Shairport Sync

Shairport Sync is an AirPlay audio player for Linux, FreeBSD and OpenBSD. It plays audio streamed from Apple devices and from AirPlay sources such as OwnTone (formerly forked-daapd).

Shairport Sync can be built as an AirPlay 2 player (with some limitations) or as "classic" Shairport Sync – a player for the older, but still supported, AirPlay (aka "AirPlay 1") protocol.

When built for AirPlay 2, Shairport Sync can play stereo and multichannel (5.1 and 7.1) audio, including lossless 24-bit stereo at 48,000 frames per second.

Metadata such as artist information and cover art can be requested and provided to other applications. Shairport Sync can interface with other applications through MQTT, an MPRIS-like interface and D-Bus.

Shairport Sync does not support AirPlay video or photo streaming.

Quick Start

• If you are updating from a previous version of Shairport Sync, please visit the release notes for possible breaking changes.
• A building guide is available here.
• A Docker image is available on the Docker Hub. Also see docker/README.md.
• Next Steps and Advanced Topics are here.
• Runtime settings are documented here.
• Build configuration options are detailed in CONFIGURATION FLAGS.md.
• The man page, detailing command line options, is here.
• Some advanced topics and developed in ADVANCED TOPICS.

Features

• Outputs AirPlay audio to ALSA, sndio, PipeWire, PulseAudio, to a unix pipe or to STDOUT. It also has limited support for libao.
• Metadata — Shairport Sync can deliver metadata supplied by the source, such as Album Name, Artist Name, Cover Art, etc. through a pipe or UDP socket to a recipient application program — see https://github.com/mikebrady/shairport-sync-metadata-reader for a sample recipient. Sources that supply metadata include iTunes and the Music app in macOS and iOS.
• An interface to MQTT, a popular protocol for Inter Process Communication, Machine-to-Machine, Internet of Things and Home Automation projects. The interface provides access to metadata and artwork, with remote control for Classic AirPlay clients.
• Digital Signal Processing facilities – please see the DSP Wiki Page Guide. (Thanks to Yann Pomarède for the code and to Paul Wieland for the guide.)
• An MPRIS-like interface, partially complete and very functional, including access to metadata and artwork, and partial remote control for Classic AirPlay clients.
• A native D-Bus interface, including access to metadata, artwork and system settings. Remote control is also available for Classic AirPlay clients.
• Better Volume Control — Shairport Sync 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 Sync is modelled. See also the diagram of the volume transfer function in the documents folder. In addition, Shairport Sync can offer an extended volume control range on devices with a restricted range.
• Flexible output rates, formats and channels with built-in transcoding.
• Remote control commands -- such as play, pause, and volume-setting commands -- can be sent to Classic AirPlay clients via the D-Bus, MPRIS and MQTT interfaces. Switch to the development branch to get experimental support for remote control with AirPlay 2 clients.

Some features require configuration at build time – see CONFIGURATION FLAGS.md.

Status

Shairport Sync was designed to run best on stable, dedicated, stand-alone low-power "headless" systems with ALSA as the audio system and with a decent CD-quality Digital to Analog Converter (DAC).

Shairport Sync runs on recent (2018 onwards) Linux systems, FreeBSD from 12.1 onwards and OpenBSD. It requires a system with the power of a Raspberry Pi B or better.

Classic Shairport Sync runs on a wider variety of Linux sytems, including OpenWrt and Cygwin and it also runs on OpenBSD. Many embedded devices are powerful enough to power classic Shairport Sync.

Heritage and Acknowledgements

The functionality offered by Shairport Sync is the result of study and analysis of the AirPlay and AirPlay 2 protocols by many people over the years. These protocols have not been officially published, and there is no assurance that Shairport Sync will continue to work in future.

Shairport Sync is a substantial rewrite of the fantastic work done in Shairport 1.0 by James Wah (aka abrasive), James Laird and others — please see this list of the contributors to Shairport 1.x and Shairport 0.x. From a "heritage" point of view, Shairport Sync is a fork of Shairport 1.0.

For the development of AirPlay 2 support in Version 4.x, special thanks are due to:

• JD Smith for really thorough testing, support and encouragement.
• ejurgensen for advice and code to deal with pairing and encryption.
• ckdo for pointing the way, particularly with pairing and encryption protocols, with a functional Python implementation of AirPlay 2.
• invano for showing what might be possible and for initial Python development.
• Charles Omer for Docker automation, repository management automation, testing, encouragement, enthusiasm.

Much of Shairport Sync's AirPlay 2 functionality is based on ideas developed at the openairplay airplay2-receiver repository. It is a pleasure to acknowledge the work of the contributors there.

Thanks to everyone who has supported and improved Shairport Sync over the years.

More about Shairport Sync

The audio that Shairport Sync receives is sent to the computer's sound system, to a named unix pipe or to STDOUT. By far the best sound system to use is ALSA. This is because ALSA can give direct access to the Digital to Analog Converter (DAC) hardware of the machine. Audio samples can be sent through ALSA directly to the DAC, maximising fidelity, and accurate timing information can be obtained from the DAC, maximising synchronisation.

Synchronised Audio

Shairport Sync offers full audio synchronisation. Full audio synchronisation means that audio is played on the output device at exactly the time specified by the audio source. To accomplish this, Shairport Sync needs access to audio systems – such as ALSA on Linux and sndio on FreeBSD and OpenBSD – that provide very accurate timing information about audio being streamed to output devices. Ideally, Shairport Sync should have direct access to the output device used, which should be a real sound card capable of working with 44,100 or 48,000 samples ("frames") per second, interleaved PCM stereo of 8, 16, 24 or 32 bits. Shairport Sync will choose a suitable output rate, format and channel count. This can be done manually or automatically.

Shairport Sync works well with sound servers such as PipeWire and PulseAudio, widely used sound servers found on many desktop Linuxes. While the timing information is not as accurate as that of ALSA or sndio, it is often impractical to bypass these systems.

For other use cases, Shairport Sync can provide synchronised audio output to a unix pipe or to STDOUT, or to audio systems that do not provide timing information. This could perhaps be described as partial audio synchronisation, where synchronised audio is provided by Shairport Sync, but what happens to it in the subsequent processing chain, before it reaches the listener's ear, is outside the control of Shairport Sync.

Latency, "Stuffing", Timing

AirPlay protocols use an agreed latency – a time lag or delay – between the time represented by a sound sample's timestamp and the time it is actually played by the audio output device, typically a Digital to Audio Converter (DAC). Latency gives players time to compensate for network delays, processing time variations and so on. The latency is specified by the audio source when it negotiates with Shairport Sync. AirPlay sources set a latency of around 2.0 to 2.25 seconds. AirPlay 2 can use shorter latencies, around half a second.

As mentioned previously, Shairport Sync implements full audio synchronisation when used with ALSA, sndio, PipeWire or PulseAudio audio systems. This is done by monitoring the timestamps present in data coming from the audio source and the timing information coming back from the audio system itself. To maintain the latency required for exact synchronisation, Shairport Sync will perform interpolation -- effectively shortening or lengthening the stream of audio to exactly match the output rate to the input rate. If the output device is running too slow or too fast relative to the source, Shairport Sync will resample sequences of audio frames to add or remove frames as needed. Higher quality resampling can be achieved with libsoxr support, but this requires a good deal of processing power — most embedded devices probably can't support it. If your computer is fast enough, Shairport Sync will automatically choose this method.

Interpolation is not done for partial audio synchronisation – the audio samples are simply presented at exactly the right time to the next stage in the processing chain.

Timestamps are referenced relative to the source computer's clock – the "source clock", but timing must be done relative to the clock of the computer running Shairport Sync – the "local clock". So, Shairport Sync synchronises the source clock and the local clock, usually to within a fraction of a millisecond. In AirPlay 2, this is done with the assistance of a companion application called NQPTP using a PTP-based timing protocol. In classic AirPlay, a variant of NTP synchronisation protocols is used.