AC-4 IMS (Immersive Stereo) is a compact delivery mode of Dolby AC-4 that packs an immersive, Atmos-style experience into a low-bitrate stream suitable for broadcast and streaming, even down to mobile-friendly rates. Where E-AC-3 carries Atmos via Joint Object Coding on top of a 5.1 core, AC-4 IMS uses AC-4's more modern, more efficient object-based coding to deliver immersive height and placement at a fraction of the bandwidth, rendered to whatever speakers or headphones the listener has. It is part of the next-generation audio toolkit adopted by ATSC 3.0 and DVB. For services balancing immersive ambition against tight bitrate budgets, AC-4 IMS is the efficient path to Atmos.
