Dolby AC-4 is an audio compression technology developed by Dolby Laboratories. Dolby AC-4 decoders are required to decode 5.1 surround sound. Dolby AC-4 bitstreams can contain audio channels and/or audio objects. Dolby AC-4 has been adopted by the DVB project and standardized by the ETSI.
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History
On March 10, 2015, the Advanced Television Systems Committee (ATSC) announced that Dolby AC-4 was one of the three standards proposed for the audio system of ATSC 3.0.
On July 14, 2015, Dolby Laboratories announced that Sony Visual Products and Vizio would support Dolby AC-4.
On April 14, 2016, Dolby Laboratories announced that Samsung would ship TVs with support for Dolby AC-4 in 2017.
Technical details
Dolby AC-4 can have up to 5.1 core audio channels which all Dolby AC-4 decoders are required to decode. Additional audio channels may be encoded as side signals which Dolby AC-4 decoders can optionally support which would allow for the delivery of 7.1.4 channel audio. Side signals may also contain audio objects. Dolby AC-4 has two different channel based encoding tools with Advanced Joint Channel Coding (A-JCC) used for low bit rates and Advanced Coupling (A-CPL) used for high bit rates. A-JCC doesn't support side signals and is limited to 5.1 channel audio while A-CPL does support side signals. Dolby AC-4 supports up to 7 audio objects with a core decoder and can optionally support additional audio objects with a more advanced decoder. The use of different decoders allows Dolby AC-4 to support lower cost devices while also allowing for more advanced decoders for AV receivers.
Dolby states that Dolby AC-4 provides a 50% reduction in bit rate over Dolby Digital Plus. When Dolby AC-4 was tested by the DVB the MUSHRA score was 90 at 192 Kbit/s for 5.1 channel audio. When tested for ATSC 3.0 the bit rates needed for the required audio score was 96 Kbit/s for stereo audio, 192 Kbit/s for 5.1 channel audio, and 288 Kbit/s for 7.1.4 channel audio.
Dolby AC-4 is extensible and audio substreams allow for new features to be added to Dolby AC-4 while maintaining compatibility with older decoders.
Patent licensing
Dolby AC-4 is covered by patents and requires a license from Dolby Laboratories. Dolby AC-4 has a consumer royalty rate of US$0.15 to US$1.20 depending on the type of device and volume of sales. The professional royalty rate is up to US$50 for an eight channel transcoder.