Amir's
post above is correct. Here is the long version if you want the history.
THX began as a purported certification program for movie theaters. As the story goes, George Lucas was upset about the poor quality of movie theater projection and sound in theaters where
Star Wars films were being shown. In the early 1980s, he met Tomlinson Holman, then known for the
Apt Holman preamplifier. Holman had some ideas about using subwoofers in movie theaters, and THX--actually an abbreviation for Tomlinson Holman's Crossover (Xover)--was born. (It was by all accounts just a coincidence that THX was part of the title of the title of a George Lucas short film later remade as the feature
THX 1138.)
I know nothing about the projection or sound requirements for movie theaters that THX supposedly imposed on its certified theaters, but I do recall going to a THX-certified theater in Washington, DC as a child. That theater had much better sound than the non-THX theaters in the small town where I grew up.
Later THX expanded into home theater certification. I believe that the THX home theater program initially was a holistic set of standards for a home theater: to have a THX home theater, you had to buy all THX equipment through a THX installer. There supposedly were specific requirements for every piece of equipment in a THX system, including cables and the projection screen. Such equipment usually included a CRT front projector costing more than $10,000 (Runco was a common brand); a speaker system usually consisting of three MTM left-center-right speakers, two "dipole" surrounds mounted on the wall to bounce sound all over the room, and two subwoofers (common brands were M&K, Snell, and Atlantic); a separate processor with specific processing modes and EQ features (often made by Lexicon); and moderately powerful amplifiers (I remember that certain NAD, Parasound, and Rotel amplifiers had the certification). The dipole surrounds were the most distinctive part of the system because they were designed to bounce sound all over the room, in contrast to what surround channels were supposed to do later, when discrete surround codecs and multichannel music came into existence. (The notion of bouncing sound all over the room lives on in Tomlinson Holman's latest product, the Apple HomePod, which has rear-facing tweeters designed to make the sound more spacious.)
There were some kids in the town where I grew up whose dad had a "media room" built. The media room housed a real, certified THX home theater. In our little social circle, that media room was a big deal in the early 1990s, and we had a lot of fun watching
Star Wars,
Die Hard, etc. on LaserDisc in the media room. The sound in that media room was really loud, but it was definitely not high-fidelity. The picture on the CRT projector was okay but of course worse in all respects (except perhaps color depth) than what one can get from any inexpensive HDTV today.
THX also certified the sound and picture on certain videotapes and discs. I guess those media tended to be on the higher-quality side, but I don't know what was involved technically, if anything, other than adding the loud THX sound effect before the movie.
Later, perhaps around 2000, THX began certifying downmarket A/V receivers that one could buy in mass market stores. I recall the definite sense that they had begun just to stamp their logo on gear made by companies that paid for the privilege. There were different levels--THX Select, THX Ultra, and THX Ultra 2--but those levels did not seem to correspond to the quality of what was certified. A relative of mine had a THX Ultra 2 Onkyo receiver that was clearly an underpowered piece of junk.
I'm not sure when THX began to develop--or perhaps buy the patent rights to--amplifier technologies, such as those used in the Benchmark power amp. Licensing a state-of-the-art amplifier technology seems like a more substantial contribution to audio than THX's other activities over the years have been.
