"Dolby Digital Plus builds on the original Dolby Digital specifications, allowing for higher bit rates and more channels. Dolby Digital Plus has a maximum bit rate of 6mbps and supports 13.1 channels. In comparison, Dolby Digital caps out at 640kbps and 5.1 channels. So Dolby Digital Plus essentially provides 10 times the bandwidth of the original Dolby Digital. The new format also allows for extremely low bit rate multichannel sound for streaming on the Web or over the air. The benefits of the Dolby Digital Plus codec include transient prenoise processing, enhanced channel coupling, adaptive hybrid transform processing, and channel and program extensions.
Whereas the original Dolby Digital signal was sent over either optical or coaxial cables, Dolby Digital Plus uses the new HDMI connector. Content encoded with Plus will be backwards compatible, but the resulting sound won't be as detailed. "