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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Web Media Application Developer Guidelines</title>
<script src="https://www.w3.org/Tools/respec/respec-w3c" class="remove" defer></script>
<script class="remove">
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name: "Joel Korpi",
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company: "CastLabs",
companyURL: "http://www.castlabs.com"
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</script>
<style>
.head > a.logo {
float: right;
margin: 0.4rem 0 0.2rem .4rem;
}
.head > a.logo {
border: none;
text-decoration: none;
background: transparent;
}
table {
border-spacing: 0;
border-collapse: collapse;
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}
td {
border: 1px solid gray;
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</head>
<body class="informative">
<section id="abstract">
<p> This specification is a companion guide to the <a
href="https://w3c.github.io/webmediaapi/" target="_blank">Web
Media API spec</a>. While the Web Media API spec is targeted
at device implementations to support media web apps in 2018,
this specification will outline best practices and developer
guidance for implementing web media apps. This specification
should be updated at least annually to keep pace with the
evolving web platform. The target devices will include any
device that runs a modern HTML user agent, including
televisions, game machines, set-top boxes, mobile devices and
personal computers. </p>
</section>
<section id="sotd">
<p style='border: 5px solid red; border-radius: 10px; padding: 1em; margin: 1em; text-align: center; font-weight: 300; font-size: 120%;'>🚩
<br>This document is deprecated and <strong>MUST NOT</strong> be used for further technical work.</p>
<p style='position: fixed; bottom: 1em; right: 1em; padding: 0.5em; border-radius: 5px; background-color: red; color: white;font-weight: bold; font-size: 150%;writing-mode: vertical-rl;
text-orientation: mixed;'><span style='background-color: white;padding: 4px; border-radius: 3px;z-index: 1000;'>🚩</span> Deprecated document. Do not use.</p>
<p>This is a deprecated document and is no longer updated. It is inappropriate to cite this document as other than deprecated work.</p>
</section>
<section>
<h2>Introduction</h2>
<ol class="ednote" title="Notes on v1 draft specification:">
<li>This document is directed towards application developers.
Its content will contain best practices for building media
applications across devices, it will not provide direction to device
manufactures or User Agent implementers. </li>
<li>This is a companion spec put forth by the Web Media API
Community Group.</li>
</ol>
<section>
<h3>Scope</h3>
<p>The scope of this document includes general guidelines, best
practices, and examples for building media applications across
web browsers and devices. </p>
<p>The target audience for these guidelines are software
developers and engineers focused on building cross-platform,
cross-device, HTML5-based applications that contain
media-specific use-cases.</p>
<p>The focus of this document is on HTML5-based applications,
however the use-cases and principles described in the guidelines
can be applied to native applications (applications that have
been developed for use on a particular platform or device). The
examples in this document provide a starting point to build
your media application and includes example implementations from
various providers and vendors. This document also includes
sample content and <a>manifests</a> as well as <a>encoding</a> guidelines to
provide hints on achieving the best quality and
efficiency for your media applications.</p>
</section>
<section>
<h3>Accessibility</h3>
<p>These guidelines will cover making your applications compliant
with accessibility requirements from the perspective of
delivering and consuming media. However, to make sure your
entire application is accessibility-friendly, please see the W3C Web Content
Accessibility Guidelines [[WAI-WEBCONTENT]].</p>
</section>
<section>
<h3>Glossary of Terms</h3>
<p>The following lexicon provides the common language used to build this document and communicate concepts to the end reader.<br>
</p>
<table>
<tr>
<td><b>Term</b></td>
<td><b>Definition</b></td>
</tr>
<tr>
<td><b>360 Video</b></td>
<td>Video content where a view in every direction is
recorded at the same time, shot using an
omni-directional camera or a collection of cameras.
During playback the viewer has control of the viewing
direction like a spherical panorama.</td>
</tr>
<tr>
<td><dfn>AAC</dfn><br>
</td>
<td>Advanced Audio Coding is a proprietary
audio coding standard for lossy digital audio
compression. Designed to be the successor of the MP3
format, AAC generally achieves better sound quality than
MP3 at the same <a>Bit rate</a>.<br>
</td>
</tr>
<tr>
<td><dfn>ABR</dfn><br>
</td>
<td>Adaptive Bitrate Streaming is a method to optimize
video playback by automatically selecting a specific bitrate rendtion
of a video file within the video player. Some ABR algorithms use client bandwidth throughput to determine
which <a>rendition</a> to play. For example, if a client tries to play a video on a slow internet connection, the
player could automatically select the lower bitrate <a>rendition</a> to maximize playback speed.<br>
</td>
</tr>
<tr>
<td><b>AVOD</b></td>
<td>Advertising-supported Video on Demand. AVOD services
monetize their content by serving ads to users, as
opposed to other business models such as paid
subscription or pay-per-title.</td>
</tr>
<tr>
<td><dfn>Bit rate</dfn><br>
</td>
<td> Bit rate, (also known as data rate), is
the amount of data used for each second of video. In the
world of video, this is generally measured in kilobits
per second (kbps), and can be constant or variable. </td>
</tr>
<tr>
<td><dfn>codec</dfn><br>
</td>
<td>a codec is an algorithm defining the compression
and decompression of digital video or audio. Most codecs
employ proprietary coding algorithms for data
compression. MP3, <a>H.264</a>, and <a>HEVC</a> are examples of
codecs.<br>
</td>
</tr>
<tr>
<td><dfn>CDN</dfn></td>
<td>A content delivery network (CDN) is a system of
distributed servers (network) that deliver pages and
other media content to a user, based on the geographic
locations of the user, the origin of the webpage and the
content delivery server.</td>
</tr>
<tr>
<td><dfn>Chunk</dfn> or <b>Chunking</b></td>
<td><a>ABR</a> technologies typically break a video or audio
stream into chunks to make transmission more compact and
efficient. These chunks are typically 2-10 seconds in
length and contain at least one <a>I-Frame</a> so that the
video player has complete information for which to
render the video from that point in the manifest. </td>
</tr>
<tr>
<td><dfn>Closed Captions</dfn></td>
<td>Used for accessibility and usability, closed captions
are a visual representation of the audio content of a
media file stored as metadata file or track and
displayed as an overlay in the video player in
synchronization with the video/audio track. Typical
formats for closed captioning are WebVTT and SRT.<br>
</td>
</tr>
<tr>
<td><dfn>DRM</dfn></td>
<td>A combination of encryption, authentication and access
control technologies that are used to prevent
unauthorized users from consuming copyrighted media
content. The most widely used DRM solutions today are
Microsoft PlayReady, Google Widevine, and Apple FairPlay
Streaming.<br>
<br>
There are also open technologies that use standards such
as Common Encryption [[MPEGCENC]] and Media Source
Extensions (MSE) [[MEDIA-SOURCE]] to create a secure environment without
third-party products.<br>
</td>
</tr>
<tr>
<td><dfn>Embed</dfn></td>
<td>Most video players in a web page use what's called an
"Embed" or "Embed Code" that is placed in the code of
your HTML page or app that will render the video
playback environment. </td>
</tr>
<tr>
<td><dfn>Encoding</dfn></td>
<td>Also referred to as compression, this is the process
of removing redundancies from raw video and audio data,
thereby reducing the amount of data required to deliver
the media across a network, on a physical disc, etc. The
process can result in reduced visual or auditory quality
of the encoded media, but the loss is usually
imperceptible to the user. <a>H.264</a> and <a>HEVC</a> are examples
of <a>codec</a>s that use compression during <a>transcoding</a>. </td>
</tr>
<tr>
<td><dfn>EME</dfn> (Encrypted Media Extensions)</td>
<td>Encrypted Media Extensions (EME) is a recommended W3C
specification for providing a communication channel
between web browsers and digital rights management (<a>DRM</a>)
agent software. This allows the use of HTML5 video to
play back <a>DRM</a>-wrapped content such as streaming video
services without the need for third-party media plugins
like Adobe Flash or Microsoft Silverlight. The use of a
third-party key management system may be required,
depending on whether the publisher chooses to scramble
the keys.<br>
</td>
</tr>
<tr>
<td><dfn>Container</dfn> (Format)<br>
</td>
<td>A format or "container format" is used
to bind together video and audio information, along with
other information such as metadata or even subtitles.
For example, .MP4, .MOV, .WMV etc. are all container
formats that contain both audio, video, and metadata in
a single file. Formats contain tracks that are encoded
using <a>codec</a>s. For example an .MP4 might use the <a>AAC</a>
audio <a>codec</a> together with the <a>H.264</a> video <a>codec</a>.
</td>
</tr>
<tr>
<td><dfn>H.264</dfn></td>
<td>Also known as MPEG-4 AVC (Advanced Video Coding) it is
now one of the most commonly used recording formats for
high definition video. It offers significantly greater
compression than previous formats. </td>
</tr>
<tr>
<td><dfn>HEVC</dfn><br>
</td>
<td>High Efficiency Video Coding is one of
the newest generation video <a>codec</a>s that is able to
achieve efficiency up to 4x greater than <a>H.264</a>, but it
requires the HEVC <a>codec</a> to be present on the device.
HEVC typically takes considerable more processing power
to encode and decode than older <a>codec</a>s such as <a>H.264</a>.<br>
</td>
</tr>
<tr>
<td><dfn>HLS</dfn></td>
<td>HTTP Live Streaming (HLS) is an adaptive streaming
technology created by Apple that allows the player to
automatically adjust the quality of a video delivered to
a web page based on changing network conditions to
ensure the best possible viewer experience. </td>
</tr>
<tr>
<td><dfn>I-Frame</dfn> (video)</td>
<td>In video compression, an I-Frame is an independent
frame that is not dependent on any future or previous
frames to present a complete picture. I-Frames are
necessary to provide full key frames for the
encoder/decoder. </td>
</tr>
<tr>
<td><dfn>In-stream</dfn> (Advertisement)</td>
<td>A video advertisement that accompanies the main video content. Examples include pre-rolls, mid-rolls, and post-rolls which play before, during, or after a main piece of content, respectively.</td>
</tr>
<tr>
<td><dfn>Live Streaming</dfn></td>
<td>Live streaming is a type of broadcast that is
delivered over the Internet where the source content is
typically a live event such as a sporting event,
religious service, etc. Unlike <a>VOD</a>, viewers of a live
stream all watch the same broadcast at the same time. </td>
</tr>
<tr>
<td><dfn>Manifest</dfn></td>
<td>A manifest is a playlist file for adaptive streaming
technologies (such as <a>HLS</a>, DASH, etc.) that provides the
metadata information for where to locate each segment of
a video or audio source. Depending on the configuration,
some technologies have two types of manifests: one
"master" manifest that contains the location of each
<a>rendition</a> manifest and one <a>rendition</a> manifest for each
<a>rendition</a> that contains the location (relative or
absolute) of each <a>chunk</a> of a video or audio source. </td>
</tr>
<tr>
<td><dfn>Media Renderer</dfn></td>
<td>A process that takes as input file-based video bytes and renders those bytes
to the client display, such as a computer monitor or television display. The media renderer is
responsible to ensure that the video is displayed accurately, per the encoded profile.</td>
</tr>
<tr>
<td><b>Metadata Track</b></td>
<td><a>ABR</a> streaming technologies contain the ability to
include not only video and audio tracks within the
stream, but also allow for metadata tracks for
applications such as <a>Closed Captions</a>, advertising cues,
etc.<br>
</td>
</tr>
<tr>
<td><dfn>Mezzanine File</dfn></td>
<td>An intermediate file typically created after post-production that is used for distribution or
for input into another workflow process, such as transcoding. Mezzanine files
are typically very high quality to ensure the minimum
quality degredation. Often times, a lossless <a>codec</a> is used
to achieve this high quality.<br>
</td>
</tr>
<tr>
<td><dfn>MPEG-DASH</dfn></td>
<td>Dynamic Adaptive Streaming over HTTP (DASH), also
known as MPEG-DASH, is an adaptive bitrate streaming
technique that enables high quality streaming of media
content over the Internet delivered from conventional
HTTP web servers. </td>
</tr>
<tr>
<td><b>Outstream Advertisement</b></td>
<td>A video advertisement that stands alone, without accompanying main video
content. Examples include a video ad that is <a>embed</a>ded within a text news article page that contains text and images as the main content medium.</td>
</tr>
<tr>
<td><dfn>Player</dfn><br>
</td>
<td>A video or audio media player that is used to render a
media stream in your application environment. There are
commercially available and open source video players and
SDKs for almost every platform. </td>
</tr>
<tr>
<td><dfn>Rendition</dfn></td>
<td>A specific video and audio stream for a target quality
level or bitrate in an adaptive streaming set or
manifest. For example, most <a>HLS</a> or DASH adaptive
streaming manifests contain multiple renditions for
different quality/bitrate targets so that the viewer
automatically views the best quality content for their
specific Internet connection speed. </td>
</tr>
<tr>
<td><dfn>SDK</dfn></td>
<td>A Software Development Kit is a set of tools that
allow the creation of applications for a certain
framework or development platform. Typically they are
the implementation of one or more APIs to interface to a
particular programming language, and include debugging
utilities, sample code, and documentation. </td>
</tr>
<tr>
<td><dfn>Streaming</dfn></td>
<td>Delivering media content to a viewer via Internet protocols such as HTTP or
RTMP.</td>
</tr>
<tr>
<td><b>SVOD</b></td>
<td>Subscription-supported Video on Demand. SVOD services
monetize their content through paid subscriptions from
users, as opposed to other business models such as
advertising or pay-per-title.</td>
</tr>
<tr>
<td><dfn>Transcoding</dfn></td>
<td>The process of converting a media asset from one <a>codec</a> to another.</td>
</tr>
<tr>
<td><dfn>Trick Play/Mode</dfn></td>
<td>Trick mode, sometimes called trick play, is a feature of digital video systems including Digital Video Recorders and Video on Demand systems that mimics the visual feedback given during fast-forward and rewind operations that were provided by analogue systems such as VCRs.</td>
</tr>
<tr>
<td><b>URL Signing</b></td>
<td>URL signing is a mechanism for securing access to
content. When URL Signing enforcement is enabled on a
property; requests to a content server or content
delivery network must be signed using a secure token.
The signing also includes an expiration time after which
the link will no longer work.</td>
</tr>
<tr>
<td><dfn>VOD</dfn></td>
<td>Video on demand is video that is served at the request
of a user. Delivery is typically through a content
delivery network to optimize delivery speed and
efficiency.<br>
</td>
</tr>
<tr>
<td><b>VR</b></td>
<td>Virtual Reality is a realistic, immerse,
three-dimensional environment, created using interactive
software and hardware, and experienced or controlled by
movement of the body. VR experiences typically require
wearing a head mounted display (HMD).</td>
</tr>
</tbody>
</table>
<p>For a more detailed list of relevant terms, please see the
glossary of terms for the vendors or technologies you use in
your workflow. </p>
</section>
</section>
<section>
<h2>Media Playback Use Cases</h2>
<section>
<h3>General Description</h3>
<p>Material (typically in video or audio content) is made
available by a content provider via a web-enabled application
and delivered by a content distribution network. There are
three distinct interlocking processes: generation, delivery
and consumption / playback. </p>
<section>
<h3>Content Generation</h3>
<p> <a>Mezzanine File</a> content is normally delivered to the service
provider as a file with near lossless compression.
Specifically, for High Definition content at 1080p with 24, 30, 50, or
60 frames-per-second, the bitrate of the original content is
typically 25Mbps to 150Mbps for 1080P HD and 150Mbps - 300 Mbps for 4K UHD. </p>
<p><a>ABR</a> <a>streaming</a> content is generated by <a>transcoding</a> the <a>Mezzanine File</a> against an <a>transcoding</a> profile. Firstly, the mezzanine file is transcoded into a set of versions, each with its own average bitrate. Secondly, each version is packaged into segments of a specified duration. </p>
<p> The first process is defined by an <a>transcoding</a> profile. A
profile describes the set of constraints to be used when video
is being prepared for consumption by a range of video
applications. The description includes the different bitrates
to be generated during the <a>transcoding</a> process that will allow
for the same content to be consumed on a wide variety of
devices on different networks from cellular to LAN. </p>
<p> The second process is performed by a packager which segments
the different bitrates. Next, the output is packaged into a
transport <a href='#dfn-container'>format</a> such as transport streams (.ts) or fragmented
MP4s (.m4s). Lastly, they are optionally encrypted with a <a>DRM</a> that is
suitable for the environment where the content is going to be
played out. The packager is also responsible for the
generation of a <a>manifest</a> file, typically a DASH (.mpd), <a>HLS</a>
(.m3u8) or possibly Smooth (.ism) or HDS (.f4v), that
specifies the location of the media and its <a href='#dfn-container'>format</a>. </p>
</section>
<section>
<h4>Content Delivery</h4>
<p> After the content has been generated the resulting segments
of video and corresponding <a>manifest</a> files are pushed to an
origin server. However, the assets are rarely delivered
directly from the origin. </p>
<p> At this stage, the control in the chain switches to the
client's web video application. The content provider supplies the client with the URL of a manifest file located on a <a>CDN</a> rather than the origin. The manifest URL is typically passed to a video <a>player</a> for playback. The player makes a HTTP GET request for the manifest from <a>CDN</a> edge. If the <a>CDN</a> edge does not currently have the manifest available, the <a>CDN</a> requests a copy of the file from the origin and the file is cached at the edge for later use. The CDN edge then returns the the manifest to the requesting player. <i>(NOTE: There are different levels of caching; popular <a>VOD</a> content is kept closer to the edge in the <a>CDN</a> network in this way it can be delivered to customers faster than an edge server that isn't tuned for high volume delivery.)</i> </p>
</section>
<section>
<h3>Content Playback</h3>
<p> As mentioned in the Content Generation section, the <a>player</a> uses a tag within the <a>manifest</a> to determine the playout type. In <a>HLS</a>, if there is a type with the value of <a>VOD</a>, the player will not reload the manifest. This has important consequences if there are changes in availability after the session as commenced. In DASH, the difference between a live and VOD playlist are more subtle. At this point the behavior
differs between players found on the different devices
depending on the transport formats. However, broadly, it
behaves in the following way: <br>
</p>
<ol>
<li>While the player's <a>ABR</a> algorithm determines which
<a>rendition</a> to use during playback, for instance based on
the currently measured network throughput or the current buffer,
the algorithm might not have enough information at playback start
to use such metrics. The initial decision depends on the player's
<a>ABR</a> algorithm but is often based on prior knowledge. This
can for instance be previous measurements of network throughput, a
simple default value that is used if not enough data is available
to estimate the current network throughput, or a custom algorithm to
determine the initial bandwidth estimation.</li>
<li>Once the player has this information it can then compare
this with the metadata from the manifest that describes the
different qualities that the content provider is supplying.
It picks the quality level with an average bitrate that is
as close to the calculated throughput but within its bounds to
avoid a situation where a consistent experience is
interrupted as the player requires more data than the
current network bandwidth can supply, where the player’s
buffer is emptying faster than it is being filled. </li>
<li>It then requests a segment from a location on the edge
server that typically relative to the location of the
manifest. </li>
<li>The downloaded segment is then added to the video decoder
pipeline for playback. For player's that are based on Media
Source Extensions [[MEDIA-SOURCE]] the segment is appended to the
corresponding media source buffer. Note that the segment is
appended as is even if the content is encrypted. The
underlying <a>DRM</a> system is usually integrated with the
decoder pipeline and will decrypt the samples prior to decoding.
</li>
<li>The <a>media renderer</a> pulls the video data from the buffer and
passes it to the video surface where it is rendered. </li>
<li>If the available bandwidth remains constant the player
will continue to request segments from the same bitrate
stream, pulling down chunks and filling its video buffer. In
the event a change in network availability the player will
make a decision about the need to either drop to a lower
<a>rendition</a> or request a higher bitrate. In case of <a>HLS</a>,
switching the <a>rendition</a> will trigger a download of a new
child manifest prior to the download of the associated segment. In case
of <a>MPEG-DASH</a> the player can start downloading the associated
segment directly.</li>
</ol>
<i>NOTE: This workflow may have additional steps if <a>DRM</a> is used.</i>
</section>
</section>
<section>
<h3>On-Demand Streaming (VOD)</h3>
<p> Despite the almost identical mechanics, content generation, delivery, and play out used for <a>VOD</a> and live, a large organization will typically maintain two distinct workflows as there are subtle but important ways in which they differ. </p>
<section>
<h4>Content Generation</h4>
<p>For VOD the source is typically static file-based rather than a feed. The
<a>encoding</a> profiles are also subtly different. A greater
priority can be placed on high quality as the latency, time to
live, is not a requirement. There are important
differences in the <a>manifests</a> created. In <a>HLS</a> there is a tag
that tells the <a>player</a> whether the playlist is describing on
demand material: #EXT-X-PLAYLIST-TYPE:VOD. As we will see
shortly this is used by the player. There are also client side
restrictions where certain profiles are blocked due to rights
restrictions and network consumption capped bitrates on movies
and entertainment while being allowed on sports content.
Fragments size will also effect playout as a player's <a>ABR</a> can
be more responsive if the <a>chunk</a> is smaller, e.g. 2 seconds
rather than 10 seconds. </p>
</section>
<section>
<h3>Content Delivery</h3>
<p> The <a>CDN</a> configuration and topology for delivering <a>VOD</a>
content is also different to live. There are different
levels of caching; popular <a>VOD</a> content which is kept closer to
the edge in the <a>CDN</a> network in this way it can be delivered to
customers faster than an edge server that isn't tuned for high
volume delivery. Older and less popular content is retained in
mid-tier caching while the long tail content is relegated to
a lower tier. </p>
</section>
<section>
<h4>Content Playback</h4>
<p> As mentioned in the content generation section the <a>player</a>
uses a tag within the <a>manifest</a> to determine the playout type.
In <a>HLS</a> if there is a type with the value of VOD, then the player will not
reload the manifest. This has important consequences if there
are changes in availability after the session as commenced. In
DASH the difference between a live and <a>VOD</a> playlist are more
subtle but one of the primary identifier for liven content in <a>MPEG-DASH</a>
is the <code>profiles</code> attribute of the <code>MPD</code> element which could
for example reference the <code>urn:mpeg:dash:profile:isoff-live:2011</code> profile
to indicate live content.
</p>
<p> There are other differences in playout as well. Unlike
live, a VOD asset has a predefined duration, information
around duration and playback position can be used to update the UI
to provide feedback to the user on the proportion
of the asset watched. </p>
<p>Additionally, the UX requirements are different between <a>VOD</a> and live. Unlike live content, which lends itself to being browsed within an EPG (electronic program guide), VOD content is typically navigated using tiles or poster galleries. There is also a <a href='#dfn-trick-play-mode'>trick play</a> bar to view the current playback position and seek to other points within the content. </p>
</section>
<section>
<h4>VOD Pre-caching</h4>
<p>In the previous sections, we outlined the use cases
associated with video <a>streaming</a>. In this section, we give some
examples of use-cases that are specific to on-demand streaming
and are mainly related to strategies employed on the clients
to improve performance in some way. </p>
<p>Pre-caching is a strategy used in on-demand streaming and is important to ensure the best user experience.
By buffering the beginning segments of a video, you can make playback as close to instantaneous as possible.
This is important because buffering (the state
of the video application when the <a>player</a> has insufficient
content within its framebuffer to continuously play content),
has a direct relationship to engagement and
retention. For every second of buffering
within a session a certain amount of users abandon a video stream.
Web video application developers will use points within an
application's UX to pre-cache content. For example, when entering a mezzanine/synopsis page, the application might preemptively cache the content by filling the player’s video buffer or, alternatively, storing the <a>chunk</a>s locally.. Pre-caching allows the video to commence playing without buffering and provides the user with a more responsive initial playback experience. This technique is used by Netflix, Sky
and the BBC in the case of on-demand content being watched in
an in-home context. This technique is not used for cellular sessions where the user’s mobile data would be consumed on content that they do not watch. </p>
</section>
</section>
<section>
<h3>VOD – further considerations</h3>
<section>
<h4>Byte range requests in context of web video application</h4>
<p>
An HTTP range request allows a client to request a portion of a larger
file. In the <a>VOD</a> scenario where a user wants to access a specific
location, a range request allows the <a>player</a> access content at a
specific location without downloading the entire transport <a>chunk</a>.
</p>
<p>
Both the player and the server need to support range requests. The
player needs to be able to playback a source buffer and the server must
be configured to serve ranges. The exchange begins when a client makes
an HTTP HEAD request. If range requests are supported, the server will
then respond with a header that includes <code>Accept-Ranges:
bytes</code> and the client can issue subsequent requests for
partial content. The returned bytes are added to an
<code>ArrayBuffer</code> and then appended to the
<code>SourceBuffer</code> which in turn is used as the SRC parameter of
the of the video HTML5 tag/element.
</p>
</section>
<section>
<h4>HDCP security requirement for HDMI</h4>
<p>
Some content has limitations placed on its distribution by the
owners. There are different ways of protecting the owner’s rights. The
most common is <a>DRM</a> (digital rights management) which prevents the
content being watched on a client device if the user cannot ‘unlock’ it
with a key. However, once the content is unlocked the service provider
should make every effort to prevent the user from re-distributing this
content.
</p>
<p>
A technical solution to prevent interception as a signal travels from a
device to a television or projector is termed High Bandwidth Content
Protection (HDCP). If implemented by a device manufacturer, HDCP takes
the form of a key exchange between devices. If the exchange fails, the
signal is not transmitted to the display and the device is responsible
for notifying the end user of the error.
</p>
</section>
<section>
<h4>Watermarking</h4>
<p>
Support for real-time watermarking is becoming an important consideration
for distributors of high-value content. A service provider’s right to
distribute content is linked to their ability to protect it with studios and
sports channels insisting on the capacity for a service provider to detect a
breach of copyright on a stream by stream basis. A service provider can
include a vendor's <a>SDK</a> in the client that can add a watermark at run-time.
Normally, the watermark is invisible to the consumer and takes the form of a
digital signal, this is referred to as forensic water marking. Part of the
service provided by the watermarking vendor is to monitor the black
market for re-distributed material. Illicit streams or files are intercepted and
screened for the digital fingerprint inserted on the client. If a suspect
stream is found the vendor directs the service provider(s) to the source
of the misappropriated stream.
</p>
<p>
While watermarking is not an issue that developers will often be faced
with, the processing requirements can dictate the choice of platform
for the content distribution. The watermarking process might require
processing power and low-level functionallity through ring-fenced
resources that platforms such as Android, iOS or Roku provide. The
options for watermarking in the browser are limited to overlays and
this limitation is then reflected in a distributor’s choice of platform
for their web video application.
</p>
</section>
</section>
<section>
<h3>Live Streaming</h3>
<p>
Even though Live and On-Demand <a>streaming</a> scenarios have a lot in
common, there are a few distinct differences.
</p>
<section>
<h4>Content Generation</h4>
<p>
In contrast to <a>VOD</a> content generation, the typical input is a live
feed of data. Usually there is also a higher priority on
low-latency and time to live, which is in turn reflected in smaller
segments.
</p>
<p>
A big difference between <a>VOD</a> and Live content generation can be
found in the differences between <a>manifests</a> for the two content
types.
Besides a general profile that tells the <a>player</a> if the manifest
represents live content, there are a few other, important properties
that define how a player will be able to playback the live content,
when updates will be fetched, and how close to the live edge playback
starts.
These properties are pre-defined and expressed in the manifest during
content preparation, but play an important role in content playback
and view experience.
</p>
</section>
<section>
<h4>Content Delivery</h4>
<p>
The content delivered through a <a>CDN</a> is generally very similar to the
<a>VOD</a> playback use-case. There might be slight differences when it
comes to caching and distribution of segments, and you might
observe higher load on the origin when playback is configured to be
very close to the live edge.
</p>
</section>
<section>
<h4>Content Playback</h4>
<p>
Even though playback and playback parameters between Live and <a>VOD</a>
playback are very similar, there are critical differences. Maybe
the biggest difference is the playback start position. While <a>VOD</a>
playback session usually start at the beginning on the stream, a
Live playout usually starts close to the live edge of the stream,
with only a few segments between the playback start position and
the end of the current <a>manifest</a>.
</p>
<p>
The other main difference is that the live content changes over
time and the <a>player</a> needs to be aware of these changes. In both
DASH and <a>HLS</a>, this results in regular manifest updates. Depending
on the format, it is possible to define the interval for the
manifest updates.
</p>
<p>
What happens during a typical playback session is:
</p>
<ol>
<li>Player loads the manifest for the first time</li>
<li>Playback starts at the live-edge defined in the manifest</li>
<li>Player re-loads the manifest in regular, pre-defined, intervals.</li>
<li>The new manifest is used to update playback state information and the segment list</li>
</ol>
<p>
This is a typical loop that the player goes through on each
manifest update cycle.
</p>
</section>
<section>
<h4>Potential Issues</h4>
<p>
One of the most common cases that an application needs to be
prepared for is the <a>player</a> falling behind the live window. Assume
for example that you have a buffer window of 10 seconds on your
live stream. Even if the user interface does not allow explicit
seeking, buffering events can still stall the playout and push the
playback position towards the border of the live window and even
beyond it. Depending on the player implementation the actual error
might be different, but implementation should be aware of the
scenario and be prepared to recover from it.
</p>
<p>
Another common problem that can occur both in Live and VoD playback
sessions are missing segment data or timestamp alignment issues. It
might happen that a segment for a given quality is not available on
the server. At the same time, misalignments in segment timestaps
might occur. Although both of these issues migh be critical, how
they are handled depends on the player implementation. A player can
try to jump over missing segments or alignment gaps. A player can
also try to work around missing segments by downloading the segment
from a different <a>rendition</a> and allow a temporary quality switch.
At the same time, a player implementation might also decide to
treat such issues as fatal errors and abort playback. Applications
can try to manually restart playback sessions in such scenarios.
</p>
</section>
</section>
<section>
<h3>Thumbnail Navigation</h3>
<p>
Thumbnails are small images of the content taken at regular time
intervals. They are an effective way to visualize scrubbing and
seeking through content.
</p>
<p>
There is currently no default way to add thumbnail support to a playback
application. And there is not out-of-the-box browser support. However,
since thumbnails are just image data, the browser has all the capabilities
for a client to implement thumbnail navigation in an application.
</p>
<p>
The most common way to generate thumbnails is to render a set of images out of
the main content in a regular time interval, for example every 10 seconds.
The information about the location of these images then needs to be passed
down to the client, which can then request and load an image for a given
playback position. For more efficient loading, images
are often merged into larger grids (sometimes called sprites). This way, the client
only needs to make a single request to load a set of thumbnails
instead of a request per image.
</p>
<p>
Unfortunately, neither DASH or <a>HLS</a> do currently specify a way to
reference thumbnail images directly from <a>manifests</a>. However, the
DASH-IF Guidelines [[DASHIFIOP]] describe an extension to
reference thumbnail images. The thumbnails would be exposed as single
or gridded images. All parameters required to load and display the
thumbnail images are contained in the Manifest. This approach
also works for live Manifests that are updated regularly by the <a>player</a>.
The following example shows how thumbnails can be referenced according
to the DASH-IF Guidelines:
</p>
<pre class="example xml" title="Thumbnail Reference in DASH-Manifest">
<AdaptationSet id="3" mimeType="image/jpeg" contentType="image">
<SegmentTemplate media="$RepresentationID$/tile$Number$.jpg" duration="125" startNumber="1"/>
<Representation bandwidth="10000" id="thumbnails" width="6400" height="180">
<EssentialProperty schemeIdUri="http://dashif.org/guidelines/thumbnail_tile" value="25x1"/>
</Representation>
</AdaptationSet>
</pre>
</section>
</section>
<section>
<h2>Media Playback Methods</h2>
<section>
<h3>Device Identification</h3>
<p>Device identification is required both at the level of device
type and family and also to uniquely identify a device.
Different techniques are used in different environments.</p>
<section>
<h4>Device Type</h4>
<p>In the context of a mobile client the broad device type is
already known - you can’t install an Android client on an iOS
device. However, operating systems evolve and are extended.
Within the application layer you will still need to
determine the level of support for feature X and branch your
code accordingly.</p>
<p>
If the application is hosted or the same application is
deployed via different app stores then the clients runtime
could be more ambiguous. The classic approach is to use the
<code>navigator.userAgent</code>. While this returns a string that does
not explicitly state the device by name a regular expression
match can be used to look for patterns that can confirm the
device family. As an example the name ‘Tizen’ can be found in
the user agent strings for Samsung and ‘netcast’ for LG
devices. In the Chrome browser the strings will be different
on Windows, Mac, Android and webview.</p>
<p>
Another method similar to feature detection is to look for
available APIs, many of these are unique to a device, for
example <code>if(tizenGetUser){ then do X }</code>.</p>
<p>Besides the mentioned feature switches, device type identification
can be crucial to determine which content should be loaded on a
client device. A typical scenario for DRM protected content is to
have different content types for different platforms based on the
platforms playback capabilities. One might for instance package
<a>HLS</a> and <a>MPEG-DASH</a> variants of the same content but
using different encryption and DRM schemes. In that case the client
application needs to decide which content to load based on the device
type and its capabilities.</p>
</section>
<section>
<h4>Unique device Identifier</h4>
<p>After you have determined what type of device you are on you
may need to identify that device uniquely. Most device
manufacturers provide an API so once you have discovered what
type of device you are on you then know what API’s will be
available to you. Each manufacturer provides a string
constructed in a different way, some use the serial number of
the device itself while others use lower level unique
identifiers taken from the hardware. In each case the
application layer should attempt to namespace this in some way
to avoid an unlikely, but theoretically possible, clash with
another user in any server-side database of users and devices.<br>
In a classic PC/browser, rather that mobile or STB (set-top-box),
environment there are technical issues with using unique
identifiers as a user can access these stored locally and
either delete them or reuse them in another context, this
could allow them to watch content on more devices than their
user account allows. For this reason some vendors prefer to
use <a>DRM</a> solutions that both create and encrypt unique
identifiers in a way that obscures them from the user.</p>
<p>As mentioned above one typical use case where a unique device
identifier is needed is to restrict the number of devices that a user
can use. This could refer to overall devices or the number of devices
a user can use in parallel. In both cases a unique device identifier
will be required.</p>
</section>
</section>
<section>
<h3>Device Content Protection Capabilities</h3>
<p>A web video application needs to determine the content
protection capabilities of the device that is being used to
playback the content. The method of doing this will vary from
device to device and between <a>DRM</a> systems.
</p>
<p>
Regardless of a device's capability to play back a stream
encrypted with a specific <a>DRM</a> it is worth noting that a
content provider will be aware of this capability in advance
and consequently encrypt the streams to target a specific
device. Beyond technical feasibility: can device X play back
content encrypted with <a>DRM</a> Y? the provider will have a number
of considerations when choosing a <a>DRM</a> for playback on a
specific device; cost, utility, complexity and content value
(the last consideration being mapped to contractual
obligations). As a consequence, in most situations the API
call that requests the stream from a back end service will
either be to a service that is only configured to return
streams with a suitable encryption or the server will use data
from the request headers or key-value pairs in the request
payload to determine which streams to return.</p>
<p>
In the context of the browser the major vendors broadly
dictate the <a>DRM</a>s available, Apple’s Safari browser and Apple
TV support Apple’s FairPlay <a>DRM</a>. Microsoft's Internet Explorer
and Edge browsers only support Microsoft's PlayReady out of
the box while Google’s Widevine Modular is supported by
Google’s Chrome browser but is also included the browser
developers not tied to a significant hardware providers; Opera
and Firefox.</p>
<p>
Browser <a>DRM</a> detection capabilities are tested via the <a>EME</a>
(Encrypted Media Extensions) API [[ENCRYPTED-MEDIA]]. The API is an extension to
the HTMLMediaElement. The process of determining the available
system is broadly as follows: </p>
<ol>
<li>The stream is passed to the VIDEO or AUDIO HTML5 tag/media element</li>
<li>The browser detects the stream is encrypted</li>
<li>The media event <code>encrypted</code> is thrown</li>
<li>A check is made to see if there are already MediaKeys
associated with the element</li>
<li>If there are no keys already associated with the element
then use the <a>EME</a> API <code>navigator.requestMediaKeySystemAccess()</code>
to determine which <a>DRM</a> system is available. This is done by
passing a key value pair that includes a string
representation of each MediaKey system to the above method
which in turn returns a boolean</li>
<li>Use the <code>MediaKeySystemAccess.createMediaKeys()</code> to return a
new MediaKeys object.</li>
<li>Then use <code>HTMLMediaElement.setMediaKeys(mediaKeys)</code></li>
</ol>
<p>
Please note that the browser will throw the encrypted event if it
detects <a>DRM</a> init data or identifies an encrypted stream by other means.
That does not mean that a client can establish encrypted playback only
once the encrypted event was triggered. An encrypted session can also
be established manually, without relying on the browser to detect
encrypted content first.
</p>
<p>The remaining steps are covered in a later section on using EME
but it’s worth noting at this stage that the
navigator.requestMediaKeySystemAccess() is not uniformly
implemented across all modern browsers that support EME. As an
example Chrome returns true for <code>com.widevine.alpha</code> however IE
and Safari throw errors and Firefox returns null. A possible
solution to this is offered <a href="https://stackoverflow.com/questions/35086625/determine-drm-system-supported-by-browser">here</a>.
</p>
</section>
<section>
<h3>Ad Insertions</h3>
<p>There are many ways to incorporate media advertisements into an application. The most
common types are:</p>
<ol>
<li><a>In-stream</a>: Video advertisement that is played before, mid, or after main content
(such as a clip or episode)</li>
<li>Outstream: Video advertisement stands alone (without other video content) and is
placed natively within other parts of the application</li>
</ol>