In a nutshell, the original Dolby Surround recorded sound from the front center, left and right of an environment, plus sound from the rear. Those four channels were combined so they could be transmitted via a traditional stereo signal, and then were decoded at the receiving end to be sent to left, right and rear speakers (the center signal was a “phantom” one created from the left and right signals). It sounded better, but the rear sound reproduction wasn’t very realistic.
New iterations of the technology added extra speakers, greater channel separation and directionality, finally leading us to today’s advanced surround sound. Don’t let all of the formats like Digital, THX, EX, Pro Logic, Atmos, DTS and others confuse you. They all work on the same basic principle.
Most modern systems have four speakers in front of the central listening position: left, right, center and a subwoofer which reproduces low frequencies separately. There are then several “surround” speakers which are usually placed alongside but just behind the listening position, with some systems having even more speakers placed further to the rear. The numbers you often see like 5.1, 6.1 and 7.1 simply refer to the number of speakers; the first digit refers to the number of active speakers, while the “.1” refers to the subwoofer.
What makes this all work is the fact that today’s audio technology allows many different channels of sound to be encoded in a traditional “stereo” signal. That means producers can carefully determine the audio to be placed in each channel, and modern components can decode those signals to send a different audio track to each of the surround sound system
speakers. In that way, the realistic audio effects of a concert hall, a gun battle or a plane soaring overhead – including the directionality as well as the ambient and reflected sound – can all be reproduced faithfully.