Specifications

SYNC I/O Guide70
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
similar, 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 SYNC I/O can use as its clock
reference, at nearly any speed including
still/paused.
SYNC I/O uses the phase relationship between
the two square waves to determine the device’s
direction (forward or reverse). However, this is
relevant only when SYNC I/O 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. SYNC I/O 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. Using SYNC I/O Setup software utility’s
Pulse Per Frame setting, you can set SYNC I/O to
match the PPF rate of the external device’s Bi-
Phase/Tach encoder.
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 SYNC I/O 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, SYNC I/O 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 equip-
ment, 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 I/O 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 I/O 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