AC4 Format Guide

Available Conversions

Convert to AC4

About AC4 Format

AC-4 (Audio Codec 4), also known as Dolby AC-4, is a next-generation audio codec developed by Dolby Laboratories, standardized as ETSI TS 103 190. It was designed from the ground up to meet the requirements of modern broadcasting, streaming, and immersive audio delivery. AC-4 is the mandatory audio codec for ATSC 3.0 (Next Gen TV), the next-generation terrestrial television broadcasting standard being deployed in the United States, South Korea, and other countries. The codec achieves approximately 50% better compression efficiency compared to its predecessor Dolby Digital Plus (E-AC-3), delivering equivalent audio quality at half the bitrate. AC-4 natively supports immersive audio including Dolby Atmos object-based audio, enabling three-dimensional sound experiences through both speaker systems and headphones. The format also introduces advanced features such as dialogue enhancement (allowing viewers to adjust dialogue levels independently), audio description mixing, and dynamic range control, all designed to improve the viewer experience across different playback environments from mobile phones to home theater systems.

History of AC4

Dolby began development of AC-4 in the early 2010s as a successor to the Dolby Digital family of codecs (AC-3 from 1991 and E-AC-3/Dolby Digital Plus from 2004). The codec was designed to address the evolving needs of the broadcast and streaming industries, where bandwidth efficiency, immersive audio, and personalization were becoming critical requirements. AC-4 was first published as an ETSI standard (ETSI TS 103 190) in 2014, with subsequent revisions adding features and refining the specification. A major milestone came in 2016 when the ATSC (Advanced Television Systems Committee) selected AC-4 as one of the mandatory audio codecs for the ATSC 3.0 standard, ensuring its deployment in next-generation television broadcasting across North America. South Korea became the first country to deploy ATSC 3.0 services in 2017, with AC-4 audio support. In the United States, ATSC 3.0 broadcasts began rolling out in major markets from 2020 onward. Dolby also integrated AC-4 support into their Dolby Atmos ecosystem, enabling the same immersive audio content to be delivered across broadcast, streaming, and packaged media. Major TV manufacturers including Samsung, LG, Sony, and Hisense began incorporating AC-4 decoders into their smart TVs from 2018 onward. The codec has also been adopted by streaming services as a delivery format for Dolby Atmos content alongside the more established E-AC-3+JOC format.

Key Features and Uses

AC-4 achieves its compression efficiency through a combination of advanced coding techniques including modified discrete cosine transform (MDCT) with adaptive window switching, sophisticated psychoacoustic modeling, and advanced entropy coding. The codec supports sample rates of 44.1 kHz and 48 kHz with bit depths up to 24 bits. Channel configurations range from mono through 7.1.4 (seven channels, one LFE, four height channels) for immersive audio layouts. A distinctive feature of AC-4 is its native support for audio objects alongside channel-based audio, enabling Dolby Atmos immersive audio within the broadcast stream. The codec includes built-in dialogue enhancement technology that allows end users to adjust the relative level of dialogue versus background audio, improving intelligibility without affecting the overall mix. AC-4 supports multiple audio presentations within a single stream, enabling broadcasters to offer different language tracks, audio descriptions for visually impaired viewers, and alternative commentary tracks efficiently. The codec's dynamic range control system adapts audio output to the playback environment, ensuring content sounds appropriate whether played on TV speakers, soundbars, headphones, or full home theater systems. AC-4 also supports metadata for loudness management compliant with ITU-R BS.1770 standards.

Common Applications

AC-4's primary application is in ATSC 3.0 next-generation television broadcasting, where it serves as the audio backbone for over-the-air TV transmission in the United States, South Korea, and other adopting countries. Smart TVs from major manufacturers decode AC-4 natively, enabling viewers to experience immersive Dolby Atmos audio from broadcast signals. Streaming services use AC-4 as a delivery format for Dolby Atmos content, particularly on platforms that support next-gen audio experiences. The codec is used in mobile broadcasting scenarios where its bandwidth efficiency is particularly valuable, delivering high-quality audio over cellular networks and mobile broadcast systems. Content production facilities use AC-4 in their delivery pipelines when preparing content for ATSC 3.0 broadcast distribution, encoding final masters into AC-4 alongside video codecs like HEVC. The automotive industry is exploring AC-4 for in-car entertainment systems, where its personalization features (dialogue enhancement, dynamic range control) are valuable in the challenging acoustic environment of a vehicle. AC-4 is also being adopted in digital signage and public address systems where efficient audio delivery with loudness management is important. Professional broadcast equipment manufacturers have integrated AC-4 encoding and decoding into their product lines for broadcast infrastructure deployment.

Advantages and Disadvantages