[Mp4-tech] Frame rate in h.264

[Glenn Connery]

John,
Thanks for the help, somehow skipped over this at first read.
As you suggest, some samples did not have VUI in them.  I'll have to 
come up with an alternative.
However, a sample I made with the Main Concept h.264 encoder did have 
the VUI info in it.  It is an NTSC sample and has num_units_in_tick = 
900,900 and time_scale equal to 27,000,000.  Which gives you a FRAME 
RATE of 29.97.  Seems correct to me, but based on your email you 
suggest this should be the field rate instead.
>From my copy of h.264 "ITU-T Rec. H.264 (03/2005) - Prepublished 
version", I include the following:
"For example, when the clock frequency of a video signal is 30000 / 1001 
Hz, time_scale may be 30 000 and num_units_in_tick may be 1001."
Now this clearly seems to be a reference to the NTSC frame rate, and 
suggests the numbers are correct to me, though the text is less clear 
than one might like.
On Thursday 14 July 2023 10:07 am, John Cox wrote:
> Glenn
>
> You can get the info from the VUI (in SPS) from the timing_info
> fields (num_units in tick, time_scale, fixed_frame_rate_flag). 
> Unfortunately these fields are optional and very few encoders fill
> them in.  Indeed in the only recent example I've seen where they were
> filled in the encoder had got the value wrong by a factor of two.
> num_units_in_tick/time_scale should yield field time (or half frame
> time if progressive) rather than frame time.
>
> John Cox
> SJ Consulting Ltd
>
> >In MPEG-2 you could just parse the SEQ header and pull out some bits
> > to determine if the frame rate is 29.97/NTSC or 25/PAL or whatever.
> >
> >I don't see any obvious way to determine this from a .264 stream,
> > other than by figuring it out based on the number of frames and the
> > time elapsed, but given that there could be repeats that might be
> > more complex than I'd like.
> >
> >Thanks,
> >Glenn
> >
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