supysonic/docs/transcoding.md

Transcoding

Transcoding is the process of converting from one audio format to another. This allows for streaming of formats that wouldn't be streamable otherwise, or reducing the quality of an audio file to allow a decent streaming for clients with limited bandwidth, such as the ones running on a mobile connection.

Transcoding in Supysonic is achieved through the use of third-party command-line programs. Supysonic isn't bundled with such programs, and you are left to choose which one you want to use.

If you want to use transcoding but your client doesn't allow you to do so, you can force Supysonic to transcode for that client by going to your profile page on the web interface.

Configuration

Configuration of transcoders is done on the [transcoding] section of the configuration file.

Transcoding can be done by one single program which is able to convert from one format directly to another one, or by two programs: a decoder and an encoder. All these are defined by the following variables:

• transcoder_EXT_EXT
• decoder_EXT
• encoder_EXT
• trancoder
• decoder
• encoder
• default_transcode_target

where EXT is the lowercase file extension of the matching audio format. transcoders variables have two extensions: the first one is the source extension, and the second one is the extension to convert to. The same way, decoders extension is the source extension, and encoders extension is the extension to convert to.

Notice that all of them have a version without extension. Those are generic versions. The programs defined with these variables should be able to transcode/decode/encode any format. For that reason, we suggest you don't use these if you want to keep control over the available transcoders.

Supysonic will take the first available transcoding configuration in the following order:

1. specific transcoder
2. specific decoder / specific encoder
3. generic decoder / generic encoder (with the possibility to use a generic decoder with a specific encoder, and vice-versa)
4. generic transcoder

All the variables should be set to the command-line used to run the converter program. The command-lines can include the following fields:

• %srcpath: path to the original file to transcode
• %srcfmt: extension of the original file
• %outfmt: extension of the resulting file
• %outrate: bitrate of the resulting file

One final note: the original file should be provided as an argument of transcoders and decoders. All transcoders, decoders and encoders should write to standard output, and encoders should read from standard input.

The value of default_transcode_target will be used as output format when a client requests a bitrate lower than the original file and no specific format.

Suggested configuration

Here are some example configuration that you could use. This is provided as-is, and some configurations haven't been tested.

[transcoding]
transcoder_mp3_mp3 = lame --quiet --mp3input -b %outrate %srcpath -
transcoder = ffmpeg -i %srcpath -ab %outratek -v 0 -f %outfmt -
decoder_mp3 = mpg123 --quiet -w - %srcpath
decoder_ogg = oggdec -o %srcpath
decoder_flac = flac -d -c -s %srcpath
encoder_mp3 = lame --quiet -b %outrate - -
encoder_ogg = oggenc2 -q -M %outrate -
default_transcode_target = mp3