AC-1, AC-2, and AC-3
Dolby AC-1 was the first digital coding technology introduced in 1987 with the beginning development of what would become HDTV. Later in 1989, Dolby introduced AC-2, which had a better audio quality and a reduced bit-rate. Finally, in 1991, Dolby introduced AC-3, which includes 5.1 channels and operates at 320 kbps.
Also see: Sound definitions