JPH0416971B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a multipath interference detection device in an FM (frequency modulation system) receiver.

The multipath interference detection device of the present invention is used, for example, in a car-mounted FM stereo broadcast receiver, and the multipath interference detection signal is received by diversity reception, which switches the receiving antenna when multipath interference occurs, for example. It is used as a signal to detect the occurrence of multipulse interference in a circuit or a multipath distortion removal circuit that performs signal processing to remove multipath distortion.

[Conventional technology]

Multipath interference in FM broadcasting is generally caused by interference between direct waves from the broadcasting station and reflected waves reflected by mountains, buildings, etc. When this multipath interference occurs, harmonic components and side spectrum components of the sum signal, difference signal, and pilot signal (pilot subcarrier) are generated in the composite signal. Therefore, conventional detection devices for detecting multipath interference include those that detect the third harmonic (57kHz) of the pilot signal, or those that detect high-frequency noise components contained in the composite signal after demodulation. etc. have been proposed.

[Problem that the invention seeks to solve]

The former method of detecting third-order harmonics has the problem that it cannot be used in Western countries because subcarriers with a frequency of 57 kHz are used in the ARI system that sends traffic information and identification signals for SCA services.

In addition, the latter method of detecting high-frequency noise components tends to cause malfunctions when a multipath interference detection device is used in an on-vehicle receiver because the ignition noise emitted from the vehicle's ignition system is largely distributed in the high-frequency range. There is a problem.

[Means for solving problems]

In order to solve the above problems, the multipath interference detection device in the frequency modulation receiver of the present invention extracts the side spectrum component of the pilot subcarrier from the composite signal after detecting the frequency modulated wave. The apparatus includes an extraction means and a comparison means for detecting multipath interference by comparing the level of the extracted side spectrum component with a reference value.

[Effect]

When multipath interference occurs in the FM received wave, a side spectrum component is generated in the pilot subcarrier in the composite signal. Therefore, this side spectrum component is extracted by an extraction means,
The occurrence of multipath interference can be detected by detecting by the comparison means that the value has exceeded a certain reference value.

〔Example〕

A multipath interference detection device in an FM receiver as an embodiment of the present invention will be described below.

FIG. 1 is a block diagram showing an embodiment of a multipath interference detection device used in an FM stereo broadcast receiver. In Figure 1, input terminal 1 has
A composite signal S1 demodulated by an FM detector is input. This composite signal S1 is a sum signal (L+R) of left and right channels, and a difference signal (L-R).
This includes both sidebands, pilot signals, SCA signals, etc.

The composite signal S1 is then input to a low pass filter 2. This low-pass filter 2 has a cutting frequency
It is 20kHz. Output signal S from low-pass filter 2
2 is further guided to a band elimination filter 5 via a band filter 3 and an amplifier 4. Band filter 3 is
It has a pass band width of about 19 kHz±1 kHz, while the band elimination filter 5 has a stop band width of about 19 kHz±250 Hz. In this way, the stop band characteristic of the band elimination filter 5 needs to be sharper than the pass band characteristic of the band filter 3.

The output signal S3 from the band elimination filter 5 is guided to an envelope detector 6, and its detection output signal S4 is guided to a comparison input terminal of a comparator 7. The reference input terminal of the comparator 7 receives the DC reference voltage E from the reference power supply 8.
(r) is derived. The comparison result output of the comparator is sent to the output terminal 9 as a multipath distortion detection signal S5.

The operation of the FIG. 1 apparatus will now be described.

The modulation conditions are stereo L=R. Therefore, the (LR) component is 0%. First, the main signal (L+
The demodulated waveform for the R signal is shown in FIG. The demodulated waveform is a waveform in which a 19kHz pilot signal is superimposed on the L+R signal. The frequency spectrum of this demodulated signal is shown in FIG. As is clear from FIG. 3, only the spectra of the L+R signal and the pilot signal appear in the normal state.

Next, the demodulated waveform in the case of multipath interference is shown in FIG. Multipath distortion occurs noticeably when the phase between the direct wave and the reflected wave of the FM wave reaches π radians, and in this case, the amplitude level of the pilot signal changes with the frequency period of the L+R signal. FIG. 5 shows the pilot signal waveform at the time of multipath interference. As shown in FIG. 5, paying attention to the pilot signal, it can be considered that the pilot signal is amplitude-modulated with the period of the L+R signal. Further, FIG. 6 shows the spectrum of the demodulated signal when there is multipath interference. In this case, harmonics of the L+R signal, harmonics of the pilot signal, and side spectra are generated.

It is now assumed that the composite signal S1 after FM detection when multipath interference occurs is input to the apparatus shown in FIG. 1, and the operation of the apparatus shown in FIG. 1 at this time will be described below with reference to FIG. Figure 7 is the first
1 is a signal waveform diagram of each part in the device, 1 is the output signal S2 of the low-pass filter 2, 2 is the output signal S3 of the band elimination filter 5, 3 is the detected output signal S4 of the detector 6, and 4 is the output signal of the comparator 7. The output signals S5 are shown respectively.

The input composite signal S1 is passed through a low-pass filter 2, where unnecessary high-frequency components of 20 kHz or more are removed to obtain an output signal S2 in a band including the L+R signal and the pilot signal. This output signal S2 is the fourth
As shown in the figure, the pilot signal is distorted near each peak of the L+R signal.

The output signal S2 of the low-pass filter 2 is then input to a band filter 3 to extract the pilot signal and side spectrum components in the vicinity thereof, further amplified by an amplifier 4, and then input to a band elimination filter 5. Band-stop filter 5 attenuates the pilot signal component in order to improve detection sensitivity to side spectrum components.

An example of frequency versus gain characteristics of a filter circuit including the bandpass filter 3, amplifier 4, and bandstop filter 5 is shown in FIG. In Figure 8, the vertical axis represents gain [dB], the horizontal axis represents frequency [Hz], and the overall characteristic curve represents the pilot signal frequency (19k
Hz) part is suppressed. The output signal S3 that has passed through this filter circuit becomes a signal containing many side spectrum components, and has a waveform in which the distorted portion of the pilot signal is particularly emphasized, as shown in FIG. 72. Note that the band elimination filter 5
By selecting an appropriate attenuation amount, it is possible to make the rising edge of the waveform of the distorted portion steep as shown in FIG. 7, thereby improving the response when multipath interference occurs. I can do it.

The filter output signal S3 is envelope-detected by a wave detector 6 to obtain a detection output signal S4, which is input to a comparator 7 and converted to a reference voltage E(r).
compared to As a result, an output signal of a certain level or higher is detected as shown by the broken line in FIG.
5 to the output terminal 9.

FIG. 9 shows an application example in which the multipath interference detection device of the present invention is used in a diversity type FM receiver. This application example device includes two antennas 11a and 11b for diversity, an antenna switching circuit 12 for selecting one of the antennas, a front end circuit 13, an IF amplification/demodulation circuit 14, a stereo demodulation/amplification circuit 15, and two left and right antennas. 16 speakers
1, 16r, a multipath interference detection circuit 17 according to the present invention into which the composite signal from the IF amplification and demodulation circuit 14 is input, and the antenna switching circuit 12 is switched according to the multipath distortion detection signal from the detection circuit 17. It is configured to include a switching control circuit 18 for controlling.

In this application example device, when multipath interference occurs in the FM received wave, the IF amplification and demodulation circuit 14
The multipath distortion component contained in the composite signal output from the receiver is detected by the multipath interference detection circuit 17, and the receiving antenna is switched by the switching control circuit 18 and antenna switching circuit 12 to create so-called spatial diversity. realized,
Thereby, the influence of multipath interference can be reduced.

Various modifications are possible in implementing the invention. For example, the present invention is not limited to the embodiment shown in FIG. 1, and may be modified, for example, by reversing the positions of the bandpass filter and band-stopping filter in FIG. 1, or by removing the low-pass filter or amplifier. Modifications are also possible. Furthermore, the overall performance of the filter circuit is not limited to that shown in FIG.

〔Effect of the invention〕

According to the present invention, the occurrence of multipath interference can be detected without using the third harmonic (57kHz) of the pilot signal or high-frequency noise components.
It can also be used as a receiver for broadcast reception such as ARI systems and SCA services, and even when the receiver is installed in a car, there will be no malfunction of multipath interference detection due to ignition noise, etc. Furthermore, since the waveform for multipath distortion detection can be made steep, the response characteristics when multipath disturbance occurs can be improved and detection can be performed with a shorter detection delay time.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a multipath interference detection device in an FM receiver as an embodiment of the present invention, Fig. 2 is a demodulation waveform diagram in a normal state, and Fig. 3 is a frequency spectrum diagram of the waveform shown in Fig. 2. , Figure 4 is a demodulation waveform diagram when there is multipath interference, Figure 5 is a waveform diagram showing the waveform in Figure 4 focusing only on the pilot signal, Figure 6 is a frequency spectrum diagram of the waveform in Figure 4, Fig. 7 is a signal waveform diagram of each part of the device shown in Fig. 1;
FIG. 8 is a general characteristic diagram of the filter circuit in the device shown in FIG. 1, and FIG. 9 is a block diagram showing an example of application of the present invention. 2...Low pass filter, 3...Band filter, 4
... Amplifier, 5 ... Band elimination filter, 6 ... Detector, 7 ... Comparator, 8 ... Reference power supply.

Claims (1)

[Scope of Claims] 1. A multipath interference detection device in a frequency modulation receiver, which includes extraction means for extracting a side spectrum component of a pilot subcarrier from a composite signal after detecting a frequency modulated wave; 1. A multipath interference detection device for a frequency modulation system receiver, comprising comparison means for detecting multipath interference by comparing the level of the extracted side spectrum component with a reference value. 2. The extracting means includes a band filter that passes a band including the pilot subcarrier and its side spectrum components, and a band cancellation filter that removes the pilot subcarrier component, and the band filter and the band cancellation filter 2. The device of claim 1, wherein the device is configured to allow a composite signal to be passed through.