30 September 1988
b. To ensure the proper operation of the tape machine color-framing
circuits, the SC/H phase relationship of the video recorded on tape and
house video must match. Proper uniformity of the SC/H phase is defined by
RS170A. It is important that all recorded video have a constantly correct
SC/H phase relationship.
Also, the reference input to the tape machine
should be a stable SC/H phase source.
Subcarrier timing in a studio is a well-understood concept in the
television broadcast industry; if timing is not correct, there will be color
hue shifts between sources.
If sync timing is not correct, horizontal
shifts will occur at the video switcher. The concept of SC/H phasing in a
studio requires a higher level of knowledge regarding each element within
a. First, and most obvious, is the house sync generator. If, the sync
generator cannot generate consistent SC/H phased outputs, maintaining SC/H
phase in the plant will never be possible. It is equally important that all
the sync generators in a multiple sync generator facility maintain correct
SC/H phase and color frame relationships.
b. Once SC/H phase has been established by the sync generator, none of
the elements in the system should alter the SC/H phase. Some elements are
obvious, like the video processor which regenerates sync and burst. If the
phase of the regenerated sync or burst is different from the incoming video,
the SC/H phase is altered.
Less obvious are sources which derive timing
from externally applied sync and subcarrier.
If sync and subcarrier are
fanned out through DAs, then their phase can be altered independently.
c. To avoid altering of phase, the output of each source device (SC/H
phased) must be timed prior to, or at, the input of the switcher. There are
many distortions which make the determination of color frame and SC/H phase
difficult. The most prominent is sync-to-subcarrier time base error. This
can be generated by many devices, such as sync generators (with noise in the
horizontal sync circuits), linear and regenerative pulse DAs (which suffer
from pick-off jitter or low frequency response problems), or any device that
has separate sync and subcarrier regeneration circuitry.
d. Noise, low frequency smear, hum, and power glitches are distortions
that may occur in signal transmissions. If these are not removed prior to
sync separation, determination of the exact 50-percent point of sync will be
e. Video time base error is different than sync to subcarrier time base
error. Sync to subcarrier time base is seen when triggering a scope on the
leading edge of sync and viewing color burst. What should be seen are two
overlapping cycles of subcarrier that are not blurred. An example of sync
to subcarrier time base error is shown in Figures 3-4 and 3-5.