JPH0347614B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a multipath interference cancellation circuit for removing impulsive pulse noise caused by multipath interference. In general, radio waves emitted from broadcasting stations take multiple radio wave paths (multipaths) due to the influence of obstacles such as terrain and buildings, and distortion and noise occur within the receiver due to mutual interference between direct waves and indirect waves. occurs. There are various types of interference caused by multipath, but the present invention aims to achieve good reception by removing pulse noise. In a vehicle-mounted receiver, the reception environment changes from moment to moment as the vehicle moves. Therefore, when a car passes through a point where multipath interference is particularly severe, the 19KHz stereo pilot signal included in the received signal is interfered with, as shown in Figure 1, and its amplitude increases in a pulse-like manner.
The phase is also disturbed. Therefore, when a pilot signal subjected to interference as shown in FIG. 1 is applied to a stereo multiplex circuit, a large pulse noise is generated, which causes discomfort to the listener. This is because saturation occurs in the signal path due to the increase in the level of the pilot signal itself, generating noise in the audible band, and because the phase of the pilot signal is disturbed, the 38KHz switching created from the pilot signal This is because the phase of the signal is also disturbed, a beat occurs between the pilot signal and the switching signal in the stereo multiplex circuit, and noise is generated due to this beat. The present invention has been made in view of the above points, and aims to remove pulse noise caused by multipath interference by detecting the occurrence of multipath interference and removing the pilot signal during that period. be. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In Figure 2, 1 is an antenna for receiving FM stereo broadcast signals, and 2 is an antenna for receiving FM stereo broadcast signals.
RF (radio frequency) amplification circuit; 3 is a mixing circuit that mixes the output signal of the RF amplification circuit 2 and the output signal of the local oscillation circuit 4 to generate an IF (intermediate frequency) signal; 5 is a mixing circuit that generates the IF signal; IF amplification circuit for amplification, 6
is an FM that detects the output signal of the IF amplifier circuit 5.
a detection circuit; 7 is a stereo multiplex circuit that separates the low frequency signal obtained at the output terminal of the FM detection circuit 6 into left and right stereo signals; 8 is a coil 9 and a capacitor 10;
1 is a series resonant circuit tuned to the pilot signal in order to remove the pilot signal in the signal passing through the signal transmission path when it is on; 12 is connected to the FM detection circuit 6, and the signal passes through the FM detection circuit 6; Approximately 100KHz to 500K to extract high frequency components in the signal
13 is a first buffer amplifier circuit; 14 is a level detection circuit for detecting the level of the output signal of the buffer amplifier circuit 13; 15 is a second buffer amplifier circuit;
is a differentiating circuit for differentiating the output signal of the second buffer amplifier circuit 15, which is composed of a capacitor 17 and a resistor 18, and is configured so that the output of the differentiating circuit 16 drives the control transistor 11. Assuming that the received signal is not subject to multipath interference, the RF signal received by antenna 1 is
It is amplified by the RF amplification circuit 2, converted to an IF signal by the mixing circuit 3, and then amplified by the IF amplification circuit 5.
The detection circuit 6 performs FM detection. In this case, since there is no multipath interference, there are few high-frequency noise components in the received signal, and there are few signals extracted by the bandpass filter 12 , and the level detection circuit 14
The output signal of is small, and its differential output cannot drive the control transistor 11, so the control transistor 11 is turned off. For that reason,
The series resonant circuit 8 does not operate, and all output signals from the FM detection circuit 6 are sent to the stereo multiplex circuit 7.
is applied to Stereo multiplex circuit 7
includes a decoder section 19 and a PLL (phase lock) that creates a switching signal from a pilot signal.
It consists of a switching signal generating section 20 of the Droop type, and outputs left and right stereo signals to left and right output terminals 21 and 22, respectively, but since it is a well-known circuit, a detailed explanation thereof will be omitted. Therefore, good stereo reception can be achieved in a state where there is no multipath interference. On the other hand, as the car moves, the reception conditions change and the reception signal received by antenna 1 is subject to multipath interference. The high-frequency noise component generated becomes large. Therefore, it passes through the band pass filter 12 and the level detection circuit 14.
The signal generated at the output end after level detection is
It increases rapidly, and the signal obtained at the output of the differentiating circuit 16 also becomes large. Therefore, the control transistor 11 is turned on, one end of the series resonant circuit 8 is grounded, and the series resonant circuit 8 starts to resonate at the pilot signal frequency. As a result, the impedance of the pilot signal frequency at point A of the signal transmission path decreases,
The pilot signal passing through the signal transmission path is attenuated and removed. Therefore, a pilot signal having an undesirable amplitude and phase is not applied to the stereo multiplex circuit 7, and the generation of pulse noise is prevented. At that time, as a matter of course, since the stereo multiplex circuit 7 has a switching circuit that switches to monaural operation when the pilot signal level falls below a certain level, the stereo multiplex circuit 7 does not separate the left and right stereo signals, but connects the output terminals 21 and 22 to both output terminals 21 and 22. Although a monaural signal is produced, switching to monaural reception is preferable to the listener rather than generating pulse noise. Multipath interference to the pilot signal is as shown in FIG. 1, but the width of the area affected by the interference changes depending on the speed of the vehicle in which the receiver is mounted. For example, if the speed of the car is high, the width will be narrow, and conversely, if the speed of the car is low, the width will be wide. Therefore, the period during which the series resonant circuit 8 is activated to remove the pilot signal must be determined depending on the width of the portion of the pilot signal that is disturbed . If the control transistor 11 is driven by differentiating the value, the operating period of the series resonant circuit 8 can be appropriately set. If the operating period of the series resonant circuit 8 is to be determined more strictly, it is conceivable to use a waveform shaping circuit such as a Schmitt trigger circuit at the subsequent stage of the differentiating circuit 16 . In the embodiment of FIG. 2, a series resonant circuit is used as the pilot signal removal circuit, but the present invention is not limited thereto, and various circuits suitable for removing the pilot signal can be used. In addition, in the embodiment shown in FIG. 2, multipath interference is detected by detecting the level of high frequency noise contained in the output signal of the FM detection circuit 6. Multipath interference can be detected by performing AM detection on the IF signal before the limiter is applied or by detecting phase jitter in the pilot signal. As described above, the present invention can provide a multipath interference removal circuit that can effectively remove the generation of pulse noise caused by multipath interference, and is particularly effective when used in a vehicle-mounted FM stereo receiver. It is an excellent one.

[Brief explanation of drawings]

FIG. 1 is a characteristic diagram for explaining the present invention, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. Explanation of main drawing numbers, 6...FM detection circuit, 7 ...
Stereo multiplex circuit, 8 ...Series resonant circuit, 12...Band pass filter, 14 ...Level detection circuit, 16 ...Differential circuit.

Claims (1)

[Claims] 1. A removal circuit for removing the stereo pilot signal placed between the FM detection circuit and the stereo multiplex circuit, a switch circuit for disabling the removal circuit, and a It consists of a detection circuit for detecting high frequency noise components contained in the component, and a differentiation circuit for differentiating the output signal of the detection circuit, and when multipath interference is detected by the detection circuit and the differentiation circuit, the differentiation circuit The switch circuit is driven by the output signal, and the removal circuit is activated to remove the stereo pilot signal, thereby causing the stereo multiplex circuit to operate in monaural mode in the event of multipath interference. A multipath interference removal circuit characterized by: