If you're still having trouble understanding Ambisonics, you are not alone.  Because the concepts behind the recording and playback systems using in Ambisonics are radically different from those employed in quadraphonics, Ambisonics has required its own vocabulary.  Dr. Barton, an active participant in the development of Ambisonics in England, prepared this glossary for the MCS Review.  We think you'll find it useful as you read and hear more about Ambisonics.

• A-format is the signal format used within the Soundfield microphone (and is not normally accessible to the outside world).  It consists of the four signals from the array of microphone capsules which are placed at the vertices of a regular tetrahedron.  These signals are processed electronically to produce B-format.

• B-format is the standard professional signal format.  It consists of four signals:

  • W,  a mono (pressure) signal corresponding to the output from an omni-directional microphone,
  • X,  the front-to-back directional information (a forward-pointing velocity or "figure-of-eight" microphone),
  • Y,  the side-to-side directional information (a leftward-pointing figure-of-eight), and
  • Z,  the up-to-down directional information (an upward-pointing figure-of-eight).

  In horizontal surround sound, the "Z" information is discarded, apart from some studio signal processing.

• C-format is the encoded signal format which is transmitted to the consumer.  It consists of either two, three or all four of the following signals according to the UHJ specification:

  • L,  left stereo compatible signal,
  • R,  right stereo compatible signal,
  • T,  third channel, which improves the resolution of the decoded horizontal surround sound.  It enables performance equivalent to that obtained with the original B-format signals W, X and Y, and
  • Q,  contains the height information from the Z channel of B-format.

• D-format is the decoded (speaker-feed) signal format produced by an Ambisonic decoder.  The exact specification of these signals depends on the number of loudspeakers in use and their positions in the room.

• HJ is the name given to the encoding specification jointly agreed by the BBC and NRDC as a result of the fusion of BBC Matrix H and NRDC's 45J.  It is in the form of a tolerance zone but is only defined for two-channel use.

• UHJ is a hierarchical encoding system (see C-format), the two-channel version of which falls within the HJ tolerance zone.

• Forward preference, variable preference, phasiness.  When decoding from the L and R signals of C-format, some resolution information is lost (because of the omission of the T signal).  Under some circumstances this is apparent in a phasey or blurred quality to the images.  The forward preference (sometimes called "focus") button enables the user to reduce this effect in the important forward direction at the expense of increasing it elsewhere.  Some recent decoders incorporate a variable control to allow the amount of this effect to be selected by the user, hence the term "Variable Preference Decoder".

• Periphonic is the term we use to describe with-height or "full-sphere" soundfield playback, i.e., decoding using the "Z" information as well as W, X and Y (or "Q" in addition to L, R and T).

• Ambisonics, Ambisonic technology are terms we use to describe the complete technology including the Soundfield Microphone, B-format signal processing, UHJ encoding/decoding and psychoacoustic compensation in decoders.

• Super-Stereo is a facility available on most Ambisonic decoders for use with source material not UHJ encoded.  It uses all the loudspeakers connected for surround-sound playback to lock the stereo image in place at the front.  Some decoders provide a stereo-width control which, in the "stereo decode" mode, allows the image to be stretched to extend outside the front loudspeakers.

• Quadrifontal is a term used to describe a system which attempts to produce sound images at four points around the listener, usually at the corners of a square, and coinciding with loudspeaker locations.  It is rather more precise than "quadraphonic" because of the frequent confusion of transmission channels and loudspeaker feed signals with image locations the latter term seems to cause.  Quadrifontal merely means "four-source".

• Pairwise-blending is the practice of giving an apparent source direction to a mono signal by panning between two audio channels, usually related directly to loudspeaker feeds.  This can be contrasted with the technique in Ambisonics of assigning a mono source to a direction rather than to channels related to loudspeaker feeds, the derivation of loudspeaker feed signals being a matter for the decoder according to the actual positions of the loudspeakers.

• Dominance, forward dominance, backward dominance.  Dominance is a facility available in professional Ambisonic equipment and now becoming available in domestic decoders.  The dominance control alters the balance between the direct (front stage) sound and the reverberant (rearward) sounds to produce an effect rather like the familiar "zoom" lens.  Forward dominance makes the front stage appear closer whilst backward dominance makes the front stage appear more distant and the rear closer ("wide-angle").