Dolby AC-4

What Dolby AC-4 is

Dolby AC-4 is a lossy audio codec designed for broadcast and streaming, and it is the successor to Dolby Digital (AC-3) and Dolby Digital Plus in the broadcast path. It is standardized by the European Telecommunications Standards Institute (ETSI) as TS 103 190; Part 1, covering channel-based coding, is at version 1.4.1 (July 2025). In North America the Advanced Television Systems Committee (ATSC) constrains it for NEXTGEN TV in A/342 Part 2, approved 14 April 2026. A Dolby presentation quotes ATSC A/300 on the point that broadcasters in the US, Canada and Mexico selected the A/342 Part 2 audio system, which is AC-4. Dolby states that AC-4 cuts bit rate by 50% compared with Dolby Digital Plus, according to Wikipedia's summary of Dolby's 2015 white paper. Dolby's materials describe built-in support for dialogue enhancement, loudness leveling and post-processing such as dynamic range control.

How AC-4 works

The ETSI specification supports 48 kHz, 96 kHz and 192 kHz coding; a 96 or 192 kHz stream adds a high sampling frequency extension substream that a 48 kHz decoder may skip. A distinctive trait is frame alignment with video. AC-3 uses fixed 32 ms syncframes (1,536 samples at 48 kHz), but the AC-4 frame rate index maps audio frames to video frame rates: 1,920 samples (40 ms) at 24 fps, 2,048 at 25 fps and 1,536 at 30 fps. Dolby's ATSC 3.0 handbook says the encoder can take a video reference so audio frames end exactly where video frames end, which makes switching and ad insertion clean.

For channels, Wikipedia describes two tools: Advanced Joint Channel Coding (A-JCC) for low bit rates, limited to 5.1, and Advanced Coupling (A-CPL) for high bit rates, which supports side signals for layouts such as 7.1.4. A core decoder handles up to 7 audio objects, with optional support for more. Dolby's presentation calls the consumer-facing dialogue feature Voice+, says it is cost-free, operates automatically in the encoder and is present in all decoders, and says it can extract dialogue even from legacy 5.1, stereo and mono audio. Related: dialogue enhancement and dialnorm.

AC-4 vs Dolby Digital Plus

Dolby's ATSC 3.0 handbook puts the efficiency in numbers. AC-3 is commonly used at 384 kbps for 5.1; AC-4 delivers the same 5.1 quality at 128 kbps. The handbook's recommended minimums are 64 kbps for stereo (AC-3 equivalent 192 kbps) and 144 kbps for 5.1 (AC-3 equivalent 448 kbps), a 67.9% cut for 5.1 (1 - 144 / 448). A 2-hour program at 448 kbps is 403.2 MB of audio, against 129.6 MB at 144 kbps. Wikipedia's older figures from the ATSC 3.0 tests are 96 kbit/s for stereo, 192 kbit/s for 5.1 and 288 kbit/s for 7.1.4, with a MUSHRA score of 90 at 192 kbit/s for 5.1; Dolby's own 2021 broadcaster guidance recommends 192 kbps as the minimum for 5.1 to preserve watermarking and headroom for later processing.

Quality comparisons exist for Atmos music. Production Expert reported a double-blind test with engineers comparing AC-4 at 448 kbps, DD+ JOC and a lossless 7.1.4 PCM render running at 13,824 kbps (12 channels x 24 bit x 48 kHz). With every speaker playing, DD+ JOC artifacts were slightly more noticeable than AC-4 or PCM, and AC-4 and the PCM reference were in a statistical dead heat. When listeners soloed single speakers, DD+ JOC artifacts became very noticeable and AC-4 artifacts slightly noticeable.

Where AC-4 lives

The main home for AC-4 is NEXTGEN TV, the ATSC 3.0 broadcast standard. ATSC says US ATSC 3.0 service launched in 2020 and has expanded across all major metropolitan areas, and that South Korea launched 4K ATSC 3.0 broadcasts in May 2017 reaching more than 80% of its population. For Atmos, Dolby's handbook gives 5.1.4 as a common broadcast layout, which needs 10 channels of PCM against 6 for 5.1. ATSC and Dolby do not publish a current list of TV models with AC-4 decoders in these documents, so confirm NEXTGEN TV support on the exact model; Wikipedia records adoption announcements by VIZIO and Sony in 2015 and a Samsung commitment in 2016.

In music streaming, AC-4 IMS is the variant for binaural headphone and virtualized stereo speaker playback. Production Expert says Tidal and Amazon Music have long used it for Atmos music, while speaker playback uses DD+ JOC (see Dolby Atmos Music). On HDMI, a decoded AC-4 program reaches your receiver as PCM with metadata through Dolby MAT 2.0; Dolby lists Xbox, Apple TV and the 2020 Roku 4K Ultra among MAT 2.0 sources.

Common confusions

AC-4 is not the codec behind most streaming Atmos. Production Expert calls Dolby Digital Plus with Joint Object Coding the most common Atmos delivery codec, the one heard on Apple Music, Netflix and HBO Max (DD+ JOC). AC-4 is also not a lossless format; at 448 kbps it is far below the 18.64 Mbps TrueHD ceiling on Blu-ray. And "AC-4 Atmos" is a delivery choice, not a new mix: the object scene is the same, which you can read about in audio objects vs bed channels. For setup, use the Atmos setup guide; the app and device in your chain decide which codec reaches the receiver.