User`s guide

Pro Tools Reference Guide330
Syncing Pro Tools
Pro Tools requires a clock reference—in ad-
dition to time code—in order to maintain
synchronization over time.
You can resolve Pro Tools to an external
clock reference without locking it to a posi-
tional reference. For instance, you can use a
USD to resolve the Pro Tools sample clock
to house sync, without synchronizing
Pro Tools to external time code. You
should do this if you need all your work in
Pro Tools to match the speed of all other
machines in your facility.
Let’s explore this a little further using a typ-
ical sync situation, with Pro Tools being
slaved, using a Universal Slave Driver
(USD), to a video tape recorder, with both
machines referenced to the same house
sync or black burst source:
1 When you start the videotape, time code
(LTC or VITC) is read off the tape and
routed to the USD.
2 The USD sends Pro Tools the address of
the first instance of time code it receives
(positional reference). At the same time,
based on the house sync or black burst
clock reference, the USD sends Super Clock
information to Pro Tools (clock reference).
3 Pro Tools takes the first time code address
it receives, and calculates the sample loca-
tion in the session that corresponds to the
address. Though you can work with
Pro Tools using any standard time code for-
mat, it calculates in terms of sample num-
bers—which means that it translates the
time code address to an exact sample num-
ber.
4 Assuming that the time code address cor-
responds to a sample number that is within
the Pro Tools session, Pro Tools converts
the time code address to a sample number
within the session, and begins playing back
from that point. The point that playback
starts from is the trigger point. The clock ref-
erence is used by both machines to keep
them in sync with each other’s clocks.
5 At this point, if the videotape is stopped,
rewound, and started again, the entire pro-
cess is repeated, based upon a newly calcu-
lated trigger point.
In this way, Pro Tools uses time code for
positional information, and a clock refer-
ence to maintain synchronization.
About Positional References
Time Code and Bi-phase
The “Where are we?” question refers to rel-
ative position. To describe position, many
professional audio, video, and multimedia
devices and programs use SMPTE (Society
of Motion Picture & Television Engineers)
time code.
There are two basic techniques used to
record SMPTE time code onto magnetic
tape: LTC (Linear Time Code) and VITC
(Vertical Interval Time Code). LTC is re-
corded on an audio channel or a dedicated
time code track of the audio or video de-
vice. VITC is recorded within the video sig-
nal in the video “blanking area” of each
video frame. VITC cannot be recorded on
audio tracks, so it has no application when
working with audio tape recorders, but it