TMC22091 datasheet(34/60 Pages) CADEKA

34 / 60 page

TMC22091/TMC22191

PRODUCT SPECIFICATION

SCH Phase Error Correction

SCH refers to the timing relationship between the 50% point

of the leading edge of horizontal sync and the positive or

negative zero-crossing of the color burst subcarrier refer-

ence. SCH error is usually expressed in degrees of subcarrier

phase. In PAL, SCH is deﬁned for line 1 of ﬁeld 1, but since

there is no color burst on line 1, SCH is usually measured at

line 7 of ﬁeld 1. The need to specify SCH relative to a partic-

ular line in PAL is due to the 25 Hz offset of PAL subcarrier

frequency. Since NTSC has no such 25 Hz offset, SCH

applies to all lines.

The SCH relationship is only important in the TMC22x91

when two video sources are being combined or if the com-

posite video output is externally combined with another

video source. In these cases, improper SCH phasing will

result in a noticeable horizontal jump of one image with

respect to another and/or a change in hue proportional to the

SCH error between the two sources.

SCH phasing can be adjusted by modifying BURPH and

SYSPH values by equal amounts. SCH is advanced/delayed

by one degree by increasing/decreasing the value of BURPH

and SYSPH by approximately B6h. An SCH error of 15° is

corrected with SYSPH and BURPH offsets of AAAh.

Video Test Signals

The TMC22x91 has two standard video test waveforms for

evaluating video signal integrity. They are selected and

controlled by the Format Control Register.

Setting the Format Control Register bits 0, 4, and 5 LOW

generates standard color bars at the COMPOSITE output

(Figure 11), the luminance component stair-step signal at the

LUMA output, and the chrominance component on the

CHROMA output. The six colors are 100% saturated PAL

and 75% saturated for NTSC.

The percentage color saturation is selectable via Misc.

Control Register 0E, bit 0.

The color bar test pattern comprises eight equal-width bars

during VA, the active video period. The Timing Register

value for CBL is found from:

If CBL is larger than this, the color bars are truncated at the

end of VA. If CBL is smaller than VA/8, the color bar

sequence will repeat, starting with another white bar. From

left to right color bars 1 to 8 should be white, yellow, cyan,

green, magenta, red, blue, and black.

The modulated ramp waveform is enabled by setting the For-

mat Control Register to 30h. It comprises constant-amplitude

and constant-phase subcarrier modulation superimposed on a

linear ramp which slews from black to white during the

active video portion of each horizontal line (Figure 12). This

waveform is useful in making differential gain and differen-

tial phase measurements. Differential gain is a measure of

the variation in saturation of a color as the luminance com-

ponent is varied from black to white. Differential phase is a

measure of the variation in hue of a color as the luminance

component is varied from black to white.

CBL

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Figure 8. 100% Color Bars With 100% and 75% Chrominance Saturation

24387A

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