JPS594898B2 - Pulse noise canceler - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved apparatus for canceling pulsed noise, particularly in FM radio receivers.

Conventionally, when pulse noise is present in an FM radio receiver, the pulse noise is detected and used as a control signal to shut off a gate installed in the broadcast receiving system only while the noise is present to eliminate the noise. However, during that time there was a noise canceling device that held the received signal just before the gate was shut off.

However, when this device operates due to pulse noise while receiving FM stereo broadcasting, there is a drawback that a gate cut-off sound is generated.

This phenomenon will be explained using FIGS. 1, 2, and 3.

The waveforms of the pilot signals at the output end A of the holding circuit 5 and the output end B of the audio amplifier 6 in FIG. 1 are shown in FIGS. 2 and 3.

That is, in FM stereo broadcasting, there is a 19KH2 pilot signal, but this is 38KH2 in the output after FM demodulation.
Present along with the suppressed signal of KH2.

These are combined with pulse noise for several tens of microseconds.
When the light is cut off, a cut-off sound is generated.

Assuming that gate 4 is shut off for time T, the waveform of the pilot signal at point A in FIG. 1 will be as shown in FIG. 2A.

At point B where the signal passes through the audio amplifier 6, it becomes as shown in FIG. 2B.

The waveform shown in FIG. 2B contains an audible frequency component and at the same time has a large amplitude, so that it is heard from the speaker 7 as noise.

As a method to eliminate these drawbacks, there is a method of adding a pilot signal resonance circuit to the holding circuit 5 (US Pat. No. 3,739,285), but the present invention attempts to eliminate the above drawbacks by utilizing the following phenomenon. It is something.

That is, as shown in Fig. 3A, the gate 4 is always closed at the zero level of the pilot signal.As shown in Fig. 3B, although the audible frequency component is present, the amplitude is small, so noise is generated from the speaker 7. do not.

An embodiment of the present invention shown in FIGS. 4 and 5 will be described below.

When the pilot signal at point a in FIG. 4 contains pulse noise N as shown in FIG. 5 a, the delay circuit 3
It is delayed by a half period (approximately 26.3 microseconds) of the 9KH2 pilot signal (FIG. 5b) and applied to gate 4.

On the other hand, when the output of the delay circuit 3 is applied to the pilot signal detection circuit 10, the waveform of the pilot signal at the 0 point becomes as shown in FIG. 5C.

This zero point pilot signal is detected by a zero level detection circuit 11,
In addition to the pulse shaping circuit 12, a pulse waveform as shown in FIG. 5d is obtained.

This pulse width needs to be slightly larger than the delay time of the pulse noise detection circuit 8 and the pulse shaping circuit 9.

Next, the output of the pilot signal detection circuit 10 is led to a monaural broadcast detection circuit 16, so that an output at a constant level is output only during monaural broadcasting where no pilot signal is present.

Furthermore, the output of the FM demodulation circuit 2 is transferred to the pulse noise detection circuit 8.
In addition to this, extract the pulse noise N as shown in Figure 5e and add it to the pulse shaping circuit 9 to create a pulse width that is at least a little longer than half the period of the pilot signal (26.3 microseconds). (Fig. 5f).

Next, the output from the pulse shaping circuit 12 and the output from the monaural broadcast detection circuit 16 are applied to an OR circuit 13, and this output and the output from the pulse shaping circuit 9 are applied to an AND circuit 14.

For FM stereo broadcasting, the signals d and f in Figure 5 are ANDed.
Since it is applied to the circuit 14, the output becomes as shown in FIG. 5g.

This is added to the monostable multivibrator 15 and the gate 40 control pulse (fifth
Create figure h).

This control pulse closes the gate 4, and the pilot signal has a waveform as shown in FIG. 5i.

In addition, in the case of monaural broadcasting, the output of the AND circuit 14 is the same as the output waveform of the pulse shaping circuit 9 (FIG. 5f), so the gate 40 control pulse is generated a little earlier than in the case of the above-mentioned FM stereo broadcasting. However, it does not exceed 26.3 microseconds.

At this time, since there is no pilot signal, there is no problem with its position as long as the pulse noise can be removed.

The pulse width of the gate control pulse generated by the monostable multi-by-break 15 of about 80 microseconds is set by adding the half period of the pilot signal, 26.3 microseconds, and the time width of the pulse noise.

As can be seen from the above explanation, the present invention eliminates the pulse noise by cutting off the gate when the pilot signal is at O level in synchronization with the phase of the pilot signal when pulse noise exists. It is possible to eliminate the cut-off sound.

In addition, the 19 KHz pilot signal is synchronized with the suppressed 38 KHz subchannel and the pilot signal 57 KH2 for traffic information (SK, DK) used in Europe, so the 19 KH2
If the gate is operated in synchronization with the zero level of , all of the cutoff sounds caused by these frequencies can be improved.

Furthermore, since the pilot signal detection circuit 10, the pulse noise detection circuit 8 and the subsequent gate 40 control circuits are digital circuits, there is an advantage that they can be easily integrated into integrated circuits.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional noise canceling device, Fig. 2A,
B, FIGS. 3A and 3B are diagrams explaining the operation of the same device, FIG. 4 is a block diagram of the pulse noise canceling device in one embodiment of the present invention, and FIG. 5 a-1 is a waveform for explaining the operation of the device. It is a diagram. Total: Delay circuit, 4: Gate circuit, 8
...Pulse noise detection circuit, 9, 12...
... Pulse shaping circuit, 10 ... Pilot signal detection circuit, 11 ... Zero level detection circuit, 13
...OR circuit, 14...AND circuit,
15... Monostable multivibrator, 15...
...Monaural broadcast detection circuit.

Claims (1)

[Claims] A gate circuit provided in the middle of an I FM radio reception system,
Pulse noise detection circuit that detects pulse noise and FM
A control circuit including a pilot signal detection circuit that detects a stereo pilot signal and a zero level detection circuit that detects an O level of the FM stereo pilot signal,
FM stereo pilot signal 0 when pulse noise is mixed
A pulse noise canceling device characterized in that the pulse noise is removed by cutting off the gate circuit in synchronization with the zero level. 2. The pulse noise canceling device according to claim 1, wherein the delay time of the delay circuit for timing adjustment of the gate circuit is set to a half cycle of the FM stereo pilot signal. 3. The pulse noise canceling device according to claim 1, wherein the control circuit includes a monaural broadcast detection circuit that controls a gate circuit using pulse noise detected during monaural broadcast reception.