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Update documentation with a glossary and intro page, added DASH background #94
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# Glossary | ||
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**Playlist**: This is a representation of an HLS or DASH manifest. | ||
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**Master Playlist Controller**: This acts as the main controller for the playback engine. It interacts with the SegmentLoaders, PlaylistLoaders, PlaybackWatcher, etc. | ||
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**Playlist Loader**: This will request the source and load the master manifest. It also interacts with the ABR algorithm to pick a media playlist or wraps a media playlist if it is provided as the source. There are more details about the playlist loader [here](./arch.md). | ||
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**Media Playlist**: This is a manifest that represents a single rendition of the source. | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Probably should move this up below Playlist. |
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**Segment Loader**: This controls the requesting and appending of segments on to the browser's [SourceBuffers](https://developer.mozilla.org/en-US/docs/Web/API/SourceBuffer). | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. Technically Segment Loader figures out which segment to request, and requests it via Media Segment Request. It then decrypts (if necessary) and sends the segment bytes to the Source Updater, which manages the source buffers. |
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**ABR(Adaptive Bitrate) Algorithm**: This is defined in selectPlaylist and is described more [here](./bitrate-switching.md). | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. ABR is a more generic term. We may want to point to https://en.wikipedia.org/wiki/Adaptive_bitrate_streaming and say that our "chosen" ABR algorithm is referenced by selectPlaylist (can also point to the README where it shows instructions for overriding it). |
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**Playback Watcher**: This handles seeking to live when playing a live source with a live window, or skipping over gaps in content. This is described in detail [here](). | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. May want a link here, but can more generically say that Playback Watcher tries to resolve common playback stalls caused by improper seeking, gaps in content, browser issues, and other problematic events. There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. heh, good catch. I forgot I didn't write that doc yet. 👍 |
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**Sync Controller**: This will attempt to create a mapping between the segment index and a display time on the player. |
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# Overview | ||
This project supports both [HLS][hls] and [MPEG-DASH][dash] playback in the video.js player. This document is intended as a primer for anyone interested in contributing or just better understanding how bits from a server get turned into video on their display. | ||
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## HTTP Live Streaming | ||
[HLS][apple-hls-intro] has two primary characteristics that distinguish it from other video formats: | ||
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- Delivered over HTTP(S): it uses the standard application protocol of the web to deliver all its data | ||
- Segmented: longer videos are broken up into smaller chunks which can be downloaded independently and switched between at runtime | ||
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A standard HLS stream consists of a *Master Playlist* which references one or more *Media Playlists*. Each Media Playlist contains references one or more sequential video segments. All these components form a logical hierarchy that informs the player of the different quality levels of the video available and how to address the individual segments of video at each of those levels: | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. s/contains references/contains/ |
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HLS streams can be delivered in two different modes: a "static" mode for videos that can be played back from any point, often referred to as video-on-demand (VOD); or a "live" mode where later portions of the video become available as time goes by. In the static mode, the Master and Media playlists are fixed. The player is guaranteed that the set of video segments referenced by those playlists will not change over time. | ||
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Live mode can work in one of two ways. For truly live events, the most common configuration is for each individual Media Playlist to only include the latest video segment and a small number of consecutive previous segments. In this mode, the player may be able to seek backwards a short time in the video but probably not all the way back to the beginning. In the other live configuration, new video segments can be appended to the Media Playlists but older segments are never removed. This configuration allows the player to seek back to the beginning of the stream at any time during the broadcast and transitions seamlessly to the static stream type when the event finishes. | ||
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If you're interested in a more in-depth treatment of the HLS format, check out [Apple's documentation][apple-hls-intro] and the IETF [Draft Specification][hls-spec]. | ||
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## Dynamic Adaptive Streaming over HTTP | ||
Similar to HLS, [DASH][dash-wiki] content is segmented and is delivered over HTTP(s). | ||
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A DASH stream consits of a *Media Presentation Description*(MPD) that describes segment metadata. Each segment must contain either ISO base media file format(e.g MP4) or MPEG-2 TS data. Typically the MPD will describe the various *Representations* that map to collections of segments at different bitrates to allow bitrate selection. | ||
There was a problem hiding this comment. Choose a reason for hiding this commentThe reason will be displayed to describe this comment to others. Learn more. I don't know if segment links is technically a part of "segment metadata," but it is a bit hard to glean that info from just metadata. May want to expand on the contents. |
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DASH streams can be delivered in both video-on-demand(VOD) and live streaming modes. In the VOD case, the MPD describes all the segments and representations available and the player can chose which representation to play based on it's capabilities. | ||
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Live mode is accomplished using the ISOBMFF Live profile if the segments are in ISOBMFF. There are a few different ways to setup the MPD including but not limited to updating the MPD after an interval of time, using *Periods*, or using the *availabilityTimeOffset* field. A few examples of this are provided by the [DASH Reference Client][dash-if-reference-client]. The MPD will provide enough information for the player to playback the live stream and seek back as far as is specified in the MPD. | ||
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If you're interested in a more in-depth description of MPEG-DASH, check out [MDN's tutorial on setting up DASH][mdn-dash-tut] or the [DASHIF Guidelines][dash-if-guide]. | ||
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# Further Documentation | ||
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- [Architechture](arch.md) | ||
- [Glossary](glossary.md) | ||
- [Adaptive Bitrate Switching](bitrate-switching.md) | ||
- [Multiple Alternative Audio Tracks](multiple-alternative-audio-tracks.md) | ||
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# Helpful Tools | ||
- [FFmpeg](http://trac.ffmpeg.org/wiki/CompilationGuide) | ||
- [Thumbcoil](http://thumb.co.il/): web based video inspector | ||
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[hls]: /docs/intro.md#http-live-streaming | ||
[dash]: /docs/intro.md#dynamic-adaptive-streaming-over-http | ||
[apple-hls-intro]: https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/StreamingMediaGuide/Introduction/Introduction.html | ||
[hls-spec]: https://datatracker.ietf.org/doc/draft-pantos-http-live-streaming/ | ||
[dash-wiki]: https://en.wikipedia.org/wiki/Dynamic_Adaptive_Streaming_over_HTTP | ||
[dash-if-reference-client]: https://reference.dashif.org/dash.js/ | ||
[mdn-dash-tut]: https://developer.mozilla.org/en-US/Apps/Fundamentals/Audio_and_video_delivery/Setting_up_adaptive_streaming_media_sources | ||
[dash-if-guide]: http://dashif.org/guidelines/ |
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The ABR algorithm should be called through Master Playlist Controller. Playlist Loader is just told what playlist was selected.
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changing this to
It is also instructed by the ABR algorithm to load a media playlist or wraps a media playlist if it is provided as the source