University of York: Music Technology Group; Ambisonics Decoding on a Mixer.

York University Music Technology Group

Ambisonic Decoding on a mixer

The following explanation of how to do periphonic decoding without having an Ambisonic decoder is from Lowell Cross.

You can configure a periphonic decoder with an eight-bus mixing console. Assuming that you start with the concept of eight loudspeakers located at the corners of a cube, assign W (omni) to all eight channels without polarity (phase) reversal. The X, Y, and Z are each SPLIT, through the use of "multiples" on a patch panel.

By considering the front, left, and "up" lobes of the bidirectional patterns X, Y, and Z to be "+" (IN phase with W), and the rear, right, and "down" lobes to be "-" (OUT OF phase with W), the proper channel assignments and phase relationships can easily be made. For example, the left, front, upper channel will be assigned W, X, Y, and Z IN phase. The left, front, lower channel will be assigned W, X, and Y IN phase, with Z OUT OF phase. Here are the channel and phase assignments:

Signals required to drive speakers in a cubic array
For this Speaker Position use these signals

LEFT FRONT UPPER W+ X+ Y+ Z+

RIGHT FRONT UPPER W+ X+ Y- Z+

LEFT FRONT LOWER W+ X+ Y+ Z-

RIGHT FRONT LOWER W+ X+ Y- Z-

LEFT REAR UPPER W+ X- Y+ Z+

RIGHT REAR UPPER W+ X- Y- Z+

LEFT REAR LOWER W+ X- Y+ Z-

RIGHT REAR LOWER W+ X- Y- Z-

Signals required to drive speakers in a cubic array
For this Speaker Position	use these signals
LEFT FRONT UPPER	W+ X+ Y+ Z+
RIGHT FRONT UPPER	W+ X+ Y- Z+
LEFT FRONT LOWER	W+ X+ Y+ Z-
RIGHT FRONT LOWER	W+ X+ Y- Z-
LEFT REAR UPPER	W+ X- Y+ Z+
RIGHT REAR UPPER	W+ X- Y- Z+
LEFT REAR LOWER	W+ X- Y+ Z-
RIGHT REAR LOWER	W+ X- Y- Z-

W is "in phase" to all assignments. All front channels are driven by X in phase; all rear channels are driven by X out of phase. All left channels are driven by Y in phase; all right channels are driven by Y out of phase. All upper channels are driven by Z in phase; all lower channels are driven by Z out of phase.

Lowell doesn't mention it, but if you don't have a 'phase' switch on your mixer channels (okay - I know it is really polarity, but it's always labelled phase), you can simply wire the plugs going to the balanced inputs the wrong way round on the channels that provide the inverted signals. (Of course, that's not a lot of good if your mixer only has unbalanced inputs, but...) Alternatively, if the recorder (or other sound source) you have has electronically balanced outputs (bit more difficult if they are transformer balanced, but that's unusual these days) then you have positive versions of the signal on one pin (usually pin 2 on XLR's, tip on tip-ring-sleeve jacks) and negative on another (pin 3 or ring). If you make up a splitter lead which goes from the balanced signal to two separate connectors, one having the positive version of the signal only (+ ground from pin1 or sleeve), the other having the negative version of the signal only (+ ground from pin1 or sleeve) then you can put these into separate mixer channels. Of course, if you have a computer with a multichannel soundcard, you can do it all in software!

This page is administered by Dave Malham.
Last updated 31^st March, 2004. Accessibility guidelines implemented January 2003.

If you have any suggestions, comments or requests about the design of these web pages you can reach me at dgm2@york.ac.uk .