JPH088537B2 - Multipath noise detector - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multipath for selectively detecting multipath noise, which deteriorates sound quality particularly in stereo reception in an FM stereo receiver, from high frequency noise of FM detection output. The present invention relates to a noise detection device.

2. Description of the Related Art Conventional multipath noise detection devices include a device that detects a drop in received electric field strength and a device that detects a distortion component and noise at a high frequency of an FM detection output signal. The former is
Received electric field strength signal generated by detecting the envelope of the intermediate frequency signal that is not subjected to the amplitude limit by the intermediate frequency amplifier and detecting the drop, or by synthesizing the voltage that detected the amplitude of the intermediate frequency amplifier over several stages The method of detecting the change of is used. On the other hand, the latter method uses a method of emphasizing the high frequency noise in the FM detection output signal via a high-pass filter and an automatic gain control amplifier, and detecting the noise whose amplitude exceeds the reference value. (For example, actual public Sho 62-14768, actual public Sho 62-19002, actual public Sho 62-21
087, Nikkei Electronics October 29, 1979 issue PP62-6
6, Nikkei Electronics May 19, 1986 issue PP131-13
4) Problems to be Solved by the Invention The multipath noise detection devices according to the conventional techniques described above have the following problems.

As is already known, the multipath distortion is larger as the modulation frequency is higher, and the distortion is more remarkable in stereo than in monaural. A multipath noise detector is better to detect when the distortion is large, but the method of detecting the drop in the received electric field strength is not related to the modulation frequency or monaural stereo, so it generates a lot of distortion and noise. It has a drawback that it may operate even when it does not work, and the in-vehicle receiver often has a drawback that the electric field strength often changes abruptly and malfunctions easily.

On the other hand, a method that detects distortion components and noise at high frequencies in the FM detection output signal is ideal because it is easier to detect as the occurrence of distortion or noise is larger, but it is ideal for pulse noise such as automobile ignition noise. It had the drawback of malfunctioning due to noise. To solve this drawback, the means for removing the pulse noise in the FM detection output, that is, in the FM receiver having the pulse noise removing circuit, the multipath noise is detected from the output signal of this pulse noise removing circuit. However, the pulse noise elimination circuit has a low-pass filter inserted for the purpose of delaying the signal in the signal path that passes the FM detection output signal. Is difficult to detect, and thus there is a drawback that the detection sensitivity of multipath noise is reduced.

Means for Solving the Problems The present invention provides a high-pass filter for extracting a distortion component of a high frequency included in an FM detection output signal from a distortion component of a high frequency included in an FM detection output signal, and the high-pass filter. An automatic gain control amplifier that amplifies the output signal of the filter and controls its own gain by the average value of the amplitude of its own output signal, and the output signal of the automatic gain control amplifier are input, and the peak value of the output signal is the reference When the value is exceeded, the time width is 30 (μ s
ec) A pulse detector that outputs a short-wave signal and a signal output from the pulse detector are counted, and it is detected that a predetermined number of the short-wave signals are input within a predetermined multiple of the pilot signal period. It is a multipath noise detection device including a counter circuit that outputs a signal.

Function In the multipath noise detection apparatus of the present invention, the automatic gain control amplifier (2) controls the amplitude so that the average value of the distortion components in the high frequency range is kept constant, and the pulse detector (3) causes distortion exceeding the reference value. The peak value of the waveform is detected and shaped into a rectangular wave signal with a time width of about 30 (μ sec), the rectangular wave signal is counted by the counter circuit (4) within the specified time, and it has the same period as the pilot signal frequency. Detects multipath noise generated in (about 53 μsec). Therefore, when a stereo subcarrier signal with a period of 26 (μ sec) is detected continuously, the output signal of the pulse detector (3) becomes one shaped waveform that is a series of the preceding and following rectangular wave signals. Since the circuit (4) does not make an erroneous count operation, multipath noise can be detected stably.

Examples Next, the present invention will be described in detail with reference to Examples.

As is already known, the magnitude of multipath distortion in the stereo demodulated output signal of the FM stereo receiver is several times larger in stereo reception than in monaural reception.
For example, under the condition that the total harmonic distortion rate is about 1% in monaural, even in stereo, the difference signal (L-
Even if the R signal) is zero, the total harmonic distortion rate is as high as 10%. This is mainly because the pilot signal included in the composite stereo signal is significantly distorted to generate a second harmonic component, which is converted into an audible frequency component by the demodulation operation of the stereo demodulator.

FIG. 1 is a block diagram of an embodiment of the present invention.
The FM detection output signal supplied to the high-pass filter 1
Thereby attenuates the components of the composite stereo signal and emphasizes the high frequency components. The output signal of the high pass filter 1 is input to the automatic gain control amplifier 2. The automatic gain control amplifier 2 is configured to control its own gain by the average value of the amplitude of its own output signal, and prevents the detection device from malfunctioning due to an increase in noise in the high frequency range. The output signal of the automatic gain control amplifier 2 is input to the pulse detector 3 which is adapted to generate a rectangular wave signal having an appropriate time width when the peak value of the input signal exceeds the reference value. The output signal of the pulse detector 3 is input to the counter circuit 4. The counter circuit 4 is configured to count the rectangular wave signal from the pulse detector 3 and output the signal to the terminal 6 when a predetermined number is reached within a fixed time.

In FIG. 1, the high frequency component in the FM detection output supplied to the terminal 5 is emphasized by the high pass filter 1 and the automatic gain control amplifier 2 and input to the pulse detector 3. Although the input of the pulse detector 3 includes multipath noise and other noises, the white noise generated by the weak electric field strength has its gain limited by the action of the automatic gain control amplifier 2, and thus the pulse detector 3 Does not output when the standard value set in is not reached. Pulse noise such as multi-pass noise and vehicle ignition noise is detected by the pulse detector 3 when the peak value exceeds a reference value, and is converted into a rectangular wave signal having an appropriate time width. Here, the appropriate time width is about 30 μsec, and even if the subcarrier component of the composite stereo signal exceeds the reference value of the pulse detector 3, the period of the subcarrier signal is about 26 μsec, so before and after outputting. It is possible to prevent malfunctions by making the counter circuit 4 unable to count because the rectangular wave signals of are overlapped. Now, the rectangular wave signal output from the pulse detector 3 is counted by the counter circuit 4. As mentioned above,
Since a plurality of multipath distortions in stereo continues in the period of the pilot signal, if there are N rectangular wave signals within N times or N + 1 times the period of the pilot signal, the multipath distortion can be determined. (The value of N is preferably 2 or 3.) Therefore, the counter circuit 4 counts an input signal and outputs a signal when a predetermined number N is reached, and a counter that outputs the signal at regular intervals. It is configured by a reset circuit that resets or resets after a certain time from the first counted input signal and returns to the initial state.

FIG. 2 shows the waveform of the FM detection output signal.
FIG. A shows monaural, FIG. 2B shows stereo when the left and right difference signals are zero, and FIG. 2C shows stereo when there are only left or right signals. It is an example of the waveform which generated the path distortion. Focusing on the distortion component in the waveform with multipath distortion shown in FIGS. 2B and 2C, the peak of the distortion component is the period of the pilot signal (about 53
It can be seen that multiple occurrences occur in μsec).
On the other hand, the ignition noise of an automobile has a generation cycle of at least 1 msec or more, and a plurality of ignition noises do not continue at the pilot signal cycle. Therefore, the multipath noise in stereo can be reliably distinguished from other pulse noises by detecting that a plurality of peaks of the distortion component are continuous in the period of the pilot signal.

FIG. 3 is a block diagram of an example of an FM receiver to which the multipath noise detecting device of the present invention is applied, and the inside of the multipath noise detecting device 19 uses the same symbols as those in FIG. . In FIG. 3, 13 is an antenna, 14 is a front end circuit, 15 is an intermediate frequency amplifier circuit,
Reference numeral 16 is an FM detection circuit, 17 is a pulse noise removal circuit, and 18 is a stereo demodulator. The stereo demodulator 18 can smoothly change the degree of separation by a voltage applied to a PLL (phase-lock drove) circuit 21 whose phase is synchronized with a pilot signal, a stereo demodulator 22, and a terminal 24. At least a separation control circuit 23 is included. The output signal of the multipath noise detection device 19 is supplied to the separation control circuit 23 via the peak hold circuit 20.

The inside of the multipath noise detection device 19 is as described above with reference to FIG. 1, but FIG. 3 shows one configuration example for each block. The automatic gain control amplifier 2 that amplifies the output signal of the high pass filter 1 is
The variable gain amplifier 7 is composed of a variable gain amplifier 7 and a voltage generation circuit 8 which detects the output amplitude and averages the output amplitude to generate a voltage for controlling the gain of the variable gain amplifier 7. When the peak value of the output signal of the automatic gain control amplifier 2 exceeds the reference value, the pulse detector 3 that outputs a rectangular wave signal with an appropriate time width outputs a sawtooth wave signal whose amplitude is proportional to the peak value of the input signal. It is composed of an output peak detector 9 and a Schmitt circuit 10 which is a voltage comparator having a hysteresis characteristic. Counter circuit 4 for counting the output signal of pulse detector 3
Is composed of a counter 11 and a reset pulse generation circuit 12, and the reset pulse generation circuit 12 is a PLL circuit 21.
The reference signal in synchronism with the pilot signal is received from, and a reset pulse signal having a fixed time period is generated and supplied to the reset terminal of the counter 11.

In FIG. 3, when the multipath noise detection device operates and outputs a rectangular wave signal to the terminal 6, the peak hold circuit
20 generates a sawtooth wave signal that linearly returns in a slow time of about 1 second and supplies it to the separation degree control circuit 23.

As described above, the multipath distortion after stereo demodulation is
Since it is several times larger in stereo than in monaural, the distortion can be greatly reduced by stopping the operation of the stereo demodulator and putting it in monaural, or by lowering the separation to about 6 dB or less. In FIG. 3, the multipath noise is reduced by detecting the multipath noise, lowering the degree of separation, and then slowly returning to reduce the multipath noise.

EFFECTS OF THE INVENTION As described above, when the multipath noise detection device of the present invention continuously detects a stereo subcarrier signal of 26 (μ sec) period, the output signal of the pulse detector (3) is The rectangular waveform signals are overlapped to form one continuous waveform, and the counter circuit (4) does not perform an erroneous counting operation, so that multipath noise can be detected stably.

[Brief description of drawings]

FIG. 1 is a block diagram of a multipath noise detecting device of the present invention, FIG. 2 is a waveform diagram of an FM detection output signal, and FIG. 3 is a block diagram of an FM receiver of an embodiment of the present invention. 1 ... High-pass filter, 2 ... Automatic gain control amplifier, 3 ... Pulse detector, 4 ... Counter circuit, 7 ...
… Variable gain amplifier, 8 …… Voltage generator, 9 …… Peak detector, 10 …… Schmidt circuit, 11 …… Counter, 12
...... Reset pulse generation circuit, 13 ...... Antenna, 14 ......
Front-end circuit, 15 ... Intermediate frequency amplifier circuit, 16 ...
… FM detection circuit, 17 …… Pulse noise removal circuit, 18 …… Stereo demodulator, 19 …… Multipath noise detector, 20
...... Peak hold circuit, 21 …… PLL circuit, 22 …… Stereo demodulator, 23 …… Separation control circuit.

Claims (1)

[Claims] 1. A high-pass filter for extracting a high-frequency distortion component contained in an FM detection output signal, and an output signal of the high-pass filter is amplified to control its own gain by an average value of the amplitude of its own output signal. Automatic gain control amplifier and an output signal of the automatic gain control amplifier are input, and when the peak value of the output signal exceeds the reference value, the time width 30 (μ sec)
A pulse detector that outputs a rectangular wave signal of a certain degree, counts the output signal of the pulse detector, and outputs a detection signal when a predetermined number of the short wave signals are input within a predetermined time of the pilot signal period. A multipath noise detection device having a counter circuit for outputting.