JPH0354491B2 - - Google Patents

Description

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

As a circuit of this type, the present applicant proposed a multipath reduction circuit as shown in FIG. 1 in Japanese Patent Application No. 122505/1982.

In the figure, the output of front end 1 is
It is connected to 10.7MHzBPF2 and second 10.7MHzBPF7. Further, the output of the 10.7MHzBPF2 is connected to the FM detector 3. This FM detection output is connected to one input of a differentiator 10 and an adder 12. In addition, the output of 10.7MHzBPF7 is output from AM detector 8.
is connected to the input of the AM detector 8, and the AM
AGC9 is applied to the detector.

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

Further, the output of the multiplier 11 is connected to the other input of the adder 12, and the output of the multiplier 12 is connected to the stereo demodulator 4. The outputs of the stereo demodulator 4 are an Lch output 5 and an Rch output 6.

Next, the operation of this configuration will be explained.

First, let's consider, for example, the case where the delayed signal is one wave in monaural. At this time, the output of FM detector 3 is
It looks like A in the figure. Furthermore, the IF signal that is the output of the front end 1 is as shown in FIG. 3B, and it can be seen that there is a relationship between A and B. The present invention uses this relationship to reduce multipath distortion.

In other words, by detecting the envelope shown in Figure 3 (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. This envelope signal is
Since AGC is applied, it is constant regardless of the IF signal input. This is shown in Figure 3 (c).

Also, as you can see from the distortion of the FM detection output 3 shown in Figure 3A, the distortion output is inverted every half cycle of the fundamental wave, so the AM detection output 8 in Figure 3C is inverted every half cycle. When inverted, 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 and multiplier 11 in FIG. 1 perform these functions. As shown in FIG. 2, the gain-phase characteristics of the differentiator 10 are such that the gain characteristics increase linearly with respect to the frequency of the input signal, and the phase characteristics have a constant 90° phase shift regardless of the frequency. When the output of the FM detector 3 passes through the differentiator 10, it is phase-shifted by 90 degrees and becomes the waveform shown in Figure 3 (d), and when this and the waveform shown in Figure 3 (c) are multiplied by the multiplier 11, the output as shown in Figure 3 is obtained. The cancel signal shown in E is obtained.

The multiplier 12 subtracts this cancel signal from the detection output shown in FIG. 3A, and as a result, a waveform with reduced multipath distortion is obtained at the output of the adder 12. 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 equation for multipath distortion is expressed by the following equation, assuming that there is only one delayed wave.

d=2mfpsinpτ/2・sinp(t−τ/2
)/1+1/x+cos〓/2mfsin〓/2・cosp(t
−τ/2)/x+cos〓+2mf+sinp〓/2・cosp(
t-τ/2) Here, x = DU ratio (ratio of magnitude of direct wave and reflected wave), τ = delay time, m = modulation degree, f = 75KHz,
_w0 =carrier frequency, p=modulation frequency, which are plotted in FIG. 4 with time on the horizontal axis and distortion magnitude on the vertical axis. In this figure 4, the time 2.5 msec is one cycle period of the modulation frequency 400 Hz, and assuming a COS curve from 0 msec to 400 Hz, when the waveform is downward in figure 3 A, the drop distortion is,
It can be seen that the upstream waveform corresponds to the case where jump-out distortion occurs.

Also, the output of AM detector 8 in Fig. 1 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 following equation is obtained.

This equation is plotted in Figure 5. Comparing this figure with Figure 4 shows that at a relatively large DU ratio of -10 dB, the drop at 3.5 msec and 9.2 msec and the rise at 6 msec are similar. This shows that multipath distortion can be reduced.

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.

Since this multipath distortion reduction circuit is configured as described above, the characteristics required of the differentiator 10 during stereo broadcasting are frequencies ranging from DC to 53KHz,
It is required that the phase is always shifted by 90 degrees and the gain increases with a curve of 6 dB/octave.

However, it is quite difficult to manufacture a differentiator with such characteristics, and the drawback is that if a differentiator that is optimal for the main signal region is used, the canceling effect will be reduced in the sub-signal region. It was hot. Conversely, if a differentiator that is optimal for the sub-signal region is used, the canceling effect will be reduced for the main signal region.

This invention is a further improvement of the multipath distortion reduction circuit configured as described above, and by configuring the cancel system separately for main signals and sub signals, a circuit that can more effectively exhibit the cancel effect even when receiving stereo signals is created. is intended to provide.

Hereinafter, an embodiment of the present invention will be explained based on the drawings.

In FIG. 6, the main signal cancel system has the same configuration as that in FIG. 1, and the output of the adder 12 is connected to one input of the matrix 17, so the configuration for the sub signal will be explained.

The output of the FM detector 3 is connected to the input of the differentiator 13 and one input of the adder 15. Further, the output of the differentiator 13 is connected to one input of the multiplier 14, the other input of the multiplier 14 is connected to the output of the AM detector 8, and the output of the multiplier 14 is connected to the other input of the adder 15. connected to the input of The output of adder 15 is connected to sub-signal detector 16, which is connected to the other input of matrix 17.
The outputs of the matrix 17 are Lch output 5 and Rch output 6.

In the above embodiment, the signals that generate the main signal cancel signal and the sub signal cancel signal are
Although the output of the FM detector 3 is used, it may be taken from the outputs of the first and second adders 12 and 15 after canceling the distortion of the main and sub signals, respectively, as shown in FIG. It has the effect of

Next, the operation of this invention will be explained.

As mentioned above, the main signal cancellation system is composed of the differentiator 10, the multiplier 10, the multiplier 11, and the adder 12, and performs multipath cancellation of the main signal. Since the sub signal has a high frequency, it is removed by an output filter and de-emphasis, and is connected to the matrix 17 as it is.

Next, the sub signal cancel system will be explained. The subsignal of the stereo signal is the subcarrier (L-
R) Since the signal is AM modulated, its frequency range is from 23kHz to 53kHz, and the differentiator 13 uses the second phase and gain characteristics in this frequency band.
As shown in the figure, the gain increases linearly with frequency, and the phase only needs to have a constant 90° phase shift.
For example, it can be obtained by connecting capacitors and resistors with excellent frequency characteristics in series and outputting from the connection point. Therefore, if the output of the differentiator 13 and the output of the AM detector 9 are added to the multiplier 14, as shown in FIG. 3 for the main signal, the basic waveform shown in FIG. When the output of the differentiator 13 whose phase is shifted by 90° from A (Fig. 3 D) is positive,
The output of the AM detector 9 (FIG. 3C) is output as is, and when it is negative, the output of the AM detector 9 is inverted and output, resulting in the waveform shown in FIG. 3E. By subtracting this waveform and the output of the FM detector 3 by the adder 15, distortion in the sub-signal region can be canceled and reduced.

Further, the sub-detector 16 detects only the sub-signal, and outputs a sub-signal (L-R) in which multipath distortion is canceled. Since the main signal (L+R) is obtained by the main cancel system, the matrix 17 provides Lch output and Rch output with reduced multipath distortion.

Regarding FIG. 7, the output of the differentiator 10 and
The action of the multiplier 11 to which the output of the AM detector 8 is input, and the action of the multiplier 14 to which the output of the differentiator 13 and the output of the AM detector 8 are input are the multiplications shown in FIG. It is almost the same as vessels 11 and 14,
As a result, the multipath distortion of the main signal and the sub signal is reduced, and the matrix circuit 17
or to the matrix circuit via the sub-detector 16 to reduce multipath distortion.
Output, Rch output can be obtained.

As described above, according to the present invention, since the multipath distortion canceling system is configured separately for the main signal area and the sub signal area, the configuration of the differentiators 10 and 13 is simplified, and stereo Good multipath distortion reduction effects can also be obtained for signals. Additionally, since it does not require the multi-stage delay circuit commonly used to cancel multipath distortion, distortion cancellation is not perfect, but it is possible to reduce distortion by more than 10 dB in stereo signals, which is extremely effective in practical use. You can get a great effect.

[Brief explanation of drawings]

Figure 1 is a diagram showing the circuit proposed by the applicant, Figure 2 is a diagram showing the characteristics of the differentiator in Figure 1, Figure 3 is a diagram showing waveforms of each part of the circuit in Figure 1, and Figure 4. is a diagram showing the characteristics based on the calculation results of multipath distortion, Figure 5 is a diagram showing the characteristics based on the calculation results obtained by multiplying the AM detection output of the circuit in Figure 2 by the differentiator and the force, and Figures 6 and 7 are FIG. 3 is a diagram showing an example of the present invention, respectively. 1...Front end, 2,7...Band pass filter, 3...FM detector, 5...Lch output,
6...Rch output, 8...AM detector, 9...
AGC, 10, 13... Differentiator, 11, 14...
Multiplier, 12, 15...adder, 16...sub-detector.

Claims (1)

[Scope of Claims] 1. An AM detection circuit that detects an envelope component of an 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, and an FM detection circuit that demodulates the FM signal; a first differentiator that shifts the main signal region of the output of the FM detection circuit by 90 degrees and makes the output level larger as the frequency increases; a first multiplier that multiplies the output of the differentiator and the output of the AM detection circuit to generate a first cancel signal; and a first multiplier that adds the first cancel signal and the output of the FM detection circuit. an adder, a second differentiator that shifts the sub-signal region of the output of the FM detection circuit by 90 degrees and makes the output level larger as the frequency increases; and the output of the second differentiator. a second multiplier that multiplies the output of the AM detection circuit to generate a second cancel signal; a second adder that adds the second cancel signal and the output of the FM detection circuit;
a sub-detector that detects the sub-signal region of the output of the second adder; and a sub-detector that receives the main signal output from the first adder and the sub-signal output from the sub-detector, and detects the left and right sub-detectors. A multipath distortion reduction circuit for stereo signals, comprising a matrix circuit for obtaining each output.