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

[tim nicholson]

On 01/06/15 19:53, Michael Niedermayer wrote:
> 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

The spec actually says "Up to 50Mb/s" and "Up to 208541" that being the
highest value that comes out below the the 50Mb/s. So I tend to agree
with Michael.

> 
> check yourself: 208541 * 8 * 30000/1001
> 
>  
> [...]
> 
> 

-- 
Tim.
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