Sound Devices PIX 220i - Manual (Page 41)

Synchronization and Timecode
This section gives a concise overview of timecode and video synchronization. This information ap-
plies primarily to the PIX 240i and its advanced timecode and synchronization options; however it is
useful information for PIX 220i and PIX 240i users alike. For information about the PIX 220i’s time-
code capabilities. ( See “PIX 220i Timecode Features”, page 41 )
Synchronization of video, audio, and associated timecode while recording video and audio has long
been a problematic area. Situations where several cameras are used can complicate issues further.
There may be sync issues between two (or more) cameras and/or between audio recorders because of
a) offset and b) drift. 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 record-
ers. In a post-production environment (a non-linear editor application), offset and drift are both
terms to describe a timing 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) oc-
curs 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 PIX recorders were designed with
these problems in mind and include the most comprehensive and powerful synchronization features
available in a portable recorder. These features can be used to alleviate or eliminate offset and drift
problems in both production and post-production. While the PIX recorder’s design makes setting the
parameters as easy as possible, a good understanding of synchronization is still necessary to prop-
erly operate the PIX recorder and ensure a trouble-free workflow.
In a camera, the shutter, video circuitry, audio sampling, and timecode all run off of one ‘heartbeat’
from its master internal clock. If this master clock is slightly fast, then the shutter, audio sampling,
and timecode will be slightly fast, and if the master clock is a slightly slow, the shutter etc will be a
bit slightly slow also.
When a PIX recorder’s video input is connected via HDMI or SDI to the camera, this heartbeat is
passed through the HDMI or SDI, and the PIX is synchronized to the camera’s internal clock as well.
The PIX video and audio circuitry is clocked off of this incoming video. The PIX recorder writes files
based on this clock as well - each audio sample and video frame written out to the file is synchro-
nized to the incoming video. This way, the PIX recorder is always completely in sync with the con-
nected camera - there can never be any drift of audio or video between the camera and the PIX.
Drift problems in a workflow can arise when more than one camera is used on a shoot, as each
camera is driven off of its own internal master clock. Since internal master clock speed will always
vary from camera to camera (and vary based on temperature and time), each camera used will run
at a slightly different shutter speed, audio sample rate and timecode rate. This is also true when
recording video on a camera and recording audio on a separate recorder, as each device has its own
internal master clock. Particularly problematic are long takes where there may be significant drift
from camera to camera 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 Genlock (or “Sync”) Input on a
camera which takes over the camera’s internal master clock. Wiring several cameras’ Genlock Inputs
together forces all of the cameras to run at the exact same rate with no drift. This same concept ap-
plies to audio recorders using the Word Clock input. Genlock inputs are available only on higher-
end cameras and Word Clock 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.
Sync
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onization & T
imecode
PIX 220i and PIX 240i Video Recorder User Guide
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