[MPlayer-dev-eng] [PATCH] af_ambisonic

Sebastian Olter qduaty at gmail.com
Wed Apr 18 05:55:04 CEST 2007


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
2. Decoder can use all available speakers.
3. New functions for audiophiles.
4. Helps with MPlayer's HRTF (try w=<num>)

The filter can operate automatically. Use -af amb=autoencode to encode from
5.1 to 3-channel Ambisonics (other input formats are ignored here), -af amb
to automatically decode Ambisonics into 5.0 if needed. All available options
and overall filter behavior can be displayed with -af amb=help.

Patch consists of the .amb file extension support as well. I will upload
some critical samples; currently it is possible to order them via torrent
from www.ambisonia.com.

Possible bugs and todo:

1. There may be some issues with channel mapping, especially on audio
outputs other than dsound.
2. 3D decoding with HRTF and periphonic setups. Current version can already
encode three-dimensional sound.
3. Optimization for small rooms (shelf filters), support for arbitrary
speaker arrangement with distance compensation.
4. Detection of 6 and 8-channel Ambisonics and 4-channel quadraphonic files.
5. Update Vorbis decoder to support mapping 1, which was devoted to use with
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