Avid Technology Pro Tools SYNC HD, Manual

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Pro Tools | SYNC HD Guide 58
Example of VITC Timing Rule
As an example, if you are using LTC as a posi-
tional reference from a 3/4-inch U-Matic VTR,
then that VTR should be referenced to the same
video signal that you are applying to the
SYNC HD. As another example, in Generator Pre-
set Mode (Positional Reference = Generate), a
clock reference of Internal is not a good choice,
simply because the SYNC HD internal crystal runs
asynchronously with respect to the supplied video
signal, and thus repeated or skipped frame ad-
dresses are sure to eventually occur.
LTC Signals
Because it’s an analog audio signal, LTC can
sometimes be susceptible to either tape dropouts
(tape shedding), or to level mismatches between
the LTC source and the LTC input. The SYNC HD
Freewheeling feature allows you to compensate
for brief timecode dropouts. However, if you have
serious dropouts, you may not be able to sustain
accurate synchronization.
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 ensure 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.
SYNC HD reads LTC most reliably when fed with
a LTC signal of at least –12 dBu (and preferably 0
dBu to +3 dBu.)
LTC Servo Gain
You can adjust the servo gain of the SYNC HD
LTC input from the SYNC HD front panel con-
trols and from the Pro Tools Session Setup win-
dow. See “Servo Gain” on page 48 for more infor-
mation.
Working with Analog Machines
It is good practice on a 24-track analog tape ma-
chine to record timecode 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 otherwise adjacent audio tracks.
Timecode (which is a mid-frequency alternating
pitch square wave) is very sensitive to crosstalk
from adjacent tracks, and conversely you don’t
want audible timecode leaking onto your audio
tracks.
If your ATR is under the control of a synchronizer,
you must make sure that the synchronizer and the
SYNC HD are both locked to the same reference
source (such as a video black burst generator.)
Auto-Switch LTC/VITC
Auto-Switch LTC/VITC lets the SYNC HD auto-
matically select between these two (timecode)
sources.
LTC and VITC both provide useful and unique ca-
pabilities. For instance, it is impossible to read
LTC off a paused videotape. Consequently, using
only LTC, there’s no way you can use Pro Tools to
perform Auto-Spotting of clips when the tape is
paused. However, VITC continues to be read as
long as the picture remains visible, so it
can be
used as a positional reference 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 fre-
quency response of the ATR or VTR.