HDMI audio formats
The introduction of HDMI opened up great possibilities for audio. HDMI transmits many audio formats over the same interface - but not at the same time. The devices must agree on a format, for example Dolby Digital. The HDMI handshake ensures that an end device receives the best and most compatible sound. At least that's the plan. This works well when an HDMI source and an HDMI sink are connected. However, the difficulties begin when another HDMI sink (television or sound system) is added and these devices can handle different audio and video formats. More on that here .
The special thing about HDMI is that it can transport very different audio formats together with the video signal. The medium (as Blu-Ray Disc or data stream) can contain several formats. But HDMI only outputs one format of it. Blu-ray discs often offer 3 or more formats ranging from stereo to 3D high-definition sound. Computer games, streaming services and television also offer a selection.
The Dolby Digital (AC-3) codec is a widely used audio standard for DVDs and digital television. Dolby Digital supports various audio channel configurations, but the most common are 5.1 channel and stereo. Incidentally, the notation “5.1” refers to the number of audio channels. This corresponds to the number of possible loudspeakers that receive targeted sound data. With 5.1 there are 5 speakers (left, right, middle, back left, back right) and a subwoofer for the bass. Dolby Digital 5.1 channel audio typically encodes at bit rates between 384 and 640 kbps, while stereo Dolby Digital (2.0) audio encodes at 192 kbps.
Dolby Digital Plus
Dolby Digital Plus (E-AC-3) offers up to twice the efficiency of Dolby Digital and adds new features like 7.1-channel audio and support for Dolby Atmos (more on that later). Streaming and broadcast services often use Dolby Digital Plus to deliver surround sound audio at lower bit rates. 5.1-channel audio in Dolby Digital Plus typically encodes at bit rates between 192 and 256 kbps, while stereo audio in Dolby Digital Plus uses a bit rate between 96 and 128 kbps. Dolby Digital Plus bitstreams are not backward compatible with Dolby Digital decoders. But Dolby Digital Plus decoders can decode Dolby Digital bitstreams.
The theoretical maximum is 6144 kbps. Netflix uses up to 640 kbps for 5.1 sound and up to 768 kbps for Dolby Atmos. The data rate is dynamic or adaptive. Therefore, it decreases in quiet scenes.
Many HD and UHD Blu-ray discs use the Dolby TrueHD audio codec, also known as MLP. Dolby TrueHD supports up to 24-bit audio and sample rates from 44.1 kHz to 192 kHz. Dolby TrueHD uses up to 7.1 audio channels as well as Dolby Atmos. Since Dolby TrueHD is a lossless audio codec, the data rate is variable. For example, Dolby TrueHD bit rates on Dolby Atmos at 48 kHz average around 6,000 kbps, with peak data rates of up to 18,000 kbps for high sample rate content.
What about Dolby Atmos?
Recently, there has been more content and devices that support Dolby Atmos . But Dolby Atmos is not a codec! It is an immersive audio format that supports multiple audio codecs including Dolby Digital Plus and Dolby TrueHD (but Not Dolby Digital) can be provided. Blu-ray discs deliver Dolby Atmos with Dolby TrueHD (with Dolby Digital Plus available as an alternative), and broadcast and streaming services deliver Dolby Atmos with Dolby Digital Plus. To ensure compatibility with existing devices, Dolby Atmos is implemented in these codecs as a backward compatible extension. Dolby Atmos data is hidden in the bitstream and can be decoded with a Dolby Atmos-compatible A/V receiver, soundbar or television. Only this end device generates the separate channels for all loudspeakers. Non-Dolby Atmos compatible devices will decode a 5.1ch or 7.1ch version from the Dolby Digital Plus or Dolby TrueHD bitstreams. There's new HDMI signaling, whereby an HDMI sink indicates Dolby Atmos support.
Media players such as Apple TV 4K and Xbox One S/X decode a Dolby Digital Plus or Dolby TrueHD audio track, enrich it with their own sounds and encode them again. The sound system then receives up to 8 PCM channels.
Advantage: The audio track can be overlaid with system sounds from the player ("Siri"). And the processing in the sound system is faster, which is important when gaming.
Disadvantage: The audio signal requires a lot of bandwidth without improving the quality. An output via HDMI-ARC is not possible. It requires eARC on the TV or a direct HDMI connection to the sound system. Some AV receivers then only show "Multichannel PCM" on their display, no longer the original codec.
These PCM channels can contain metadata for Dolby Atmos. This form of transmission is therefore called “Dolby Metadata-enhanced Audio Transmission” – Dolby MAT for short.
Digital Theater Systems ( DTS ) is a multi-channel audio system designed for use in cinemas, laser discs and DVDs. Like Dolby Digital, it transmits up to 5.1 channels in a compressed format. The data rate is typically 754 kbps. It is not streamable and can be found on Blu-ray discs and "backup" copies thereof.
DTS High Definition (DTS-HD for short) is a further development of the DTS sound system and is often used on Blu-Ray discs. It consists of the lossless DTS HD Master Audio (maximum 24,500 kbps) and the lossy DTS HD High Resolution Audio (maximum 6,000 kbps).
Under the name DTS:X, DTS offers a technology in which audio objects are distributed to the loudspeakers in real time. As with Dolby Atmos, countless objects are calculated from the 7.1 sound channels by the sound system. DTS:X is transported over the two DTS-HD audio formats.
More 3D audio formats
Aurora 3D a channel-based audio system. This means that the optimum speaker setup is predetermined. It can include 9.1, 10.1, or 13.1 speakers. Auro 3D can be found as an additional audio track on some Blu-Rays and in cinema films.
Tempest is the 3D audio format of the Playstation 5 and wants to compete with Dolby Atmos. Using psycho-acoustic processes, Tempest outputs 3D sound through stereo headphones and gaming TVs.
HDMI and HDMI ARC
All of these formats are supported today on the forward HDMI connection between a source device and a sink device. So from the HDMI output to the HDMI input, just like the video signal. However, a television also delivers sound without an external source, e.g. from the receiver for cable television or satellite or smart TV apps. The HDMI return channel HDMI-ARC is used to transmit such audio data. With a return HDMI connection, e.g. from the TV to the soundbar, there are 2 protocols: ARC (Audio Return Channel) and eARC (Enhanced Audio Return Channel). Only the new eARC supports all audio formats.
TV or HDMI source can also output the sound digitally via S/PDIF. S/PDIF (Sony/Philips Digital Interconnect Format) allows digital audio signals to be sent over an optical or coaxial connection. The optical connection via fiber optic cables with Toslink connectors is the most common. It has always been a popular type of connection to transmit digital audio signals to both soundbars and A/V receivers. S/PDIF supports two channels of uncompressed PCM audio or compressed audio in Dolby Digital 2.0 and 5.1 (AC3) and DTS. S/PDIF supported Not Dolby Digital Plus, Dolby TrueHD or DTS-HD. S/PDIF does not have enough bandwidth to carry the high bit rates. The conductor is plastic, not fiberglass.