The answer, not surprisingly, is to keep the surround channel separate from the other channels throughout the entire process. This approach creates discrete channels for each sound streamnothing gets mixed together, nothing has to be extracted. Left is left, right is right, center is center, and rear is rearfrom start to finish.
In the mid-1990s, the Dolby engineers got to work on the problemwhich was primarily a problem of storage space. Where videotapes didn't have space for extra audio tracks, the new DVD medium did. All the information on a DVD is recorded in digital format, so allocating a few extra bits for another channel or two was no big thing. Two tracks, four tracks, five tracks, sixall they had to do was spec it out.
Which is what they did.
To start, the Dolby guys turned to the format that was working for them in the movie theaters. Since 1992, the theatrical Dolby Digital Surround system had been encoding six channels of information onto a film's optical tracks. Of these six channels, five were used to create the surround sound fieldleft front, center front, right front, left surround, and right surround. The sixth track was used to carry the low-frequency information typically fed to a subwooferwhat the lab guys call a low frequency effects (LFE) channel. The result was referred to as a 5.1-channel systemthe five normal channels, and the ".1" LFE channel.
Seeing no reason to mess with something that workedand realizing how easy it would be to turn a 5.1-channel movie soundtrack into a 5.1-channel DVD soundtrackthe lab guys took the route of least resistance and created a 5.1-channel specification for home DVD. All six tracks (the five normal channels plus the LFE channel) are digitally encoded onto the DVD. When the DVD is played on a home DVD player, the audio information is fed into a Dolby Digital decoder, which reads all six tracks and feeds them into six separate amplifiers contained in an audio/video receiver. (Well, five amplifiers, since the LFE channel is typically passed through to a self-powered amplifier, but you get the idea.)
It's actually a simpler approach than the older matrix method. The left channel always carries left-channel information; nothing more, nothing less. The right channel is the right channel, the center channel is the center channel, and the left and right surrounds are the left and right surrounds. There's never any center channel information mixed in with the left and right channels; the surround channels always stay separate from the others, and from each other. In fact, a discrete surround decoder doesn't have to work that hard, and really doesn't have anything to decode; all it has to do is read the individual audio tracks that are laid down on the DVD.
The result: Near-perfect channel separation, with all channels delivering a full frequency response. (This wasn't always the case with matrix surround; the "mixed-in" channels frequently suffered a frequency loss in the translation.) If a director wants to have a telephone ringing to the left rear of the audience, that telephone rings in the left surround channelthe same place, every time. Precise sound positioning had arrived.
The Dolby engineers imaginatively named this new home surround system Dolby Digital becausewell, because it was. Where the Dolby Pro Logic matrix technology worked with analog audio tracks, Dolby Digital used the DVD's digital audio tracks. The analog/digital difference did more than just make effective surround sound possible; it also resulted in higher-quality sound, period.
Not to confuse things, but a competing company, Digital Theater Systems, came up with a discrete surround system named DTS, which works pretty much the same as Dolby Digital. It offers slightly better sound, however, because it records its digital information at slightly higher bit rates. The basic surround approach, however, is similar to that used by the Dolby folks. See my earlier article "The History of Surround Sound" for details on the development of the various systems.