Sound Devices 970 - Manual (Page 31)

970 User Guide
25
Sync
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onization & T
imecode
Synchronization and Timecode
In a production environment (during recording), offset and drift are both terms to describe a timing
problem between the timecode signal from two or more audio or video recorders. In a post-produc-
tion environment (a non-linear editor application), offset and drift are both terms to describe a tim-
ing problem between two or more audio or video files.
Offset (matching the beginning or “head” of a take) occurs because the beginning of takes are not
aligned due to the absence or misuse of timecode. Drift (matching the end or “tail” of a take) occurs
because different recorders (audio or camera) run at different rates - the beginning of files from each
may have zero offset, but by the end they drift apart. The 970 was designed with these problems in
mind and includes the most comprehensive and powerful synchronization features available in a
production recorder. These features can be used to alleviate or eliminate offset and drift problems in
both production and post-production. While the 970’s design makes setting the parameters as easy
as possible, a good understanding of synchronization is still necessary to properly operate the 970
and ensure a trouble-free workflow.
The 970 audio circuitry can be clocked off of incoming signal on either MADI or AES channels 1 and
2. In other words, the 970 references this clock and synchronizes its own clock to it. The 970 writes
files based on this clock as well - each audio sample written out to the file is synchronized to the
incoming signal. This way, the 970 is always completely in sync with the connected source - there can
never be any drift between the source and the 970. ( See “Sync Reference”, page 25 )
Drift problems in a workflow can arise when more than one recording device is used on a shoot,
as each device is driven off of its own internal master clock. Since internal master clock speed will
always vary from device to device (and vary based on temperature and time), each device used will
run at a slightly different audio sample rate and timecode rate (and shutter speed in the case of cam-
eras). Particularly problematic are long takes where there may be significant drift between the many
devices from the beginning to end of the take’s file. Even if the heads match, the tails of a take may
not.
These drift problems can be mitigated during recording by using a Wordclock input on an audio re-
corder which takes over the recorder’s internal master clock. Wiring several audio recorders’ Word-
clock inputs together forces all of the recorders to run at the exact same sample rate with no drift.
Wordclock inputs are available on higher-end audio recorders.
Offset problems are easier to overcome than drift problems. By feeding each recording device with
the same timecode signal, the files from different recording devices can all be in sync and there will
be no offset in post-production.
The 970 can address the synchronization issues of both offset and drift (heads and tails) via its built-
in Ambient® Lockit and Wordclock I/O. The 970’s Ambient® Lockit features an internal clock which
has an accuracy of +/-0.2ppm (½ frame per 24 hours). Numerous 970 recorders can be used to main-
tain extremely tight synchronization.
Sync Reference
The following table lists the available sources for sync reference.
Signal Connection Main View
Wordclock SYNC IN WCK
AES channels 1-2 Digital Audio I/O (DB-25) AES1-2
MADI MADI IN (BNC or Optical) MADI