Chrominance-luminance separator

#This invention relates to television apparatus, and, more particularly, to apparatus for the separation of the luminance and chrominance components of an NTSC color television signal.

#Color decoders employed in the NTSC system of color television require as a first step the separation of the luminance and chrominance components of the video signal, this function occasionally being performed by a comb filter. In a typical comb filter, the incoming video signal is separated by suitable delay lines into a main (M) line component and two adjacent line components, usually referred to as "top" (T) and "bottom" (B) signals, the main and top signals being delayed by one and two horizontal lines, respectively, relative to the bottom signal. Because the chrominance signal reverses in phase from line to line, when the main (M), top (T) and bottom (B) signals are added together in the amplitude ratios of T/4 + B/4 + M/2, the chrominance component of the video signal is cancelled thereby to derive the luminance component. By the same token, when M/2 is subtracted from the sum of B/4 + T/4, the luminance component is cancelled so as to produce the negative of the chrominance component, theoretically free of any luminance signal.

#As a practical matter, however, the above-described cancellation obtains only if there have been no changes in the picture as it is "scanned" from top to bottom through the three lines; that is, cancellation of chrominance is complete only if it is assumed that the chrominance is identical on all three lines. This assumption is usually true for large areas of the picture, but should a color transition take place vertically, such as might occur with a change in hue, so that the chrominance on the top line does not have the same phase as the chrominance on the bottom and main lines, some of the chrominance signal is not cancelled, and spurious chrominance signals appear in the luminance signal. Because it is the luminance component that determines the brightness of the picture, the uncancelled chrominance component with its 3.58MHz subcarrier superimposed on the luminance signal causes a very noticeable scintillation in the display along the line of color transition, particularly in large screen television displays where every line is quite marked.

#A better understanding of the invention will be had from the following detailed description, taken in conjunction with the accompanying drawing, the single FIGURE of which is a block diagram of a comb filter embodying the invention.

#In the system illustrated in FIG. 1, the NTSC video signal, which contains luminance and chrominance components, is applied simultaneously to the input terminal of a two-line delay line 10 through a conductor 12, to the input terminal of a one-line delay line 14 through a conductor 16 and through a conductor 18, which may include a resistor 20, to one input terminal of a summing circuit 22. The two-line delay line 10 delays the input video signal for a period of two lines or 127 microseconds, and the one-line delay line 14 delays the input video signal for a period of 63.5 microseconds or one line. By introducing time delays corresponding to one and two lines of scanned area, three adjacent line components of the video signal are aligned with respect to time. For example, lines 5, 3 and 1 are present simultaneously at the conductor 18 and at the output terminals of delay lines 14 and 10 respectively, followed by lines 7, 5 and 3, and then lines 9, 7 and 5, during each odd field scansion and lines 6, 4 and 2 and then 8, 6 and 4, etc., are present simultaneously at the conductor 18 and at the output terminals of the delay lines 14 and 10, respectively, during each even field scansion. The once-delayed component of the video signal becomes the main line (M) component, the twice-delayed component becomes what is typically termed the "top" (T) component and the undelayed component at conductor 18 is known as the "bottom" (B) component.

#Thereupon, the B, M and T components are coupled to the summing circuit 22 through respective attenuators 24, 26 and 28 having values such that the components are added in summing circuit 22 in the amplitude ratios of B/4 + T/4 + M/2. Because the chrominance signal reverses in phase from line to line, when the components are added in this ratio, the chrominance component of the video signal is cancelled and the luminance component is obtained at the output of summing circuit 22. By the same token, by subtracting M/2 from the sum of B/4 + T/4 in a second summing circuit 30, the luminance component of the video signal is cancelled so as to produce at the output of summing circuit 30 the negative of the chrominance component, theoretically free of any luminance signal.

#It will be evident that the above-described cancellation of the chrominance component to obtain the luminance component is achieved only if there has been no vertical color change in the picture in the time interval occurring between the three lines involved in the cancellation process; that is, cancellation is complete only if it is assumed that the chrominance is vertically identical on all three lines. Although this assumption is realistic for large areas of most scenes, there frequently are vertical color transitions, such as might occur with a change in hue, such that the chrominance of the "top" line would not have the same phase as the chrominance component on the bottom and main lines, with the result that the chrominance cancellation is incomplete, causing spurious chrominance signals to appear in the luminance component. The uncancelled chrominance component with its 3.58MHz subcarrier superimposed on the luminance component causes a very noticeable scintillation in the display at the point of color transition.