[MPlayer-dev-eng] [PATCH] af_ambisonic

Nico Sabbi nicola_sabbi at fastwebnet.it
Sat Apr 21 17:59:08 CEST 2007

Sebastian Olter wrote:
> Hi,
> I implemented an audio filter for Ambisonics, as it was mentioned on
> ffmpeg-devel a month ago. Ambisonics is a method of coding high quality 3D
> surround sound, which takes directions into account, instead of speakers.
> The ideas of Ambisonics are very elegant; for example the channel order is
> derived from theory and thus will never change.
> Benefits of using Ambisonics in MPlayer-related software:
> 1. Usual 5.1 tracks can be encoded into 3 channels without losing the
> surround effect. This implies more effective compression--my tests show 
> that
> a movie sound track can be encoded with Vorbis at 130 kb/s and it sounds
> better than 5.1 AAC at 256 kb/s. With 1CD rip, surround effect won't be 
> lost
> anymore.
> 2. Decoder can use all available speakers.
> 3. New functions for audiophiles.
> 4. Helps with MPlayer's HRTF (try w=<num>)

premitting that in order to listen dvd tracks I always use:
-af pan=2:1:0:0:1:1:0:0:1:1:1:1:1 ( 6 -> 2 channels with all lefts 
mapped to lefts, all rights ro right and center+lfe repeated to both 
channels, the only way for me way to listen dialogs) I tried to play a 
little with this filter applied to a 5.1 ac3 file to hear how it works 
as downmixer.
Following the instructions in DOCS I used

1) mplayer -channels 6 -af amb=sources=5.1ac3 -ao pcm:file=sound.wav
2) mplayer -af amb=sinks=2 sound.wav

but the result sounds a little like -channels 2 without filters 
applied (that is, using the default liba52's downmixer) : too much
information seems to be discarded (and dialogs are almost inaudible).

BTW, doesn't the above sequence of commands correspond to
  -channels 6 -af amb=sources=5.1ac3:sinks=2 ?

Can you suggest some better combination of options for my needs?
Possibly explain the meaning of the options in clearer terms?
What do you mean by encoder and decoder in the context of the filter?
i.e. isn't ambosonic a pure filter?

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