OTT Streaming Delivery of Dolby Atmos
The basic premise of over the top (OTT) streaming delivery is Adaptive Bit Rate Streaming (ABR). Video content is encoded at different resolutions and at different bit rates within each resolution. Audio is encoded as Mono, Stereo, and 5.1 at various bit rates per channel width using HE-AAC and Dolby Digital Plus.
Encoded media is segmented into individual files not exceeding 30 seconds. A manifest is created that describes the “ladder” of video and audio encodes. All of the media and the manifests are pushed to a Content Delivery Network (CDN), then out to “edge servers” depending on how popular the content is and where it is being requested from.
Client players request playback of content from the CDN, providing information on the capabilities of the playback device and the bandwidth currently available for streaming. The goal is to provide uninterrupted playback best suited to the device while adjusting for fluctuations in available bandwidth.
These days, there are two main streaming protocols – Apple HLS and MPEG-Dash. Between the two protocols, all playback devices are represented. Different file containers are used by each protocol, but starting in 2016, both can use fragmented MP4 (fMP4). fMP4 can be used as the packaging source so that the content does not need to be encoded twice.
Packaging is also where Digital Rights Management (DRM) occurs to prevent piracy. There are three main brands of DRM – Apple FairPlay, Google Widevine, and Microsoft PlayReady. These three DRM solutions cover all playback devices.
Dolby Atmos encoded as Dolby Digital Plus JOC is packaged at the top tier of any audio ladder and is often a completely separate tier bundled with 4K Video and Dolby Vision. The bit rate of Dolby Digital Plus JOC Dolby Atmos content on most streaming services reflects it premium status. It is generally encoded at 768kbps (using 16 elements in Spatial Coding), which is far above the minimum viable bit rate of 385kbps.
Dolby Atmos is only available to devices that are capable of decoding and rendering it. In the case of a phone or tablet with virtualized Dolby Atmos playback, the device will request a bit stream encoded with Dolby Atmos. A Dolby Atmos stream will also be provided if there is a device enabled with Dolby Atmos connected to a streaming device via HDMI. HDMI connections use Extended Display Identification Data (EDID) to signal their playback capabilities to the player application. If a device such as an AVR or soundbar enabled with Dolby Atmos is connected via HDMI to a Digital Media Appliance (DMA) device, e.g., Apple TV 4k, Roku 4, or Amazon Firestick/Cube, the streaming service player application will know to request the Dolby Atmos bit stream from the edge server.
Below is a simplified graphic of the delivery path.
Dolby Atmos OTT Streaming Delivery Signal Flow
Blu-ray Delivery of Dolby Atmos
Delivery of Dolby Atmos via Blu-ray is likewise often considered a premium offering and is often used for UHD Blu-ray releases.
Dolby Atmos is encoded as lossless Dolby TrueHD for Blu-ray. The number of elements used during Spatial Coding is 12, 14, or 16 and depends on the “bit budget” for the disk. There is a finite amount of capacity on a Blu-ray disc, so the amount of space allocated for audio varies depending on the length of the title, the encoded bit rate of the video, and how many “extras” the disc author needs to include. In addition to Dolby Atmos TrueHD, there are 7.1, 5.1, and stereo versions included. 5.1 is encoded Dolby Digital to ensure backwards compatibility.
Dolby Atmos Blu-ray Delivery Signal Flow
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