As part of its ongoing effort to advance the state of the art, the audio industry developed two new digital surround formats that deliver 6.1 channels instead of 5.1 channels. The formats are Dolby Digital Surround EX (Extended), which appeared on DVD for the first time in late 1999, and DTS ES (Extended Surround), which appeared on DVD for the first time in 2000. The DVD releases followed the two formats’ movie-theater debuts.

Both 6.1 formats add a back-center channel to a 5.1-channel home theater system, letting content providers accurately place sound effects behind you, not just to the sides and front. This allows for true “fly-over” and fly-around” effects in which jets sound as if they’re flying over your head or circling a full 360-degrees around you. One to two speakers behind the viewers deliver the sixth channel, complementing the two traditional surround speakers to the viewer’s left and right.

As of early 2005, 76 DVD titles were available in ES in the U.S., said inventor Digital Theater Systems. The number of DVD titles with Dolby Digital EX was unknown.

Dolby Digital EX soundtracks also can be delivered to consumers by local DTV stations, digital cable operators and satellite operators if the content is mixed in EX. How much content has been delivered that way is uncertain.

However they’re delivered, EX and ES soundtracks are compatible with existing 5.1-channel home theater systems, but you’ll need a new surround decoder or receiver (and at least one extra speaker) to reproduce the sixth channel.

In 1999, the first high-end preamplifier/processors that decoded 6.1-channel soundtracks hit the market, and in 2000, the first high-end surround receivers appeared with that capability. In 2004, receivers with 6.1-channel decoding are commonplace and available at less than $200.

At the mid- and high-ends of the market, many processors and A/V receivers deliver back-surround effects through two back-channel speakers rather than one to further enhance the surround experience. These systems, with a total of seven full-range speakers and a subwoofer, reduce the potential for so-called “combing” effects that reinforce certain frequencies when only one back-channel speaker is used. The additional speakers also make it easier to distinguish whether a particular sound is coming from the front-center-channel speaker or a back-center-channel speaker. Some seven-speaker systems use additional processing to enhance the seamlessness of sounds panned around you.

In 2005, a handful of suppliers were expected to offer HTiB systems equipped with EX/ES at opening prices of less than $179.