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The Audio & Video drift out of sync (A/V sync). Part 1.

Views: 3884
Votes: 0
Posted: 01 Feb, 2008
by: Admin A.
Updated: 01 Feb, 2008
by: Admin A.
Please read technical document 10226 first, regarding audio and video sync issues
with analog sources.  If you are running digital sources and using the digital
inputs of the Mediapress board, then continue below.

If you are using a good digital video source and you are still getting a drift
between the audio and video files, first checkout how you are validating your
streams.  You want to make sure the drift you see is caused by the streams and
not the player.  You could end up chasing the wrong problem.

Due to how Quicktime player handles MPEG files, you will never get consistent
audio and video synchronized playback.  Sometimes the audio and video will be in
sync.  Then they could drift or jump out of sync.  Sometimes the files will
playback out of sync right from the beginning.  The files could be perfectly in
sync but Quicktime Player may not be playing them back correctly.

A better way to check for proper A/V sync is to put the audio and video files
into DVD Studio Pro and Preview the DVD image.  This will give you a good idea
what the DVD will look like in a settop dvd player.  Using DVD Studio Pro is a
very good way to verify sync between audio and video files and it does NOT
utilize Quicktime to playback the MPEG streams.  DVDSP uses the same playback
code as the DVD Player application, which works much better than Quicktime's
playback code.

If you are still seeing A/V drift using DVDSP, then the streams may be bad.
Check the following items to see if they improve the A/V sync:

1) Do NOT preview while encoding.  We use the same preview code as in DVDSP, but
we use up a lot of the cpu power doing the encode process.  Running both preview
and encoding simultaneously can cause a random dropped video frame in the encoded
file.  If you run a long time, then the odds increase on how many dropped frames
you will have in the file.  This will cause a A/V sync problem.

2) Make sure your computer has a LOT of RAM.  Our documentation states we will
run with 512M of RAM, but this is BARE MINIMUM.  The way OSX is structured, it
uses a lot more RAM than OS9.  And if there is not enough RAM, it uses virtual
memory for the encoding process.  For those of you that operated on OS9, you may
remember that we must have virtual memory turned off in OS9 for proper long
encodes.  But in OSX you cannot turn off virtual memory.  In order to insure that
there is almost no virtual memory being used, it is required that you have a LOT
of RAM.  All of our G4 macs here have a minimum of 1.5G of RAM, and that solved a
lot of strange behavior with our encoding application.  As an added bonus, the
Macs all operate a lot quicker with this much RAM.  We highly recommend it.
Besides, RAM is cheap now.

3) On the digital inputs, both video and audio, make sure they are not re-clocked
or passed through a router.  Some routers do not have audio and video clocks
locked, so when you encode from those sources the A/V sync will drift.  For a
test, connect up the Mediapress SDI and AES/EBU inputs directly to the VTR deck
and see if the files are in sync.


Document: 10255
Created: 4/25/04
Modified: 4/25/04
Product: MediaPress
Version: 2.4.7z, MediaPress X 1.x
Mac OS: 9.x, 10.x
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