What is the difference between digital audio formats?
The difference between digital audio formats can be boiled down to the number of speakers and the method used to decode the digital signal.
Note -- In all numerical formats the ".1" designates the subwoofer or low frequency channel. Also - some newer AV receivers feature two subwoofer outputs or a ".2" designation for better/smoother low-frequency room coverage.
5.15.1 digital surround sound is the most common configuration in home theater. It actually employs six speakers -- a center, two for left and right channels, two for the right and left rear channels and a powered subwoofer.
6.16.1 surround sound employs an additional center rear speaker to fill the gap between the right and left rear surround speakers. Its signal information is decoded and calculated by the receiver's logic circuitry due to the fact that DVD audio -- even HD systems -- only provide 6 audio signals.
7.17.1 digital audio incorporates an additional rear speaker/channel for a total of eight speakers. The additional speakers are more of a fill for the rear as you now have a right and left side speaker and again have a right and left rear channels. 7.1 is usually most effective in large home theater rooms. Note - not a lot of media is available in "true" 7.1 format -- most 7.1 is synthesized via Dolby PLIIx decoding. Megamind on Blu-ray is the first disc to encode the theatrical 7.1 mix on disc.
9.1(.2)/11.1(.2)The multi-speaker surround scene has migrated to the extent of having the capability of "front height" or "front width" speaker capability with some mid to high-end AV receivers. Dolby PLIIz synthesis is employed for such ... as no current media is natively encoded for such "audio esoterica".
Decoding MethodsDolby Laboratory's Dolby Digital and Dolby Pro-Logic are probably the most familiar formats currently available. Pro-Logic is more commonly utilized by television broadcasts and almost all DVD movies utilize Dolby Digital tracks. Blu-ray discs usually have either or both -- Dolby TrueHD or DTS-HD lossless encoding available.
DTS is Dolby's "direct competitor". Some audiophiles swear by DTS citing its claims of lower compression rates and a better signal to noise ratio. Unless you have really good audio gear and/or "golden ears" you probably will not notice a difference between Dolby and DTS encoding formats.
Also note that THX is not a decoding method -- but merely Lucas Films "stamp of approval" for its standards of high-fidelity audio reproduction.
For further info on the difference between digital audio formats you may want to refer to the section on digital audio formats.