JPS634975B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse noise suppression device for effectively removing pulse noise in a vehicle-mounted FM receiver.

Such a conventional device is constructed as shown in FIG. In the figure, 1 is a low-pass filter (hereinafter abbreviated as LPF) that acts as a signal delay means.
The signal is then delayed by the amount of time necessary to process the noise. Gate 2 works to momentarily interrupt the signal only when pulse noise is input, and level hold circuit 3 maintains its output level at the signal level immediately before the interruption while the signal is interrupted by gate 2. work like that.

4 is a high-pass filter (hereinafter abbreviated as HPF) having a cutoff frequency of about 100 KHz, which functions to extract only noise components from the detection output from the FM detector (not shown). The noise amplifier 5 with AGC amplifies the output of the HPF 4, and when the noise energy exceeds a certain level, the AGC operates and lowers the gain. This is to prevent the gate 2 from being operated frequently and the signal being interrupted when the white noise component increases due to a weak electric field or the like. A monostable multivibrator (hereinafter abbreviated as monostable multi) 6 generates a pulse signal of a constant width for driving the gate 2 when the output of the noise amplifier 5 becomes larger than a certain constant value.

In the above-described noise suppression device, when the signal from the FM detector is of a relatively low audible frequency, for example, around the voice frequency, as shown in FIG. The output of the level hold circuit 3 is as shown in Fig. 2b.
As shown in FIG. 2, a signal is generated that maintains the level when gate 2 is closed. In this case, the level hold circuit 3 functions without any problem and no noise is generated.

However, if there is a 19KHz stereo pilot signal as shown in Figure 3a, if gate 2 is momentarily cut off during the gate off period shown by the broken line, gate 2 will be shut off as shown in Figure 3b. A signal held at the DC level for the period of time is obtained at the output of the level hold circuit 3, the waveform becomes discontinuous, the pilot signal is also cut off, and MPX demodulation becomes impossible, causing distortion and noise. Become. This is because the frequency of the pilot signal is high, so the time that gate 2 is cut off is half a wavelength or one wavelength.
This is because the level hold circuit 3, which simply performs correction using DC potential, cannot effectively correct the voltage.

The present invention has been made in view of the above points, and
The object is to provide a pulse noise suppression device that can remove pulse noise from an audible signal including a stereo pilot signal without generating noise.

The present invention will be explained below with reference to FIG. 4 and subsequent figures.

FIG. 4 is a block diagram showing an embodiment of the pulse noise suppression device according to the present invention.
Gate 2, level hold circuit 3, HPF4,
The noise amplifier 5 with AGC and the monostable multi 6 are the same as those in the conventional device shown in FIG. 1, and operate in the same manner as described in connection with FIG.

Therefore, the output point A of the first level hold circuit 3 is the same as that obtained at the output of the device of FIG. The output waveform shown is obtained. This output waveform is obtained by closing gate 2 from time t ₁ to _{t 2} due to the appearance of pulse noise, removing pulse noise from the composite signal from the FM detection output, and combining the audio frequency signal and the pilot signal. Consisting of

The output of the level hold circuit 3 is a first subtraction signal in which a pilot cancel signal (hereinafter referred to as a pilot cancel signal) as shown in FIG. 5b is added to the - input terminal (point B). circuit 7
is added to the + input terminal of In this way, since the signal containing the pilot signal and the signal for the piquancy are applied to the input terminals having opposite phases, a signal from which the pilot signal has been removed is obtained at the output point C of the subtraction circuit 7. The output signal of this subtraction circuit 7 is applied to the - input terminal of a second subtraction circuit 8.

The + input terminal (point D) of the subtraction circuit 8 is connected to ground via a switch circuit 9 which is turned off in synchronization with the gate 2 by a pulse signal from the monostable multi 6. ing. Since the switch circuit 9 is normally in the on state, when there is no pulse noise, the point D falls to the ground in an alternating current manner, and the subtraction circuit 8 functions simply as an inverting amplifier.
At the output point E of the subtraction circuit 8, a signal obtained by removing the pilot signal from the composite signal from the FM detection output is obtained.

A second level hold circuit 10 is connected between the signal line for the Pikeyan signal and the + input terminal of the subtraction circuit 8. This level hold circuit 10 functions to transmit the waveform of the signal for Pikeyan to point D when the switch circuit 9 is turned off and the impedance at point D rises.

By the way, the level hold circuit 3 specifically consists of a series circuit of a resistor 3a and a capacitor 3b connected between the signal line and the ground, as shown in FIG. 6, and when the gate 2 is closed (cut off). At this time, the level at point A is maintained at the level charged in capacitor 3b until then. On the other hand, the level hold circuit 10 operates at points B and D as shown in FIG.
A resistor 10a and a capacitor 10 connected between
The switch circuit 9 is configured by a series circuit with b.
is turned off, the resistor 10a and capacitor 10
A signal for Paikyan is transmitted from point B to point D through b.

With the above configuration, if pulse noise is superimposed on the composite signal, the monostable multi 6 generates a pulse signal in response to this, closes the gate 2, cuts off the transmission of the input signal, and closes the switch circuit. Turn off 9. The composite signal is L.
Since it is delayed by P.F1, the time when the gate 2 is closed and the switch circuit 9 is turned off is before the pulse noise passes through the gate 2. As a result, the output waveform of the level hold circuit 3 between time points _t1 and _t2 when the gate 2 is closed becomes as shown in FIG. 5a.

At this time as well, since the Pikeyan signal is added to the − input terminal of the subtraction circuit 7, the output of the subtraction circuit 7 when the gate 2 is closed is from time t ₁ to
At times other than _t2 , a waveform in which the normal pilot signal is canceled is obtained, and between time _t1 and _t2 , the waveform is held by the level hold circuit 3 as shown in FIG. 5a.
The output of the difference between the signal between t ₁ and _{t 2} and the Paikian signal between time t ₁ and _{t 2} of FIG.
As shown in the figure, the signal corresponding to the extra Paikyan signal is superimposed. However, due to the switch circuit 9 being turned off at this time, the signal for the piston is transmitted to point D via the level hold circuit 10, and the signal for switching to point D is transmitted to point D, which is the same as when the switch circuit 9, which was AC grounded, was turned on. The fifth level starts from the DC level of only the DC component, which is grounded in the AC manner.
A waveform in which the signal for Pikeyan is superimposed appears only between time points t ₁ and _{t 2} as shown in FIG. d. For this reason,
In the subtraction circuit 8, the signal appearing at this point D cancels out the extra Pikeyan signal included in the output of the subtraction circuit 7, and the output of the subtraction circuit 8 has a signal as shown in FIG. 5e. The pilot signal is canceled, and an output whose level is held by the level hold circuit 3 is obtained only between the time points _t1 and _t2 of the audio signal. Since the output of the subtraction circuit 8 does not include a pilot signal, the pilot signal is bypassed from the output of LPF 1 for circuits that require a pilot signal, and is used in the PLL circuit of the MPX demodulation circuit and the pilot signal cancel signal generation circuit. Used for etc.

As can be seen from FIG. 5e, the output waveform of the subtraction circuit 8 is the one obtained by canceling the pilot signal together with the pulse noise, so that the pilot signal that is cut off midway is not included. That is, the noise that occurs in the apparatus shown in FIG. 1 due to the pilot signal being cut off midway is no longer generated.

Note that gate and switch circuits are generally on/off.
It is known that spike noise is generated when the switch is turned off, but in the case of the present invention, the spike noise generated in the gate 2 and the spike noise generated in the switch circuit 9 are canceled out in the subtraction circuit 8, so that these can be canceled out. It never appears in the output.

Furthermore, in the above embodiment, explanation of the subcarrier component of the stereo composite signal is omitted; however,
Its existence does not matter as long as it serves the purpose of removing pulse noise and canceling the pilot signal.

As described above, in the present invention, after the pulse noise is removed by shutting off the gate, the pilot signal is removed by the cooperation of a pair of arithmetic circuits (subtraction or addition circuits) and a switch circuit. A pilot signal that is cut off in the middle no longer appears. Therefore, the generation of waveform distortion in the MPX demodulation circuit due to the pilot signal being cut off midway and the generation of noise associated with it are eliminated, and together with the simple circuit configuration, this has a very practical effect. play. Furthermore, since spike noise generated by gate and switch circuits is removed at the same time, it has excellent performance.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an example of a conventional device.
2a and b and 3a and b are waveform diagrams for explaining the operation of the device shown in FIG. 1, FIG. 4 is a block diagram showing an embodiment of the device according to the present invention, and FIG. 5a 6 to 7e are waveform diagrams showing waveforms at various points in the device shown in FIG. 4, and FIGS. 6 and 7 are circuit diagrams showing specific circuits of a part of the device shown in FIG. 4, respectively. 1...Low pass filter (delay means), 2...
Gate, 3... (first) level hold circuit,
4... High pass filter, 5... Noise amplifier with AGC, 6... Monostable multivibrator, 7...
...(first) subtraction circuit (arithmetic circuit), 8... (second) subtraction circuit (arithmetic circuit), 9... switch circuit (switch means), 10... (second) level hold circuit.

Claims (1)

[Claims] 1. A delay means for delaying an input signal, a gate for cutting off the output signal of the delay means, a first level holding circuit for holding the output level immediately before the cutting while the gate is cut off, and a noise detection section that detects noise and generates a pulse signal of a constant width for driving the gate; and a pilot canceler that cancels the output signal of the first level hold circuit and the pilot signal in this output signal. a first subtraction circuit to which the signals for use are input to input terminals having mutually opposite phases, a second subtraction circuit to which the output signals of this subtraction circuit are input, and an output of this first subtraction circuit. A switch means is connected in an alternating current manner between an input terminal having a phase opposite to that of a terminal into which a signal is input, and earth, and is normally turned on and turned off in response to a pulse signal from the noise detection section, and a switch means for turning off the switch means. a second level hold circuit that transmits the pilot cancel signal to a negative phase input terminal of the second subtraction circuit during the period of time.