THX Select and THX Ultra are sets of performance standards developed by the engineers at Lucasfilm. To get THX certification, a home theater receiver must meet Lucasfilm's rigorous standards for audio performance, user interface, construction, and compatibility with other THX-certified products.
A THX Select-certified receiver is designed to deliver superbly cinematic performance in rooms up to 2,000 cubic feet. The standards necessary for THX Ultra certification are even more stringent, since THX Ultra receivers are required to provide a premium home theater experience in rooms up to 3,000 cubic feet. THX Ultra 2 is the highest certification, with the most rigorous standards and proprietary surround sound post-processing technologies.