JPH0336153Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Technical field of the invention The invention relates to an image receiving device, and in particular,
Switches the image signal band in response to changes in the N ratio,
The present invention relates to a noise suppression circuit suitable for use in improving image discrimination.

(b) Background of the technology Normally, noise that enters an image receiving device includes external noise and constant thermal noise generated at the input end of the image receiving device. In the case of thermal noise, since the generation level is approximately constant, the signal-to-noise ratio (hereinafter abbreviated as S/N ratio) is proportional to the input level of the image signal. Therefore, if the input level of the image signal decreases due to a decrease in output on the transmitting side, interference with the propagation path, generation of an interference signal, etc., the S/N ratio deteriorates.

In addition, external noises include those generated by the transmitter itself, interfering signals caused by cross-modulation, and causes such as lightning. This is possible and the S/N ratio deteriorates.

Therefore, there is a need for the development of means for increasing the degree of image discrimination in response to the degree of deterioration of the S/N ratio.

(c) Conventional technology and problems Conventionally, when the S/N ratio of the input image signal is poor, image quality interference such as falling snow occurs on the received screen, making it difficult to decipher the image. .

(d) Purpose of the invention The purpose of the invention is to develop a noise suppression circuit that narrows the reception band of image signals in response to the deterioration of the S/N ratio, improves the S/N ratio, and increases the degree of image discrimination. It is about providing.

(e) Structure of the invention According to the invention, in an image receiving apparatus that receives and demodulates a color image carrier wave and then outputs an image signal subjected to a predetermined basic band limitation, the color image carrier wave is A signal-to-noise ratio detecting means 15 for detecting a signal-to-noise ratio, and a first detecting means corresponding to the respective thresholds when the output of the signal-to-noise ratio detecting means 15 exceeds predetermined high and low thresholds. an identification circuit 16 that converts the image signal into a second switching control signal, a first image signal output system that branches the image signal into two and outputs one directly, and regulates the upper limit of the pass band of the other of the two branches. The output of the narrow-band low-pass filter 9 that controls the output of the narrow-band low-pass filter 9 and the output of the narrow-band filter 10 that regulates the passband width of only the color signal included in the other of the two branches to a predetermined narrow band width are combined into the second A second image signal output system is formed by combining the output of the second switching means 11, which is connected/disconnected by a switching control signal, and each output of the first and second image signal output systems is combined with the output of the first and second image signal output systems. The first screen is selected using the switching control signal of
This is achieved by providing a noise suppression circuit for an image receiving apparatus characterized by having a switching means 13 of.

(f) Example of implementation of the idea Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a block diagram of a noise suppression circuit of an image receiving apparatus according to the present invention.

In the figure, 1 is a receiving antenna, 2 is a frequency conversion circuit, 3 is a local oscillator, 4 is an intermediate frequency amplification stage, and 5
is a demodulation circuit, 6 is a low-frequency amplifier for the basic band, 7 and 8
and 12 are resistor hybrids, 9 is a narrowband low-frequency wave generator, 10 is a narrowband wave generator, 11 and 13 are relays, 11A, 11B, 13A, 13B, and 13C are contacts, 14 is an output terminal, and 15 is a detector. , 16 is an identification circuit composed of comparators 16A and 16B.

In the figure, when a carrier wave modulated by an image signal having a color signal component is input to a receiving antenna 1, it is converted to a required intermediate frequency by a frequency conversion circuit 2 having a local oscillator 3 according to a known image receiving circuit. It is converted into an image signal by a demodulator 5 via an intermediate frequency amplification stage 4, and further converted into an image signal having a band of about 5 MHz by a low frequency converter 6 which secures the normal fundamental band. hybrid 7
The first branch forms a first image signal output system that outputs directly without any restrictions, and normally the first switching means 13 is the contact 1 of the relay 13.
It is output to the output terminal 14 via 3A and 13C,
The output is displayed on a cathode ray tube (not shown).

The other branch output of the resistor hybrid 7 is further branched into two at the next resistor hybrid 8, and the first
The branch is connected to one of the composite terminals of the next resistor hybrid 12 after the band is sufficiently narrowed (for example, about 1 MHz) by a narrow band low frequency filter 9.

The other branch output of the resistor hybrid 8 is a subcarrier that constitutes a color signal included in the image signal.
Narrowband filter 10 with 3dB attenuation centered at 3.58MHz
(for example, a passband width of about 1 MHz) and the relay contacts 11A and 1 of the relay 11, which becomes the next second switching means 11.
1B to the other composite terminal of the next resistor hybrid 12, the composite output forms the second image signal output system, and is connected to the contact 13B of the relay 13.
It is connected to the.

Further, as means for detecting the input level of the carrier wave from within the intermediate frequency amplification stage 4, that is, the S/N ratio, for example,
A detector 15 connected to an AGC circuit (not shown) or the like is provided to convert the input level of the color image carrier wave into a DC component, which is branched into comparators 16A and 16B of the next identification circuit 16. Reference voltage levels 1 and 2 are applied to the comparators 16A and 16B, respectively, as comparison thresholds (not shown). Level 1 is set in a state where the input level of the carrier wave has decreased and the required S/N ratio has begun to deteriorate, and the output of the comparator 16A that is below level 1 is referred to as a first switching control signal. In addition, level 2 is set to the ultimate state where the S/N ratio is further reduced and the S/N ratio is considerably degraded.
The output of comparator 16B, which is output below, is
This is called the switching control signal.

Therefore, the output of the comparator 16A drives the contacts 13B and 13C of the relay 13 in the direction of connecting them, and the output of the comparator 16B drives the relay 1
Contacts 11A and 11B of No. 1 are connected so as to be driven in the cutting direction.

The operation of the noise suppression circuit according to the present invention will be described below. When the input level of the carrier wave at antenna 1 is sufficiently high, the S/
The N ratio becomes good. In other words, the noise decreases in inverse proportion to the input level. Therefore, the S/N ratio can be detected by detecting the input level. It is a known method to know the input level of the carrier wave by detecting the output of an AGC circuit (not shown) with the detector 15. If it is sufficiently high, neither the comparators 16A nor 16B output it, and therefore the image signal demodulated by the demodulator 5 is output from the low frequency converter 6 which secures the fundamental band, and is sent to the relay with its original good bandwidth. 13 contacts 13
It is output to the output terminal 14 via A and 13C.

Now, when the input level of the carrier wave decreases and the required S/N ratio begins to deteriorate, the input voltage of the comparator 16A reaches level 1, and the output of the comparator 16A changes between the contacts 13B and 13 of the relay 13.
Drive C in the direction of connection. The relay 13 operated by the output of this comparator 16A is the first switching means 13, and the image signal of the output of the resistor hybrid 7 having the basic bandwidth is further transmitted via the resistor hybrid 8 to the narrow band Range filter 9
narrowed to a narrow bandwidth (e.g., about 1MHz),
On the other hand, the color signal is narrowed to a narrow bandwidth (approximately 1 MHz with 3 dB attenuation) centered on the subcarrier 3.58 MHz, and connected to the resistive hybrid 12 via relay 11.
It is combined with the output of the bandpass filter 9 and output to the output terminal 14 via the relay 13. In this case, if the normal bandwidth of about 5 MHz is narrowed to 2 MHz, the S/N ratio can be improved by about 4 dB with respect to thermal noise.

Next, when the input level of the carrier wave further decreases and the required S/N ratio deteriorates considerably, and the input voltage of the comparator 16B reaches level 2,
The output of comparator 16B is contact 1 of relay 11
Drive in the direction to cut 1A and 11B. Relay 11 operated by the output of this comparator 16B
is the second switching means 11, and the output of the bandpass filter 10 is stopped. Therefore, only the output of the narrow band low frequency filter 9 is taken out to the output terminal 14. In this case, as described above, since the change in bandwidth from the normal case can be reduced from about 5 MHz to about 1 MHz or less, it is possible to simply improve the S/N ratio by about 7 dB or more with respect to thermal noise.

Narrowing the bandwidth means sacrificing the resolution of the screen, and the output of the low frequency converter 9 results in a black and white screen display, but the S/N ratio can be automatically improved.

In this embodiment, the S/N ratio is evaluated by detecting the incoming signal level, but at the output of the demodulator, only the noise is extracted using a narrow band waver at a frequency higher than the image signal band, and the noise is separated and output. It may be a method.

In addition, in this embodiment, level 1 and level 2
Band switching was performed for each stage, but the arrangement of comparators and waveformers for multiple stages can be easily considered.

It should be noted that any narrowband transducer can easily be made into a variable bandwidth mechanism.

(g) Effects of the invention As explained in detail above, the noise suppression circuit of the image receiving device of the invention automatically switches the passband width of the image signal in response to the degree of deterioration of the S/N ratio and maximizes the passband width of the image signal. This has the effect of improving the S/N ratio by about 7 dB or more.

[Brief explanation of the drawing]

FIG. 1 shows a block diagram of a noise suppression circuit of an image receiving apparatus according to the present invention. In the figure, 1 is an antenna, 4 is an intermediate frequency amplification stage, 5 is a demodulator, 6 is a low-frequency amplifier for the fundamental band, and 7
, 8 and 12 are resistor hybrids, 9 is a narrow band low frequency wave generator, 10 is a narrow band wave generator, 11 and 13 are relays, 15 is a detector, and 16 is an identification circuit.

Claims (1)

[Claims for Utility Model Registration] In an image receiving device that receives and demodulates a color image carrier wave and then outputs an image signal subjected to a predetermined fundamental band limit, a signal for detecting the signal-to-noise ratio of the color image carrier wave. a noise-to-noise ratio detection means 15; and when the output of the signal-to-noise ratio detection means 15 exceeds predetermined high and low thresholds, first and second switching control signals corresponding to the respective thresholds are activated. a first image signal output system that branches the image signal into two and outputs one directly; and a narrowband low-pass filter that regulates the upper limit of the passband of the other of the two branches. 9 and the output of a narrow band filter 10 that regulates the pass band width of only the color signal included in the other of the two branches to a predetermined narrow band width, are connected/disconnected by the second switching control signal. Second to do
a second image signal output system which is formed by combining the output of the switching means 11; and a first image signal output system which selects each output of the first and second image signal output systems using the first switching control signal. 1. A noise suppression circuit for an image receiving apparatus, comprising a switching means 13.