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Chapter: Television and Video Engineering - Essentials of Color Television

Grey Scale Tracking

It may be recalled that red, green and blue lights must combine in definite proportions to produce white light. The three phosphors have different efficiencies. Also the three guns may not have identical I p /V gk characteristics and cut-off points.

GREY SCALE TRACKING

 

It may be recalled that red, green and blue lights must combine in definite proportions to produce white light. The three phosphors have different efficiencies. Also the three guns may not have identical I p /V gk characteristics and cut-off points.

 Therefore, it becomes necessary to incorporate suitable adjustments such that monochrome information is reproduced correctly (with no color tint) for all settings of the contrast control. In practice, this amounts to two distinct steps:

Adjustment of Low Lights

 

Non-concident I p /V gk characteristics and consequent difference in cut-off points of the three guns, results in appearance of colored tint instead of pure dark grey shades in areas of low brightness. To correct this it is necessary to bring the cut-off points in coincidence. This is achieved by making the screen grid (i.e., 1st anode) voltage different from each other. Potentiometers are normally provided for this purpose in the dc voltage suppl y to the three screen grids.

 

Adjustment of High Lights

 

It is equally necessary to ensure that all other levels of white are also correctly reproduced.This amounts to compensating for the slightly different slopes and also for the substantially different phosphor efficiencies.

 

This is achieved by varying the video (luminance) signal drive to the three guns. Since the red phosphor has the lowest efficiency, maximum video signal is fed to the red cathode and then by the use of potentiometers video signal amplitudes to the green and blue guns are varied (see Fig.) to obtain optimum reproduction of high lights

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Television and Video Engineering - Essentials of Color Television : Grey Scale Tracking |


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