JPH04286211A - Automatic frequency control circuit - Google Patents

Abstract

PURPOSE:To prevent the deterioration in S/N ratio by comparing the DC level of an input signal with the DC level after frequency demodulation of a frequency-modulated picture signal, and controlling center frequency by feeding back this level difference. CONSTITUTION:The input signal 1 is frequency-modulated by a frequency modulator 3, and a frequency-modulated signal 2 is outputted. Further, the frequency- modulated signal 2 is frequency-demodulated by a frequency demodulator 4. A DC component is taken out of the frequency-demodulated picture signal by a low pass filter 9. Besides, the DC component is taken out of the input picture signal by the low pass filter 8. Then, the DC level of the frequency- demodulated picture signal obtained by the low pass filter 9 and the DC level of the input picture signal obtained by the low pass filter 8 are compared with each other by a comparator 7, and this difference is amplified and outputted. Namely, by comparing the output of the comparator with the level difference of the input picture signal, and outputting a result, automatic frequency control is executed by giving negative feedback.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic frequency control circuit for image signals.

0002

2. Description of the Related Art As shown in FIG. 2, a conventional circuit includes a frequency modulator 3 that modulates an input signal 1 into a frequency modulated signal 2, a frequency demodulator 4 that demodulates the frequency modulated signal 2, and a frequency demodulator 4 that demodulates the frequency modulated signal 2. A low-pass filter 5 is provided for extracting the DC component from the signal demodulated by the filter 4. If there is an error in the center frequency of the frequency modulator 3 from the reference, a DC component is generated by inputting the output signal of the frequency demodulator 4 to the low-pass filter 5, and the generated DC component is passed to the frequency modulator 3. After returning to Japan, automatic frequency control was carried out.

0003

[Problem to be solved by the invention] This conventional automatic frequency control circuit generates a DC component and performs feedback only when there is an error in the center frequency, so it can be used for audio signals etc. that do not have a DC component. , it could not be used for image signals containing DC components. In addition, a conventional automatic frequency circuit can be used by removing the DC component from the image signal through a capacitor, but by removing the DC component, the image signal with a dynamic range of 1V becomes approximately 1.4V in the case of white and black image signals. This results in a dynamic range of 100%, which necessitates the need to reduce the amplitude, which has the disadvantage of causing deterioration of the S/N ratio.

SUMMARY OF THE INVENTION The present invention aims to eliminate such drawbacks and provides an automatic frequency control circuit that can automatically frequency control an image signal without deteriorating the S/N ratio.

[0005]

[Means for Solving the Problems] The present invention provides an automatic frequency control circuit equipped with a frequency modulator that frequency-modulates an input image signal and outputs a frequency-modulated signal. , a first low-pass filter that extracts a DC component from the image signal frequency demodulated by the frequency demodulator, a second low-pass filter that extracts the DC component from the input image signal, and the first low-pass filter that extracts the DC component from the input image signal. A first comparison in which the DC level of the frequency-demodulated image signal taken out by the low-pass filter is compared with the DC level of the input image signal taken out by the second low-pass filter, and the level difference is amplified and output. The frequency modulator is characterized by comprising a circuit means for negatively feeding back the output level of the first comparator to the frequency modulator.

[0006]

[Operation] The DC level of the input image signal is compared with the DC level of the frequency-modulated image signal, and the level difference is fed back to the frequency modulator to control the center frequency. This results in
Automatic frequency control of an image signal including a DC component can be performed without deteriorating the S/N ratio.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of this embodiment. That is, as shown in FIG. 1, this embodiment includes a frequency modulator 3 that frequency-modulates an input signal 1 and outputs a frequency-modulated signal. Frequency demodulator 4 that demodulates the frequency of
, a low-pass filter 9 that extracts the DC component from the image signal frequency demodulated by the frequency demodulator 4, a low-pass filter 8 that extracts the DC component from the input signal 1, and a frequency demodulator extracted by the low-pass filter 9. The DC level of the image signal and the low-pass filter 8
A comparator 7 that compares the DC level of the input image signal taken out with the DC level of the input image signal, amplifies and outputs the level difference, and this comparator 7
A comparator 6 is provided as a circuit means for negatively feeding the output level of the frequency modulator 3 back to the frequency modulator 3.

Next, the operation of this embodiment will be explained. A frequency modulator 3 frequency-modulates an input signal 1 and outputs a frequency-modulated signal 2. A frequency demodulator 4 demodulates the frequency modulated signal 2 . A low-pass filter 9 extracts the DC component from the frequency demodulated image signal. Further, a low-pass filter 8 extracts a DC component from the input image signal. A comparator 7 compares the DC level of the frequency-demodulated image signal obtained by the low-pass filter 9 with the DC level of the input image signal obtained by the low-pass filter 8, and amplifies and outputs the difference. The comparator 6 compares the output of the comparator 7 with the level difference between the input image signal and outputs the result, thereby applying negative feedback and performing automatic frequency control.

[0009]

Effects of the Invention As explained above, the present invention compares the DC level of an input image signal with the DC level of a frequency-modulated image signal after frequency demodulation, and controls the center frequency by feeding back the level difference. Therefore, it is possible to automatically control the frequency of the image signal containing the DC component.

[Brief explanation of the drawing]

FIG. 1 is a block configuration diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a block configuration diagram showing the configuration of a conventional example.

[Explanation of symbols]

1 Input signal 2 Frequency modulation signal 3 Frequency modulator 4 Frequency demodulator 5 Low-pass filter 6, 7 Comparator 8, 9 Low-pass filter

Claims (1)

[Claims] Claim 1: An automatic frequency control circuit comprising a frequency modulator that frequency-modulates an input image signal and outputs a frequency-modulated signal, the frequency demodulator demodulating the frequency of the frequency-modulated signal; a first low-pass filter that extracts the DC component from the demodulated image signal; a second low-pass filter that extracts the DC component from the input image signal; and frequency demodulation extracted by the first low-pass filter. a first comparator that compares the DC level of the input image signal extracted by the second low-pass filter with the DC level of the input image signal extracted by the second low-pass filter, and amplifies and outputs the level difference; and circuit means for negatively feeding back the output level of the frequency modulator to the frequency modulator.