The ALSA Loopback sound card is a virtual soundcard that is created once the ALSA kernel module snd-aloop is loaded. This virtual soundcard device, as its name indicates, sends back the output signal of applications using it back to itself, so one has a chance to e.g. record this signal from the same device. Simply imagine that you have a physical link between one OUT and one IN of the same device. By default, the Loopback sound card consists of 2 devices, each composed of 8 subdevices.
I required the ALSA Loopback device to re-route sound from one application to another when streaming Airplay to a Logitech Squeezebox see here. Read on to see how I installed the ALSA loopback on Ubuntu 10.04...
UPDATE 2017: THIS ARTICLE IS NOW OUT OF DATE. USE THE SHAIRTUNES2 PLUGIN AVAILABLE IN THE LOGITECHMEDIASERVER PLUGINS PAGE - FOR FIREWALL SETTINGS SEE HERE:
http://confoundedtech.blogspot.co.uk/2018/01/linux-firewall-settings-for-apple.html
This post focuses on playing music from iTunes or iOS devices to a Logitech Squeezebox using AirPlay. The concept is to stream music from iTunes or iOS devices > Ubuntu server > Squeezebox. This requires three key tools Shairport, WaveInput, ecasound and some patience.
Shairport authored by James Laird in 2011 emulates an AirPort Express for the purpose of streaming music from iTunes and compatible iPods. It implements a server for the Apple RAOP protocol, it does not support AirPlay v2 (video and photo streaming). It is a reverse engineer of the RAOP protocol and relies upon the ALAC decoder released by David Hammerton, which is reproduced within Shairport. The project can be found at https://github.com/albertz/shairport
WaveInput authored by BPA is a Logitech Squeezebox plugin that enables PC audio to be captured and played through a Squeezebox. There are version of Windows and Linux and the project can be found here http://code.google.com/p/bpaplugins/
ecasound is a multitrack processing tool for playback, recording, format conversions, effects and mixing. Its use here is to transcode the raw stream from Airplay/Shairport into a format the Squeezebox can understand. I use it to introduce some LAME MP3 compression to the stream to help with my network bandwidth and to reduce playback stutter.
