[FFmpeg-devel] [PATCH] doc/encoders: add libfaac encoder documentation

[Stefano Sabatini]

On date Friday 2013-11-01 10:13:02 -0700, Timothy Gu encoded:
> On Fri, Nov 1, 2013 at 4:27 AM, Stefano Sabatini <stefasab at gmail.com> wrote:
> > On date Thursday 2013-10-31 17:56:08 -0700, Timothy Gu encoded:
> >> On Thu, Oct 31, 2013 at 1:42 AM, Stefano Sabatini <stefasab at gmail.com> wrote:
> >> > On date Wednesday 2013-10-30 19:53:29 -0700, Timothy Gu encoded:
> >> >> On Oct 30, 2013 6:26 AM, "Stefano Sabatini" <stefasab at gmail.com> wrote:
> >>
> >> >> > + at item q (@emph{-q})
> >> >> > +Enable VBR mode, and set constant quality value as an integer.
> >> >> > +
> >> >> > +This option is valid only using the @command{ffmpeg} command-line
> >> >> > +tool. The value is expressed in QP units, the higher the value the
> >> >> > +higher the quality. A reasonable range for the option value is
> >> >> > +[10-500].
> >> >>
> >> >> The valid range for...
> >> >
> >> > Not exactly, since it will accept any non-negative value, but
> >> > reasonable quality starts with 10. I didn't check libfaac
> >> > documentation properly, I think this is already (much) better than
> >> > nothing.
> >>
> >> FAAC only supports 10-500.
> >> http://manpages.ubuntu.com/manpages/raring/man1/faac.1.html
> >
> > Values from 0 to 1,000,000 are accepted, but values greater than 500
> > produces the same output size.
> >
> > A value of 0 is accepted, and produces very low quality. A negative
> > value will not set VBR mode, since it will not enable the +qscale flag.
> >
> > The following outputs have been generated with the command:
> > ffmpeg -i slow.flv -vn -c:a libfaac -profile:a aac_low -q:a $qp -y out$qp.m4a
> >
> > ls -lh out*.m4a
> > -rw-r--r-- 1 stefano stefano 257K nov  1 11:57 out+0.m4a
> > -rw-r--r-- 1 stefano stefano 2,3M nov  1 10:22 out+10000.m4a
> > -rw-r--r-- 1 stefano stefano 2,3M nov  1 10:22 out+1000.m4a
> > -rw-r--r-- 1 stefano stefano 1,3M nov  1 10:22 out+100.m4a
> > -rw-r--r-- 1 stefano stefano 257K nov  1 10:21 out+1.m4a
> > -rw-r--r-- 1 stefano stefano 395K nov  1 10:21 out+20.m4a
> > -rw-r--r-- 1 stefano stefano 2,3M nov  1 10:22 out+500.m4a
> > -rw-r--r-- 1 stefano stefano 899K nov  1 10:22 out+50.m4a
> >
> > I'll push soon with a few more changes. Feel free to suggest more
> > tweaks.
> 

> If you test with 0, 1, and 10, they will produce the same effect as
> faac does not support 0-9 (it just clips to 10).

Do you have any evidence of this?

-rw-r--r-- 1 stefano stefano  262541 nov  1 21:59 out-0.m4a
-rw-r--r-- 1 stefano stefano  262173 nov  1 22:00 out-1.m4a
-rw-r--r-- 1 stefano stefano  262344 nov  1 22:00 out-2.m4a
-rw-r--r-- 1 stefano stefano  284164 nov  1 22:00 out-5.m4a
-rw-r--r-- 1 stefano stefano  288391 nov  1 22:00 out-8.m4a
-rw-r--r-- 1 stefano stefano  293588 nov  1 22:00 out-9.m4a
-rw-r--r-- 1 stefano stefano  293335 nov  1 22:00 out-10.m4a

While I confirm that any value greater than 500 issues the same
result (but I couldn't find this documented anywhere but in the faac
manual).
-- 
FFmpeg = Furious & Foolish Monstrous Portentous Evil Gangster