GB2057223A - Color signal processing circuit for a color television receiver - Google Patents

Abstract

Disclosed is a color signal processing circuit for a color television receiver for carrying out ACC control and colour killer control on the basis of respective mutually independent signal levels of the color burst signal, and which includes a means (17) which detects a peak value of the burst signal as amplified by a first variable amplifying means (11) to thereby ACC control said first amplifying means (11) and control color saturation, and an independent color killing means detecting (18) the amplitude of the burst signal as amplified by a second amplifying means (19) with a fixed gain. By synchronous detection (18) of a burst signal which has not been subjected to ACC control, improved noise discrimination is achieved. <IMAGE>

Description

SPECIFICATION Color signal processing circuit for a color television receiver The present invention relates to a color signal processing circuit for a color television receiver and, more particularly, to an automatic color saturation control circuit (referred to as an ACC circuit) and a color killer circuit in the color television receiver (referred to as a color TV).

A color signal processing circuit of the color TV includes a color killer circuit for killing a color to convert a color picture into a monochromatic picture when a color signal becomes small.

It is a common practice that the color killing is performed by detecting an amplitude of a color burst signal. In this case, an automatic color saturation control is active since a color burst signal is amplified, together with a chroma signal in a usual color TV.

Particularly, when the automatic color saturation control is performed by detecting a crest value of the color burst signal, the ACC operates due to noise in a weak electric field. As a result, the amplitude of the color burst signal detected for the color killing operation becomes smaller than that of the color burst signal when it is used for its own purpose, so that the color killing circuit early operates.

Accordingly, an object of the present invention is to provide a color signal processing circuit for a color TV which provides an easy-to-see picture on the screen when a color noise is large and stabilizes an operating point for the color killing.

The present invention may be summerized as a color signal processing circuit for a color television receiver comprising a color signal input terminal, means connected to the input terminal for automatically controlling color saturation by detecting a crest value of a color burst signal containing noise component, an amplifier with a fixed gain connected to the terminal, and means connected to the amplifierfor killing color by detecting the amplitude of the burst signal.

The above and other objects of the present invention will be apparent from the following description taken in connection with the accompanying drawings, in which: Figure 1 is a block diagram of a color signal processing circuit which provides a background of the invention; Figure2 is a graphical representation of an input/output characteristic useful in explaining the color signal processing circuit shown in Figure 1; and Figure 3 is a block diagram of an embodiment of the present invention.

Before preceding with the present invention, a conventional signal processing circuit will be described in detail referring to Figures 1 and 2, for a better understanding of the invention. In Figure 1, a carrier chrominance signal containing a color burst signal is applied to an input terminal 1. The chrominance carrier signal is applied to a first chrominance cinnil smnlifior 11 and a second chrominance signal amplifier 12 where it is successively amplified. The second chrominance signal amplifier 12 is provided with a connection terminal 2 connected to a bypass capacitor 21. The carrier chrominance signal sufficiently amplified by the two amplifiers 11 and 12 is applied to a separation circuit 13 where it is separated into a carrier chrominance signal and a color bust signal.The carrier chrominance signal is applied to a color saturation controller 14 with a color saturation control terminal 3 where it is controlled to have a proper amplitude, and then is led to exterior via an output circuit 15 and a color signal output terminal 4.

The color burst signal separated from the carrier chrominance signal is led to blocks for performing ACC, color killing and automatic phase control (APC). Figure 1 is illustrated with omission of the APC block. The color burst signal is fed to a crest value detector 17 for detecting a crest value of the color burst signal. The crest value detector 17 is provided with a low-pass filter connecting terminal 5 which is connected to a burst signal crest value hold filter including a capacitor 22 and a resistor 23. The crest value of the color burst signal detected by the crest value detector 17 is amplified by an amplifier 16 to have a given level and then is applied to the color signal amplifier 11. Then, the gain of the first color signal amplifier 11 is controlled so that the amplitude of the color burst signal is constant.

The above-mentioned operation graphically depicted is as shown in Figure 2. In Figure 2, an ordinate represents an amplitude of the carrier chrominance signal while an abscissa represents an amplitude of the color burst signal at the input terminal 1. When the signal level of the carrier chrominance siganl applied to the input terminal 1 is small, the gain of the color signal amplifier 11 is at maximum, so that the output signal from the amplifier increases proportional to the input signal.

When the crest value of the color burst signal is detected by the crest value detector 17 with increase of the input signal, the gain of the color signal amplifier 11 decreases, so that the output signal of the amplifier becomes constant.

Accordingly, a relation between the innut and output signals of the circuit shown in Figure 1 is as indicated by a solid line a shown in Figure 2.

A synchronizing detector 18 multiplies the color burst signal applied from the separation circuit 13 by a color subcarrier signal applied to a color subcarrier input terminal 6, thereby to detect the amplitude of the color burst signal. The synchronizing detector of which the connection terminal 7 is connected to a capacitor 24, applies an output signal to the color saturation controller 14 when the amplitude of the burst signal becomes extremely small. thereby to reduce the output level of the carrier chrominance signal to zero and effect the color kiilii;8. This is depicted by an arrow portion of the solid line a shown in Figure 2 and is provided to prevent color noise from appearing in the picture when the input signal is very small or the receiver is in the monochromatic broadcasting receiving mode.

Note here that the amplitude of the color burst signal is detected in both the ACC and color kille circuits, but the methods for the detection is different from each other in those circuits. In the ACC portion, the crest value is used for the detection of the color burst signal. Therefore, when the color burst signal is small in a weak electric field in an actual color television system, noise with larger amplitude than that of the color burst signal is superposed on the color burst signal and the crest value is detected. For this reason, in case where the amplitude of the color burst signal is capable of providing the maximum gain of the color signal amplifier 11, the noise component restricts the gain of the color signal amplifier 11,with the resultthatthe amplitude of the color signal at the output terminal 4 becomes smallerthan that when no noise exists.In other words, the color signal containing much noise is small and thus the color noise on the picture of the color television set also is small to provide an easy-to-see picture. A dotted line b shown in Figure 2 indicates a chrominance signal output containing no noise of this case.

The detection of the amplitude of the bust signal for the color killer has a problem of an erroneous operation due to noise. To avoid this, a synchronizing detector is commonly used which is highly resistive to the noise. In the circuit construction shown in Figure 1, as the noise becomes larger, the amplitude of the color burst signal of its own applied to the synchronizing detector 18 also decreases correspondingly. As a result, the killing operation of color is more rapid. When the synchronizing detection is used for the detection in the ACC portion, its proper characteristic by the solid line a is obtained.

In this case, the color noise also is amplified as it is, so that a picture obtained is bad in quality with much color noise.

The present invention provides a color signal processing circuit eliminating the above-mentioned disadvantages. An embodiment of the color signal processing circuit is illustrated in Figure 3.

The circuit construction of Figure 3 is the same as in Figure 1 except the following features. The circuit shown in Figure 1 separates the color burst signal from the carrier chrominance signal by the separation circuit 13 and applies the separated color burst signal to the synchronizing detector 18. The circuit of Figure 3 amplifies the color burst signal applied to the input terminal 1 by an amplifier 19 with a fixed amplification to a given signal level and then applied it to the synchronizing detector 18. With the provision of the amplifier 19, the color burst signal is always amplified with a fixed rate. Further, since the amplitude of the color burst signal is detected by the synchronizing detector 18 which is stable against the noise, the operating point of the color killing is invariable against the noise. Additionally, the same circuit as the conventional one is employed for the ACC, the gain of the color signal amplifier 11 is restricted to be low even if the color noise is large, so that the color noise appearing on the screen thereby to obtain a high quality of picture.

As described above, when the color is killed by detecting the amplitude of the color burst signal, the synchronizing detection is performed by applying the color burst signal to the amplifier with a fixed amplification. Therefore, the conventional defect is eliminated in which the noise component reduces the color burst signal thereby to quicken the color killing operation. As a result, the operating point of the color killing is stabilized.

Claims (3)

1. A color signal processing circuit for a color television receiver having an automatic color saturation control means including input terminal (1) to which a carrier chrominance signal containing a color burst signal is applied, first amplifying means 11 for amplifying said carrier chrominance signal applied to said input terminal, separating means 13 for separating a color burst signal from said carrier chrominance signal amplified by said first amplifying means, and a crest value detecting means 17 for controlling an amplification of said first amplifying means by detecting a crest value of the color burst signal separated by said separating means, said color signal processing circuit comprising: second amplifying means 19 for amplifying a color burst signal applied to said input terminal with a fixed gain; and color killing means for killing color by detecting 18 the amplitude of the color burst signal amplified by said second amplifying means and restricting 14the signal level of said carrier chrominance signal derived from said automatic color saturation control means when the amplitude detected is below a given value.

2. A color signal processing circuit according to Claim 1, in which said color killing means includes a synchronizing detecting means for detecting the amplitude of said color burst signal by comparing said color burst signal with a color sub-carrier signal.

3. Acolorsignal processing circuit substantially as herein described with reference to Figure 3 of the accompanying drawings.