Audio starts off in sync then becomes unsynced

[maleeyams]

Hi!!

So we recently shot a film using the RED Scarlet and have just begun post production. However, there is a major issue with the audio that no one seems to know how to fix: the audio will start of synced with the slate and then as the clip goes on it'll become exponentially more unsynced. We did find a somewhat solution: slowing down the audio by a few small percentages seems to do the trick, but I'd love to know if there's a better way to fix this issue/why it's happening in the first place.

Thanks a ton!!!!

[bouldersoundguy]

Did you record the audio on a separate device? Most likely the clocks were divergent.

[maleeyams]

Did you record the audio on a separate device? Most likely the clocks were divergent.

The clocks? Like internal timecodes? Could you elaborate on this? We did record on separate devices.

[bouldersoundguy]

The clocks? Like internal timecodes? Could you elaborate on this? We did record on separate devices.

Digital audio and video devices have clocks in them that they use to determine when to capture an audio sample or video frame. They're generally very precise (as in a particular clock doesn't vary over time), but they're not all perfectly accurate (as in one clock might measure a second slightly differently from another clock). So if you record audio on something like a Zoom recorder and video on a camcorder, there's going to be drift. How long it takes before the drift becomes noticeable depends on how closely the two clocks match. With my particular audio recorder (Zoom H5) and cameras (4 Sony Handycams and 2 GoPro Hero3), I can generally go at least an hour. I mostly record live music and the typical set is an hour long, so that works out.

There are some devices with known issues. The Zoom H4 and H4n (non-Pro models) had inaccurate clocks at the 44.1 kHz sample rate, so with those it was always best to use 48 kHz, which is the standard for video anyway.

Once in a while I seem to have some amount of noticeable drift. I use Vegas Pro 18 for all my audio and video work, and it can easily stretch or squeeze audio to match. To do this, it's super helpful to have a slate at the start and end of the clip, though that's often not possible with live events.