More Information
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-Shairport Sync works by using timing information and timestamps present in data coming from the audio source (e.g. an iPhone) to play audio at exactly the right time. It uses this to monitor and control the *latency* — the time-gap between when a sound frame is supposed to be played, as specified by its `timestamp`, and the time when it is actually played by the audio output device, usually a Digital to Audio Converter (DAC).
+Shairport Sync works by using timing information and timestamps present in data coming from the audio source (e.g. an iPhone) to play audio at exactly the right time. It uses the information to monitor and control the *latency* — the time-gap between when a sound frame is supposed to be played, as specified by its `timestamp`, and the time when it is actually played by the audio output device, usually a Digital to Audio Converter (DAC).
The latency to be used is specified by the source when it negotiates with Shairport Sync. Most sources set a latency of exactly two seconds. Recent versions of iTunes and forkedDaapd use a latency of just over 2.25 seconds.
-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, another thing Shairport Sync has to do is to synchronize the source clock and the local clock, usually to within a fraction of a millisecond, using a variant of NTP synchronisation protocols.
+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, another thing Shairport Sync has to do is to synchronize the source clock and the local clock, and it does this usually to within a fraction of a millisecond, using a variant of NTP synchronisation protocols.
To maintain the exact latency required, if an output device is running slow relative to the source, Shairport Sync will delete frames of audio to allow the device to keep up. If the output device is running fast, Shairport Sync will insert frames to keep time. The number of frames inserted or deleted is so small as to be almost inaudible on normal audio material. Frames are inserted or deleted as necessary at pseudorandom intervals. Alternatively, with `libsoxr` support, Shairport Sync can resample the audio feed to ensure the output device can keep up. This is less obtrusive than insertion and deletion but requires a good deal of processing power — most embedded devices probably can't support it. The process of insertion/deletion or resampling is rather inelegantly called “stuffing”.
Shairport Sync is designed for audio back ends that offer accurate timing and synchronisation information, including `alsa` on Linux and `sndio` on FreeBSD. It must have direct access to the output device, which must be a real sound card capable of working with 44,100, 88,200 or 176,400 samples per second, interleaved PCM stereo of 8, 16, 24 or 32 bits. The default is 44,100 samples per second / 16 bits (you'll get a message in the logfile if there's a problem).
-Shairport Sync works well with PulseAudio, a widely used sound server found on many desktop Linuxes. While the timing and synchronsiation information is not as accurate as with `alsa` or `sndio`, removing or disabling PulseAudio so that Shairport Sync can have direct access to a sound card via `alsa` is often impractical.
+Shairport Sync works well with PulseAudio, a widely used sound server found on many desktop Linuxes. While the timing and synchronsiation information is not as accurate as that of `alsa` or `sndio`, removing or disabling PulseAudio so that Shairport Sync can have direct access to a sound card via `alsa` is often impractical.
For more about the motivation behind Shairport Sync, please see the wiki at https://github.com/mikebrady/shairport-sync/wiki.
Status
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-Shairport Sync works on a wide variety of Linux devices and FreeBSD. It works on standard Ubuntu laptops, on the Raspberry Pi with Raspbian Wheezy and Jessie, Arch Linux and OpenWrt, and it runs on a Linksys NSLU2 and a TP-Link 710N using OpenWrt. It works with built-in audio and with a variety of USB-connected audio amplifiers and DACs, including a cheapo USB "3D Sound" dongle, a first generation iMic and a Topping TP30 amplifier with a USB DAC input. It will not work properly — if at all — with a PulseAudio (pseudo-)output device.
+Shairport Sync works on a wide variety of Linux devices and FreeBSD. It works on standard Ubuntu laptops, on the Raspberry Pi with Raspbian Wheezy and Jessie, Arch Linux and OpenWrt, and it runs on a Linksys NSLU2 and a TP-Link 710N using OpenWrt. It works with built-in audio and with a variety of USB-connected audio 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 Sync runs well on the Raspberry Pi on USB and I2S cards. It can drive the built-in sound card – see the note below on configuring the Raspberry Pi to make best use of it.
+Shairport Sync will work with PulseAudio, which is installed in many desktop Linuxes.
-At the time of writing, OpenWrt trunk does not support USB audio well on the Raspberry Pi.
+Shairport Sync runs well on the Raspberry Pi on USB and I2S cards. It can drive the built-in sound card – see the note below on configuring the Raspberry Pi to make best use of it.
Shairport Sync runs natively on FreeBSD using the `sndio` sound system.
Shairport Sync runs on Ubuntu, OpenWrt, Debian, Arch Linux, Fedora and FreeBSD inside VMWare Fusion on a Mac, but synchronisation in inaccurate — possibly because the sound card is being emulated.
-Shairport Sync will route audio to a PulseAudio sound server, which is installed in many desktop Linuxes. It appears as another audio "Application".
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-Shairport Sync will output to `alsa` and `sndio` cards, to standard output and to pipes using appropriate backends. You can try compiling additional backends in as you wish, but it definitely will not work properly with them. Maybe someday...
+In addition, Shairport Sync can output to standard output, pipes, `soundio` and `ao` devices using appropriate backends.
For information about changes and updates, please refer to the RELEASENOTES.md file in the distribution.
Building And Installing the Development Version
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-The following procedures will install the shairport-sync application into your system. Before continuing, you should check to see if shairport-sync is already installed – you can use the command `$ which shairport-sync` to find where it is located, if installed. If it is installed you should delete it – you may need superuser privileges. After deleting, check again in case further copies are installed elsewhere.
+The following procedures will install the `shairport-sync` application into your system. Before continuing, you should check to see if `shairport-sync` is already installed – you can use the command `$ which shairport-sync` to find where it is located, if installed. If it is installed you should delete it – you may need superuser privileges. After deleting, check again in case further copies are installed elsewhere.
To build Shairport Sync from sources on FreeBSD please refer to [FREEBSD.md](https://github.com/mikebrady/shairport-sync/blob/development/FREEBSD.md).
projects / thirdparty / shairport-sync.git / commitdiff
