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.

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.