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CHAPTER
9
2
Degrees of Accuracy
BEFORE YOU GO ANY FURTHER…
Digital audio devices found in today’s studio
support varying degrees of accuracy when it comes
to synchronization. The Digital Timepiece supplies
the best possible accuracy for each type of device
that it supports. Here is a brief overview of several
basic categories of accuracy to which most digital
audio devices belong, starting with the highest
(best). After you look them over, think about each
device in your studio and the category it belongs to.
Doing so will help you make better decisions when
installing the Digital Timepiece, which supports a
wide range of synchronization scenarios.
SAMPLE-ACCURATE SYNC
When two devices achieve sample-accurate sync,
the master device and slave device are in
continuous, sample-accurate synchronization with
each other. Not only are their sample clocks
continuously aligned (
phase-locked ), sample for
sample, but they also locate to —and start playing
or recording on — exactly the same sample every
time. For example, if the master device cues to
sample number 49,856,237 in a recording, the slave
device will cue to exactly the same sample.
Resolution is 44.1 or 48 thousandths of a second. A
slight bit of skew might be introduced due to
analog filter delays. But if so, the skew will be
consistent, so it will not cause phasing.
An example of devices that support this level of
synchronization is a chain of ADAT tape recorders
via their proprietary ADAT sync protocol. If you
record a stereo track pair from one ADAT to
another in two separate record passes, the stereo
image would remain in perfect phase. The Digital
Timepiece has the ability to drive a chain of ADATs
at this level (sample-accurate locating with
continuous phase-lock). But even more signifi-
cantly, the Digital Timepiece can also make a stack
of ADATs synchronize with a stack of DA-88s
(which also support sample-accurate sync) at this
level. The Digital Timepiece is the first
synchronizer to be able to do this.
FRAME-ACCURATE SYNC WITH PHASE-
LOCK
When two devices achieve frame-accurate sync
with phase-lock, master and slave device play back
in continuous phase-lock with each other, sample
for sample, with no phasing or drifting over time.
However, the timing resolution at which the
devices locate — and begin recording or playing
back — is equivalent to the SMPTE time code
frame rate being used (e.g. 30 frames per second).
Digidesign Pro Tools is a classic example of a
system that synchronizes at this level of accuracy.
When slaving externally, Pro Tools locates and
begins recording or playing according to MIDI
Time Code it receives, which has a has a quarter
frame resolution of 30 frames per second times
four — or approximately a 120th of a second (or
whatever time code frame rate is being used).
Because MTC suffers from general MIDI delays
and skewing, Pro Tools and systems like it also
employ a software averaging scheme which helps
with accuracy even further.
Note, however, that Pro Tools — as a slave — also
requires a sample clock (Digidesign refers to it as
“superclock” or “slave clock”) to keep it phase-
locked with its master once it starts. While
“superclock” maintains phase lock at normal
digital audio sample rates (44.1 or 48 thousand
times a second), it does not provide single-sample
locating as described in the previous section: it
simply keeps the Pro Tools sample clock in phase