[FFmpeg-devel] [PATCH] libavformat/mxfenc: Allow more bitrates for NTSC IMX50

[Baptiste Coudurier]

Hi Tomas


> On Aug 22, 2019, at 3:47 AM, Tomas Härdin <tjoppen at acc.umu.se> wrote:
> 
> ons 2019-08-21 klockan 12:47 -0700 skrev Baptiste Coudurier:
>> Hey guys,
>> 
>> 
>>> On Aug 19, 2019, at 9:54 AM, Thomas Mundt <tmundt75 at gmail.com> wrote:
>>> 
>>> Am Fr., 16. Aug. 2019 um 23:31 Uhr schrieb Tomas Härdin <tjoppen at acc.umu.se
>>>> :
>>>> tor 2019-08-15 klockan 13:55 +0200 skrev Thomas Mundt:
>>>>> Am Do., 15. Aug. 2019 um 11:01 Uhr schrieb Tomas Härdin <
>>>> tjoppen at acc.umu.se
>>>>>> :
>>>>>> ons 2019-08-14 klockan 22:18 +0200 skrev Thomas Mundt:
>>>>>>> 
>>>>>>> New patch attached.
>>>>>> 
>>>>>> Looks OK. I'll push in a few days if no one else has any comments
>>>>>> 
>>>>> 
>>>>> Thanks. Would you mind porting it to branches 4.1 and 4.2?
>>>> 
>>>> I'm not quite sure what the process is for that. I have confirmed that
>>>> the problem exists in 4.1 and 4.2 and that your patch fixes it.
>>>> 
>>>> I think we also might want to put a note somewhere in the documentation
>>>> how to make NTSC IMX50 files.
>>>> 
>> 
>> Yeah, it’s been an issue for quite some time, s356m mentions:
>> "When used as a signal source for the type D-10 recording format, the
>> bit stream is carried by SDTI-CP, as defined in SMPTE 326M, using
>> recommended operating point bit rates as defined in this annex. Other
>> bit rates may be used. However, users are cautioned that other system
>> design parameters within the studio may not support all bit rates.
>> 
>> Table A.1 indicates recommended operating points to simplify studio
>> operations and to provide users with a tool to be used in designing
>> systems."
>> 
>> Then specifies the exact value of the sequence_header bit_rate_value,
>> 50mit/s being “1E848h”, "To be used when compliant with EBU D84 and
>> D85"
> 
> mpeg12enc.c does this, if I read it correctly. 49999840 / 400 =
> 124999.6 which gets rounded up to 125000.

Oh, that’s good actually.

>> I don’t think it is a good idea to produce files with the wrong
>> bit_rate value, and I know for a fact that many file analyzers in use
>> today will simply reject the d-10 essence.
> 
> Would writing 50000000 in mxfenc for values in the range
> [49999840,50000000] make it pass?

If the d-10 mpeg-2 essence has the right bit_rate value, the analyzer works.
Thinking about this, and assuming the seq header bit_rate_value is correctly set,
Could we infer the D-10 UL after the first frame according to the mpeg-2 essence ? 
That would guarantee correctness and eliminate the hackish bitrate check.

Thanks!

— 
Baptiste