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CyTube Custom Content Metadata

Last updated: 2022-02-12

Purpose

CyTube currently supports adding custom audio/video content by allowing the user to supply a direct URL to an audio/video file. The server uses ffprobe to probe the file for various metadata, including the codec/container format and the duration. This approach has a few disadvantages over the officially supported media providers, namely:

  • Since it accepts a single file, it is not possible to provide multiple source URLs with varying formats or bitrates to allow viewers to select the best source for their computer.
    • It also means it is not possible to provide text tracks for subtitles or closed captioning, or to provide image URLs for thumbnails/previews.
  • Probing the file with ffprobe is slow, especially if the content is hosted in a far away network location, which at best is inconvenient and at worst results in timeouts and inability to add the content.
  • Parsing the ffprobe output is inexact, and may sometimes result in detecting the wrong format, or failing to detect the title.

This document specifies a new supported media provider which allows users to provide a JSON manifest specifying the metadata for custom content in a way that avoids the above issues and is more flexible for extension.

Custom Manifest URLs

Custom media manifests are added to CyTube by adding a link to a public URL hosting the JSON metadata manifest. Pasting the JSON directly into CyTube is not supported. Valid JSON manifests must:

  • Have a URL path ending with the file extension .json (not counting querystring parameters)
  • Be served with the Content-Type header set to application/json
  • Be retrievable at any time while the item is on the playlist (CyTube may re-request the metadata for an item already on the playlist to revalidate)
  • Respond to valid requests with a 200 OK HTTP response code (redirects are not supported)
  • Respond within 10 seconds
  • Not exceed 100 KiB in size

Manifest Format

To add custom content, the user provides a JSON object with the following keys:

  • title: A nonempty string specifying the title of the content. For legacy reasons, CyTube currently truncates this to 100 UTF-8 characters.
  • duration: A non-negative, finite number specifying the duration, in seconds, of the content. This is what the server will use for timing purposes. Decimals are allowed, but CyTube's timer truncates the value as an integer number of seconds, so including fractional seconds lends no advantage.
  • live: An optional boolean (default: false) indicating whether the content is live or pre-recorded. For live content, the duration is ignored, and the server won't advance the playlist automatically.
  • thumbnail: An optional string specifying a URL for a thumbnail image of the content. CyTube currently does not support displaying thumbnails in the playlist, but this functionality may be offered in the future.
  • sources: A nonempty list of playable sources for the content. The format is described below.
  • audioTracks: An optional list of audio tracks for using demuxed audio and providing multiple audio selections. The format is described below.
  • textTracks: An optional list of text tracks for subtitles or closed captioning. The format is described below.

Source Format

Each source entry is a JSON object with the following keys:

  • url: A valid URL that browsers can use to retrieve the content. The URL must resolve to a publicly-routed IP address, and must the https: scheme.
  • contentType: A string representing the MIME type of the content at url. A list of acceptable MIME types is provided below.
  • quality: A number representing the quality level of the source. The supported quality levels are 240, 360, 480, 540, 720, 1080, 1440, and 2160. This may be extended in the future.
  • bitrate: An optional number indicating the bitrate (in Kbps) of the content. It must be a positive, finite number if provided. The bitrate is not currently used by CyTube, but may be used by extensions or custom scripts to determine whether this source is feasible to play on the viewer's internet connection.

Acceptable MIME Types

The following MIME types are accepted for the contentType field:

  • video/mp4
  • video/webm
  • video/ogg
  • application/x-mpegURL (HLS streams)
    • HLS is only tested with livestreams. VODs are accepted, but I do not test this functionality.
  • application/dash+xml (DASH streams)
    • Support for DASH is experimental
  • rtmp/flv
    • In light of Adobe phasing out support for Flash, and many browsers already dropping support, RTMP is not supported by this feature. RTMP streams are only supported through the existing rt: media type.
  • audio/aac
  • audio/mp4
  • audio/mpeg
  • audio/ogg

Other audio or video formats, such as AVI, MKV, and FLAC, are not supported due to lack of common support across browsers for playing these formats. For more information, refer to MDN.

Audio Track Format

Each audio track entry is a JSON object with the following keys:

  • label: A label for the audio track. This is displayed in the menu for the viewer to select a text track.
  • language: A two or three letter IETF BCP 47 subtype code indicating the language of the audio track.
  • url: A valid URL that browsers can use to retrieve the track. The URL must resolve to a publicly-routed IP address, and must use the https: scheme.
  • contentType: A string representing the MIME type of the track at url. Any type starting with audio from the list above is acceptable. However the usage of audio/aac is known to cause audio syncrhonization problems for some users. It is recommended to use an m4a file to wrap aac streams.

Important note regarding audio tracks:

Because of browsers trying to be too smart for their own good, you should include a silent audio stream in the video sources when using separate audio tracks. If you do not, the browser will automatically pause the video whenever the browser detects the page as not visible. There is no way to instruct it to not do so. You can readily accomplish the inclusion of a silent audio track with ffmpeg using the anullsrc filter like so: ffmpeg -f lavfi -i anullsrc=channel_layout=stereo:sample_rate=48000 -i input.mp4 -c:v copy -c:a aac -shortest output.mp4 It is recommended to match the sample rate and codec you intend to use in your audioTracks in your silent track.

Text Track Format

Each text track entry is a JSON object with the following keys:

  • url: A valid URL that browsers can use to retrieve the track. The URL must resolve to a publicly-routed IP address, and must the https: scheme.
  • contentType: A string representing the MIME type of the track at url. The only currently supported MIME type is text/vtt.
  • name: A name for the text track. This is displayed in the menu for the viewer to select a text track.
  • default: Enable track by default. Optional boolean attribute to enable a subtitle track to the user by default.

Important note regarding text tracks and CORS:

By default, browsers block requests for WebVTT tracks hosted on different domains than the current page. In order for text tracks to work cross-origin, the Access-Control-Allow-Origin header needs to be set by the remote server when serving the VTT file. See MDN for more information about setting this header.

Example

{
  "title": "Test Video",
  "duration": 10,
  "live": false,
  "thumbnail": "https://example.com/thumb.jpg",
  "sources": [
    {
      "url": "https://example.com/video.mp4",
      "contentType": "video/mp4",
      "quality": 1080,
      "bitrate": 5000
    }
  ],
  "textTracks": [
    {
      "url": "https://example.com/subtitles.vtt",
      "contentType": "text/vtt",
      "name": "English Subtitles",
      "default": true
    }
  ]
}

Permissions

The permission node to allow users to add custom content is the same as the permission node for the existing raw file support. Custom content is considered as an extension of the existing feature.

Unsupported/Undefined Behavior

The behavior under any the following circumstances is not defined by this specification, and any technical support in these cases is voided. This list is non-exhaustive.

  • Source URLs or text track URLs are hosted on a third-party website that does not have knowledge of its content being played on CyTube.
  • The webserver hosting the source or text track URLs serves a different MIME type than the one specified in the manifest.
  • The webserver hosting the source or text track URLs serves a file that does not match the MIME type specified in the Content-Type HTTP header returned to the browser.
  • The manifest includes source URLs or text track URLs with expiration times, session IDs, etc. in the URL querystring.
  • The manifest provides source URLs with non-silent audio as well as a list of audioTracks.