Alexa Guide Track Sync

Posted by JoeE 
Alexa Guide Track Sync
August 23, 2011 03:53PM
Has anyone noticed that the sync on the Alexa ProRes dailies files are out by one frame? I've done two consecutive projects using that camera and on both the audio was ahead of the picture by one frame. Next one I'll talk to the sound man ahead of time and maybe he can solve this somehow....
Re: Alexa Guide Track Sync
August 24, 2011 02:00AM
yes, we discovered this on our last job.
we were about two frames out.

the picture takes that much time to be written apparently.
a small delay unit between the sound recordist and the alexa might fix it.
it'd need a bit of testing to get just right.

i suggested a small Behringer mixer with in-built fx,
but they came up with an alternate solution probably not pertinent to you.

our story is, it was a musical, super low budget recording on a 2-tk zoom,
and the idea was to record the payback feed direct to the Alexa. bad idea, as it was out of sync and we had no real reference as to how far out it was.
caused some problems for the first few days
especially when they were looking back at takes to judge performance, and sync.
solution was to split the playback track, offset one track by two frames, and feed that track to the camera

Re: Alexa Guide Track Sync
August 24, 2011 11:42AM
Thanks for the confirmation. We just took a look at some stuff that was shot at the end of the shoot at a different setting. Because it was an element for main titles cg, we shot it at the highest res you can put on an Alexa SxS card which is ProRes 4444 instead of 422 HQ which we used for the standard scenes. On that the takes are dead sync. Should of had them shoot that the whole way. I have plenty of disk space.....
Re: Alexa Guide Track Sync
August 24, 2011 02:33PM
Hey Joe

Could you just confirm:

You have no issue with AV sync on Alexa SxS using ProRes 444 codec?

But there are issues on the ProRes 422?

Has anyone fed this back to Arri?

For instant answers to more than one hundred common FCP questions, check out the LAFCPUG FAQ Wiki here : []
Re: Alexa Guide Track Sync
August 24, 2011 03:43PM
Quite funny you bring an Arri to a low cost shoot, and more you bring a Zoom for sound.

Nevertheless, i'll build in a sync offset option in QTchange tomorrow, so you can batch fix this kind of files.

Re: Alexa Guide Track Sync
August 24, 2011 07:28PM
"Quite funny you bring an Arri to a low cost shoot, and more you bring a Zoom for sound."

crazy, i know.
the camera was practically free. guess the DP had more pull.

for the record, all our shooting was Prores 4x4, and we seemed to be 1.5 to 2 frames out of sync.

my guess is its not consistent.

Ok, here is one solution...
August 29, 2011 12:04PM
Took me a bit longer than suspected, but it's done.
QTchange now can offset a Quicktime sound track, either positive or negative, and way more subtile than just per frame.
(You can enter the value in Milliseconds)

Get QTchange here:


(and yes, there is a free demo)

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