SS0607-8
30 September 1988
4.
The single line reference signal for this master/source-synchronizing
generator system can be color black or encoded subcarrier. State of the art
broadcast equipment has an encoded subcarrier to improve and simplify the
locking of source-synchronizing generators.
The encoded subcarrier signal
consists of a continuous 3.579545 MHz sine wave that contains two phase-
inverted cycles (one per color frame).
This brief phase inversion is
precisely positioned on the front porch of blanking, preceding line 11 on
field 1, of the four-field sequence (fig 2-4).
The phase inversion thus
communicates horizontal, vertical, and color frame information to the
source-synchronizing generators.
Encoded subcarrier provides a number of
advantages over color black as a locking signal. Subcarrier does not have
to be generated from the periodic color burst, so jitter is avoided. The
use of a single frequency encoded subcarrier eliminates the normal group
delay problem (usually encountered when color black travels through coaxial
cable).
NOTE: Jitter is the up/down unstable movement of a video picture
when it is not properly locked.
Figure 2-4.
V1 color frame pulse
5.
A color black reference sync generator (fig 2-5) must first regenerate
subcarrier from the color burst. Jitter can result if this regeneration is
not done precisely.
Second, it must precisely compare the regenerated
subcarrier with the exact 50-percent point on the leading edge of horizontal
and vertical sync to determine color frame.
If this process is not done
precisely, the result may be SC/H phase instability, jitter, and independent
lock to sync and subcarrier.
An output SC/H phase will track reference
input SC/H phase error.
Sometimes SC/H error indicators are provided to
help overcome these deficiencies.
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