JPS5811144B2 - noise suppression circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse noise suppression circuit in an FM stereo receiver or the like.

Figure 1 shows the configuration of a general FM stereo demodulation circuit, where 1 is a terminal to which the output of the frequency discriminator (composite signal) is applied, 2 is the pilot signal included in the composite signal applied to terminal 1, and the frequency is divided by 2. 3 is a low-pass filter; 4 is a voltage-controlled oscillator whose oscillation frequency is controlled by the output of the low-pass filter 3; and 5 is a frequency divider by two.

7 is a 2-frequency divider which divides the output of the 2-frequency divider 5 by 2 and outputs a signal having the same period as that of the 2-frequency divider 6 and whose phase is delayed by π/2;
8 is a phase comparator that compares the phase of the pilot signal and the output of the 2-frequency divider 7; 9 is a low-pass filter; 10 is a DC amplifier;
Reference numeral 11 denotes a stereo detection circuit which determines that a stereo broadcast has been received when the output level of the DC amplifier 10 is above a certain value, and determines that a monaural broadcast has been received when it is below a certain value.

12 is a phase inversion/waveform conversion that inverts the phase of the output of the frequency divider I by 2, converts the waveform from a rectangular wave to a sine wave, and outputs an opposite phase signal with the same period and the same level as the pilot signal in the composite signal. A gate circuit 13 is controlled by the output of the stereo detection circuit 11 and is opened during stereo broadcasting and closed during monaural broadcasting; 14 is a gate circuit 13;
This is a pilot signal removal circuit that calculates the sum of the output and the composite signal and removes the pilot signal component.

15 is a stereo demodulation circuit that separates and outputs left and right channel audio signals during stereo broadcasting, and outputs the frequency discriminator output as is as left and right audio signals during monaural broadcasting; 16 and 17 are output terminals for left and right stereo audio signals, respectively; 18 is a terminal connected to a stereo display lamp.

Figure 2 shows the signal waveforms of each part, where a is the stereo signal included in the composite signal, b is the pilot signal included in the composite signal, C is the output of the voltage controlled oscillator 4, and d, e, and f are each 2 Outputs of frequency dividers 5, 6, 7,
g is the output of circuit 12.

Note that the demodulation circuit described in FIGS. 1 and 2 is a general one, and a description of its operation will be omitted.

When removing pulse noise in this type of FM stereo demodulation circuit, a noise suppression circuit shown in FIG. 3 is provided between the frequency discriminator and the demodulation circuit.

In FIG. 3, 19 is a terminal connected to a frequency discriminator, and 20 is a terminal connected to a stereo demodulation circuit.

21 is a noise detection circuit that detects pulse noise; 22 is a gate control signal generating circuit; 23 is a delay circuit that delays the output of the frequency discriminator for a certain period of time; and 24 is a gate that closes when the gate control signal of circuit 22 is applied. A circuit 25 is a clamp that holds the signal level immediately before the gate circuit 24 is closed while the gate circuit 24 is closed, and sends the output of the gate circuit 24 as it is to the buffer circuit 26 while the gate circuit 24 is open. It is a circuit.

In this circuit, if pulse noise is included in the output of the frequency discriminator, the noise detection circuit 21 detects the noise and drives the circuit 22 to generate a gate control signal corresponding to the noise.

Further, the composite signal containing noise is delayed by a delay circuit 23 and applied to a gate circuit 24 in a state in which the timing is matched with the gate control signal.

The gate circuit 24 is closed only during the period when the gate control signal 24 is applied, and the clamp circuit 25 continues to output the output immediately before the gate circuit 24 was closed, thereby preventing the signal from being interrupted.

However, legal issues arise in this case.

In other words, even though the signal level is maintained at a constant value in the clamp circuit 25 during the period when noise is added, the pilot signal removal circuit provided in the FM stereo demodulation circuit generates a signal with the same period and amplitude as the pilot but with an opposite phase. Since this signal is added, this signal is superimposed on the demodulated left and right channel audio signals and output, creating new noise.

The present invention solves this problem, and examples thereof will be described below.

In Figure 4, the numbers 2-12, 14-19, 21-26
correspond to the same reference numerals in Figures 1 and 3, respectively.

27 is a gate circuit for controlling the transmission of the output of the phase inversion/waveform conversion circuit 12, and 28 is a clamp circuit.

Next, the operation of the main parts of this embodiment will be explained.

When a stereo signal is received and pulse noise is not detected by the detection circuit 21, the gate circuit 2
7 will be opened.

Therefore, the output of the phase inversion/waveform conversion circuit 12 is applied to the clamp circuit via the gate circuit 27, and is output as is and applied to the stereo demodulation circuit 15 for normal stereo demodulation operation.

However, when a monaural broadcast is being received or when pulse noise is detected by the detection circuit 21, the gate circuit 27 is closed.

Therefore, the classic circuit 28 holds and outputs the signal immediately before the gate circuit 21 closes.

In this case, when a monaural broadcast is received, it is not necessarily necessary to hold the previous level in the clamp circuit.

In this way, since the signal for removing the pilot signal included in the composite signal is not applied to the pilot signal removal circuit when noise occurs, the generation of noise can be prevented.

As is clear from the above description, according to the present invention, it is possible to reliably remove noise contained in a composite signal, and also to remove unnecessary components mixed in by a conventional pilot signal generation circuit during noise suppression. .

[Brief explanation of drawings]

FIG. 1 is a block diagram of an FM stereo demodulation circuit, FIG. 2 is a signal waveform diagram thereof, FIG. 3 is a block diagram of a conventional noise suppression circuit, and FIG. 4 is a block diagram of a noise suppression circuit according to an embodiment of the present invention. It is a diagram. 12... Phase inversion/waveform conversion circuit, 14...
... Pilot signal removal circuit, 21 ... Noise detection circuit, 23 ... Delay circuit, 24, 27 ...
・Gate circuit, 25.28...clamp circuit.

Claims (1)

[Claims] 1. A detection circuit that detects pulse noise included in the output of the frequency discriminator, a delay circuit that delays the output of the frequency discriminator for a certain period of time, and transmission of the output of the delay circuit when the detection circuit detects noise. a first gate circuit that stops the first gate circuit; when this first gate circuit is closed, it outputs while maintaining the signal level immediately before closing, and when it is open, a first gate circuit that allows the output of the first gate circuit to pass through; a clamp circuit, a generation circuit that generates a signal for canceling the pilot signal included in the output of the frequency discriminator, a second gate circuit that stops transmission of the output of the generation circuit when the detection circuit detects noise; a second clamp circuit that holds and outputs the signal immediately before closing when the second gate circuit is closed; and a second clamp circuit that allows the output of the second gate circuit to pass when the second gate circuit is open; A noise suppression circuit comprising a pilot signal removal circuit inputting the output of the first clamp circuit and the output of the first clamp circuit.