Discrete surround sound keeps each channel separate from start to finish. This improved channel separation, whichever format is used, results in a more realistic surround effect. In the home, all discrete surround formats have always been digital in naturewhich also contributes to improved sound quality.
The most popular discrete surround format today is Dolby Digital. As the name implies, it's a completely digital process, as opposed to the analog matrix technology used in the older Dolby Pro Logic format.
Dolby Digital is used in most commercial DVDs as well as some satellite and cable programming (especially by HBO, Showtime, and other pay channels), and is the format specified for all HDTV broadcasts. In short, it's the surround format of choice for most forms of home entertainment programming.
There are actually several variations of Dolby Digital, depending on how many channels are used. That's rightnot all Dolby Digital soundtracks utilize a full array of surround channels. Dolby Digital is actually a flexible format that supports up to 5.1 channels. So mono films are often recorded in Dolby Digital 1.0 (one center channel, no subwoofer), and older stereo films in Dolby Digital 2.0 (left and right front channels, no subwoofer).
The most common format, however, is Dolby Digital 5.1. This format includes left front, center front, right front, left surround, and right surround channels, plus a separate low frequency effects (LFE) channel that is fed to your system's subwoofer. Add that up and you get the 5 main channels plus the ".1" LFE channel. (Those rare soundtracks without an LFE channel are technically designated "5.0"the "0" noting the lack of an LFE channel.)
While Dolby Digital can deliver fewer than 5.1 channels. to get more than 5.1, you have to move up to a new formatDolby Digital EX, which we'll discuss in a moment.
The Digital Theater Systems company markets a discrete surround format, called DTS, that competes head-to-head with Dolby Digital. Like Dolby Digital 5.1, the DTS format includes five main channels (left front, center front, right front, left surround, and right surround) plus a separate LFE channel. Although technically possible, one seldom (if ever) sees DTS used for mono or stereo recordings.
DTS works similarly to Dolby Digital, but with higher data rates. This results, to some ears, in slightly better sound quality. (I personally find that DTS soundtracks have a noticeably wider dynamic range than similar Dolby Digital soundtracksdo a side-by-side test on the Twister DVD to judge for yourself.)
That said, DTS is an optional format in the home theater environment, and is not included on many DVDs. There are far more DVDs with Dolby Digital soundtracks than with DTS soundtracks. Most A/V receivers include both Dolby Digital and DTS decoders, however, and will automatically detect DVDs with DTS soundtracks.
Dolby Digital EX
Dolby Digital EX is a new variation of the Dolby Digital format. EX adds a "surround rear" channel in addition to the regular left and right surrounds. This rear channel is placed behind the listener/viewer, while the left and right surrounds are placed to the sides. Note, however, that this surround channel is matrixed, not discrete, so it's not overly directional.
There are actually two variations of Dolby Digital EXthe original 6.1-channel format (with a single matrixed rear channel) and the newer 7.1-channel format (with matrixed rear left and rear right channels). Neither format is widely used as yet, although more and more mid- and high-end A/V receivers are coming with 6.1 or 7.1 EX decoders instead of the older Dolby Digital 5.1 decoders.
By the way, Dolby Digital EX is sometimes referred to as THX Surround EX, as it has THX certification. (Learn more about THX here.)
Like Dolby Digital EX, DTS ES is a 6.1 technology with an added surround rear channel. There is also a 7.1-channel version, with two surround rear channels. Both versions use matrix technology to create and decode the rear channel(s).
DTS ES Discrete
If matrixed surround channels aren't ideal to your ears, check out the DTS ES Discrete system. Unlike both Dolby Digital EX and DTS ES, DTS ES Discrete uses discrete technology for its single rear channel. The rear channel in ES Discrete is thus more distinct from the side surrounds than with the other 6.1-channel formats.