Dolby Digital (AC-3)
Definition
A perceptual audio coding format supporting up to 5.1 surround sound (six discrete channels), developed by Dolby Laboratories. The standard audio format for DVD, Blu-ray, broadcast television, and cinema since the 1990s. Uses lossy compression at typical bitrates of 384–640 kbps.
In Simple Terms
The surround sound format you've been hearing in movie theaters and on DVDs for decades. It delivers 5.1 channels — five speakers plus a subwoofer — in a compressed package. It's the baseline standard before Atmos, and still required as a compatibility layer in most deliveries.
In Practice
A film mixed in Dolby Atmos also requires a Dolby Digital 5.1 downmix for DVD release and broadcast compatibility. The Atmos renderer generates this downmix automatically from the spatial mix.