Basics of Dolby Digital Surround Sound
Dolby Digital is an all-digital surround sound format that handles audio compression, so it’s available only for digital content. DVDs and HDTV use Dolby Digital (DVDs may also contain some other system, such as DTS — Digital Theater Systems).
Dolby Digital 5.1 represents the current minimum level of performance that you should require from your home theater system. Also keep in mind that any receiver with Dolby Digital decoding can also decode Dolby Surround Pro Logic.
Although Dolby Digital is the more widely known consumer moniker, it also goes by the more techie name of AC-3, which is actually the name used in the official part of the DVD video standard (for “regular” non-HD-DVDs), as well as the ATSC (Advanced Television Standards Committee) standard for DTV and HDTV. AC-3 is also part of the standards for HD-DVD and Blu-ray discs. Dolby Digital is everywhere, and you definitely want your home theater gear to support it. Just about every home theater receiver built since the late 1990s supports Dolby Digital.
What’s so great about Dolby Digital is that it encodes six discrete audio channels. The older Dolby Surround encoded four channels onto two-channel soundtracks, which often resulted in all sorts of bleedovers between channels and less-than-clear demarcations in the sound details.
Because Dolby Digital has six clean channels, your receivers and controllers can precisely control the different elements of your sound mix. More importantly, the rear surround speakers are each fed by their own independent channels, enabling true spatial separation for that rear sound field. With this setup, when you hear that bullet whiz by or that starship warp overhead, the sound moving across your entire speaker system is smooth and controlled — and digital.
Dolby’s ability to encode and decode information is only as strong as its source data. If it’s working with a two-channel stereo movie, you may see something like “Dolby Digital 2.0” on the package, designating that it is a stereo signal being encoded and decoded using Dolby Digital. All fine and dandy, but it’s still a stereo signal. However, if your receiver includes Dolby Pro Logic, Dolby Pro Logic II, or even Dolby Pro Logic IIx, the stereo signal on the disc can still be listened to as a multichannel surround signal, thereby allowing you to achieve full playback on your system.