[FFmpeg-user] The Magic Microsecond

[Roger Pack]

> We are still using one of Sherpya's FFmpeg builds to transcode
> recordings on a SD card. Due to security precautions (and restrictions
> by sysadmins to -not- access files on the PC of the transcoder) we are
> doing the transcoding entirely on SD cards.
>
> Compare the four following command lines:
>
> ffmpeg-r32754              -i /SD_VIDEO/MOV001.MOD -vf scale=384:288 \
>         -aspect 4:3 -sws_flags bicubic -vcodec flv -b:v 320k \
>         -strict -2 -acodec aac -ab 96k -ar 44100 output.flv
>
> ffmpeg-r32754 -ss 0.000001 -i /SD_VIDEO/MOV001.MOD -vf scale=384:288 \
>         -aspect 4:3 -sws_flags bicubic -vcodec flv -b:v 320k \
>         -strict -2 -acodec aac -ab 96k -ar 44100 output.flv
>
> ffmpeg-20120706              -i /SD_VIDEO/MOV001.MOD -vf scale=384:288 \
>         -aspect 4:3 -sws_flags bicubic -vcodec flv -b:v 320k \
>         -strict -2 -acodec aac -ab 96k -ar 44100 -threads 0 \
>         output.flv
>
> ffmpeg-20120706 -ss 0.000001 -i /SD_VIDEO/MOV001.MOD -vf scale=384:288 \
>         -aspect 4:3 -sws_flags bicubic -vcodec flv -b:v 320k \
>         -strict -2 -acodec aac -ab 96k -ar 44100 -threads 0 \
>         output.flv
>
> You would expect them all to do the transcoding in more ore less the
> same amount of time. Well, when the output file is on a SD card, it
> ain't so.
>
> The first command line ends with:
> frame= 6012 fps= 22 q=24.7 Lsize= 12814kB time=00:04:00.48 \
>         bitrate= 436.5kbits/s
>
> The 22 fps was completely unacceptable and we moved to MEncoder, which
> did not have the problem. By pure chance, I discovered a few weeks ago
> how to speed up ffmpeg-r32754 dramatically: skip the first microsecond!
> -ss 0.000001.
>
> Result:
> frame= 6010 fps=330 q=23.8 Lsize=   12809kB time=00:04:00.48 \
>         bitrate= 436.3kbits/s
>
> Fps: 330. Now we are talking! 15 times as fast.

Maybe this is the equivalent of analyzeduration or probesize 0 or something...
-r