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Appendix A: Additional Synchronization Information 69
If a dropout occurs, the USD waits until the
freewheel duration has expired before at-
tempting to switch over to the opposite
source. If neither source is available, the
USD will stop reading time code.
Digital Clock Signal Types
A reference clock signal is part of any digi-
tal recording system. It is required because
whenever digital audio information is
mixed together or passed between devices,
the playback samples must be aligned with
the recording samples. In some cases (such
as with AES/EBU or S/PDIF digital inter-
faces), the clock signal is embedded in the
data stream itself. In other cases, such as
SDIF, the clock signal is carried as an en-
tirely separate signal from the digital audio
sample data.
The USD is able to resolve to three different
types of digital audio clock information:
AES/EBU, Word Clock, and Super Clock
(256x Slave Clock).
Word Clock
Many professional digital audio products—
including open-reel multitrack tape record-
ers, digital mixing consoles, and the Tas-
cam DA-88 modular digital multitrack—
have Word Clock (1x sample rate) connec-
tors.
Word Clock allows the DA-88 (and other
Word Clock-compatible devices) to send or
receive external clock information which
controls the sample rate, which in turn
(where applicable) controls the play and
record speed.
Using just Word Clock, it is possible to cre-
ate a “chain” of digital devices in your stu-
dio by picking one source as the Word
Clock master, and configuring other
sources as Word Clock slaves.
AES/EBU
Some professional digital audio products
use AES/EBU “null clock” (which is an
AES/EBU data stream that contains only
clock information only and no audio infor-
mation) as a system clock reference source.
These systems rely upon a single AES/EBU
master clock source that is distributed
throughout a digital audio facility, in much
the same way that house sync is distributed
throughout a video facility. If you are con-
necting the USD to such a system, you will
want to use the USD AES/EBU input as the
clock reference connection, so that all sys-
tem components are referenced to the
same time base.
In some cases (such as using the USD as a
standalone clock resolver or time code gen-
erator without a digital audio workstation),
you may wish to use an audio DAT ma-
chine (or other similar device) as a source
of AES/EBU null clock, and resolve your
system to this reference source. In this case,
the audio sample data in the AES/EBU data
stream is stripped off, and only the clock
information is used.
Super Clock
Super Clock (or Slave Clock) is Digidesign’s
proprietary hi-speed Word Clock that runs
at 256x the sample rate. It is required by
Digidesign Pro Tools systems to synchro-
nize audio interfaces (up to the limits of
each system) to a single, highly-accurate