What is THX? And how does it relate to Dolby Digital and DTS Digital Surround?
First, it is important to realize that THX is a quantitative measure of quality that works in conjunction with the different surround sound formats (e.g., Dolby Digital and DTS) to bring the quality of the sound presentation to the highest standards.
To ensure the highest possible quality in home cinema, THX defines stringent picture and sound track criteria for film-to-DVD (and film-to-VHS) transfers. Such DVD titles are labeled as "THX Digitally Mastered for superior sound and picture quality" or simply "THX Certified". THX also defines stringent performance standards for audio/video source and processing components. These include strict performance standards for DVD players, receivers, preamplifiers, power amplifiers, speakers, interconnect cables, speaker cables, and even the room's acoustic characteristics (for dedicated home theater environments).
For the "everyday consumer", the "THX" label means that the highest standards have been used, whether the product is a DVD-Video title, or an audio/video component.
What is the difference between "THX Select" and "THX Ultra"?
For receivers and loudspeakers, the THX Select certification is for components that are certified to perform in a mid-sized home theater environment of up to 2,000 cubic feet. The volume of a room, expressed in cubic feet, is computed by multiplying the length by the width by the height of the room. (For example, a room that is 18 feet wide by 14 feet wide by 9 feet high, has a volume of 18 x 14 x 9 = 2,268 cubic feet.) The more stringent THX Ultra certification (which is equivalent to the original, plain THX certification) is given to components that meet the THX performance standards for larger home theater environments (up to 3,000 cubic feet). Theoretically, if you have a home theater environment that is 2,000 cubic feet or less, THX Select receivers and loudspeakers should deliver about the same presentation standards as the higher-end THX Ultra components.
More Info: Read the THX web page
on the differences between THX Ultra and THX Select.
For other components such as preamplifiers, power amplifiers, DVD players, the certification is just known as THX Ultra (or equivalently just THX) certification. There are no THX Select certification for these types of components.
What are some "THX Certified" DVD titles?
There are many -- they include: "The Mask of Zorro", "Titanic", "True Lies", "Tarzan", "Mulan", and many, many others.
So what does "THX" stand for?
Some say that "THX" was named after George Lucas' first movie, "THX 1138" (1971). Others say that it was an acronym for Tomlinson Holman's eXperiment. Tomlinson Holman was the premiere audio engineer that George Lucas hired to develop the THX standard.