JPS5911220B2 - 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. If you think about it, countermeasures will need to be taken within the receiver.

In order to reduce this multipath distortion, a method as shown in FIG. 1 has been proposed. 1 in the figure is an intermediate frequency amplifier,
2 is a variable delay device, 3 is a variable attenuator, 4 is a variable phase shifter, 5
is a subtracter, and 6 is a detector. Now, the carrier frequency ω. ,
If the FM wave of the modulation signal μ() has a reflected wave with an amplitude twice (<1) and a delay time τ, its composite signal i()
isexpj(ω.

(−τ)+ft−”,,dt) to exp(−j)(ω
0τ+f)-rttdt))・・・・・・・・・(1)
Then, the transfer characteristic f(Q) becomes f(Q)=1+exp
It can be expressed as (-jQ)...(3).

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

On the other hand, in the configuration shown in Fig. 1, the direct wave and the delayed wave are subtracted, so the transfer characteristic fl(Qゞ) has the intermediate frequency as Ct)i,
Delay time T of delay device 2, transmission ratio r of attenuator 3, phase shifter 4
However, if the phase shift amount a is set as .

Now, if a is adjusted so that the relationship Ct)IT+a+2n7C=00T holds true, then it becomes Q-Q゜, and in the end f(Q
) and FV(Q) are connected in series, transfer characteristic f(Q)・f
ゞ(Q) becomes, and if R2<1, multipath interference can be removed.

However, as is clear from the description, r, R. It is necessary to optimally set the three parameters of CT, and the high frequency signal must be arbitrarily delayed up to several tens of ps, which poses many technical and cost difficulties. In view of these drawbacks, the present invention implements countermeasures for the detected signal, which is relatively easy to handle.Moreover, the higher the frequency of the modulation signal, the more susceptible it is to FM multipath distortion. The object of the present invention is to realize an FM multipath distortion reduction device with a simple configuration and easy adjustment by focusing on the fact that large distortion occurs in stereo reception even though it is hardly a problem in monaural reception.

The principle of the present invention will be explained below. Now, when an FM signal passes through a transmission path with the characteristics of equation (3), assuming that the amplitude characteristics are not affected by the limiter effect in the receiver, the FM wave 1 after passing will be, and this transverse wave output 0 will be Here Q (Q) is Q.

If we perform Taylor expansion around (Q.=fl-,,00dt-00T), then from equation (9) and 01, if we assume a sine wave where the modulation signal Tt(t)=muCl)COSpt, we get the equation Q1). By substituting , the fourth term and subsequent terms in equation 0 all correspond to harmonic distortion terms.

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

On the other hand, B, is obtained from equation (5) by substituting equation 04) into equation 03), then rearranging 1--1P-1 under the condition of R2《1, that is, Tt(t)-mu0) COSpt. The amplitude of the modulated signal is (1+2rc0sQ) due to the multipath phenomenon.
) 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 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-C
OsPr), which is the reciprocal of the amplitude influence characteristic given by equation 06). Therefore, Q Muku style is 0
By multiplying by equation 6), the original COsPtf) angle will be restored. 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 frequency KHz of 38 KHz,
Even if P in the (1-OsPt) power is represented by 38 KHz, a practically sufficient effect can be obtained. FIG. 2 shows a configuration for realizing the above contents.

Of the composite signal output from the detector 6, the main signal and pilot signal pass through a low pass filter 7, and the sub signal passes through a band pass filter 8 having a center frequency of 38 KHz. On the other hand, the intermediate frequency signal has its time-average amplitude made constant by an automatic gain control circuit 10, and then its envelope is detected by an envelope detector 11 and input to a waveform converter 12. This waveform converter 12 gives an output Xa to an input X, and from equation (alternative), a-
Since it is 1-COsPτ, it is sufficient if it can be adjusted within the range of 0≦a≦2. The sub-signal that has passed through the bandpass filter 8 and the output signal of the waveform converter 12 are input to an analog multiplier 9, and the product signal is taken out as a corrected sub-signal. This corrected sub-signal, the main signal and the pilot signal that have passed through the low-pass filter 7 are combined again in an adder 13, and are provided to a stereo demodulator 14 as a corrected composite signal. What about figure 3? This is an example of a configuration for realizing a waveform converter 12 that outputs an output signal.

15 is a logarithmic circuit, 16 is a variable magnification circuit, and 17 is an antilogarithmic circuit.

When the input signal is x, the output signal y is as follows.

As the variable magnification circuit 16, a combination of a variable resistor and an amplifier circuit, a voltage controlled variable gain amplifier circuit, etc. are used. Also, if the delay time becomes large, not only the sub signal but also 19KH
The amplitude of the z pilot signal is also affected.

Recent FM receivers often employ a pilot signal cancellation circuit that uses a 19KHz signal that is stably generated by the stereo demodulator's PLL circuit, but when the amplitude of the pilot signal changes like this, the cancellation effect is lost. Significantly decreased. Therefore, FIG. 4 is an extension of the configuration shown in FIG. 2, in which the amplitude of the pilot signal is stabilized when resynthesizing the composite signal. In the figure, 18 is a low-pass filter that passes the main signal, 19 is a 19KHz band-pass filter that passes the pilot signal, and 20
is an amplitude limiter. Since the pilot signal subjected to amplitude modulation is waveform-shaped by the limiter 20 and then applied to the adder 12, the above-mentioned problem can be avoided. As is clear from the above, according to the present invention, adjustment can be performed simply by properly initializing the automatic gain control circuit 10. It is only necessary to set a in one place in the waveform converting circuit 12 to give . Furthermore, since no delay device is required for either the high frequency signal or the detected signal, the configuration can be extremely simplified.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a conventional FM multipath distortion reduction device, FIG. 2 is a block diagram showing an embodiment of the FM multipath distortion reduction device of the present invention, and FIG. 3 is a block diagram of the main parts of the device. FIG. 4 is a block diagram showing another embodiment of the FM multipath distortion reduction device of the present invention. 6...Detector, 7,18...Low pass filter, 8...Band pass filter, 9...
... Analog calculator, 10... Fractional gain control circuit, 11... Envelope detector, 12...
... Waveform converter, 13... Adder, 14.
... Stereo demodulator, 15 ... Logarithmic circuit, 16 ... Variable magnification circuit, 17 ... Anti-logarithm circuit, 19 ... 19KHz band Passage filter, 20... amplitude limiter.

Claims (1)

[Claims] 1 means for selecting a sub-signal from a composite signal obtained by detecting an FM signal modulated by a stereo signal; a means for detecting a carrier envelope of the FM signal; and an output signal of the envelope detecting means as an input. comprising exponentiation means whose output is the a-th power (0≦a≦2) of the input signal; and multiplication means whose output is the output of the exponentiation means as one input and the selected sub-signal as the other input; FM multipath characterized in that a corrected sub-signal is obtained as the output of the multiplication means, and the corrected sub-signal is added to the addition means together with the main signal and the pilot signal to be recombined as a composite signal. Distortion reduction device.