JPH0155631B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a color video signal processing circuit, and relates to a color video signal processing circuit for processing a color video signal whose S/N ratio is deteriorated due to repeated copying or passing through an electronic circuit with poor characteristics. An object of the present invention is to provide a color video signal processing circuit that performs processing such that image quality can be substantially improved without significantly affecting visual perception.

The S/N of the conveyed color signal may be affected by repeated copying or passing through an electronic circuit with poor characteristics.
In order to improve the image quality of NTSC color video signals which have deteriorated in quality, conventionally this has been done by eliminating unnecessary signals using a filter having the characteristics shown in FIG. In other words, as is well known, in the NTSC color video signal, the luminance signal and the carrier color signal are selected to interleave frequencies and are band-sharing multiplexed, and as shown in FIG. The spectral distribution of the carrier color signal shown by the broken line is located in the gap in the spectral distribution of . Note that in the figure, _H indicates the horizontal scanning frequency.

As can be seen from Figure 1, the spectrum of the luminance signal is distributed around frequencies that are integral multiples of the horizontal scanning frequency _H , while the spectrum of the carrier color signal is an odd number multiple of 1/2 of the horizontal scanning frequency _H. They are distributed around the frequency of . Therefore, conventionally the third
As shown in FIG. A comb filter for luminance supplies the luminance signal to the output terminal 4 from the adding circuit 3, and an output terminal from the subtracting circuit 5 that subtracts the output signal of the 1H delay circuit 2 from the input color video signal as shown in FIG. 3B. 6, a color signal comb filter that separates and extracts the carrier color signal in the input color video signal is used to eliminate unnecessary signals.

However, in the conventional processing circuit using the above-mentioned comb filter, the 1H delay circuit 2 requires an expensive 1H delay line, a delay line drive circuit, a delay line output demodulation circuit, etc., and the overall circuit is complicated and expensive. It had some drawbacks.

The present invention eliminates the above-mentioned drawbacks, and one embodiment thereof will be described below with reference to FIGS. 4 to 6.

FIG. 4 shows a block system diagram of an embodiment of the color video signal processing circuit according to the present invention. In the figure, a band-sharing multiplexed signal of a luminance signal and a carrier color signal, such as an NTSC color video signal, which enters an input terminal 7 is sent to a buffer amplifier circuit 8 to compensate for the insertion loss of this electronic circuit. After the influence on the previous stage is prevented, the signal is supplied to the YC separation circuit 9, where the luminance signal and the carrier color signal are separated. The YC separation circuit 9 consists of a band filter 10 and a subtraction circuit 11, and extracts a carrier color signal and a color burst signal with a color subcarrier frequency of 3.58 MHz from the band filter 10, and extracts a luminance signal from the subtraction circuit 11. Of the carrier color signal and the color burst signal, the color burst signal is extracted by a burst extraction circuit 12 and supplied to an input terminal 13c of a low level component detection circuit 13 and a mixing circuit 15, which will be described later. Further, the carrier color signal is supplied to an input terminal 13a of a low-level component detection circuit 13 and an input terminal 14a of a level control circuit 14, which will be described later.

The low level component detection circuit 13 has a configuration as shown in FIG. 5, and outputs a low level component detection signal to the output terminal 13d when the carrier color signal level and the luminance signal level are smaller than the color burst signal level.
output to the terminal 14b of the level control circuit 14,
The level of the carrier color signal is suppressed. This is due to consideration of human visual characteristics. In other words, as is generally well known, the visual characteristics of humans are such that the color of an object appears different as its area becomes smaller, and as the area gradually becomes smaller and the viewing angle becomes about 2 minutes, all colors become red-orange. It can only be seen in the color system and the blue-green color system. As the area of the object becomes smaller, the human eye cannot perceive differences in color and can only perceive brightness. Even if the brightness is low, the human eye cannot distinguish between colors and can only perceive brightness.

Therefore, a low level component detection circuit 13 shown in FIG. 5 generates a control signal that suppresses low signal levels that have low color perception for humans. In the figure, the carrier color signal that has entered the input terminal 13a is applied to one input terminal of the level comparator 19 via the rectifier circuit 17. Further, the luminance signal from the subtraction circuit 11 that has entered the input terminal 13b is sent to the rectifier circuit 18.
The signal is applied to one input terminal of the level comparator 20 via. Further, the color burst signal that has entered the input terminal 13c is rectified by the rectifier circuit 21 and then branched into two branches, which are then sent to the other input terminals of the level comparators 19 and 20 via the level adjusters 22 and 23, respectively.
It is applied as a first reference voltage.

The level comparators 19 and 20 output a low level detection signal and supply it to the level comparator 24 only when the carrier color signal and the luminance signal are lower in level than the reference voltage. Level comparator 24 is level comparator 19
and 20, and when a low level detection signal is taken out from the level comparators 19 and 20, a gain attenuation control signal is supplied to the level control circuit 14 via the output terminal 13d, and the level comparator A gain attenuation control signal is also output when a lower level detection signal than 20 is extracted (however, the gain attenuation control signal has a smaller amount of gain attenuation than in the above case). Furthermore, when a low level detection signal is extracted from only the level comparator 19, the level comparator 24 outputs a control signal that sets the gain to 1. Therefore, the control signal supplied from the output terminal 13d of the low-level component detection circuit 13 to the input terminal 14b of the level control circuit 14 reduces the gain of the level control circuit 14 when the luminance signal level is lower than the color burst signal level. and when the gain is high, the gain is controlled to a predetermined constant value (1 in this case). Here, since the NTSC color video signal is negatively modulated and transmitted, the amplitude is reduced in bright areas of the screen, but the demodulated color video signal is input to the input terminal 7 in FIG. It is also assumed here that the signal level during the video period is lower as it approaches the pedestal level.

As shown in FIG. 6, the level control circuit 14 includes a gate circuit 25 that allows only the carrier color signal to pass among the carrier color signals and color burst signals inputted to the input terminal 14a, and a gate circuit 25 that outputs a signal from the output signal of the gate circuit 25. A band filter 26 that waves only carrier color signal band components, and a S/N ratio by this band filter 26.
It consists of a gain control amplifier circuit 27 that variably controls the level of the carrier color signal that is well extracted. In the gain control amplifier circuit 27, the carrier color signal is attenuated by 3 dB each by the bandpass filters 10 and 26, a total of 6 dB, so to compensate for this, the gain is normally set to 1.
(=6dB). Here, input terminal 14
The output control signal of the low-level component detection circuit 13 described above is input to b, and the luminance signal level is at the first level.
When the level of the luminance signal and carrier color signal is lower than the first and second reference signal levels, the gain of the gain control amplifier circuit 27 is controlled to be smaller than 1. For example, set it to 0 so that the amount of attenuation is maximum.

As a result, the output terminal 14 of the level control circuit 14
A carrier color signal whose level is controlled in accordance with the luminance signal level is taken out from c and is supplied to a mixing circuit 15 shown in FIG. The mixing circuit 15 receives the luminance signal from the YC separation circuit 9 and the burst sampling circuit 1.
The color burst signal from 2 and the carrier color signal from the level control circuit 14 are mixed and multiplexed to form a color video signal, which is output to the output terminal 16. This output color video signal is reproduced on a monitor, but if the reproduced image is a dark image with low brightness and low color perception, the carrier color signal is suppressed, so the visual image quality can be improved. . A similar effect can also be obtained with respect to noise in a relatively narrow portion such as chromaticity noise mixed into a luminance signal.

Note that in the above embodiments, the levels of the luminance signal and the carrier color signal are compared based on the color burst signal level, but the reference level can be set arbitrarily.

As described above, the color video signal processing circuit according to the present invention processes the brightness signal and the carrier color signal from a color video signal obtained by band-sharing multiplexing the brightness signal and the carrier color signal, respectively, using a band filter and a subtraction circuit. a separating circuit, which compares the level of the separated luminance signal with a first reference level to obtain a first level detection signal, and compares the level of the separated carrier color signal with a second reference level to obtain a second level detection signal; a comparison means for obtaining a level detection signal; and a first detection signal from the comparison means.
and a second level detection signal, when the luminance signal and the carrier color signal are at a lower level than the first and second reference levels, respectively, the first value is set, and only the luminance signal is lower than the first reference level. A circuit that generates a level control signal that is a second value corresponding to the brightness signal level at that time when the level is the same, and a third value otherwise, and a circuit that generates a level control signal that is the first value when the level control signal is the first value. Attenuates the separated carrier color signal by the maximum amount of attenuation, when the second value, attenuates the carrier color signal by a larger attenuation amount as the luminance signal level is lower according to the second value; When the value is , a level control circuit that makes the amount of attenuation for the carrier color signal zero, and a color video signal that is processed by mixing the separated luminance signal and the output carrier color signal and color burst signal of the level control circuit, respectively. Because you get a mixed circuit and more,
Without using expensive circuit components such as a 1H delay line, the carrier color signal level is suppressed in areas where the luminance signal level is low, the luminance signal level is low, and the carrier color signal level is low, and the luminance signal is Since the carrier color signal can be extracted without attenuation for the portion where the level and the carrier color signal level are above a certain value, the image quality of the portion that cannot be perceived as color can be improved with an inexpensive configuration without significantly affecting the visual sense. In addition, in the case of a dark screen where only the luminance signal is lower than the first reference level, the amount of attenuation of the carrier color signal can be analogously adjusted according to the luminance signal level at that time, that is, according to the darkness of the screen. Because it is variable, it is possible to obtain the optimal image quality improvement effect according to the degree of color perception rather than simply suppressing the carrier color signal, and also to suppress noise in relatively thin parts such as chromaticity noise mixed in the luminance signal. Similarly, the image quality can be improved, and from the above, when the carrier color signal and the luminance signal are recorded separately and applied to a VTR for playback, it has the feature that the VTR playback image can be significantly improved. be.

[Brief explanation of drawings]

Figure 1 is a diagram showing the frequency spectrum of an NTSC color video signal, Figure 2 is a diagram showing an example of the frequency characteristics of a comb filter, and Figures A and B are comb filters for luminance signals and carrier color signals, respectively. FIG. 4 is a block system diagram showing the configuration of the filter; FIG. 4 is a block system diagram showing an embodiment of the device of the present invention; FIGS.
Each figure is a block system diagram showing an embodiment of each essential part of FIG. 4. 7...Color video signal input terminal, 9...YC separation circuit, 10, 26...Band filter, 11...Subtraction circuit, 13...Low level component detection circuit, 14...Level control circuit, 15...Mixing circuit, 16...Color video signal Output terminal, 19, 20, 24...level comparator,
27...Gain control amplifier circuit.

Claims (1)

[Scope of Claims] 1. A circuit that separates the luminance signal and the carrier chrominance signal from a color video signal obtained by band-sharing multiplexing the luminance signal and the carrier chrominance signal, respectively, using a bandpass filter and a subtraction circuit; The level of the separated carrier color signal is compared with a first reference level to obtain a first level detection signal, and the level of the separated carrier color signal is compared with a second reference level to obtain a second level detection signal. a comparison means; and based on the first and second level detection signals from the comparison means, when the luminance signal and the carrier color signal are at a lower level than the first and second reference levels, respectively, a first level detection signal is detected. level control, which is a second value corresponding to the luminance signal level at that time when only the luminance signal is at a lower level than the first reference level, and a third value otherwise. a circuit for generating a signal; attenuating the separated carrier color signal by a maximum attenuation amount when the level control signal is at the first value;
When the value is, the carrier color signal is attenuated by a larger amount of attenuation as the luminance signal level is lower according to the second value, and when the value is the third value, the amount of attenuation for the carrier color signal is zero. and a mixing circuit that obtains a processed color video signal by mixing the separated luminance signal, the output carrier color signal and the color burst signal of the level control circuit, respectively. color video signal processing circuit.