[FFmpeg-user] IMX50 NTSC framesize differs to the SMPTE 356M-2001 Standard - why?

[Michael Niedermayer]

On Mon, Jun 01, 2015 at 04:43:12PM +0200, Christoph Gerstbauer wrote:
> Hello
> 
> I am testing the standard "conformness" of ffmpegs IMX50 (Mpeg2
> I-frame, MXF) encoder.
> 
> I mentioned via ffprobe or via encoding that ffmpeg does encode a
> NTSC framesize of: "208542".
> But the SMPTE Standard 356M-2001 specifies this vlaue with "208541".
> For PAL there is no problem, both framesizes are "250000".
> 
> So at the encoding params I have to set the following:
> 
> -bufsize 2000000  (set ratecontrol buffer size (in bits))
>         SMPTE for PAL: max coded frame size  = 250,000 BYTES = 2000000 bits
>         SMPTE for NTSC: max codec frame size = 208,541 BYTES = 1668328 bits
> -rc_init_occupancy 2000000   (number of bits which should be loaded
> into the rc buffer before decoding starts)
>         SMPTE for PAL: max coded frame size  = 250,000 BYTES = 2000000 bits
>         SMPTE for NTSC: max codec frame size = 208,541 BYTES = 1668328 bits
> 
> But at the NTSC encoding I get get there errors:
> 
> C:\Users\gersti>ffmpegnew -i "C:\Users\gersti\D-10_NTSC.mxf" -map
> 0:v -map 0:a -c:v mpeg2video -r 30000/1001 -pix_fmt yuv422p -aspect
> 4:3 -minr
> ate 50000k -maxrate 50000k -b:v 50000k -g 1 -flags

yes, this happens because 50mbit/sec is not correct
a max framesize of 208541 results in a bit rate of max
49.999840 mbit/sec, IIUC thats what the spec means by 50mbit/sec

check yourself: 208541 * 8 * 30000/1001


[...]

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

I know you won't believe me, but the highest form of Human Excellence is
to question oneself and others. -- Socrates
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