AC-4 is Dolby's next-generation codec, built for next-generation broadcast and streaming and adopted by ATSC 3.0 and DVB. Unlike AC-3, it is object-based from the ground up, carrying immersive Dolby Atmos efficiently enough for terrestrial TV and mobile. It bakes in features the older codecs bolt on: dialogue enhancement that lets viewers raise speech over the mix, robust loudness and dynamic-range metadata, and a compact immersive mode (AC-4 IMS) for low bitrates. AC-4 is far more efficient than E-AC-3 at the same quality, but its newer footprint means decoder support is still expanding compared with the ubiquitous AC-3/E-AC-3 base.
