Historical

Sidebar: The Mathematics of Dolby's Mono Surround Sound

Even though it may come from two loudspeakers, the surround signal on Dolby-encoded recordings is a single channel—monophonic. To encode it into the main stereo channels, it is subtracted from each channel by inverting its electrical polarity and then adding it to the front-channel signals. (Adding a minus to a plus subtracts it.)

But it's not quite that simple. Since the surround effects often contain the same frequencies as front-channel sounds, simply subtracting the surround from the fronts would impair the sound in front. So a compromise is struck. Instead of reversing the surround's polarity—that is, changing its phase by 180 degrees—the surround signal is split into two identical channels. Each is "phase-shifted" by half that amount, or 90 degrees, and in opposite directions. One is phased +90 degrees relative to the fronts, the other is phased –90 degrees relative to the fronts.

They are, thus, 180 degrees out of phase relative to each other, but only halfway out relative to the fronts, which drastically reduces their detrimental effect on the front signals. Instead of taking a deep bite out of the front sounds, they just take a nibble. To decode the surround signal, the process is reversed. The left and right channels are then split off to yield two pairs of identical left and right signals. One of these pairs is then subtracted from the other, and what's left is the original surround signal, more or less intact.—J. Gordon Holt