Dolby Pro Logic

Dolby Pro Logic is the older, uglier way of doing surround. Sometimes it's called "Dolby Surround." It's essentially just stereo sound--two channels. However, extra information is added to the sides, which a Dolby Pro Logic decoder (99% of the home theater amps/receivers on the market) can use to approximate surround sound. While the amp is only fed two audio channels, it can use those channels to fill in sound for center and rear speakers. A later version of the standard, Dolby Pro Logic II, does an even better job of this. One nifty thing about Pro Logic is that it still sounds good in stereo mode, without decoding the extra surround information.

Dolby Digital Surround

Dolby Digital is the most common surround sound on DVDs. It sometimes goes by the name AC3. It's also part of the ATSC standard for high definition TV. Whereas Dolby Pro Logic only contains 2 channels of sound, Dolby Digital actually has separate, discrete channels for Center, Right, Left, Rear Right, Rear Left, and a subwoofer (or LFE--low frequency effects). This makes it "true" surround sound. The channels are far more distinct than with Pro Logic. However, the audio also takes up a lot more space; it is common for AC3 tracks to run at 448kbps (about 75 kbps per channel).

Digital Theater System (DTS)

Digital Theater Sound is slightly newer than Dolby Digital. It's a rival standard that is not quite as common on DVDs. Its selling point is higher audio fidelity. While both AC3 and DTS are lossy compression schemes, DTS is less so. When given sufficient bandwidth, it can produce far higher quality audio than AC3. Unfortunately, it is common for DVDs to contain both AC3 and DTS audio. When this happens, the bitrate of the DTS track is reduced. When DTS gets over a megabit a second to play around with, it produces beautiful sound. But when the bitrate drops down to 768kbps, or even lower, AC3 is a strong competitor. For this reason, you are often better off with the AC3 track. It will be smaller and sound just as good.