Specifications

SYNC I/O Guide68
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 SYNC I/O.
As another example, in Generator Preset Mode
(Positional Reference = Generate), a clock refer-
ence of Internal is not a good choice, simply be-
cause the SYNC I/O internal crystal runs asyn-
chronously 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 drop-
outs (tape shedding), or to level mismatches be-
tween the LTC source and the LTC input. The
SYNC I/O freewheeling feature allows you to
compensate for brief time code dropouts. How-
ever, 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 posi-
tional reference), you will need to ensure that
your LTC is recorded at as high a level as possi-
ble without distortion, and that there are no
dropouts longer than 1/80th of a frame.
SYNC I/O reads LTC most reliably when fed
with a LTC signal of at least –12 dBu (and pref-
erably 0 dBu to +3 dBu.)
LTC Servo Gain
You can adjust the servo gain of the SYNC I/O
LTC input using the front panel controls. See
“Servo Gain” on page 60 for more information.
Working with Analog Machines
It is good practice on a 24-track analog tape ma-
chine to record time code on Track 24 at a refer-
ence 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. Time code (which is a mid-frequency al-
ternating pitch square wave) is very sensitive to
crosstalk from adjacent tracks, and conversely
you don’t want audible time code leaking onto
your audio tracks.
If your ATR is under the control of a synchro-
nizer, you must make sure that the synchronizer
and SYNC I/O are both locked to the same refer-
ence source (such as, typically, from a video
black burst generator.)
Auto-Switch LTC/VITC
Auto-Switch LTC/VITC lets SYNC I/O automati-
cally select between these two (time code)
sources.
LTC and VITC both provide useful and unique
capabilities. For instance, it is impossible to read
LTC off a paused videotape. Consequently, us-
ing 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 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 broad-
cast-quality VTRs); LTC can be read at fast wind-
ing speeds, as long as its signal remains within
the high-end frequency response of the ATR or
VTR.