Avid Technology Digidesign USD, Руководство пользователя, Страница: 72 / 96

Universal Slave Driver 68
If you plan to use LTC as a Clock Reference
(whether or not you are also using it as a
Positional Reference), you will need to en-
sure that your LTC is recorded at as high a
level as possible without distortion, and
that there are no dropouts longer than
1/80th of a frame.
The USD reads LTC most reliably when fed
with a LTC signal of at least -12 dBu (and
preferably 0 dBu to +3 dBu.)
Working with Analog Machines
It is good practice on a 24-track analog tape
machine to record time code on Track 24 at
a reference level of -10 dBu (or lower), with
Track 23 left blank as a “guard” track. This
practice avoids crosstalk “bleed” that can
occur between the time code track and oth-
erwise adjacent audio tracks. Time code
(which is a mid-frequency alternating
pitch square wave) is very sensitive to
crosstalk from adjacent tracks, and con-
versely you don’t want audible time code
leaking onto your audio tracks.
If your ATR is under the control of a syn-
chronizer, you must make sure that the
synchronizer and USD are both locked to
the same reference source (such as, typi-
cally, from a video black burst generator.)
Auto-Switch LTC/VITC
Auto-Switch LTC/VITC lets USD automati-
cally select between these two (time code)
sources.
LTC and VITC both provide useful and
unique capabilities. For instance, it is im-
possible to read LTC off a paused video-
tape. Consequently, using only LTC,
there’s no way you can use Pro Tools to
perform Auto-Spotting of regions when the
tape is paused. However, VITC continues to
be read as long as the picture remains visi-
ble, so it can be used as a positional refer-
ence when the VTR is paused. On the other
hand, VITC cannot be read at fast winding
speeds (except by broadcast-quality VTRs);
LTC can be read at fast winding speeds, as
long as its signal remains within the high-
end frequency response of the ATR or VTR.
Examples of Auto-Switch LTC/VITC
◆
The USD will switch to LTC for posi-
tional reference during hi-speed searching
and cueing, for example, or whenever the
tape speed is too high to read VITC.
◆
The USD will switch to VITC if LTC stops
or is unavailable. This will include, for ex-
ample, if a tape is paused or parked.
◆
If both LTC and VITC are available, the
USD chooses which one to use based on
the speed of playback. The switch-over
point is approximately 75% of full 1x play-
back speed. Above 75% playback speed,
LTC is favored; below 75% speed, VITC is
favored.