Avid Technology Digidesign USD, Руководство пользователя, Страница: 74 / 96

Universal Slave Driver 70
clock from USD. Pro Tools audio interfaces
must always be connected to this separate
Super Clock signal chain.
Bi-Phase/Tach
Strictly speaking, Bi-Phase/Tach signals are
clock reference signals, and do not contain
positional information of their own. How-
ever, they do contain enough information
for the USD to calculate positional infor-
mation.
Bi-Phase/Tach signals use two square waves
to generate pulses that can function as a
clock reference. The two square waves are
90° out-of-phase, in a pattern that resem-
bles this:
With a Bi-Phase signal, the USD can deduce
the direction (forward or reverse) of the sig-
nal based upon which wave is read “high”
relative to the other. For instance, with
some film equipment, when the device is
running forward, it will generate a Bi-Phase
signal where the “A” wave leads the “B”
wave—that is, where the A wave peaks be-
fore the B wave peaks. When the device is
in reverse, the B wave will lead the A wave.
However, some film equipment works in
the opposite manner, which is why the
USD’s Input Signals option lets you make
the appropriate selection (Fwd = A leads B,
or Fwd = B leads A).
Calculating the direction of a Tach signal is
slightly different. As you may recall, Tach
also uses two signals. The “A” signal is a
square wave that provides clock informa-
tion; the “B” signal is in a steady state (high
or low) that indicates the direction. Unfor-
tunately, not all Tach-generating equip-
ment uses the B signal in the same way.
Fortunately, the USD’s Input Signals op-
tions allows you to choose the appropriate
method (Tach: Fwd = B is Low, or Tach:
Fwd = B is High).
This explains how the USD can use a Bi-
Phase/Tach signal to deduce the direction,
and how it also uses the signal as a clock
reference—as long as USD is told the start-
ing frame of the first clock signal.
Pilot Tone
The USD can resolve to an external Pilot
Tone signal for synchronizing to (or trans-
ferring audio from) certain types of open-
reel audio tape recorders.
In general, Pilot Tone is a sine wave refer-
ence signal running at the “line frequency”
or “mains frequency,” meaning the same
frequency transmitted by the AC line volt-
age from the local power utility.
Pilot Tone is used on location film shoots
to establish a common sync reference be-
tween a film or video camera with a porta-
ble 1/4” analog ATR (such as those made by
Nagra or Stellavox). On location, Pilot
Tone is derived by clock referencing the
camera to the local AC line frequency
(which is 60Hz or 50Hz depending on the
country of origin), and this same frequency
Square Wave A
Square Wave B