All 5.1 is usually recorded in Dolby AC3. That is 448kbps 6 channel (5 sat +1 sub). THX is just a standart which gives the quality of sound. THX has some std rules for 5.1 if these are met then the sound quality is said to be at THX std. THX was George Lucas's first movie. So the name was given to the std because the 5.1 was first implemented by him, in his movie. 7.1 is a newer standard. Not supported by DVD yet. But most probably will be a part of HD.