JPS61126829A - Fm demodulation circuit - Google Patents

Abstract

PURPOSE:To obtain an excellent picture quality by differentiating a demodulation signal of an FM demodulation circuit to apply boundary discrimination of black/white levels, bringing the demodulation signal to a prescribed voltage when the signal is an impulsive noise generated on the boundary and holding the demodulation signal just before the impulsive noise when the signal is the impulsive noise at the outside of the boundary. CONSTITUTION:Only a luminance component of a demodulation signal shown in waveform (a) is extracted by a low-pass filter 8 from an output of a PLL type FM demodulation circuit 4. A differentiation signal shown in a waveform (b) is obtained by a differentiation circuit 9, and each of waveform (c) (from black to white level) and a waveform (d) (from white to black level) representing the boundary of white/ black levels is outputted to a voltage generating circuit 11 by using a proper threshold value 16 at a waveform shaping circuit 10. A switch control circuit 7 uses an output of a waveform shaping circuit 6 to cut off the signal at a switch 13 and a switch 14 is cut off by an impulsive noise at an AND between the waveform shaping circuits 6 and 10, that is, at the white/black level boundary. The voltage generating circuit 11 uses an output signal of the waveform shaping circuit 10 to generate a white level voltage from the waveform (c) and generate a black level voltage from the waveform (d).

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an FM demodulation circuit that demodulates an FM modulated signal, and particularly to an FM demodulation circuit suitable for a satellite broadcast receiver.

Conventionally, as a method for reducing impulsive noise in FM receivers, as shown in Patent Publication No. 16451/1980, impulsive noise is detected, and based on this, the impulsive noise section is divided into areas just before the impulsive noise. It is known to hold the demodulated voltage or signal frequency to prevent signal disturbance due to impulsive noise. The example shown here is related to external noise such as ignition noise of a car, but a similar method can also be used for impulsive noise caused by the demodulation circuit being out of synchronization in a weak electric field in a PLL type FM demodulation circuit. It is being

However, when receiving television signals, large impulsive noises tend to occur at the boundaries between white to black and black to white on the TV screen, and if the information immediately before the impulsive noise is held for the period of noise, noise will be generated at the boundaries above. There is a problem with adding .

[Purpose of the invention]

The purpose of the present invention is to eliminate the drawbacks of the above-mentioned FM demodulation circuit,
An object of the present invention is to provide an FM demodulation circuit that removes impulsive noise with a simple configuration.

[Summary of the invention]

In the present invention, the occurrence of impulsive noise is detected by monitoring the entire level of the demodulated signal of the FM demodulation circuit, and at the same time, by differentiating the demodulated signal, it is determined whether or not it is at the boundary between black and white. When there is impulsive noise other than the border between black and white, the demodulated signal immediately before the impulsive noise is held, and when there is impulsive noise at the border, the demodulated voltage is set to the white level if it changes from black to white, and the demodulated signal is set to the white level if it changes from white to black. Impulse noise is removed by setting the black level.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be explained with reference to FIG. The demodulated output of the PLL type FM demodulation circuit 4, which is composed of a phase detector 1, a loop filter 2, and a voltage controlled oscillator 3, is branched through an emphasis circuit 19t, one of which is led to an impulsive noise detection circuit 5. If the supply value exceeds a certain threshold value, it is determined that impulsive noise has occurred.

The output of the detection circuit 5 is led to a switch control circuit 7 via a waveform shaping circuit 6. Another branched demodulated output is guided to a low-pass filter 8, differentiated by a differentiating circuit 9, and guided to a switch control circuit 7 and a voltage generation circuit 11 via a waveform shaping circuit IQt-. On the other hand, the demodulated output is delayed by a delay circuit 12 and then switched on and off by switches 15 and 14 controlled by the output of the switch control circuit 7 described above. The delay time of the delay circuit 12 is set so that impulsive noise and switch disconnection coincide. switch 13
A hold capacitor 15 is arranged to hold the entire demodulated voltage immediately before the switch is turned off while the switch is turned off. While the switch 14 is turned off, the output of the voltage generating circuit 11 becomes a demodulated output. When the detection circuit 5 is activated and it is determined that impulsive noise has occurred, the switch 14 is turned off and the output of the voltage generation circuit 11 is substituted for the demodulated signal if the noise is on the boundary between black and white.
If it is outside the boundary, the voltage of the previous demodulated signal is substituted to remove impulsive noise. FIG. 2 is a diagram illustrating black-white boundary determination. From the output of the PLL type FM demodulation circuit 4, a low-pass filter 8 extracts only the luminance component of the demodulated signal of the waveform α, a differentiation circuit 9 obtains a differential signal of the waveform, and a waveform shaping circuit 10 divides the signal into black and white using an appropriate threshold value 16. Waveform C (black → white) showing the boundary.

Each of the waveforms d (white→black) is output to the voltage generation circuit 11. The waveform shaping circuit 10 also connects the switch control circuit 7.
A signal obtained by calculating the logical sum of waveform C and waveform d is output. The switch control circuit 7 is disconnected by a switch 13 according to the output of the waveform shaping circuit 6, and the waveform shaping circuits 6, 10 are disconnected by a switch 13.
By the logical product of
Turn switch 14 off. The voltage generation circuit 11 generates a white level voltage from the waveform C and a black level voltage from the waveform d, according to the output signal of the waveform shaping circuit 10. As a result, at the boundary between black and white, new noise is not added due to the removal of impulsive noise, and at other points other than the boundary, impulsive noise is removed by holding the immediately preceding information, and image quality deterioration can be reduced. FIG. 3 shows an embodiment of the main parts of the switch control circuit 7, the switches 13 and 14, and the voltage generation circuit 10. The switch control circuit 7 is composed of a simple logic circuit, and the switches 13 and 14 are analog switches such as FETs, which are disconnected when the voltage applied from the switch control circuit 7 is stopped. Voltage generation circuit 1
0, the output of the waveform shaping circuit 9 is guided to the white level voltage generation circuit 17 and the black V-pel voltage generation circuit 18, respectively, and the respective voltages are generated by the output signal of the waveform shaping circuit 9 and the switch 14 disconnection signal. When no voltage is applied, the output of the voltage generating circuit 10 is held at high impedance. In this embodiment, the PLL method is used for the FM demodulation circuit, but it is clear that similar effects can be obtained by using other demodulation methods such as slope detection.

〔Effect of the invention〕

According to the present invention, all boundary determination between black and white is performed by differentiating the decoded signal of the FM'41 modulation circuit, and when impulsive noise occurs on the boundary, the demodulated signal is set to a constant voltage, and impulsive noise other than the boundary is In this case, by holding all demodulated signals immediately before impulsive noise, good image quality with impulsive noise removed can be obtained.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the FM demodulation circuit according to the present invention, FIG. 2 is a waveform diagram explaining the present invention, and FIG. 5 is a circuit diagram of an embodiment of the FM demodulation circuit according to the present invention. It is. 5...Detection circuit 6, 10...Waveform shaping circuit 7...Switch control circuit 8...Low pass filter 9...Differentiating circuit 11...Voltage generation circuit 12...Delay circuit 13.14 ...Switch 15...Capacitor')? ′ （、・ ゝ＼〜）

Claims (1)

[Claims] A demodulation circuit that demodulates an FM-modulated television signal, and is connected to the demodulation circuit, receives the output signal of the demodulation circuit, and generates a noise detection signal when the output signal of the demodulation circuit exceeds a predetermined range of values. An impulsive noise detection circuit that generates a An FM demodulation circuit including a hold circuit that holds the output signal of the demodulation circuit at the signal immediately before the noise detection signal when the detection signal is input, the FM demodulation circuit is connected to the demodulation circuit and receives the output signal of the demodulation circuit. a low-pass filter; a differentiation circuit connected to the low-pass filter and differentiating the output signal of the low-pass filter;
A waveform that generates a first output signal when the output signal of the differentiating circuit exceeds a predetermined first value, and generates a second output signal when the output signal exceeds a predetermined second value. A shaping circuit is connected to the waveform shaping circuit, and generates a predetermined first voltage when a fourth output signal is input from the waveform shaping circuit, and generates a predetermined first voltage when a second output signal is input. A voltage generation circuit that generates a predetermined second voltage, an AND circuit connected to the waveform shaping circuit and the impulsive noise detection circuit, and connected to the hold circuit and the voltage generation circuit, and also connected to the AND circuit. 1. An FM demodulation circuit comprising: a switch circuit for selectively switching between an output signal of a hold circuit and an output signal of a voltage generation circuit according to an output signal of an AND circuit.