Skip to main content

Dolby Digital

Under Maintenance

The content in this entry is incomplete & is in the process of being completed.

Dolby Digital is a family of both lossless and lossy audio compression algorithms and technologies.

Format Overview


Originally known as Dolby Digital, AC-3 was first released in 1991 to provide digital 5.1 sound in cinemas from 35mm film reels. AC-3 is notable for being the first audio codec to make use of the “Modified Discrete Cosine Transform” algorithm. The codec has seen widespread use an adoption, due to its prevalence in DVDs, TV, and Blu-rays as a surround codec.

Dolby Digital Surround EX

Like Dolby’s earlier Pro Logic technology, Dolby Digital Surround EX matrixes a sixth, centre back surround channel into the left and right surround channels of a 5.1 stream, allowing for a 6.1 mix to be unfolded when played on a 6.1 or 7.1 system with EX decoding. This technology is fully backwards compatible with existing AC-3 decoders, producing the standard 5.1 stream. Surround EX was first introduced in 1999 with the release of “Star Wars: Episode I – The Phantom Menace”.


Often referred to as “Dolby Digital Plus”, E-AC-3 is the successor to Dolby’s earlier AC-3 codec, featuring support for higher bitrates (6,144kbps vs 640kbps), more channels (15 vs 5), and additional coding tools allowing for more efficient encoding. E-AC-3 can be found in the short-lived HD-DVD format, Blu-ray discs, and as the main surround codec for most streaming services, particularly if Dolby Atmos is used. Contrary to popular belief, E-AC-3 is not backwards compatible with AC-3, rather Dolby mandates that all E-AC-3 decoders can also decode standard AC-3 content. As E-AC-3 is an optional codec on Blu-ray, all discs encoded with E-AC-3 encode the first 5.1 channels as AC-3, with the additional rear channels/Atmos content being encoded as E-AC-3.


Dolby’s TrueHD is a lossless multi-channel audio codec based on Meridian’s Lossless Packing (MLP) codec, although the two aren’t compatible with each other. TrueHD is mainly used on Blu-ray and supports Dolby Atmos’s spatial audio data. The TrueHD specification supports up to 16 audio channels (although the Blu-ray specification limits this to 7.1) with a sample rate of 192KHz and a bit depth of 24 bits. As TrueHD is an optional codec on Blu-ray, each TrueHD steam includes a backup AC-3 stream encoded alongside it for compatibility purposes. Since 2010, Dolby TrueHD has seen a decline in usage in favour of DTS-HD Master Audio on Blu-ray discs, but has seen a slight resurgence as the codec used for Dolby Atmos audio, but DTS-HD MA is still more common on non-Atmos titles.


To be added.


To be added.