Ambisonic decoding

This is really the Ambisonic Speaker Decoder page. The Ambisonic B-format WXYZ signals define what you should hear. How these signals are presented to you by the speakers for best results, depends on how many speakers you have and where they are. Ambisonics treats directions where no speakers are placed with as much importance as speaker positions. The last thing we want, is to be aware that the sound is coming from 5 or 7 speakers.

We present some simple decoding equations known to give good results for common speaker arrangements.

But Ambisonic Speaker Decoders can use much more information about the position of your speakers including their exact position and distance from the listener. Because human beings use different mechanisms to locate sound, Classic Ambisonic Decoders modify the speaker feeds at each frequency to present the best information using Shelf Filters.

The complexities of Shelf Filters and Distance Compensation are explained in "Ambisonic Surround Decoders" and "SHELF FILTERS for Ambisonic Decoders" in the External links.

There are specialised decoders for large audiences in large spaces.

Hardware decoders have been commercially available since the late 1970s; currently, Ambisonics is standard in surround products offered by
Meridian Audio, Ltd.. Ad hoc software decoders are also available (see External links).

More to come.

There are four main types of decoder.

Diametric decoders

This design is intended for a domestic, small room setting, and allows speakers to be arranged in diametically opposed pairs.

Regular Polygon decoders

This design is intended for a domestic, small room setting. The speakers are equidistant from the listener and lie equally spaced on the circumference of a circle. The simplest Regular Polygon decoder is a Square with the listener in the centre. At least four speakers are required. Triangles do not work, exhibiting large "holes" between the speakers. Regular Hexagons perform better than Squares especially to the sides.

For the simplest (two dimensional) case (no height information), and spacing the loudspeakers equally in a circle, we derive the loudspeaker signals from the B-format W, X and Y channels::$P\_n\; =\; W\; +\; X\; cos\; heta\_n\; +\; Y\; sin\; heta\_n$where $heta\_n$ is the direction of the speaker under consideration.

The most useful of these is the Square 4.0 decoder. This works well with the majority of ITU 5.1 systems; especially if the listener moves forward so he is in the middle of the square. Centre Front is not used. In most 5.1 systems, the centre speaker is very different from the others drawing attention to itself and spoiling the illusion.

If a centre speaker feed must be used, it should be at a very low level. Centre = X / 10 has been used successfully.

Alternatively, a 5.1 set of speaker feeds can be generated by decoding for a Regular Pentagon (and a 7.1 feed by using a Regular Heptagon) but the speakers really need to be equally spaced in a circle.

More sophisticated 5.1 decodes allow the listener to move back from the centre but so far, these have inferior performance to the simple decodes above.

Readers should note that the coordinate system used in Ambisonics follows the right hand rule convention with positive X pointing forwards, positive Y pointing to the left and positive Z pointing upwards. Horizontal angles run anticlockwise from due front and vertical angles are positive above the horizontal, negative below.

Auditorium decoders

This design is intended for a large, public space setting.

"Vienna" decoders

These are so named because the paper introducing them was presented at the 1992 AES conference held in Vienna. This design is covered by Cite patent|US|5757927 and can be licensed from Trifield Productions (see External links). The technology provides one approach to the decoding of Ambisonic signals to irregular loudspeaker arrays (such as ITU) commonly used for 5.1 surround sound replay.

See also

*Ambisonics
*
*Meridian Audio, Ltd. (manufacturer of hardware decoders)

External links

* [http://members.tripod.com/martin_leese/Ambisonic/faq_latest.html Ambisonic Surround Sound FAQ] (Sections 17 and 18 for hardware decoders)
* [http://www.ambisonia.com/wiki/index.php/Playback_Software Ambisonia website] Bruce Wiggins's WAD decoders for 4.0, 6.0 and 8.0 are nearly Classic Ambisonic Decoders and easy to use plugins for Windows Media Player.
* [http://www.radio.uqam.ca/ambisonic/b2x.html B2X Plug-Ins] B2D, B2G and B2Stereo software decoders, in VST and Audio Unit formats, for Mac OS X
* [http://www.ambisonia.com/Members/ricardo/ Shelf Filters and Distance Compensation] "Ambisonic Surround Decoder" and "SHELF FILTERS for Ambisonic Decoders" explain these important features of Classic Ambisonic Decoders for those designing software decoders
* [http://www.trifield.co.uk/Ambisonics.swf Trifield Productions] (for "Vienna" decoder licensing)