Dolby AC-4

Overview

Dolby AC-4 is a modern audio compression format designed for broadcast and streaming delivery. Standardized by the European Telecommunications Standards Institute (ETSI) in April 2014 as TS 103 190, AC-4 was proposed as one of three audio standards for ATSC 3.0 in March 2015. Unlike its predecessors AC-3 (Dolby Digital) and E-AC-3 (Dolby Digital Plus), AC-4 achieves significantly improved compression efficiency, delivering up to 50% better bitrate performance than E-AC-3 while maintaining or improving audio quality.

Technical Foundation

AC-4 uses an improved Modified Discrete Cosine Transform (MDCT) audio coding algorithm, the same mathematical foundation employed in MP3 and AAC formats, but with enhanced coding tools optimized for broadcast and immersive audio delivery. The codec supports bitrates ranging from 24 to 1536 kbit/s, providing flexibility across stereo, surround, and immersive configurations.

For ATSC 3.0 applications, AC-4 bitrate requirements are: 96 kbit/s for stereo audio, 192 kbit/s for 5.1 channel surround audio, and 288 kbit/s for 7.1.4 channel audio with Dolby Atmos. These requirements represent the bitrates needed to achieve a MUSHRA (Mean Opinion Score) of 90, a standard perceptual quality benchmark.

Channel and Content Structure

AC-4 can deliver up to 5.1 core audio channels that all AC-4 decoders are required to decode, ensuring universal compatibility across conforming devices. AC-4 bitstreams can contain audio channels and/or audio objects, providing content creators with flexible delivery options. Traditional bed channels enable legacy compatibility, while object-based audio enables immersive rendering on capable systems.

A distinctive feature is dialogue enhancement, which allows decoders to adjust speech level independently from music and effects based on encoder-generated parametric metadata. This is particularly useful for viewers with hearing impairment or those watching at low volumes, who can increase dialogue clarity without amplifying explosions or music to uncomfortable levels.

Efficiency Comparison

AC-4's compression performance significantly outpaces earlier Dolby formats. AC-3 supports 32–640 kbit/s, while E-AC-3 extends to 32–6144 kbit/s across all configurations. AC-4's 24–1536 kbit/s range is narrower but optimized for the practical bitrates used in broadcast. At 192 kbit/s for 5.1 channel audio, AC-4 achieves a MUSHRA score of 90, demonstrating approximately 50% efficiency improvement over Dolby Digital Plus at equivalent quality levels.

Real-World Deployment

ATSC 3.0, the primary deployment vehicle for AC-4 in North America, was available to approximately 75% of the U.S. population across over 80 media markets as of February 2025. However, adoption of AC-4-capable consumer hardware remains limited: ATSC 3.0 tuners were available in approximately 10% of new TVs shipped in 2024. South Korea remains the leading ATSC 3.0 deployment region globally, having launched full-time services in May 2017. Brazil officially adopted the standard as "DTV+" in August 2025, with commercial services planned during the 2026 FIFA World Cup.

Backward Compatibility and Hardware Considerations

AC-4 is not backward compatible with older AC-3 devices. Hardware manufactured before approximately 2017 cannot decode AC-4 bitstreams, requiring broadcast operators and content distributors to implement fallback strategies for mixed device fleets. This incompatibility is a significant barrier to widespread consumer adoption and necessitates careful planning in transition scenarios where legacy and next-generation devices coexist in the marketplace.

Standards Document

AC-4 for ATSC 3.0 is formally documented in ATSC A/342, with the most recent approved version being A/342:2026-04, dated April 14, 2026.