Specifications

Pro Tools Reference Guide542
“Frame rate” is the rate of speed that the time
code is ascending through the frames. For exam-
ple, when someone refers to a time code of
29.97, they usually are referring to “using a
frame count of 30 frames, but counting each
frame at the speed of 29.97 frames per second.”
LTC (Longitudinal or Linear Time
Code)
LTC is time code that is recorded and played
back, or generated, in the form of an analog au-
dio signal. LTC is supported by many audio and
video tape recorders.
LTC Speed Usage
LTC can be read at high tape shuttle speeds, al-
lowing a machine’s time code reader to commu-
nicate with synchronizers at rewind or fast for-
ward speeds exceeding 50 times playback speed
(provided the tape recorder is able to reproduce
the time code at this speed). However, LTC can-
not be read at very slow shuttle speeds (such as
when you are “crawling” the tape frame by
frame) or when the machine is paused. With
LTC, the VTR must be running (usually at a min-
imum speed of about 1/10th normal playback
speed) in order to capture a SMPTE time address.
VITC (Vertical Interval Time Code)
VITC is a type of time code that is recorded and
played as an invisible part of a video signal.
VITC is commonly used in professional video
editing and audio-for-picture applications. Be-
cause VITC is recorded as part of each video
frame, it must be recorded at the same time as
the video signal—it cannot be added later as
LTC can. Since VITC cannot be recorded on au-
dio tracks, it is never used to synchronize audio-
only recorders. Instead, LTC is most often used
in audio-only applications.
VITC Speed Usage
VITC’s ability to be read when moving a VTR
transport at slow speeds or when the VTR is
paused makes it more useful than LTC in these
situations. When VITC is used, Pro Tools can
capture the current SMPTE time from the VTR
when it is paused or in “crawl” mode. However,
if you are using additional external transport
synchronizers in your setup, most synchroniz-
ers cannot read VITC at speeds exceeding ap-
proximately 10 times playback speed, prevent-
ing slaved machines from maintaining
synchronization during rewind and fast for-
ward.
LTC/VITC Auto-Switching
Many synchronizers and devices support auto-
matic switching between LTC and VITC, de-
pending on the speed, to get the best of both
worlds (both the SYNC I/O and USD support
auto-switching). For example, VITC might be
used when a VTR is paused, or crawling frame-
by-frame, while the synchronizer might auto-
matically switch to LTC when fast-forwarding.
Bi-Phase/Tach
This electronic pulse stream is used by film mag
recorders, film editing stations, and film projec-
tors. You can use this format to synchronize
Pro Tools if you have a SYNC I/O (or USD). Un-
like time code, Bi-Phase/Tach doesn’t actually
contain absolute location information. It simply
supplies speed (based upon the frequency of the
pulses) and direction, and therefore, relative po-
sition. Since the SYNC I/O can “count” both the
speed and direction of the stream of pulses, it
can use a Bi-Phase/Tach source to deduce posi-
tional information from a starting “address
point.” The difference between Bi-Phase and
Tach formats is that Bi-Phase encodes rate and
direction on a pair of signals using a format