DigiDesign SYNC HD, Manual

Page: 77 / 103

Appendix A: Additional Synchronization Information 71
Examples of Auto-Switch LTC/VITC
‹
The SYNC HD will switch to LTC for posi-
tional reference during hi-speed searching and
cueing, for example, or whenever the tape speed
is too high to read VITC.
‹
The SYNC HD will switch to VITC if LTC stops
or is unavailable. This will include, for example,
if a tape is paused or parked.
‹
If both LTC and VITC are available, the
SYNC HD chooses which one to use based on
the speed of playback. The switch-over point is
approximately 75% of full 1x playback speed.
Above 75% playback speed, LTC is favored; be-
low 75% speed, VITC is favored.
If a dropout occurs, the SYNC HD waits until the
freewheel duration has expired before attempt-
ing to switch over to the opposite source. If nei-
ther source is available, the SYNC HD will stop
reading time code.
Digital Clock Signal Types
A reference clock signal is part of any digital re-
cording 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 dig-
ital interfaces), the clock signal is embedded in
the data stream itself. In other cases, such as
SDIF, the clock signal is carried as an entirely
separate signal from the digital audio sample
data.
The SYNC HD is able to resolve to AES/EBU and
Word Clock.
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 information) as a system
clock reference source. These systems rely upon
a single AES/EBU master clock source that is dis-
tributed throughout a digital audio facility, in
much the same way that house synchronization
is distributed throughout a video facility. If you
are connecting the SYNC HD to such a system,
you will want to use the SYNC HD AES/EBU in-
put as the clock reference connection, so that all
system components are referenced to the same
time base. (Note that AES/EBU does not support
176.4 kHz and 192 kHz sample rates.)
In some cases (such as using the SYNC HD as a
standalone clock resolver or time code generator
without a digital audio workstation), you may
wish to use an audio DAT machine (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.
Word Clock
Many professional digital audio products—in-
cluding open-reel multitrack tape recorders, dig-
ital mixing consoles, and the Tascam DA-88
modular digital multitrack—have Word Clock
(1x sample rate) connectors.
Word Clock allows the DA-88 (and other Word
Clock-compatible devices) to send or receive ex-
ternal clock information which controls the
sample rate, which in turn (where applicable)
controls the play and record speed.