The present invention relates to a pure red color detection circuit and a color compensation circuit compensating color differential signals by using it and emphasizing the red color.
Television receivers having a high additional value have been developed according to their increased screen sizes and high quality color television receivers having a good red color reproducibility are desired.
Block diagrams of a red color detection circuit and a color compensation circuit in accordance with the prior art disclosed in Japanese Patent Laid-Open No.5-233667 are shown in FIGS. 6 and 8, respectively. The function of a red color detection circuit is explained below, referring to FIGS. 6 and 7.
FIG. 6 shows a block diagram of a red color detection circuit 101 of the prior art and FIG. 7 shows signal waveforms at various points of red color detection circuit 101. Red color detection circuit 101 includes an absolute value outputting circuit 101b, a subtracter 101c and a limiter 101d. In FIG. 7, waveforms A and B are color differential signals (R-Y) and (B-Y), respectively which are input to red color detection circuit 101 and the color differential signal (R-Y) is behind the color differential signal (B-Y) by 90 degrees in phase. An absolute value B-Y is made from the input color differential signal (B-Y) at absolute value outputting circuit 101b and is output (waveform C) and is input to subtracter 101c. Subtracting the output of absolute value outputting circuit 101b B-Y (waveform C) from color differential signal (R-Y) (waveform A) at subtracter 101c (waveform D) and only a positive part is outputted from limiter 101d (waveform E).
Because this signal exists near the phase of 90 degrees, the detected output is a signal corresponding to a red color in the input chrominance signal.
The function of a color compensation circuit of the prior art is explained below, referring to FIG. 8. The color compensation circuit includes red color detection circuit 101, an amplitude control circuit 102 for a color differential signal (R-Y) and another amplitude control circuit 103 for a color differential signal (B-Y). A color differential signal (R-Y) and red color detected signal (waveform E) in FIG. 7 output from red color detection circuit 101 are supplied to amplitude control circuit 102 and the color differential signal (R-Y) is controlled by the red color detected signal so that the amplitude decreases. Also a color differential signal (B-Y) and the red color detected signal output from red color detection circuit 101 are supplied to amplitude control circuit 103 and also the color differential signal (B-Y) is controlled by the red color detected signal so that the amplitude decreases. Thus, suppressing a signal having a large red component prevents red color from saturation.
This circuit, however, aims to prevent red color saturation and a different kind of apparatus is necessary for improving red color reproducibility which is intended in the present invention. The present invention detects a signal closer to a pure red color in a red color detection circuit and decreases a Y/C ratio, emphasizes a red color without saturating the red color and as a result, improves red color reproducibility by decreasing the luminance signal Y by the red color detection signal and bringing a color phase of the chrominance signal C closer to a red color by the red color detection signal in the color compensation circuit.
A pure red color detection circuit of the present invention slices a subcarrier modulated color differential signal (R-Y) at a designated level at a slice circuit, generates an absolute value signal of a subcarrier modulated color differential signal (B-Y) at an absolute value outputting circuit, subtracts the absolute value B-Y from the sliced signal at a subtracter, takes out only a positive part of the subtracted signal at a limiter and outputs. The output signal is a signal having a narrow phase range near 90 degrees in the chrominance signal and as a result, a pure red color is detected.
A color compensation circuit of the present invention includes the above mentioned pure red color detection circuit, a Y signal compensation block, an (R-Y) signal compensation block and a (B-Y) signal compensation block and decreases a Y/C ratio, makes the chrominance signal containing a red color signal higher than a designated level close to a pure red color, emphasizes the red color without saturation and can improve red color reproducibility by outputting a luminance signal Yxe2x80x2 which is made by subtracting the pure red signal detected at the pure red detection circuit from the input luminance signal Y, at the Y signal compensation block, outputting a color differential signal (R-Y)xe2x80x2 which is made by adding the pure red signal detected at the pure red color detection circuit to the input (R-Y) signal, at the (R-Y) signal compensation block and outputting a color differential signal (B-Y)xe2x80x2 which is made by subtracting the pure red signal detected at the pure red detection circuit from the input color differential signal (B-Y), at the (B-Y) signal compensation block.
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