On 2021-02-23 00:41, Carl Zwanzig wrote:
-snip-
If you're starting with mpeg-ps or -ts, ...
There's no such thing as PTS in mpeg-ts. The transport stream sets the SCR (System Clock Reference)
(aka TB) but the PTSs are in the presentation stream, stored as integer ticks of the SCR.
I've been told (at doom9.org) that MKV (which is a TS) stores PTSs but I find
that hard to believe.
... 'Presentation time stamps have a resolution of 90kHz", so at
29.97fps the PTSs should be 3003.003... apart. Since they're whole numbers, that would be 3003,
6006, etc with an extra +1 every 333 frames.
Are those MPEG SCR counts or ffmpeg TB counts? Let's see...
1001/30000 = 33.3[6..]ms.
1/90000 = 0.011[1..]ms.
33.3[6..]/0.011[1..] = 3003 (exactly), so you are a little bit off.
Jim says: The PTS resolution is slightly under 1/30,000 second.
That suggests ffmpeg is only using 1/3 the resolution available to the mpeg formats, of am I
misunderstanding something? (I don't have time to dive into the code at the moment.)
There's also some interesting stuff at
http://dranger.com/ffmpeg/tutorial05.html
http://cseweb.ucsd.edu/classes/sp03/cse228/ (old, but should be useful)
and
"Understanding Timelines within MPEG Standards" (looks technically intense)
https://ir.cwi.nl/pub/23650/23650B.pdf
which I'll read in the next few days.
Later,
z!
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