Surround Sound and Formats
From the 1950s, many movies started to use several tracks of audio to give the audience a sense of being surrounded by sound played back from speakers to the front, side and rear of the audience.
Dolby Pro Logic
In 1987, Dolby Laboratories introduced the the Dolby Surround Pro Logic encoding format. Pro Logic allows encoding of four channels of sound into two channels stored on a medium. During playback, a Dolby Pro Logic receiver converts a Pro Logic encoded two-channel signal back into four channels.
Pro Logic is a simple but effective format, which provides four channels of sound: three full-range channels which play through the front right, front left, and center speakers, and a fourth channel of limited-bandwidth sound which is shared by two surround speakers.
The stereo sound channels in modern television programs are also usually Pro Logic encoded. In my experience, this is the most evident in commercials, which make full use of the four channels offered by Pro Logic. Also, many sitcoms feature audience laughter in the rear surround channel while the actual (usually not-at-all-funny) dialog is played back by the front speakers.
Dolby Pro Logic is a matrixed multichannel system, which means that some channels are actually derived from other channels. In this case, center and the single rear surround channel sounds are created from parts of the front right and left channels. The single matrixed rear surround channel is split and sent to both the surround speakers, so both surround speakers play the same sounds.
Although Dolby Digital, strictly speaking, is a particular method of encoding audio information digitally, the term is often used to refer to its most popular form: 5.1-channel audio.
Unlike Dolby Pro Logic, Dolby Digital 5.1-channel audio is a discrete multichannel surround sound system. Discrete means that the sound information contained in each of the six available channels is distinct and independent from the others. These six channels are described as a "5.1-channel" system, because there are five full-bandwidth channels with 3-20,000 Hz frequency range for front, center, and surround, plus one "low frequency effects" (LFE) channel devoted to frequencies from 3-120 Hz. This last channel (the LFE channel) takes up approximately 1/10th of the bandwidth required by the other five full-range channles, hence the ".1" in "5.1".
The LFE Channel is sometimes erroneously referred to as the "Subwoofer" channel. The fact is that modern dolby-digital receivers direct low-frequency sounds from all channels to the subwoofer, and the LFE channel helps the receiver to figure out what to send to the subwoofer more precisely.
Dolby Digital uses data compression to reduce the amount of disc and broadcast space required by 6 discrete channels of sound. Because of its efficiency and high sound quality, Dolby Digital has been chosen as the multichannel digital audio format for DVDs, high-end laserdiscs, and HDTV (High Definition Television) broadcasts.
Digital Theater Sound (DTS)
Like Dolby Digital, DTS provides a 5.1-channel digital audio system. However, DTS uses less compression than Dolby Digital. As a result, some say that the sound produced by DTS is slightly more accurate than the sound produced by Dolby Digital.
On the flip side, DTS requires much more space due to lower compression. Hence, and most DVDs with a DTS soubd-track have very limited room for extra features like commentaries, foreign languages, and multiple versions of the movie.
However, most Dolby Digital receivers, including the Sony STR-DE845, also include a DTS decoder.
THX Surround EX
THX Surround EX isn't actually a new format. It is collaboration between THX and Dolby Labs designed to give surround sound improved directionality. THX adds another one or two surround speakers to the two already present in a 5.1-channel system. However, those channels, rather than being discrete, share a matrixed channel of sound composed of information included in the existing surround channels.
However, whereas the upgrade from Dolby Pro Logic to Dolby Digital constitutes a huge improvement, THX Surround EX's enhancement over Dolby Digital is relatively subtle. For the time being, it looks like Dolby Digital will continue to reign as the dominant home theater surround format. Nevertheless, it's certainly exciting to keep an eye on THX Surround EX!
Comparison Table for Different Sound Formats