From Edison's first wax-cylinder machine to the mid-1950s, home audio was always mono. Stereo took off in the '60s, and the first surround format to take hold and flourish was Dolby Surround in 1982. Dolby Surround is a matrix format; that is, its surround effects are encoded in two stereo channels. Many of today's surround formats are discrete and feature six separate tracks.

Some DVDs feature multiple surround formats--for example, Dolby Surround, Dolby Digital, and DTS--on a single disc. This brings up another angle to the surround story; to enjoy the full sound potential of, say, a DTS-ES-encoded DVD, you need a receiver that processes DTS-ES. In other words, formats exist in both software and hardware. Other formats, such as Dolby Pro Logic II and DTS Neo:6, are found only on hardware--receivers, HTIBs, DVD players, and so on--and are applied to processing software or music sources, such as CDs, DVDs, MP3 files, and radio.

When we designate the maximum number of speakers/channels for each format, such as 5.1 or 6.1, we're referring to the number of front, center, rear, and possibly center-rear speakers (the .1 refers to the subwoofer) that the format can use. Stereo, a.k.a. 2.0, is a two-channel format. Check out the diagram below to see how they are set up in a room. Note that even though 7.1 systems use two back-surround speakers, the same channel of information goes to both of them.

Dolby Digital is the most popular surround format and can be found on virtually every DVD, including almost all DVD music discs, and many HDTV programs. Pretty much every A/V receiver and HTIB on the market features Dolby Digital processing.

First, Pro Logic II isn't one of those synthetic "jazz club" and "stadium" modes and doesn't add synthesized reverberation or echoes. It's a hardware-based format that's compatible with all stereo and matrix-encoded surround software. Pro Logic II is an update of the original Pro Logic and can be used to freshen up Dolby Surround-encoded videotapes and DVDs. It also works wonders with stereo CDs and radio broadcasts.

Dolby EX is a 6.1-channel format, adding one (or two) rear center-surround channel speakers to the standard 5.1 array. EX can deliver more enveloping surround effects than standard Dolby Digital. EX processing can be used to improve the sound of regular Dolby Digital-encoded DVDs. Unlike DTS ES, Dolby EX features an extra rear channel that's matrix rather than discrete. That's why ES can provide a more precise location for the rear-effects soundstage.

An extension of Dolby Digital (DD), the Dolby Digital Plus (DD+) has been chosen as the mandatory surround format for Blu-ray and HD-DVD. It supports up to 13.1 discreet channels with a maximum data rate of 6Mbps. To support the high data rate, HDMI and IEEE1394 (FireWire) have been selected as the new digital interface for DD+ connectivity. The latest Dolby format is backward compatible with existing DD decoders via down-mixing capability and SPDIF digital audio output. DD+ will be available from late 2005.

Dolby Headphone synthesizes five-channel surround effects over any brand of headphone. It offers up to three processing "flavors" that simulate rooms of varying sizes and acoustics. While Dolby Headphone technology offers a substantially more spacious sound compared to the standard stereo headphone experience, it does not synthesize quasisurround from conventional stereo sources such as CDs, MP3s, or FM radio. Right now, you'll find Dolby Headphone processing on the latest generation of higher-end receivers, but we expect the feature will soon find its way onto more moderately priced components.