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

Timothy Gu timothygu99 at gmail.com
Fri Nov 1 23:55:56 CET 2013


On Nov 1, 2013 2:07 PM, "Stefano Sabatini" <stefasab at gmail.com> wrote:
>
> 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?

See faac-1.28/libfaac/frame.c line 268.

>
> -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).

Weird.

Timothy


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