The multichannel digital system known as Dolby Digital Surround uses an algorithm called AC-3 (stands for "Audio Code number 3") which consists of a sequence of data that enters the converter via an optical, coaxial or radio frequency input and is decoded by a digital card that returns the content of the transmission to its original state.
Dolby Digital is available in different formats: 6.1, 5.1, 4.1, 3.1, 2.1, 5.0, 4.0, 3.0, 2.0, 1.0, 1.1: the numbers before the point represent the number of channels, those after the point the presence of an LFE element (for the emission of low frequencies – in practice the subwoofer), the separation between the channels can reach 90dB. The speaker arrangement for the 5.1 which is currently the most widespread consists of 2 right front speakers and left, one front center and two rear speakers (all for reproducing frequencies over a very wide range) in addition to the subwoofer that deals with low frequencies.
But at present no Mac has an optical digital output and the only possibility is the use of USB as a bridge to digital.
Already at MacWorld in New York M-Audio had presented a low-cost USB-Digital interface suitable for the purpose: Sonica.Sonica supports 24-bit / 96kHz and SRS TruSurround XT technology.
The interface is now supported by Jaguar and a version of the VideoLan freeware player (version 4.3) is available that can support AC3 audio with the M-Audio product.
The software can be downloaded from this page.