiflex circuit, 6, 7...Output terminal, 8...
Envelope detection circuit, 9, 12, SCC... Signal conversion circuit, 10, M... Indicator, 11... High pass filter, BPFi... First band filter, BPF2... Third 2 bandpass filter, HAL...high gain amplitude limiting amplifier,
SUB...Arithmetic circuit.

Claims (1)

[Claims] 1 means for supplying the FM demodulated signal obtained from the FM demodulator to a first bandpass filter having a narrow band with a center frequency value of 19 KHz; means for converting the output signal from the high-gain amplitude-limiting amplifier into a square wave signal having a constant amplitude by applying the signal to an amplitude-limiting amplifier; and a second bandpass filter having a narrow band with a center frequency value of 19 KHz for the output signal from the high-gain amplitude-limiting amplifier. means for obtaining a sine wave signal from the second bandpass filter;
means for supplying each output signal from the bandpass filter to an arithmetic circuit and performing subtraction between the two signals in the arithmetic circuit to obtain an interference distortion component; and a means for supplying the interference distortion component to a signal conversion circuit. A method for displaying the amount of interference distortion caused by multipath interference of FM radio waves, comprising means for converting into a signal suitable for driving an indicator, and means for providing an output signal from the signal conversion circuit to the indicator.