James Zhou 2000

Reference (by Rich Felker ) : https://lists.libav.org/pipermail/ffmpeg-user/2005-September/001042.html

"You set timebase to the fundamental time unit you want to work with,
in seconds. Then you set the timestamp of each frame to the time (in
timebase units) at which that frame should be shown. For example
suppose you have a system in which frames may take place on 1/120
second granularity, with a mix of 24fps and 30fps content. You could
use a timebase of 1/120 with timestamps:

00 05 10 15 20 25 30 34 38 42 46 50 54 58 62 66 70 ...
\_________________/\_____________________________/
  24 fps sequence          30 fps sequence

If your frames don't come in on any regular interval, but at
completly random times (supposedly some webcam devices are random..)
then you can just set a timebase of 1/1000 or something and use
timestamps in milliseconds taken from the actual (system clock) times
at which you sampled the frames."