JPH0354493B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit for reducing multipath distortion in an FM tuner.

Conventionally, there has been a circuit of this type as shown in FIG.

In the figure, the output of front end 1 is
10.7M band pass filter (hereinafter referred to as BPF)
is connected, its output is connected to the input of FM detector 3, its output is connected to the input of stereo demodulator 4, and its output is Lch output 5, Rch output
The output is output 6, and a series connection of a capacitor C and a switch SW is connected between these two outputs.

In such a configuration, when multipath occurs and noise increases and it becomes difficult to hear, the Lch output 5
and Rch output 6.
By turning on the SW, a harsh Lch output,
The high frequency component with the opposite phase in the Rch output is capacitor C.
Since the signal is shot at , it is canceled and the noise is reduced to the auditory sense.

Conventional circuits are configured as described above, so the higher the frequency of the signal component, the more
The disadvantage was that the mixing ratio of Rch increased and the separation (degree of separation) deteriorated significantly.

Also, when the level of the reflected wave is higher than the level of the direct wave, that is, when the DU ratio, which is the level ratio of both waves, is reversed, the way multipath distortion occurs is
Since the distortion is different from the case where the DU ratio is not reversed, the effect of reducing multipath distortion has been reduced.

This invention was made in order to eliminate the drawbacks of conventional circuits, and provides a circuit that can reduce multipath distortion by using a signal with the phase of the cancel signal reversed when the DU ratio is reversed. It is intended to provide.

Hereinafter, one embodiment of the present invention will be described based on the drawings.

In Figure 2, the output of front end 1 is
Connected to the inputs of 10.7MHzBPF2 and 10.7MHzBPF7. Further, the output of the 10.7MHzBPF2 is connected to the input of the FM detector 3.

Further, the output of the FM detector 3 is connected to an input of a differentiator 10, one input of an adder 12, and one input of an adder 14. Also, the output of 10.7MHzBPF7 is
It is connected to the input of AM detector 8, and AGC 9 is applied from the output of AM detector 8 to the AM detector.

Next, the output of the differentiator 10 is connected to one input of the multiplier 11, and the output of the AM detector 8 is connected to the other input of the multiplier 11.

Next, the output of the multiplier 11 is connected to the other input of the adder 12 and the input of the inverter 13.
The output of is connected to the other input of adder 14.
Further, the output of the adder 12 is connected to one input of the switch 15 and the input of the 57KHz level detector 16, and the output of the adder 14 is connected to the other input of the switch 15 and the input of the 57KHz level detector 17. Connected to input.

Further, the output of the 57KHz level detector 16 is connected to one input of the comparator 18, the output of the 57KHz level detector 17 is connected to the other input of the comparator 18, and the output of the comparator 18 is connected to the input of the switch driver 19. The switch 15 is controlled by the switch driver 19. The output of the switch 15 is sent to the stereo demodulator 4.
The output is connected to the input of Lch output 5, Rch
The output will be 6.

Next, the operation of this invention will be explained. first
Considering the case when the DU ratio is not reversed,
For example, when the delayed signal is monaural and has one wave, due to multipath distortion, the output of the FM detector 3 will have a protruding whisker for each half wave of the sine wave, which is the FM detected fundamental wave, as shown in Fig. 4 A. The waveform is made up of alternating lines and whiskers. Furthermore, the IF signal that is the output of the front end 1 is as shown in FIG. 4B, and it can be seen that A and B are related. The present invention uses this relationship to reduce multipath distortion.

In other words, by detecting the envelope shown in Figure 4 (b) of the output of the front end 1, using it to create a distortion cancel signal, and subtracting it from the FM detection output, the distortion will be reduced.

First, the output of the front end 1 is passed through a 10.7 MHz BPF 7 filter with a flat amplitude characteristic within the band to remove unnecessary interference. Next, the envelope of the IF signal is detected by passing it through an AM detector 8 including an AGC 9. Since this envelope signal is subjected to AGC, it remains constant regardless of the IF signal input. This is shown in Fig. 4 (c).

Also, as you can see from the distortion of the FM detection output 3 shown in Figure 4A, the way the distortion appears is reversed every half cycle of the fundamental wave, so the AM detection output 8 in Figure 4C
By inverting every half cycle, a cancel signal is created. Furthermore, the higher the modulation frequency, the greater the distortion, so the higher the modulation frequency, the larger the cancel signal must be.

The differentiator 10 in Fig. 2 performs these operations.
and a multiplier 11. As shown in Fig. 3, the gain-phase characteristics of the differentiator 10 increase linearly with the frequency of the input signal, and the phase characteristics have a constant 90° phase shift regardless of the frequency. have. Therefore, the output of the FM detector 3 is the differentiator 10
As shown in FIG. 4D, a waveform whose phase is shifted by 90 degrees from the fundamental waveform with distortion shown in FIG. 4A is obtained. Next, when the output of the AM detector 8 (FIG. 4 C) and the output of the differentiator 10 (FIG. 4 D) are applied to the multiplier 11, when the output of the differentiator 10 is positive, the multiplier The output of 11 is the output of AM detector 8 as it is, and when the output of differentiator 10 is negative, it is output from multiplier 11.
The output of the AM detector 8 is inverted and output. Therefore, the output of the multiplier 11 is a distorted waveform whose phase is inverted every half wave of the fundamental wave, as shown in FIG. 4E. The waveform of this distortion becomes a cancel signal, and the adder 12 subtracts this cancel signal from the output of the FM detector 3 in Fig. 4A. As a result, multipath distortion is reduced in the output of the adder 12. A waveform is obtained.

The above is an explanation of the cancel effect caused by waveforms.

Next, we will show how distortion can be canceled using a calculation formula.
The formula for multipath distortion is, assuming that there is one delayed wave, the FM direct wave I _FM is I _FM = Isin (ωot + Δω/p sinpt), whereas the reflected wave delayed by time τ is I ₀
I ₀ =I′sin(ω ₀ (t−τ) +Δω/p·sinp(t−τ)), and the combined wave of these two equations generates distortion due to phase modulation. Here, I, I' are the respective amplitudes of the FM signals, ω ₀ is the carrier frequency, Δω
is the angular frequency shift and p is the modulation frequency.

Therefore, the equation for distortion due to multipath is expressed by the following equation.

d=2mpsinpτ/2・sinp(t−τ/2
)/1+1/x+cos〓wo〓+2mfsinp〓/2・cosp(t
-τ/2)/x+cos〓+2mfsinp〓/2・cosp(t
-τ/2) Here, x = DU ratio (ratio of magnitude of direct wave and reflected wave), τ = delay time, m = modulation degree, = 75K
Hz, W ₀ = carrier frequency, p = modulation frequency, which is plotted in Figure 5 with time on the horizontal axis and distortion magnitude on the vertical axis.
If we draw a COS curve for this 400Hz fundamental wave from 0msec for 2.5msec during one period of Hz, we get the 4th
Similar to the waveform in Figure A, whiskers that jump in and whiskers that pop out appear alternately. It also appears that the smaller the DU ratio, the larger the distortion.

Also, the output of AM detector 8 in Fig. 2 is
When the envelope of the IF signal is multiplied by the differential of the FM detection output, which is the output of the differentiator 10, the output of the FM detector 3 is f(p), and the differentiator 10
Since the output of is df(p)/dt, the distortion formula is:
The following formula is obtained.

This equation is plotted in FIG. 6, and when this figure is compared with FIG. 5, it is found that they are similar, which indicates that multipath distortion can be reduced. That is, the DU in Figures 5 and 6
For the curves with ratios of −3 dB and −6 dB, time series
There is a large difference between the sharp fall and sharp rise waveform shown in FIG. 5 and the smoothly rising and falling waveform shown in FIG. 6, and there is little hope of canceling or reducing multipath distortion. However, in Figure 5 -
The 10 dB curve has a smooth distortion waveform, chronologically up to 1.3 msec and then 2.5 msec.
Since the phase rise and fall of the distortion waveforms up to sec are almost the same in both figures, by adding these two waves to the adder 12, a certain degree of canceling effect is exerted and multipath distortion is reduced. be able to. In actual experiments, a distortion reduction effect of approximately ten-odd dB was achieved at a DU ratio of -10 dB.

Although the above description has been made using an example of monaural signal broadcasting, the same can be said when stereo signal broadcasting is performed, and multipath distortion can be reduced. Next, consider the case where the DU ratio is reversed. In this case, the multipath distortion appears in the opposite phase compared to when the DU ratio is not reversed, so if the cancel signal that is the output of the multiplier 11 is inverted, even if the DU ratio is reversed, the multipath distortion will appear in the opposite phase. Path distortion can be reduced. The inverter 13 and adder 14 perform this.

Therefore, in FIG. 2, when the DU ratio is not reversed, the output of the adder 12 is sent to the stereo demodulator 4.
input to the adder 14 when the DU ratio is reversed.
If the output of DU is input to the stereo demodulator 4, then DU
Multipath distortion can be reduced regardless of whether the ratio is reversed or not.

Additionally, the 57KHz level detector 1 automatically switches the outputs of the adders 12 and 14.
6, 17, a comparator 18, a switch driver 19, and a switch 15. This takes advantage of the fact that when there is a lot of multipath distortion, the harmonic components of the 19KHz pilot signal (57KHz in this case) increase.

First, when the DU ratio is not reversed, adder 12
Since the distortion of the output of
The output of the filter 17 is smaller, and the output of the comparator 18, switch driver 19, and switch 15
, the output of the adder 12 is connected to the input of the stereo demodulator 4.

Next, when the DU ratio is reversed, the distortion of the output of the adder 14 is less than the distortion of the output of the adder 12, so the output of the 57KHz filter 16 is smaller than the output of the 57KHz filter 17, and the comparator 18, the output of the adder 14 is output to the stereo demodulator 4 by the functions of the switch driver 19 and the switch 15.
connected to the input of

As a result of the above, the output of the adder with less multipath distortion is automatically input to the stereo demodulator 4, and multipath distortion can be reduced whether or not the DU ratio is reversed.

Note that in the above embodiment, the inverter 13 is the multiplier 11.
The output of the AM detector 8 is connected to the output of the AM detector 8 to create a signal with the phase of the cancel signal reversed.In order to create this phase reversed signal, for example, an inverter may be connected to the output of the AM detector 8. Differentiator 10
The same effect can be obtained by connecting it to the output of
However, in this case, one extra multiplier is required.

Further, although the frequency of the level detectors 16 and 17 is set to the third harmonic (57 KHz) of the pilot signal, it may be set to another harmonic frequency.

As described above, according to the present invention, since a signal obtained by inverting the multipath cancel signal is also generated, multipath distortion can be reduced even when the DU ratio is reversed. Furthermore, since the harmonic components of the pilot signal are extracted and the switching device is controlled accordingly, multipath distortion can be automatically reduced even when the DU ratio is reversed or otherwise. Furthermore, since there is no need for a delay circuit, which is generally required for canceling multipath distortion, the configuration is simple and an effective distortion reduction effect can be obtained.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a conventional circuit, Fig. 2 is a diagram showing an embodiment of the present invention, Fig. 3 is a diagram showing characteristics of the differentiator shown in Fig. 2, and Fig. 4 A to E are diagrams showing the circuit in Fig. 2. Figure 5 is a diagram showing the characteristics based on the calculation results of multipath distortion, Figure 6 is the AM of the circuit shown in Figure 2.
FIG. 7 is a diagram showing characteristics based on calculation results obtained by multiplying a detector output and a differentiator output. 1...Front end, 2,7...Band pass filter, 3...FM detector, 4...Stereo demodulator, 5...Lch output, 6...Rch output, 8...
AM detector, 9...AGC, 10...differentiator, 1
1... Multiplier, 12, 14... Adder, 13...
Inverter, 15... Changeover switch, 16, 17...
...Level detector, 18...Comparator, 19...
...Switch driver.

Claims (1)

[Claims] 1. An AM detection circuit that detects the envelope component of the FM signal subjected to multipath interference, an AGC circuit that makes the output of this AM detection circuit constant regardless of the magnitude of the FM signal input, and demodulates the FM signal. An FM detection circuit, a differentiator that shifts the phase of the output of this FM detection circuit by 90 degrees and increases the level as the frequency increases, and the output of the differentiator is multiplied by the output of the AM detection circuit. a multiplier that generates a cancel signal; a first adder that adds the cancel signal and the output of the FM detection circuit; an inverter that outputs a signal whose phase is reversed with respect to the cancel signal; and the inverter. and a switching means for selectively switching the output of the first or second adder for input to a stereo demodulator. A multipath distortion reduction circuit featuring: