Ambisonics - G-Format

G-Format

The lack of availability of Ambisonic decoders (only a handful of hardware decoder models are currently available, although software-based players are now emerging) led to the proposal that Ambisonics could be distributed by decoding the original signal (preferably B-Format but also legacy 2-channel UHJ recordings) in the studio instead of at the listening end. A professional software or hardware-based decoder is used to decode the Ambisonic signal to a conventional surround speaker array (e.g. 5.1) and the resulting speaker feeds are authored to a conventional multichannel disc medium such as DVD. This is known as "G-Format".

The obvious advantage of this approach is that any surround listener can be able to experience Ambisonics; no special decoder is required beyond that found in a common home theatre system. The main disadvantage is that the flexibility of rendering a single, standard Ambisonic signal to any target speaker array is lost: the signal is targeted towards a specific "standard" array and anyone listening with a different array may experience a degradation of localisation accuracy, depending on how much the actual array differs from the target.

In practice, Ambisonics in general has proved to be very robust, however. Examples of G-Format recently released by Nimbus Records used 2-channel UHJ decoded to a square array of four speakers (this is conventional for decoding planar Ambisonic recordings; a rectangle of sides with ratios of between 2:1 and 1:2 can be used, a square being midway between the two). The resulting 4-channel (LF, RF, LS, RS) signal was authored to DVD-Audio/Video discs and although many listeners will be listening on arrays other than a square, the results have proved very encouraging.

Some releases of G-format sourced from B-Format have also occurred, for example the album Swing Live by Bucky Pizzarelli (available on Chesky Records, DVD-A or SACD), where a B-Format SFM recording was "manually decoded" to 4.0 speaker feeds in the mixdown process.