JPS5911219B2 - FM multipath distortion reduction device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an apparatus for reducing FM multipath distortion that occurs when an FM signal is received through a plurality of transmission paths due to reflection, diffraction, etc.

Multipath interference in FM reception caused by Japan's unique topography and the rise of urban buildings is an extremely serious problem, similar to TV ghost interference. This multipath interference can be expected to be improved to some extent by using an antenna with excellent directivity, but it is difficult to improve if the difference in the angle of incidence between the direct wave and the reflected wave is small, and the optimal direction differs for each broadcast. Considering this, countermeasures must be taken within the receiver. As a countermeasure using such a circuit, it has the same characteristics (delay time, D/U ratio, phase difference when no modulation) as the multipath phenomenon that occurs during actual transmission, but has the opposite polarity (
It has been proposed to install a transmission system in the receiver (where direct waves and delayed waves are subtracted) and pass FM waves that have suffered multipath interference through this system in order to improve distortion. . However, the above method has problems in that it is extremely complicated to optimally set the three parameters during actual broadcast reception, and it is also technically difficult to arbitrarily delay the high frequency signal up to several tens of tts. In view of these drawbacks, the present invention is based on the viewpoint of implementing countermeasures for detected signals that are relatively easy to handle.Moreover, FM multi-bass distortion is characterized in that the higher the frequency of the modulation signal, the more susceptible it is to the effect, and therefore the normal Focusing on the problem of distortion during stereo reception in the field, the present invention realizes a multipath distortion reduction device with a simple configuration and easy adjustment. The principle of the present invention will be explained below.

Now, the carrier frequency ω. , modulation signal μ(), amplitude i. When a reflected wave with twice the amplitude (<1) and a delay time τ is superimposed on the FM wave of , the composite wave i becomes as follows.

l+Rexp(-j)(Q) in this equation (1).

T+Mura-Tpdt) is the transfer characteristic when multipath interference is viewed as a type of transmission path with distortion. For simplicity, the transfer characteristic f(Q) can be expressed as follows.

Also, the amplitude characteristic A(Q) and phase characteristic p of this f(Q)
(Q) will be respectively.

By the way, when an FM wave passes through a transmission path having a transfer characteristic of f(Q) above, assuming that the amplitude characteristic is not affected by the limiter effect in the receiver, the FM wave 1 after passing becomes as follows. This detection output 0 means that p(Q) is Q. (However, Q.=f+-T(!).

However, from equations (7) and (8), if the modulation signal is a sine wave with It( t ) = Q)COSpt, then equation (9) can be obtained. In the AL equation, all terms after the coefficient B correspond to harmonic distortion terms.

Therefore, the fundamental wave component 0p(t) can be expressed as.

On the other hand, b1 is obtained from equation (5) by substituting equation U'5 into equation 01). Next, under the condition of R2《1, find 111 and rearrange it. In other words, the modulated signal of Tt ( t ) = 80c0spt is Due to the multipath phenomenon, the amplitude becomes (1+COSPT
-12rc0sQ) will be doubled.

Although the sub-signals in the composite signal are subjected to carrier wave suppression amplitude modulation, this amplitude information is affected by the above-mentioned effects, causing large distortion and a decrease in the degree of separation during stereo reproduction. On the other hand, the envelope of the carrier wave of the FM signal is affected by equation (4). If this envelope signal is X, then this X is (1-
COsPT), which is the reciprocal of the amplitude influence characteristic given by equation 04). Therefore, if 0p is multiplied by xl-COsPt, the original amplitude o will be multiplied. Here, the sub-signal is not a single sine wave, but in a normal north, most of the energy in the sub-signal is concentrated in the surrounding earth number KHz of 38 KHz, so P of 1-COSPT in equation 06) is
A sufficient effect can be obtained as represented by z.

Incidentally, the above amplitude correction can of course be applied not only to the sub-signal but also to the pilot signal. That is, for sub-signal correction, xl-COSPT (where P=2r
x38 x 103), x can be used for the 19 KHz pilot signal.

Besides, A. FIG. 1 shows the changes in each delay time T of b.

As is clear from the figure, the relationship between a and b for T from 0 to 9 is repeated for all 0T, so if you memorize the relationship between a and b in Figure 1 in advance,
From the value of b that optimally corrects the pilot signal, the value of a that optimally corrects the sub-signal can be given.

The configuration of the present invention based on the above principle is shown in FIG.

The composite signal obtained by the FM detector 1 is converted into a main signal by a low-pass noilter 2, a pilot signal by a 19 KHz band-pass filter 3, and a center frequency 38.
The sub-signals are separated by a KHz bandpass filter 4. On the other hand, the intermediate frequency signal is controlled by the automatic gain control circuit 5.
After the time-average amplitude is made constant, the envelope is detected by the envelope detector 6. This envelope signal is input to a power circuit T which gives Xa and a power circuit 8 which gives Xa. Here, these A, . The value of b is given by the variable interlock setting circuit 9 so as to set it while maintaining the relationship between the two described above. Next, in the analog multiplication circuit 10, the pilot signal which is the output of the bandpass filter 3 is multiplied by the output signal of the exponentiation circuit 8, and the pilot signal is corrected. Further, in the analog multiplication circuit 11, the sub-signal which is the output of the bandpass filter 4 is multiplied by the output signal of the exponentiation circuit 7, and the sub-signal is corrected. The main signal output from the low-pass filter 2, the pilot signal after the above correction, and the sub signal are recombined into a composite signal by an adder circuit 12 and input to a stereo demodulation circuit 13. At the same time, the corrected pilot signal, which is the output of the multiplier 10, receives its amplitude modulation component from the detection circuit 14.
and its level is displayed on the display 15. If the correction to the pilot signal is optimal, the amplitude after correction will be constant and the indication on the display 15 will be minimum. Therefore, if the setting circuit 9 is controlled to minimize the indication on the display 15 and the optimum b value is given, at the same time the a value that optimally corrects the sub-signal is selected. Note that the setting circuit 9 is used for A.2π=O to 1 in FIG. This can be easily realized by using a double P variable resistor that provides the curve b, or by switching the terminals of a resistor network arranged to closely approximate the curve.

Naturally, it is also possible to determine the output level of the detector 14 using an electronic circuit and automatically control the setting circuit 9. In the above embodiment, the modulation component of the corrected pilot signal is detected and the level is displayed to determine whether the degree of adjustment is appropriate. Since it is asymmetrical, it goes without saying that the areas may be compared centering on the zero level of the recombined composite signal to determine whether the degree of adjustment is appropriate or not.

As described above, according to the present invention, there is no need for a variable delay device for either the high frequency signal or the detected signal, and furthermore, it is only necessary to make adjustments while looking at the indicator, so the configuration is simple and the operation is simple. This makes it possible to easily realize an FM stereo broadcast multipath distortion reduction device.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an example of the configuration of an FM multipath distortion reduction device according to the present invention, and FIG. 2 is an explanatory diagram of the operation of the device. 1...FM detector, 2...Low pass filter, 3...19KHz band pass filter,
4...bandpass filter, 5...depression gain control circuit, 6...envelope detector, 7,
8...Power circuit, 9...Variable interlock setting circuit, 10, 11...Analog multiplication circuit, 1
2...Addition circuit, 13...Stereo demodulation circuit, 14...Detection circuit, 15...
display.

Claims (1)

[Claims] 1. Means for selecting a pilot signal from a composite signal obtained by detecting an FM signal modulated with a stereo signal, means for selecting a sub-signal as well, means for detecting the envelope of the FM signal, and the detecting means. A first exponentiation means that raises the envelope signal obtained by raising the envelope signal to the a power (0≦a≦2), a second exponentiation means that also raises the envelope signal obtained in When representing the value of b is 1
-cos θ/2, a means for changing the relationship between a and b while maintaining the relationship; a first multiplication means that receives the selected sub-signal and the output signal of the first exponentiation means; a second multiplier which receives as input the pilot signal and the output signal of the second exponentiation means, which obtains the corrected sub-signal and pilot signal from the outputs of the first and second multiplier means, and performs the correction. By detecting and displaying the degree of amplitude modulation of the pilot signal, the sub-signal,
An FM multipath distortion reduction device characterized in that it is configured to know the degree of correction of a pilot signal.