The upmdcli UPnP front-end to MPD implements support for Linn Songcast with help from the sc2mpd auxiliary process.

What is Songcast

Songcast is a protocol for transporting audio streams across the network. This is independant from the UPnP framework which mostly deals with URLs and audio files (usually, but not necessarily, compressed).

The streams transported by Songcast are actual real-time audio data, which can go straight to an audio card for playing.

Controlling the streams (connecting, starting, stopping) is done through an UPnP service named OpenHome Receiver.

The typical use of Songcast is to have an audio driver on a Windows or OS X desktop capture and forward the audio stream to a remote Songcast device.

Any application on the desktop will (be compelled to) transparently play to the remote device, without need to know anything about Songcast.

Implementation of Songcast support in upmpdcli

upmpdcli implements the Receiver service, and uses an auxiliary process (sc2mpd) for transporting the audio data. sc2mpd is a slight modification of the sample program which comes with the Linn Songcast open-source implementation, with the addition of an HTTP interface (based on libmicrohttpd) to forward the data to mpd.

Setting up a connection happens as follows:

  • If it finds an executable sc2mpd command in the PATH when starting up, upmpdcli advertises a Receiver service.

  • The Songcast implementation from the desktop finds out about the Receiver through the normal UPnP mechanisms and can be instructed to use it. It then tells the Receiver in upmpdcli to start playing.

  • upmdpcli starts the sc2mpd process, which gets ready to receive data through Songcast, and make it available through HTTP.

  • upmpdcli instructs mpd to play the URL for the sc2mpd output.

There is currently a 10-12 S delay between the connection request and the moment the audio starts playing. This is not normal, but I could not find the reason for it, so it’s an unavoidable inconvenience at the moment.

Configuration

No configuration is necessary by default: if sc2mpd is present, upmpdcli will advertise the Songcast capability, and any host with a Songcast sender installed should be able to use it.

However, you can set a number of values in the upmpdcli configuration file (you must set the upmpdcli -c option for sc2mpd to see them, the environment variable will not work):

sclogfilename

Name of the file which will receive sc2mpd log messages. stderr by default. This can’t be the same file used by upmpdcli.

scloglevel

Log verbosity.

schttpport

HTTP port used by mpd to connect to sc2mpd. 8888 by default. This must be an available port on localhost, and it will only accept connections from localhost.

sc2mpd

Path for the sc2mpd executable file (e.g. /usr/local/bin/sc2mpd). Only useful if sc2mpd was not installed to a location in the executable $PATH set for the init scripts. Typically only /bin and /usr/bin are in there.

Building sc2mpd

There are two parts in building sc2mpd:

  • Building the Openhome libraries

  • Building scmpd proper

Building the Openhome libraries is a bit of a black art, and the sc2mpd source comes with an ohbuild.sh script which will try to clone the Openhome git repositories and build the libs:

mkdir /my/place/for/openhome
sh ohbuild.sh /my/place/for/openhome

Miscellaneous error messages will be displayed during the build. Hope for the best…

When this is done configure sc2mpd:

sh autogen.sh
./configure --prefix=/usr --with-openhome=/my/place/for/openhome
make
sudo make install

The build uses static libraries, so you can move the executable to another machine without needing the openhome directory.

Miscellaneous remarks

Songcast is probably best transported over a wired connection. If you are doing this over WIFI and experiencing glitches, the wireless is the first suspect.