[Mp4-tech] Frame rate in h.264

[Gary Sullivan]

Probably we ought to change that example in E.2.1 in the next
corrigendum.  The example in the NOTE near the end of D.2.2 is better.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of John Cox
+> Sent: Monday, July 18, 2023 3:27 AM
+> To: mp4-tech lists.mpegif.org
+> Subject: Re: [Mp4-tech] Frame rate in h.264
+> 
+> Glenn
+> 
+> The table you need to look at is E-6 (under the description of
+> fixed_frame_rate_flag).
+> 
+> Once you've worked through all the verbiage what it says is that the
+> duration of a field picture is 1x 
+> num_units_in_tick/time_scale and the
+> duration of a frame picture is 2x 
+> num_units_in_tick/time_scale. (If you
+> specify pic_struct then other options are available too for 3/2 pull
+> down etc.)
+> 
+> I agree that the example given is misleading and I guess 
+> that the sample
+> I saw came from the Main Concept encoder.
+> 
+> JC
+> 
+> >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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+> >
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