Dolby Atmos Music

What Dolby Atmos Music is

Dolby Atmos Music is the same Atmos object format used in film, applied to songs and albums. Instead of mixing to two stereo channels, an engineer places each instrument or vocal as an audio object in three-dimensional space, and the playback system renders those positions to headphones, a soundbar or a speaker array. Audient's Atmos tutorial notes that for music the bed can often be dropped entirely, because objects give pinpoint left/right, front/back and top/bottom placement that stereo cannot match. Audient says engineers drag an icon around a three-dimensional panner in a compatible DAW or in the Dolby Atmos Music Panner plugin, and the position is written as metadata that the playback system decodes in real time. One constraint carries over from the format: objects cannot feed the LFE channel, which is a cinema effects channel and not meant for bass management, so most music mixes never use it. For the underlying structure see audio objects vs bed channels and Dolby Atmos.

How Atmos Music is delivered

A studio master can hold up to 128 inputs, but streaming cannot carry that. As Dolby's Hybrik documentation explains, spatial coding reduces the beds and objects to 12 or 16 elements, rendered to a 5.1 or 7.1 core and sent as Dolby Digital Plus JOC at a minimum of 384 kbps for 12 elements or 448 kbps for 16. Wikipedia says Tidal uses E-AC-3 for this speaker path. Headphones and virtualized stereo speakers use a different codec: Production Expert reports that Tidal and Amazon Music have long used AC-4 IMS for that path, and the newer AC-4 that supports multichannel speakers (see Dolby AC-4) had hardly been heard outside the lab.

The compression is steep. Production Expert's engineers compared a lossless 7.1.4 PCM render, which runs at 13,824 kbps (12 channels x 24 bit x 48 kHz), against AC-4 at 448 kbps. That is a ratio of about 31 to 1 (13,824 / 448), and 448 kbps is 3.2% of the studio rate. In their double-blind session, DD+ JOC artifacts were slightly more noticeable than AC-4 or PCM with every speaker playing, but became very noticeable once listeners soloed individual speakers. Masking by the rest of the mix hides most of it in normal listening. The same article reminds readers that most consumers on Apple Music, Netflix and HBO Max never hear the lossless render the studio monitored. See DD+ JOC.

Services, devices and the timeline

Wikipedia dates Atmos Music adoption by Tidal and Amazon Music to December 2019, and Apple Music's announcement to 17 May 2021 with rollout from 7 June 2021. Tidal opened up TV and home theater playback in May 2020. Engadget reported that it required a Tidal HiFi subscription (about $20 a month) and a compatible streamer, such as an Apple TV 4K, Fire TV Stick 4K or NVIDIA Shield TV. What Hi-Fi? added that Sony and Philips Android TVs can play it directly, and listed the Sony HT-ST5000 and Sonos Arc soundbars and the Philips 65OLED984 TV among compatible gear. According to What Hi-Fi?, until that rollout Tidal's Atmos tracks could be heard only on the Amazon Echo Studio speaker. The launch catalogue was 50 releases, including Kraftwerk's 3D The Catalogue and Hans Zimmer's Live in Prague, before a Universal Music Group deal added thousands of tracks.

Apple brands its version Spatial Audio with Dolby Atmos. Apple's support page says supported songs play in Dolby Atmos automatically on any Apple or Beats Bluetooth headphones and on the built-in speakers of an iPhone XS or later (except iPhone SE), recent iPad Pro and iPad Air models, 2018 or later MacBook Pro and MacBook Air, the 2021 iMac and Apple Studio Display. An Apple TV 4K can send it to HomePod speakers, an Atmos soundbar, an Atmos AV receiver or a television that supports Atmos. Dynamic head tracking works on Apple Vision Pro and on supporting headphones paired with a compatible iPhone, iPad or Mac.

Atmos Music vs Atmos for film

The renderer and the home limits are shared. Home Atmos for film, television and music uses spatial coding to reduce the audio to at most 16 concurrent elements, so a music track does not escape the cluster limit just because it has no picture. What differs is authoring: film mixes lean on a 7.1.2 bed for ambience and dialogue, while music mixes are frequently object-only. Loudness is the other difference you hear. Apple's FAQ advises turning on Sound Check if Atmos tracks sound quieter than stereo tracks.

Common confusions

Atmos Music is not lossless. Production Expert says most listeners on Apple Music, Netflix and HBO Max hear a lossy, data-compressed version of the studio mix, and Wikipedia notes Apple Music announced Atmos alongside lossless audio. It is also not the same as head-tracked Spatial Audio; the object mix is the format, and head tracking is a feature of certain Apple hardware. Sony 360 Reality Audio is a different object format (see Sony 360 Reality Audio), and Apple's Spatial Audio branding is covered in Apple Spatial Audio. Because the speaker path is DD+ JOC, and Wikipedia notes that ARC supports compressed Dolby codecs up to 5.1 channels with Atmos metadata, ordinary ARC can carry it; check the stereo vs surround for music guide before rebuilding your room around it.