DVD-Audio is an audio format similar to the CD. It is commonly claimed to have higher audio quality than the CD. DVD-Audio has not become popular due to the cost and rarity of the players, and because the traditional CD is considered good enough. Tests have shown that the traditional CD already has the highest possible audio quality. This has been mathematically proven by the Nyquist-Shannon sampling theorem. DVD-Audio does, however, support surround sound, and many DVD-Audio releases are available with surround sound. The traditional CD theoretically supports surround sound too, but, with the exception of DTS and Dolby Surround-encoded CDs, no surround-capable CD player has ever been made, and no surround-formatted CD has ever been released.