Dolby Digital Plus - An enhancement over standard Dolby Digital, DD+ offers higher bit rates and more efficient compression, resulting in improved sound quality. It can also support movie soundtracks up to 7.1 discrete channels (though honestly, the vast majority of Hollywood movies are only mixed for 5.1). On Blu-ray, DD+ is encoded as an extension to a "core" Dolby Digital AC-3 track. DD+ is optional on the Blu-ray format, and not all disc players are required to support it. Many players will simply read the 640 kb/s core and disregard the extension. As a result, most movie studios prefer to use either basic Dolby Digital AC-3 or some of the other advanced formats.