5.7 KiB
ShairPort 2.0
Shairport 2.0 allows you to synchronise the audio coming from all your devices. Specifically, Shairport 2.0 allows you to set the "latency" -- the time between when a sound is sent and when it is played. This allows you to synchronise Shairport 2.0 devices reliably with other devices playing the same source.
Shairport 2.0 is a pretty substantial rewrite of Shairport 1.0 by James Laird. It is still very experimental, and only works with Linux and ALSA. Some of the support files, e.g. PKGBUILD and shairport.service files, are out of date.
What is Shairport?
Shairport emulates an AirPort Express for the purpose of streaming music from iTunes and compatible iPods and iPhones. It implements a server for the Apple RAOP protocol. ShairPort does not support AirPlay v2 (video and photo streaming).
Shairport 2.0 does Audio Synchronisation
Shairport 2.0 allows you to set a delay (a "latency") from when music is sent by iTunes or your iOS device to when it is played in the Shairport audio device. The latency can be set to match the latency of other output devices playing the music, achieving audio synchronisation. Shairport 2.0 uses extra timing information to stay in sync with the source's time signals.
Status
Shairport 2.0 is working on Raspberry Pi and Linksys NSLU2, both using OpenWrt. It works on an Ubuntu laptop. It works well with built-in audio and with a variety of USB-connected Amplifiers and DACs.
Shairport 2.0 does not work wih Raspian. It does run, but it's very glitchy. This seems to be due to a known problem with Raspian -- see http://www.raspberrypi.org/forums/viewtopic.php?t=23544. On the other hand, Shairport 2.0 runs well on the exact same hardware but using OpenWrt in place of Raspian.
Shairport 2.0 runs reasonably well on Ubuntu 13.10 inside VMWare Fusion 6.0.3 on a Mac, but you need to set the latency (option -L) to 88200 to bring it into sync with iTunes on the same machine. I don't know why :)
Please note that Shairport 2.0 only works with the ALSA back end. You can compile the other back ends in as you wish, but it definitely will not work properly with them. Maybe someday...
There are lots of little changes, e.g. volume control profile, muting, autotools build control...
Build Requirements
Required:
- OpenSSL
- Avahi
- ALSA
Debian users can get the basics with
apt-get install libtool libssl-dev libavahi-client-dev libasound2-dev
Building Instructions
If you're building ShairPort for OpenWrt, there's an OpenWrt package at https://github.com/mikebrady/shairport.
Otherwise, to build Shairport, cd into the shairport directory and execute the following commands:
$ autoreconf -i
$ ./configure --with-alsa --with-avahi
$ make
Note that shairport is not installed automatically, so where it says shairport below, you might have to enter its path name, e.g. ./shairport if you are still in the shairport directory.
Running Shairport 2.0
The -L setting is for the amount of latency -- the units are frames, with 44,100 frames to the second. Although 99,400 frames is slightly more than two seconds, it sounds good -- YMMV.
Examples
The first is an example of a standard Ubuntu based laptop:
shairport -d -L 99400 -a "Shairport 2.0" -- -d hw:0
In the following are examples of the Raspberry Pi and the NSLU2 -- little-endian and a big-endian ARM systems running OpenWrt. For best results, you should access the hardware volume controls as shown. Use alsamixer or similar to discover the name of the volume controller to be used after the -c option.
For a Raspberry Pi using its internal soundcard that drives the headphone jack:
shairport -d -L 99400 -a "Shairport 2.0" -- -d hw:0 -t hardware -c PCM
For a Raspberry Pi driving a Topping TP30 Digital Amplifier, which has an integrated USB DAC":
shairport -d -a Kitchen -L 99400 -- -d hw:1 -t hardware -c PCM
For a cheapo "3D Sound" USB card (Stereo output and input only) on a Raspberry Pi:
shairport -d -L 99400 -a "Shairport 2.0" -- -d hw:1 -t hardware -c Speaker
For a first generation Griffin iMic on a Raspberry Pi:
shairport -d -L 99400 -a "Shairport 2.0" -- -d hw:1 -t hardware -c PCM
For an NSLU2, which has no internal soundcard, to drive the "3D Sound" USB card:
shairport -d -L 99400 -a "Shairport 2.0" -- -d hw:0 -t hardware -c Speaker
Notes
If you run Shairport from the command line without daemonising it (omit the -d), and if you turn on one level of verbosity (include -v), e.g. as follows for the Raspberry Pi with "3D Sound" card:
shairport -L 99400 -a "Shairport 2.0" -v -- -d hw:1 -t hardware -c Speaker
it will print statistics like this occasionally:
Drift: -15.3 (ppm); Corrections: 21.6 (ppm), missing_packets 0, late_packets 0, too_late_packets 0 resend_requests 0.
"Drift" is the net corrections -- the number of frame insertions less the number of frame deletions made, given as a moving average in parts per million. After an initial settling period, it represents the divergence between the source clock and the sound device's clock.
"Corrections" is the number of frame insertions plus the number of frame deletions (i.e. the total number of "corrections" made), given as a moving average in parts per million. The closer this is to the absolute value of the drift, the fewer "unnecessary" corrections that are being made.
For reference, a drift of one second per day is approximately 11.57 ppm. Left uncorrected, even a drift this small between two audio outputs will be audible after a short time.
It's not unusual to have resend requests, late packets and even missing packets if some part of the connection to the Shairport device is over WiFi.