Multipath distortion reducing circuit

A multipath distortion reducing circuit particularly adapted for use with stereophonic receiving circuits in which the amount of mixing between right and left channels is reduced. In accordance with the invention, the envelope of the received FM signal is AM detected, and the envelope detected circuit is multiplied by the FM detected signal differentiated to provide a distortion cancelling signal. The distortion cancelling signal is summed with the detected FM signal to provide an output having a reduced multipath distortion component.

BACKGROUND OF THE INVENTION 
The present invention relates to a circuit for reducing the amount of 
multipath distortion in an FM tuner. 
An example of a conventional multipath distortion reducing circuit of this 
type is shown in FIG. 1. In FIG. 1, the output terminal of a front end 1 
is connected to a 10.7 MHz bandpass filter (BPF) 2, the output terminal of 
which is connected to the input terminal of an FM detector 3. The output 
terminal of the FM detector 3 is connected to the input terminal of a 
stereo demodulator 4. The latter provides a left channel output and a 
right channel output respectively at a left channel output terminal 5 and 
a right channel output terminal 6. A series circuit of a capacitor C and a 
switch SW is connected between the output terminals 5 and 6. 
When multipath distortion occurs and accordingly the noise level increases 
to make it difficult to hear the reproduced signal, the switch SW 
connected between the left channel output terminal 5 and the right channel 
output terminal 6 is turned on. In this case, high-frequency noise 
components contained in the right and left channel outputs, which are 
offensive to the ear and which are opposite in phase to one another, are 
cancelled out, being shorted by the capacitor C. Thus, the perceived noise 
level is reduced. However, the circuit is disadvantageous in that, the 
higher the frequency of the cancelled-out signal components, the larger 
the ratio of mixing of right and left channels, and the lesser the degree 
of separation between the two channels. 
SUMMARY OF THE INVENTION 
An object of this invention is to eliminate the above-described 
difficulties accompanying a conventional multipath distortion decreasing 
circuit. 
In accordance with the above and other objects, the invention provides a 
circuit in which the envelope of the IF signal is detected to form a 
distortion cancelling signal which is applied to reduce the amount of 
multipath distortion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the invention will be described with reference to 
the drawings. 
As shown in FIG. 2, in a multipath distortion reducing circuit of the 
invention the output terminal of a front end is connected to a first 10.7 
MHz BPF 2 and a second 10.7 MHz BPF 7. The output terminal of the first 
10.7 MHz BPF 2 is connected to an FM detector 3, the output terminal of 
which is connected to a differentiator 10 and one input terminal of an 
adder 12. The output terminal of the second 10.7 MHz BPF is connected to 
an input terminal of an AM detector 8. The output of the AM detector 8 is 
utilized to apply automatic gain control (AGC) to the AM detector 8 with 
the aid of an AGC circuit 9. 
The output terminal of the differentiator 10 is connected to one input 
terminal of a multiplier 11, the other input terminal of which is 
connected to the output terminal of the AM detector 8. The output terminal 
of the multiplier 11 is connected to the other input terminal of the adder 
12, the output terminal of which is connected to a stereo demodulator 4. 
The stereo demodulator 4 provides a left channel output and a right 
channel output, respectively, at a left channel output channel 5 and a 
right channel output terminal 6. 
The operation of the above-described circuit will now be described. 
The case will be considered where one reflected signal is present in the 
received FM signal and the FM signal is in the monaural mode. In this 
case, the output of the FM detector 3 is as shown in FIG. 4A and an IF 
signal outputted by the front end 1 is as shown in FIG. 4B. It can be 
understood that a certain relation exists between the signals shown in 
FIGS. 4A and 4B. In accordance with the invention, this relation is 
utilized to reduce the amount of multipath distortion. 
The envelope of the output of the front end 1 (shown in FIG. 4B) is 
detected to form a distortion cancelling signal. The multipath distortion 
is decreased by subtracting the distortion cancelling signal thus formed 
from the FM detection output. 
In more detail, first the output of the front end 1 is applied to the 
second 10.7 MHz BPF, which has a flat in-band amplitude characteristic, to 
remove unwanted interference components therefrom. The output thus treated 
is applied to the AM detector 8 under automatic gain control to detect the 
envelope of the IF signal. The envelope signal, as shown in FIG. 4C, has a 
constant amplitude irrespective of the presence or content of the IF 
signal due to the automatic gain control effect applied thereto. 
As is apparent from FIG. 4A, the distortion appearing in the FM detection 
output is inverted every half period of the fundamental wave. Therefore, a 
cancelling signal can be obtained by inverting the AM detection output 8 
(FIG. 4C) every half period. 
The amount of distortion generally increases with the modulation frequency. 
Therefore, it is necessary to increase the amplitude of the cancelling 
signal as the modulation frequency increases. For this purpose, the 
differentiator 10 and the multiplier 11 are provided as shown in FIG. 2. 
The differentiator 10 has a gain-phase characteristic as shown in FIG. 3. 
The output of the FM detector 3 is shifted by 90.degree. in phase when 
applied to the differentiator 10 to convert it to a waveform as shown in 
FIG. 4D. This signal and the signal shown in FIG. 4C are subjected to 
multiplication by the multiplier 11 to obtain the cancelling signal shown 
in FIG. 4E. The cancelling signal is applied to the adder 12 where it is 
subtracted from the detection output shown in FIG. 4A. As a result, a 
signal in which the multipath distortion has been substantially reduced is 
provided at the output terminal of the adder 12. 
The distortion reducing effect will be described mathematically. 
If a single reflection is involved in the multipath-distorted signal, the 
distortion component can be expressed as follows: 
##EQU1## 
where x=ratio in magnitude of direct wave to reflected wave, .tau.=delay 
time, m=modulation factor, f=75 KHz, .omega..sub.0 =carrier frequency, and 
p=modulation frequency. 
FIG. 5 is a graphical representation showing the multipath distortion 
characteristic described by the above expression in which the horizontal 
axis represents time and the vertical axis represents the magnitude of the 
multipath distortion. 
When the IF signal envelope outputted by the AM detector 8 is multiplied by 
the differential FM detection output outputted by the differentiator 10 as 
shown in FIG. 2, the above-described expression is converted as follows: 
##EQU2## 
A characteristic plotted according to this expression is as shown in FIG. 
6. As is apparent from a comparison of FIGS. 5 and 6, the characteristic 
of FIG. 6 is similar to that of FIG. 5. Thus, the reduction in the amount 
of multipath distortion brought about with the use of the invention has 
been proven. 
While the invention has been described with reference to a monaural signal 
broadcast, the technical concept of the invention is similarly applicable 
to the case of a stereo signal broadcast. 
In the above-described embodiment, only the multipath distortion decreasing 
circuit is shown. However, a small amount of high blending may be effected 
by connecting a small-value capacitor between the right channel output 
terminal and the left channel output terminal. Furthermore, in the 
above-described embodiment, the differentiator 10 receives the output of 
the FM detector 3. However, it may receive the output of the adder 12 as 
shown in FIG. 7. 
As is apparent from the above description, the mulipath distortion reducing 
circuit according to the invention is so designed that a multipath 
distortion cancelling signal is formed by detecting the envelope of the IF 
signal, thereby to decrease the multipath distortion. Therefore, with the 
circuit of the invention, the amount of multipath distortion can be 
decreased without lowering the degree of separation.