Dolby AC-4 is an audio compression technology developed by Dolby Laboratories. 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.
Its development started in late 2011 to create a high-quality audio format that had a certain resemblance to Dolby TrueHD destined for multiple uses, to be used in streaming services, future TV broadcasts, and mainly to replace the traditional Dolby AC-3, and in December 2014 Dolby Laboratories approved the sound format for commercial use.
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.
AC-4 uses an improved modified discrete cosine transform (MDCT) audio coding algorithm. 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. However, for 22.2 channel audio, the required bit rate may be as high as 1536 kbit/s.
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.
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. Dolby only charges for one technology per device, which means that Dolby AC-4 effectively costs nothing in devices that include existing Dolby technologies such as Dolby Digital Plus. The professional royalty rate is up to US$50 for an eight channel transcoder.
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