On 2021-02-23 13:17, Rob Hallam wrote:
On Tue, 23 Feb 2021 at 17:55, Mark Filipak (ffmpeg) <[email protected]>
wrote:
[ information on PTS ]
Interesting information, thank you for sharing those insights.
Without wanting to cast aspersions, does this mean that ffmpeg does
something different with regards to timestamps than the MPEG spec?
Hey Rob,
Without knowing the decoder and pipeline delta-PTS resolution [1], I can't say for sure. What I can
say is that ffmpeg reports "1k tbn". I'm unsure what 'tbn' means but others have said that it means
"time base number" [2]. If 'tbn' is used in calculations as a surrogate for MPEG's 90KHz time base,
then I'd say that 1ms resolution is too small to prevent errors -- it would need to be no larger
than 0.01[1..]ms but could/should be even finer (in order to prevent accumulated error in chained
calculations).
[1] MPEG keeps PTS as a 33-bit value. Because the MPEG time base clock runs at 90KHz (i.e. the 27MHz
system time clock divided by 300), then the smallest possible frame-to-frame delta-PTS represents
0.011[1..]ms in time. Real examples: Changing delta-PTS from '3003' to '3000' is a frame-to-frame
change of 33.36[6..]ms to 33.33[3..]ms (which, as frame rates, is 30fps to 29.970fps). (Note that
33.36[6..]ms = 33.33[3..]ms + 0.011[1..]ms + 0.011[1..]ms + 0.011[1..]ms.)
[2] The meanings of 'tbr', 'tbn', and 'tbc' were the subjects of my very first posting to
ffmpeg-users back on 2019-09-29, subject: "Two initial questions". I got no answer then, and I've
not gotten a precise answer since. Oh, I've since gotten vague musings, but it appears that this
whole subject is quite murky to everyone. I accepted the situation because "those ffmpeg folks must
be awfully experienced, so I'll just hang out until I learn more". Based on (bogus, abusive)
reactions here and (courteous, more informed but still murky) discussions at doom9.org, well, I
guess I've lost my patience. Sorry.
If this is the case, is it possible to quantify the difference, at least to
an order of magnitude?
Asking out of idle curiosity as the description of the hardware and spec
piqued my interest.
You have just pulled the tubes out of your arms, my friend, and you are now ready to face the
implications of "THE MATRIX".
Let me expound a little further, eh?
If 30fps produces a delta-PTS of '3000' and 29.970 produces a delta-PTS of '3003', then what does a
delta-PTS of '3001' produce? (And is it possible? And is it valid?).
Yes, it is programmatically possible.
A delta-PTS of '3001' produces time stamps as though from a
29.99000333222259246917694101966fps video.
Will such a video play properly? Probably, but maybe not if burned to a DVD and popped into a DVD
player. MPEG's 'frame_rate_code' metadata allows for: '24/1.001', '24', '25', '30/1.001', '30',
'50', '60/1.001', and '60'. It doesn't allow for '30/1.0003[3..]', but a player will probably play
30fps and ignore the PTSs (or will ignore 'frame_rate_code' and 'play' the PTSs).
Is it valid? No.
And the biggest question: If those PTSs are further manipulated in chained calculations, what will
happen? I think errors happen. I think jitter happens. I think busted transcodes happen. I think
out-of-sync audio happens. I think scripts that have worked for a year but that break on particular
use cases happen.
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