# YouTube channels as podcasts

A YouTube channel (or playlist) can be subscribed to like a podcast. Only its
sound is played, and **the server never downloads or streams a video or its
sound**: each app finds the audio stream itself.

## Server (diora-web, `podcasts/youtube.py`)

- `POST /podcasts/feeds/add/` takes a YouTube address as `rss_url`: a channel
  (`/@name`, `/channel/UC…`, `/c/…`, `/user/…`), a playlist (`?list=…`), a video
  (subscribes its channel) or the feed itself. The server reads the channel page
  once (feed address, name, picture) and stores the feed
  `https://www.youtube.com/feeds/videos.xml?channel_id=UC…` (or `?playlist_id=…`).
  `GET /podcasts/search/?q=<YouTube address>` answers with that one channel.
- From then on the feed is refreshed like any other. It carries the latest 15
  videos. **Shorts are left out.** There is no duration (`duration_seconds` 0)
  until a player knows it: `POST /podcasts/progress/save/` takes an optional
  `duration_seconds` (number, 1 s … 7 days), stored only while the episode has none.
- An episode from such a feed has
  - `audio_url` = `https://www.youtube.com/watch?v=<11-character id>`. This is
    **the** marker: a client recognises a YouTube episode by this prefix;
  - `guid` = `yt:video:<id>`, `artwork_url` = the video's thumbnail,
    `description` = the video description as HTML (links, line breaks; timestamps
    like `00:00 Intro` work as usual).
- Nothing on the server touches the sound of such an episode: no ID3 chapters,
  no transcripts, and `/podcasts/episodes/<id>/silences/` answers
  `{"ready": true, "spans": [], "error": "youtube"}` (skip silence is up to the
  app, as on Android anyway).

## Apps

A client that sees a `watch?v=` address resolves it **itself, on the device,
to an audio-only stream** (no video track), and plays or downloads that.

| Client  | How |
|---------|-----|
| Android | NewPipeExtractor: the original audio track, progressive, opus before m4a, ≤ 160 kbit/s; resolved when the stream is opened and once more on a 403; downloads in 10 MB ranges (in one go YouTube throttles to ~50 KB/s) |
| Desktop | yt-dlp in the main process (`-f bestaudio`), Electron itself as its JavaScript runtime; yt-dlp is fetched from its GitHub release on first use and kept up to date (`src/main/youtube.js`) |
| Browser | can't (YouTube doesn't let a web page read its pages, and a stream address only works for the device that asked for it): the episode is listed but says it plays in the app |

Stream addresses are bound to the device's IP address and run out after a few
hours: resolve shortly before playing, and again when the player reports an
error (podcasts-web.js reloads after a media error anyway).

Progress, queue, played state etc. work as for any episode: they belong to the
episode on the server, not to the stream address.