But it's only parts per million!

A project log for Atomic Synchronator

Beyond '67 SMPTE timecode: an affordable GPS based AUX syncing track generator for dual-system sound or multicam setups

Raymond LutzRaymond Lutz 08/05/2021 at 01:040 Comments

The atomicsynchronator encoding is precise enough to evaluate each device *effective* sample rate: my Nikon D300S sound is spec'd at 44100 Hz but is really 44099.688 Hz (all digits are significant), a discrepancy more simply expressed in ppm (here 7 ppm). My Fuji X-E1 shows an error of 33 ppm. Why is this consequential? Because when syncing sound and video from 2 distinct devices at the clip start, the tracks will drift apart slowly and for long enough takes (interviews, live shows) it will be noticeable.

How long, you ask? Take the maximum acceptable offset between video and sound to be 15 milliseconds (cf link); this small interval would be 33 ppm of the whole recording, hence T = (1e6/33)*15e-3 = 454 seconds! After about 8 minutes the drift is out of spec... The solution? Split the tracks and realign them again each time the offset is bigger than 15 ms. My software suite will have a dedrift program for correcting high drift device takes. For sure, ffmpeg based, but how? Will I have to interpolate frames? ouach.