AC-3, marketed as Dolby Digital, is the 1991 surround codec that brought 5.1 to cinema, DVD, and ATSC broadcast television. It carries up to six channels (five full-range plus LFE), typically at 384-448 kbps on disc, and introduced the metadata-driven loudness control — dialnorm — that the broadcast world relied on for decades. Though E-AC-3 has replaced it for streaming and AC-4 and MPEG-H are taking over next-generation broadcast, AC-3 is far from gone: it remains a baseline in legacy delivery, set-top boxes, optical media, and HDMI/S-PDIF passthrough, so decoders must still support it everywhere.
