Dolby Digital Plus (DD+ or E-AC-3), is a digital audio compression scheme developed specifically for the introduction of HDTV and HD DVD/Blu-ray Disc. It is a development of the earlier Dolby Digital system. Compared to Dolby Digital it supports a greater range of bitrates at higher quality and up to thirteen full range audio channels compared with Dolby Digital's five. The compression algorithm also has a number of improvements aimed at increasing quality at a given bitrate compared with Dolby Digital.Dolby Digital Plus software is not directly backward compatible with existing Dolby Digital decoders. However, Dolby Digital Plus capable players are required to be able to transform the output to a backwards compatible Dolby Digital signal.
Dolby Digital Plus offers increased bitrates (up to 6.144 Mbit/s), support for more audio channels (up to 13.1), improved coding techniques to reduce compression artifacts, and backward compatibility with existing AC-3 hardware.
The maximum number of discrete coded channels is the same for both formats: 7.1. However, HD DVD and Blu-ray impose different technical constraints on the supported audio-codecs. Hence, the usage of DD+ differs substantially between HD DVD and Blu-ray.
