Dolby AC-4 is Dolby’s latest generation audio codec for delivering immersive audio. It builds upon the capabilities of Dolby Atmos experiences today by delivering audio at higher quality and lower bitrates, delivering new experiences with binaural metadata and head tracking, enabling optimal rendering for both headphone and speaker playback, and is a platform for future innovation that will enable new features for creating and delivering Dolby Atmos content. There are multiple levels and profiles of AC-4 that are optimized for different use cases and content types. 

What are the different levels/profiles of AC-4 and how do they differ? 

There are 3 types of AC-4 bitstreams that exist in the ecosystem for delivering Dolby Atmos experiences:

• AC-4 IMS is specifically designed to deliver surround and immersive experiences to headphones and on-device stereo speakers. It is used across content types and can be found on a number of streaming services today.
• AC-4 Level 3 (L3) is widely deployed and designed to deliver high quality surround and immersive AV content across broadcast and OTT, with similar capabilities to DD+JOC at higher quality and lower bitrates.  
• AC-4 Level 4 (L4) is the most advanced version of AC-4 in market, designed to deliver immersive experiences across loudspeakers and headphones. It supports the full range of Dolby Atmos metadata for both speakers and headphones and has the highest spatial and audio fidelity available today. 

What are the benefits of AC-4 L4 

AC-4 L4 offers a number of improvements over legacy codecs (such as DD+JOC) that include: 

• Increased efficiency – higher quality at lower bitrates and perceptually lossless at higher bitrates
• Improved spatial resolution – increased object count provides greater spatial resolution in the encoded file
• One codec for headphone and speaker playback – AC-4 L4 supports Dolby Atmos binaural metadata, head tracking, and delivers creative intent across speaker and headphone playback.  

How can I encode AC-4 L4 

AC-4 L4 MP4 files can be encoded using the "Export master to .mp4" function in Dolby Atmos Renderer application as well as in supported DAWs and partner tools with integrated Dolby Atmos rendering and export (Immersive Master Pro, Apple Logic Pro).  

AC-4 L4 can also be encoded using dedicated Dolby encoding tools such as Dolby Encoding Engine.