ac3go in Evey¶
In short:
- Chrome, Firefox and Edge cannot play Dolby Digital (AC-3) or Dolby Digital Plus (E-AC-3) sound. Instead of having the server convert it, Evey's web player decodes it inside the page, with ac3go, a decoder that runs in the browser as WebAssembly.
- The sound reaches you as it is in the file, and the server does no audio conversion for it.
- You have nothing to set. Stream info in the player shows the audio as client-decoded, and Admin → Now Playing shows ac3go in the player for that track.
- If the decoder cannot run, the player switches to sound converted by the server (AAC) on its own.
The rest of this page explains when this path is taken, how the sound stays in sync with the picture, and what to check when a film plays without sound. Which audio track plays, and when it is copied or converted in general, is on Audio.
ac3go is developed as a separate library, written in Go with no dependency: github.com/gravity-zero/ac3go.
When the browser decodes the sound itself¶
All of these must hold:
| Condition | Otherwise |
|---|---|
| The browser cannot play AC-3 or E-AC-3 itself, and it runs WebAssembly. The player tests the browser for both rather than recognising it by name. | A browser that decodes Dolby Digital natively, such as Safari, receives the track as an ordinary one. |
| The audio track being played is AC-3 or E-AC-3. Other tracks of the film do not count. | Another codec follows the rules on Audio. |
| The track has at most six channels (5.1), and no more than the profile's Max Audio Channels. This path hands over the track as it is in the file, so it cannot fold the channels down. | The server converts the track to AAC, which can fold the channels for a stereo profile. A 7.1 E-AC-3 track is always converted. |
| The film plays through the server's stream (Remux or Transcode), not as a file read directly. | The track follows the decision for that playback. |
How it plays¶
The picture plays through the browser's usual player; its stream carries no audio track, and the player's volume and mute controls drive the gain of the sound engine instead. The sound comes from a separate audio stream that the server names in the playlist: the track as it is in the file, packaged by MKVGo when it can, by FFmpeg copying it otherwise. Neither decodes it.
In the page, a background worker loads ac3go and decodes each audio segment to plain sound, away from the code that draws the picture. The player then plays that sound through the browser's audio output, kept in step with the picture: the picture is the clock, and the sound is scheduled against it. A pause stops both together. A seek, or a gap after the stream stalled, re-aligns the sound to the new position.
A 5.1 track is handed to the browser in the channel order it expects, so the browser can fold it for stereo speakers or headphones.
At the start of a film, the decoder takes a moment to load. When the server also offers the track converted to AAC, that sound plays during the moment, and ac3go takes over once it is ready. The decoder is downloaded once by the browser and checked again at each use, so a server update never mixes an old and a new copy.
If ac3go cannot run¶
When the decoder fails to load, fails on a segment, or the server named no audio stream for it, the player stops using it for the rest of the title and asks the server for the normal stream, with the sound converted to AAC. The film keeps its sound; Stream info then shows the conversion instead of client-decoded.
Warning
A reverse proxy that sets its own Content-Security-Policy must allow
'wasm-unsafe-eval' in script-src, otherwise the decoder cannot start.
It must also serve the decoder files under /wasm/ like the rest of the
web app. See
Reverse proxy configurations.
On a cast receiver¶
Evey's Chromecast receiver does not use ac3go. On TV boxes with little memory, receivers decoding the sound this way stopped after a few minutes of playback. The TV gets Dolby Digital as it is when its receiver reports that it passes it through, and the server's AAC otherwise: see Casting.
The library and its licence¶
ac3go is published under the GNU AGPLv3, like Evey. It is built from source
when the Evey image is built (compiled to WebAssembly with the Go toolchain),
and its licence is served next to the decoder, at /wasm/ac3go.LICENSE.md.