Avid Technology Digidesign USD, Руководство пользователя, Страница: 43 / 96

Chapter 4: Using the USD 39
To select a positional reference using the
front panel:
■
Press the Clock Reference switch to cycle
the USD through its available clock refer-
ences.
To select a positional reference using USD
Setup:
■
Choose a reference setting from the Posi-
tional Reference pop-up.
▲
Be sure to select the appropriate frame
rates whenever you use LTC or VITC as your po-
sitional reference.
Positional Reference Options
The following sections provide additional
information and configuration instruc-
tions for each available Positional Refer-
ence format.
Linear Time Code (LTC)
LTC is often striped onto an ATR or VTR au-
dio track. Professional VTRs typically have
an address or cue track, intended for LTC. If
you are working with a standard audio
tape, you’ll almost certainly be working
with LTC; if you’re working with a video-
tape, you may be able to work with either
LTC or VITC, or both.
LTC can also be generated as an interpola-
tion of Absolute code. This is how time
code DAT machines, DA-88, and many dig-
ital VTRs work. In any case, LTC is deliv-
ered to USD as a series of audio pulses, re-
gardless of how it is stored or generated.
LTC can function simultaneously as a posi-
tional reference and a clock reference.
To use LTC as the USD’s positional reference:
From Pro Tools:
■
In the USD Setup area of the Session
Setup window, choose LTC from the Posi-
tional Ref pop.
Using the front panel controls:
■
Using the front panel Positional Refer-
ence switch, select LTC.
Using USD Setup:
■
Select Linear Time Code (LTC) under
USD Setup’s Positional Ref pop-up menu.
Ensure that you’ve selected the appropriate
clock reference, sample rate, frame rate,
and freewheel duration. Also make sure the
LTC signal is routed properly to the USD
LTC In connector.
Freewheel Duration
Freewheel duration (time code freewheel)
configures USD for a maximum number of
frames it should continue generating if
time code drops out or is otherwise inter-
rupted. Freewheel settings are ignored
when USD is in Internal/Generate mode.
Example of Time Code Freewheel
In a 30 fps Pro Tools session, if Freewheel
Duration/Time Code Freewheel is 28
frames, the USD will continue to generate
until either the incoming time code signal
is restored, or until 28 frames elapse,
whichever occurs first.