Sound Devices PIX 250i - Manual (Page 40)

PIX 250i User Guide
v. 2.10 Features and specifications are subject to change. Visit www.sounddevices.com for the latest documentation.
34
Sync
hr
onization & T
imecode
Synchronization and Timecode
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 re-
corders. 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) 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 PIX 250i 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 PIX 250i’s design makes setting the parameters as
easy as possible, a good understanding of synchronization is still necessary to properly operate the
PIX 250i 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. If the master clock is a slightly slow, the shutter, etc will also be
slightly slow.
When a PIX 250i’s video input is connected via HDMI or SDI to a camera or other video source,
this “heartbeat” is passed through the HDMI or SDI. The PIX 250i video and audio circuitry can be
clocked off of this incoming signal. In other words, the PIX 250i references this clock and synchro-
nizes its own clock to it. The PIX 250i writes files based on this clock as well - each audio sample and
video frame written out to the file is synchronized to the incoming video. This way, the PIX 250i is
always completely in sync with the connected video source - there can never be any drift of audio or
video between the camera and the PIX. ( See “Sync Reference”, page 35 )
Drift problems in a workflow can arise when more than one camera is used on a shoot, as each cam-
era 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 record-
ing video on a camera and recording audio on a separate recorder, as each device has its own inter-
nal master clock. Particularly problematic are long takes where there may be significant drift from
camera to camera from the beginning to the 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.
The PIX 250i can address the synchronization issues of both offset and drift (heads and tails) via its
built-in Ambient® Lockit with Genlock Out and Timecode Reader. The PIX 250i’s Ambient® Lockit
features an internal clock which has an accuracy of +/-0.2ppm (½ frame per 24 hours). Numerous
PIX 250i recorders can be used to maintain extremely tight synchronization.