Pro Tools | SYNC HD Guide
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Using just Word Clock, it is possible to create a
“chain” of digital devices in your studio by picking
one source as the Word Clock master, and config-
uring other sources as Word Clock slaves.
Bi-Phase/Tach
Bi-Phase and Tach are used with mag machine,16,
35, and 70 mm projectors, flatbed editing systems
and other types of motor-driven film equipment.
Bi-Phase (sometimes called Quadrature Sync) and
Tach information are similar, though they do dif-
fer.
Bi-Phase
A Bi-Phase signal consists of two square
waves, which are generated directly by a device’s
transport mechanism, and which are 90° out-of-
phase with one another. As a Bi-Phase-generating
device plays it outputs a steady stream of square
waves that the SYNC HD can use as its clock ref-
erence, at nearly any speed including still/paused.
The SYNC HD uses the phase relationship be-
tween the two square waves to determine the de-
vice’s direction (forward or reverse). However,
this is relevant only when the SYNC HD is using
the Bi-Phase signal as a positional reference.
Tach
A Tach signal is a variation of Bi-Phase.
With Tach’s two signals, one is used only as the di-
rection indicator, while the other is used as the ve-
locity, or rate indicator. The SYNC HD uses this
rate signal when resolving to Tach as a clock refer-
ence.
There are several different standards for the num-
ber of pulses-per-frame for Bi-Phase or Tach de-
vices. You can set the SYNC HD to match the PPF
rate of the external device’s Bi-Phase/Tach en-
coder from Pro Tools.
Strictly speaking, Bi-Phase/Tach signals are clock
reference signals, and do not contain positional in-
formation of their own. However, they do contain
enough information for the SYNC HD to calculate
positional information.
Bi-Phase/Tach signals use two square waves to
generate pulses that can function as a clock refer-
ence. The two square waves are 90° out-of-phase,
in a pattern that resembles this:
With a Bi-Phase signal, the SYNC HD can deduce
the direction (forward or reverse) of the signal
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 gener-
ate a Bi-Phase signal where the “A” wave leads the
“B” wave—that is, where the A wave peaks before
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 oppo-
site manner, which is why the SYNC HD Input
Signals option lets you make the appropriate selec-
tion (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 information; the “B” signal is
in a steady state (high or low) that indicates the di-
rection. Unfortunately, not all Tach-generating
equipment uses the B signal in the same way. For-
tunately, the SYNC HD Input Signals option al-
lows you to choose the appropriate method (Tach:
Fwd = B is Low, or Tach: Fwd = B is High).
Bi-Phase/Tach signals
Square
Wave A
Square Wave B
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