DigiDesign SYNC HD, Manual

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SYNC HD Guide 72
Using just Word Clock, it is possible to create a
“chain” of digital devices in your studio by pick-
ing one source as the Word Clock master, and
configuring other sources as Word Clock slaves.
Bi-Phase/Tach
Bi-Phase and Tach are used with mag ma-
chine,16, 35, and 70 mm projectors, flatbed ed-
iting systems and other types of motor-driven
film equipment. Bi-Phase (sometimes called
Quadrature Sync) and Tach information are sim-
ilar, though they do differ.
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-generat-
ing device plays it outputs a steady stream of
square waves that the SYNC HD can use as its
clock reference, at nearly any speed including
still/paused.
The SYNC HD uses the phase relationship be-
tween the two square waves to determine the
device’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
direction indicator, while the other is used as
the velocity, or rate indicator. The SYNC HD
uses this rate signal when resolving to Tach as a
clock reference.
There are several different standards for the
number of pulses-per-frame for Bi-Phase or Tach
devices. You can set the SYNC HD to match the
PPF rate of the external device’s Bi-Phase/Tach
encoder from Pro Tools, or using the SYNC
Setup software utility’s Pulse Per Frame setting
(Windows only).
Strictly speaking, Bi-Phase/Tach signals are clock
reference signals, and do not contain positional
information 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 ref-
erence. The two square waves are 90° out-of-
phase, in a pattern that resembles this:
With a Bi-Phase signal, the SYNC HD can de-
duce the direction (forward or reverse) of the sig-
nal based upon which wave is read “high” rela-
tive 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 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 op-
posite manner, which is why the SYNC HD In-
put 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 information; the “B” signal
is in a steady state (high or low) that indicates
the direction. Unfortunately, not all Tach-gen-
erating equipment uses the B signal in the same
way. Fortunately, the SYNC HD Input Signals
options allows you to choose the appropriate
method (Tach: Fwd = B is Low, or Tach: Fwd = B
is High).
Illustration of Bi-Phase/Tach signals
Square Wave A
Square Wave B