
TMC22091/TMC22191
PRODUCT SPECIFICATION
34
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 B6
h
. An SCH error of 15
°
 is 
corrected with SYSPH and BURPH offsets of AAA
h
.
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 30
h
. 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
VA
7
+
8
=
Figure 8. 100% Color Bars With 100% and 75% Chrominance Saturation
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
24387A