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

[tim nicholson]

On 16/06/15 12:20, Christoph Gerstbauer wrote:
>>> Hello Tim,
>>>
>>> for IMX30 PAL/NTSC I have the same values like you.
>>>
>>> whats your buffers/init_occupancy for IMX50 NTSC?
>>>
>> Not sure I've ever had to do it, but a colleague uses an exact 50000000
>> bit rate which gives a 1668334 buffer and frame size of 208541 if all
>> figures are rounded to integers.
>>
>> These files have generally been accepted the other side of the Atlantic.
>> As I've said before, the coder has got to reconcile a conflicting set of
>> integer values when dealing with NTSC frame rates. It would be
>> itneresting to diff frames made using the values above to those made
>> using your values derived by working back from the exact integer frame
>> size instead of forward from the bit rate.
> So I dont have to set the "buffers/init_occupancy" an general or just in
> the NTSC case?
> All what I have to set is the videobitrate itself?

No, that is not what I said, let me try again.

You requested a change to the accepted bit rate based upon working back
wards from the max integer framesize to the nearest integer bit rate to
provide more consistent values, however the parameters are still not
mathematically consistent as they cannot all be integer values.

I have suggested if you work forward from the exact bit rate you end up
with a non integer frame size (in bytes) that is within 1 byte of the
correct max value.

So which ever way you do the maths you end up with a non integer value
somewhere, so if you feed the system integer values for all parameters
(which is all it accepts) one of the values cannot be adhered to absolutely.

This is something the coder has to resolve, and I did ask if you had
done checksums of frames coded with parameters derived from the
alternative approaches to see if the coder actually resolved matters to
the same end result.

> 
> Current syntax part:
> -minrate 49999840 -maxrate 49999840 -b:v 49999840 -bufsize 1668328
> -rc_init_occupancy 1668328
> 
> Alternativ syntax:
> -minrate 50000000 -maxrate 50000000 -b:v 50000000
> 
> So there is no need to set "bufsize" and "rc_init_occupancy"?
> 

I have never suggested that, and I do not believe it to be correct, they
are important constraints that will affect the frame delay requirements.

> Best Regards
> Christoph
> 
> 
>>> [...]
> 
> _______________________________________________
> ffmpeg-user mailing list
> ffmpeg-user at ffmpeg.org
> http://ffmpeg.org/mailman/listinfo/ffmpeg-user


-- 
Tim.
Key Fingerprint 38CF DB09 3ED0 F607 8B67 6CED 0C0B FC44 8B0B FC83