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 3,000 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 (than 3,000 cubic feet) home theater environments. Theoretically, if you have a home theater environment that is 3,000 cubic feet or less, "THX Select" receivers and speakers should deliver about the same presentation standards as the higher-end "THX Ultra" components.
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.
