[FFmpeg-user] AAC conversion from 7.1 channel to 2 channel

[rohit a]

Hi,

For AAC conversion from 5.1 channels to 2 channels, the equation used in
 surround_to_stereo()  in libavodec/resample.c is

*        l = av_clip_int16(fl + (0.5 * rl) + (0.7 * c));*
*        r = av_clip_int16(fr + (0.5 * rr) + (0.7 * c));*

I was not able to find any algorithms/equations for converting 7.1 channel
to stereo.
Is there any equation for converting 7.1 channel to stereo ?

Regards,
Rohit




On Fri, May 3, 2013 at 5:47 PM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:

> rohit a <rohit.tvm.86 <at> gmail.com> writes:
>
> > For AAC conversion from 5.1 channels to 2 channels,
> > ffmpeg code uses surround_to_stereo() in
> > libavcodec/Resample.c
>
> This looks outdated, I believe you should use
> libswresample.
>
> > In that the input data was packed in the format
> > fl, fr, c, lfe, rl, rr
> >         fl = input[0];  // front left
> >         fr = input[1];  // front right
> >         c = input[2];   // center
> >         lfe = input[3]; // low frequency effects
> >         rl = input[4];  // rear left
> >         rr = input[5];  // rear right
>
> > I would like to convert AAC 7.1 channels to 2 channels.
>
> Did you try?
> Without testing, I suspect it should work.
>
> > But i have no idea about how the input data is packed.
>
> FFmpeg internally expects wav channel order.
>
> > Can anyone please tell me how the input data format
> > is arranged for AAC 7.1 channels ?
>
> In aac, different channel orderings are possible,
> and usually it is not wav-order, but the aac
> decoder should correctly order the channels.
> (If it does not, this would be a bug, reports
> are very welcome!)
>
> Carl Eugen
>
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