9.7 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. For example, synchronised multi-room audio is possible without difficulty.
Shairport 2.0 is a pretty substantial rewrite of Shairport 1.0 by James Laird and others. It is still 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 Some of the support files, e.g. PKGBUILD and shairport.service files, are out of date.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 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 synchronised with the source's time signals, eliminating "drift", where audio streams slowly drift out of synchronisation.
To stay synchronised, if an output device is running slow, Shairport 2.0 will delete frames of audio to allow it to keep up; if the device is running fast, Shairport 2.0 will insert interpolated frames to keep time. The number of frames inserted or deleted is so small as to be inaudible. Frames are inserted or deleted as necessary at pseudorandom intervals.
What else?
- Shairport 2.0 offers finer control at very top and very bottom of the volume range. See http://tangentsoft.net/audio/atten.html for a good discussion of audio "attenuators", upon which volume control in Shairport 2 is modelled. See also the diagram of the volume transfer function in the documents folder.
- Shairport 2.0 will mute properly if the hardware supports it.
- If it has to use software volume and mute controls, the response time is shorter than before -- now it responds in 0.15 seconds.
Status
Shairport 2.0 is working on Raspberry Pi with Raspian and OpenWrt, and it runs on a Linksys NSLU2 using OpenWrt. It also works on a standard Ubuntu laptop. It works well with built-in audio and with a variety of USB-connected Amplifiers and DACs, including a cheapo USB "3D Sound" dongle, a first generation iMic and a Topping TP30 amplifier with a USB DAC input.
Shairport 2.0 compiles and runs pretty well on the built-in sound card of a Raspberry Pi model B under Raspian. Due to the limitations of the sound card, you wouldn't mistake the output for HiFi, but it's really not too shabby. USB-connected sound cards work well, so long as the wired Ethernet port is not in use -- WiFi is fine if the network is not too busy. However, driving any USB-based audio output device is glitchy if you are using Ethernet at the same time or if you're on a busy WiFi network. It works, but it's very glitchy. This seems to be due to a known problem -- see http://www.raspberrypi.org/forums/viewtopic.php?t=23544 -- and it will hopefully be fixed in a forthcoming update to Raspian. Shairport 2.0 runs well on the same hardware using OpenWrt in place of Raspian and then it works properly with both Ethernet and WiFi.
Shairport 2.0 runs on Ubuntu and Debian 7 inside VMWare Fusion 6.0.3 on a Mac, but synchronisation does not work -- possibly because the soundcard is being emulated.
Shairport 2.0 works only with the ALSA back end. You can try compiling the other back ends in as you wish, but it definitely will not work properly with them. Maybe someday...
One other change of note is that the Shairport 2.0 build process uses GNU autotools and libtool to examine and configure the build environment -- very important for cross compilation. All previous versions looked in the current system to determine which packages were available, instead of looking at what packages were available in the target system.
Build Requirements
Required:
- OpenSSL
- Avahi
- ALSA
- libdaemon
- autoconf
- libtool
Debian, Ubuntu and Raspian users can get the basics with:
apt-get install avahi-daemon autoconf libtool libdaemon-dev libssl-dev libavahi-client-dev libasound2-dev
Building Instructions
If you're interested in Shairport 2.0 for OpenWrt, there's an OpenWrt package at https://github.com/mikebrady/shairport.
Otherwise, to build Shairport 2.0, download it:
git clone https://github.com/mikebrady/shairport-sync.git
Next, cd into the shairport-sync directory and execute the following commands:
$ autoreconf -i
$ ./configure --with-alsa --with-avahi
$ make
If you run $sudo make install, shairport will be installed along with an initscript which will automatically launch it at startup. The settings used are the most basic defaults, so you will want to edit the file /etc/init.d/shairport to give the service a name, use a different card, use the hardware mixer and volume control, etc.
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. For the built-in soundcard on the Raspberry Pi, a latency of about 98500 seems to be better.
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, including getting true mute and instant response to volume control and pause commands, you should access the hardware volume controls as shown. Use amixer or 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, there appears to be a bug in ALSA -- you can not specify a device other than "default". Thus:
On an NSLU2, to drive a first generation Griffin iMic:
shairport -d -L 99400 -a "Shairport 2.0" -- -t hardware -c PCM
On an NSLU2, to drive the "3D Sound" USB card:
shairport -d -L 99400 -a "Shairport 2.0" -- -t hardware -c Speaker
Latency
The latency you set with the -L option is the exact time from a sound signal's original timestamp until that signal actually "appears" on the output of the DAC, irrespective of any internal delays, processing times, etc. in the computer. Thus, to get perfect audio synchronisation, the latency should be the same for all Shairport 2.0 devices, no matter what output devices they use -- build-in audio, USB DACs, etc. In the writer's experience, this is true.
(When you're checking latency, be aware that many noise-cancelling headphones add a short but noticeable delay to their audio output.)
What is slightly curious is that the latency that most closely matches that of the Airport Express is around 99,400 frames, a little over the two seconds that most people report as the Airport Express's latency.
Some Statistics
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:
Divergence: 15.3 (ppm); corrections: 21.6 (ppm); missing packets 0; late packets 0; too late packets 0; resend requests 0.
"Divergence" is actually the net sum of corrections -- the number of frame insertions less the number of frame deletions -- 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. The example above indicates that the output DAC's clock is running 15.3 ppm faster than the source's clock.
"Corrections" is the number of frame insertions plus the number of frame deletions (i.e. the total number of "corrections"), 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.
Miscelleanous
Shairport 2.0 actively maintains synchronisation with the source. If synchronisation is lost -- say due to a very busy iTunes host or a very congested network -- then Shairport 2.0 will mute its output and resynchronise. The loss-of-sync threshold is a very conservative 30 ms -- i.e. the actual time and the expected time must differ by more than 30 ms to trigger a resynchronisation. Smaller disparities are corrected by insertions or deletions, as described above.