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Shairport Sync offers full audio synchronisation, a feature of AirPlay that previous implementations do not provide. 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 – that provide very accurate timing information about output devices. Shairport Sync must have direct access to the output device used, 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).
Alternatively, Shairport Sync works well with PulseAudio, a widely used sound server found on many desktop Linuxes. While the timing information is not as accurate as that of alsa or sndio, it is often impractical to remove or disable PulseAudio. In that case, the pa backend can be used.
For other use cases, Shairport Sync can provide synchronised audio output to a unix pipe or to standard output, 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.
For more about the motivation behind Shairport Sync, please see the wiki at https://github.com/mikebrady/shairport-sync/wiki.
Synchronisation, Latency, "Stuffing"
The AirPlay protocol uses an agreed latency – the time difference, 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). The latency to be used is specified by the audio source when it negotiates with Shairport Sync.
As mentioned previously, Shairport Sync implements full audio synchronisation when used with alsa, sndio or PulseAudio systems. This is done by monitoring the timestamps present in data coming from the audio source and the timing information from the audio system, e.g. alsa. To maintain the latency required for exact synchronisation, if the 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”.
Stuffing 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, 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 either the PTP or NTP synchronisation protocols.
What else?
- 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.
- Hardware Mute — Shairport Sync can mute properly if the hardware supports it.
- Support for the Apple ALAC decoder.
- Output bit depths of 8, 16, 24 and 32 bits, rather than the standard 16 bits.
- Output frame rates of 44,100, 88,200, 176,000 or 352,000 frames per second.
- Fast Response — With hardware volume control, response is instantaneous; otherwise the response time is 0.20 seconds with
alsa, 0.35 seconds withsndio. - 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.
- Compiles on Linux and FreeBSD.
- Outputs to
alsa,sndio, PulseAudio, JACK, to a unix pipe or toSTDOUT. It also has limited support for libao and forsoundio. - An MPRIS interface, partially complete and very functional, including access to metadata and artwork, and some limited remote control.
- An interface to MQTT, an often-used protocol in home automation projects.
- A native D-Bus interface, including access to metadata and artwork, some limited remote control and some system settings.
Heritage
Shairport Sync is a substantial rewrite of the fantastic work done in Shairport 1.0 by James Laird and others — please see https://github.com/abrasive/shairport/blob/master/README.md#contributors-to-version-1x for a 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.
Configuring Shairport Sync
To get the best from Shairport Sync, you’ll need to (a) give Shairport Sync a service name by which it will be seen in iTunes etc. and (b) specify the backend you wish to use - alsa for the ALSA backend, or pa for the PulseAudio back end, etc. If only one backend is included at compilation, or if the backend is ALSA, there is no need to explicitly specify the backend.
For the ALSA backend you may need to (c) specify the output device to use and (d) specify the name of the mixer volume control to use to control the output level. To get values for (c) and (d) you might need to explore the ALSA output devices with a program like alsamixer or similar.
Shairport Sync reads settings from a configuration file at /etc/shairport-sync.conf (note that in FreeBSD it will be at /usr/local/etc/shairport-sync.conf). When you run $sudo make install, a sample configuration file is installed or updated at /etc/shairport-sync.conf.sample (/usr/local/etc/shairport-sync.conf.sample in FreeBSD). This contains all the setting groups and all the settings available, but they all are commented out (comments begin with //) so that default values are used. The file contains explanations of the settings, useful hints and suggestions. In addition, if the file doesn't already exist, a default configuration is installed, which should work in almost any system with a sound card.
Settings in the configuration file are grouped. For instance, there is a general group within which you can use the name tag to set the service name. Suppose you wanted to set the name of the service to Front Room asd give the service the password secret, then you should do the following:
general =
{
name = "Front Room";
password = "secret";
// ... other general settings
};
(Remember, anything preceded by // is a comment and will have no effect on the setting of Shairport Sync.)
Important: You should never use an important password as the AirPlay password for a Shairport Sync player -- the password is stored in Shairport Sync's configuration file in plain text and is thus completely vulnerable.
No backend is specified here, so it will default to the alsa backend if more than one back end has been compiled. To route the output to PulseAudio, add:
output_backend = "pa";
to the general group.
The alsa group is used to specify properties of the output device. The most obvious setting is the name of the output device which you can set using the output_device tag.
The following alsa group settings are very important for maximum performance. If your audio device has a mixer that can be use to control the volume, then Shairport Sync can use it to give instant response to volume and mute commands and it can offload some work from the processor.
- The
mixer_control_nametag allows you to specify the name of the mixer volume control. - The
mixer_devicetag allows you specify where the mixer is. By default, the mixer is on theoutput_device, so you only need to use themixer_devicetag if the mixer is elsewhere. This can happen if you specify a device rather than a card with theoutput_devicetag, because normally a mixer is associated with a card rather than a device. Suppose you wish to use the output device5of cardhw:0and the mixer volume-control namedPCM:
alsa =
{
output_device = "hw:0,5";
mixer_device = "hw:0";
mixer_control_name = "PCM";
// ... other alsa settings
};
The pa group is used to specify settings relevant to the PulseAudio backend. You can set the "Application Name" that will appear in the "Sound" control panel.
Shairport Sync can run programs just before it starts to play an audio stream and just after it finishes. You specify them using the sessioncontrol group settings run_this_before_play_begins and run_this_after_play_ends. This is to facilitate situations where something has to be done before and after playing, e.g. switching on an amplifier beforehand and switching it off afterwards. Set the wait_for_completion value to "yes" for Shairport Sync to wait until the respective commands have been completed before continuing.
Note that the full path to the programs must be specified, and script files will not be executed unless they are marked as executable and have the appropriate shebang #!/bin/... as the first line. (This behaviour may be different from other Shairports.)
Shairport Sync can run a program whenever the volume is set or changed. You specify it using the general group setting run_this_when_volume_changes. This is to facilitate situations where something has to be done when the volume changes, e.g. adjust an external mixer value. Set the wait_for_completion value to "yes" for Shairport Sync to wait until the command has been completed before continuing. Again, please note that the full path to the program must be specified, and script files will not be executed unless they are marked as executable and have the appropriate shebang #!/bin/... as the first line.
Note: Shairport Sync can take configuration settings from command line options. This is mainly for backward compatibility, but sometimes still useful. For normal use, it is strongly recommended that you use the configuration file method.
Raspberry Pi
Many Raspberry Pi Models have a built-in audio DAC that is connected to the device's headphone jack. Apart from a loud click when used for the first time after power-up, it is now quite adequate for casual listening.
To make Shairport Sync output to the built-in audio DAC and use its hardware mixer, in the alsa section of the configuration file, set the output device and mixer as follows:
alsa =
{
output_device = "hw:0"; // the name of the alsa output device. Use "alsamixer" or "aplay" to find out the names of devices, mixers, etc.
mixer_control_name = "Headphone"; // the name of the mixer to use to adjust output volume. If not specified, volume in adjusted in software.
// ... other alsa settings
(Remember to uncomment the lines by removing the // at the start of each.) When these changes have been made, reboot the machine.
A problem with the built-in DAC that it declares itself to have a very large mixer volume control range – all the way from -102.38dB up to +4dB, a range of 106.38 dB. In reality, only the top 60 dB of it is in any way usable. To help get the most from it, consider using the volume_range_db setting in the general group to instruct Shairport Sync to use the top of the DAC mixer's declared range. For example, if you set the volume_range_db figure to 60, the top 60 dB of the range will the used. With this setting on the Raspberry Pi, maximum volume will be +4dB and minimum volume will be -56dB, below which muting will occur.
From a user's point of view, the effect of using this setting is to move the minimum usable volume all the way down to the bottom of the user's volume control, rather than have the minimum usable volume concentrated very close to the maximum volume.
Examples
Here are some examples of complete configuration files.
general = {
name = "Joe's Stereo";
};
alsa = {
output_device = "hw:0";
};
This gives the service a particular name — "Joe's Stereo" and specifies that audio device hw:0 be used.
For best results with the alsa backend — including getting true mute and instant response to volume control and pause commands — you should access the hardware volume controls. Use amixer or alsamixer or similar to discover the name of the mixer control to be used as the mixer_control_name.
Here is an example for for a Raspberry Pi using its internal soundcard — device hw:0 — that drives the headphone jack:
general = {
name = "Mike's Boombox";
};
alsa = {
output_device = "hw:0";
mixer_control_name = "Headphone";
};
Here is an example of driving a Topping TP30 Digital Amplifier, which has an integrated USB DAC and which is connected as audio device hw:1:
general = {
name = "Kitchen";
};
alsa = {
output_device = "hw:1";
mixer_control_name = "PCM";
};
For a cheapo "3D Sound" USB card (Stereo output and input only) on a Raspberry Pi:
general = {
name = "Front Room";
};
alsa = {
output_device = "hw:1";
mixer_control_name = "Speaker";
};
Finally, here is an example of using the PulseAudio backend:
general = {
name = "Zoe's Computer";
output_backend = "pa";
};
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.
Metadata broadcasting over UDP
As an alternative to sending metadata to a pipe, the socket_address and socket_port tags may be set in the metadata group to cause Shairport Sync to broadcast UDP packets containing the track metadata.
The advantage of UDP is that packets can be sent to a single listener or, if a multicast address is used, to multiple listeners. It also allows metadata to be routed to a different host. However UDP has a maximum packet size of about 65000 bytes; while large enough for most data, Cover Art will often exceed this value. Any metadata exceeding this limit will not be sent over the socket interface. The maximum packet size may be set with the socket_msglength tag to any value between 500 and 65000 to control this - lower values may be used to ensure that each UDP packet is sent in a single network frame. The default is 500. Other than this restriction, metadata sent over the socket interface is identical to metadata sent over the pipe interface.
The UDP metadata format is very simple - the first four bytes are the metadata type, and the next four bytes are the metadata code (both are sent in network byte order - see https://github.com/mikebrady/shairport-sync-metadata-reader for a definition of those terms). The remaining bytes of the packet, if any, make up the raw value of the metadata.
Latency
Latency is the exact time from a sound signal's original timestamp until that signal actually "appears" on the output of the audio output device, usually a Digital to Audio Converter (DAC), irrespective of any internal delays, processing times, etc. in the computer.
Problems can arise when you are trying to synchronise with speaker systems — typically surround-sound home theatre systems — that have their own inherent delays. You can compensate for an inherent delay using the appropriate backend (typically alsa) audio_backend_latency_offset_in_seconds. Set this offset to compensate for a fixed delay in the audio back end; for example, if the output device delays by 100 ms, set this to -0.1.
Resynchronisation
Shairport Sync actively maintains synchronisation with the source. If synchronisation is lost — say due to a busy source or a congested network — Shairport Sync will mute its output and resynchronise. The loss-of-sync threshold is a very conservative 0.050 seconds — i.e. the actual time and the expected time must differ by more than 50 ms to trigger a resynchronisation. Smaller disparities are corrected by insertions or deletions, as described above.
- You can vary the resync threshold, or turn resync off completely, with the
generalresync_threshold_in_secondssetting.
Tolerance
Playback synchronisation is allowed to wander — to "drift" — a small amount before attempting to correct it. The default is 0.002 seconds, i.e. 2 ms. The smaller the tolerance, the more likely it is that overcorrection will occur. Overcorrection is when more corrections (insertions and deletions) are made than are strictly necessary to keep the stream in sync. Use the statistics setting to monitor correction levels. Corrections should not greatly exceed net corrections.
- You can vary the tolerance with the
generaldrift_tolerance_in_secondssetting.
Some Statistics
If you turn on the general statistics setting, a heading like this will be output to the log file (or to STDERR if the -u command line option is chosen):
sync error in milliseconds, net correction in ppm, corrections in ppm, total packets, missing packets, late packets, too late packets, resend requests, min DAC queue size, min buffer occupancy, max buffer occupancy, source nominal frames per second, source actual frames per second, output frames per second, source clock drift in ppm, source clock drift sample count, rough calculated correction in ppm
This will be followed by the statistics themselves at regular intervals, for example:
0.15, 0.0, 0.0, 1003, 0, 0, 0, 0, 8539, 208, 225, 44099.95, 44274.21, 44099.68, 0.00, 0, -6.12
0.34, 0.0, 0.0, 2006, 0, 0, 0, 0, 8591, 206, 225, 44099.99, 44169.96, 44099.76, 0.00, 0, -5.27
0.89, 0.0, 0.0, 3009, 0, 0, 0, 0, 8571, 218, 225, 44100.01, 44152.81, 44099.85, 0.00, 0, -3.66
0.85, 0.0, 0.0, 4012, 0, 0, 0, 0, 8619, 218, 225, 44100.00, 44142.80, 44099.80, 0.00, 0, -4.62
1.29, 0.0, 0.0, 5015, 0, 0, 0, 0, 8623, 218, 225, 44100.00, 44129.39, 44099.83, 0.00, 0, -3.85
1.41, 0.0, 0.0, 6018, 0, 0, 0, 0, 8619, 216, 225, 44099.99, 44127.74, 44099.86, 0.00, 0, -2.76
1.53, 0.0, 0.0, 7021, 0, 0, 0, 0, 8603, 218, 225, 44100.00, 44109.60, 44099.81, 0.00, 0, -4.21
1.81, 0.0, 0.0, 8024, 0, 0, 0, 0, 8655, 215, 225, 44100.00, 44110.01, 44099.82, 0.00, 0, -4.16
2.00, -82.1, 82.1, 9027, 0, 0, 0, 0, 8481, 208, 225, 44100.00, 44110.69, 44099.83, 0.00, 0, -3.97
1.96, -8.5, 8.5, 10030, 0, 0, 0, 0, 8599, 209, 225, 44099.99, 44108.84, 44099.86, 0.00, 0, -3.02
1.94, 0.0, 0.0, 11033, 0, 0, 0, 0, 8583, 217, 225, 44100.00, 44109.33, 44099.87, 0.00, 0, -2.93
1.90, -5.7, 5.7, 12036, 0, 0, 0, 0, 8601, 219, 225, 44100.00, 44110.01, 44099.85, 20.47, 10, -23.88
2.02, -36.8, 36.8, 13039, 0, 0, 0, 0, 8488, 216, 225, 44100.00, 44107.18, 44099.85, 20.39, 12, -23.83
1.96, -17.0, 17.0, 14042, 0, 0, 0, 0, 8554, 218, 226, 44100.00, 44108.80, 44099.84, 20.63, 15, -24.13
1.99, -31.2, 31.2, 15045, 0, 0, 0, 0, 8592, 216, 225, 44100.00, 44106.85, 44099.85, 20.95, 18, -24.39
1.98, -28.3, 28.3, 16048, 0, 0, 0, 0, 8627, 219, 225, 44099.99, 44109.01, 44099.88, 21.23, 20, -23.90
2.01, -31.2, 31.2, 17051, 0, 0, 0, 0, 8582, 217, 225, 44100.00, 44108.86, 44099.84, 21.57, 22, -25.10
1.96, -19.8, 19.8, 18054, 0, 0, 0, 0, 8556, 215, 226, 44100.00, 44103.79, 44099.86, 21.59, 24, -24.85
1.98, -25.5, 25.5, 19057, 0, 0, 0, 0, 8563, 219, 225, 44100.00, 44102.62, 44099.86, 22.26, 25, -25.47
2.00, -28.3, 28.3, 20060, 0, 0, 0, 0, 8569, 214, 225, 44100.00, 44103.22, 44099.88, 22.57, 27, -25.41
1.98, -42.5, 42.5, 21063, 0, 0, 0, 0, 8570, 216, 225, 44100.00, 44104.14, 44099.85, 22.74, 28, -26.08
1.89, -31.2, 31.2, 22066, 0, 0, 0, 0, 8524, 218, 225, 44100.00, 44104.66, 44099.87, 22.89, 30, -25.89
1.96, 0.0, 0.0, 23069, 0, 0, 0, 0, 8650, 218, 225, 44100.00, 44105.36, 44099.88, 22.92, 32, -25.73
1.98, -34.0, 34.0, 24072, 0, 0, 0, 0, 8475, 219, 225, 44100.00, 44106.04, 44099.88, 22.90, 34, -25.70
2.07, -138.8, 138.8, 25075, 0, 0, 0, 0, 8490, 218, 226, 44100.00, 44104.93, 44099.87, 23.87, 35, -26.92
1.33, 0.0, 0.0, 26078, 0, 0, 0, 0, 8617, 216, 225, 44100.00, 44104.66, 44099.88, 23.68, 38, -26.43
1.67, -22.7, 22.7, 27081, 0, 0, 0, 0, 8562, 217, 225, 44100.00, 44106.16, 44099.87, 25.06, 40, -27.93
1.98, -28.3, 28.3, 28084, 0, 0, 0, 0, 8544, 218, 225, 44100.00, 44105.21, 44099.87, 25.91, 41, -28.84
1.99, -22.7, 22.7, 29087, 0, 0, 0, 0, 8569, 219, 225, 44100.00, 44102.30, 44099.87, 25.91, 41, -28.74
1.66, -56.6, 56.6, 30090, 0, 0, 0, 0, 8554, 218, 225, 44100.00, 44103.07, 44099.86, 26.38, 44, -29.46
1.44, 0.0, 0.0, 31093, 0, 0, 0, 0, 8642, 218, 225, 44100.00, 44102.18, 44099.87, 26.08, 46, -28.98
1.56, 0.0, 0.0, 32096, 0, 0, 0, 0, 8647, 218, 225, 44100.00, 44102.76, 44099.87, 25.79, 48, -28.64
1.68, 0.0, 0.0, 33099, 0, 0, 0, 0, 8647, 217, 225, 44100.00, 44103.41, 44099.88, 25.66, 49, -28.51
1.92, -14.2, 14.2, 34102, 0, 0, 0, 0, 8639, 217, 225, 44100.00, 44102.38, 44099.87, 25.45, 51, -28.40
2.00, -45.3, 45.3, 35105, 0, 0, 0, 0, 8593, 216, 225, 44100.00, 44103.23, 44099.86, 25.35, 53, -28.61
1.98, -17.0, 17.0, 36108, 0, 0, 0, 0, 8609, 218, 225, 44100.00, 44102.54, 44099.87, 25.21, 55, -28.12
1.99, -31.2, 31.2, 37111, 0, 0, 0, 0, 8575, 217, 225, 44100.00, 44101.98, 44099.88, 25.11, 57, -27.88
1.99, -14.2, 14.2, 38114, 0, 0, 0, 0, 8624, 218, 225, 44100.00, 44104.44, 44099.87, 24.95, 60, -27.97
1.99, -28.3, 28.3, 39117, 0, 0, 0, 0, 8582, 209, 225, 44100.00, 44104.91, 44099.86, 24.88, 62, -27.98
1.86, -17.0, 17.0, 40120, 0, 0, 0, 0, 8504, 207, 225, 44100.00, 44104.52, 44099.88, 24.66, 63, -27.39
1.96, -56.6, 56.6, 41123, 0, 0, 0, 0, 8517, 212, 226, 44100.00, 44102.29, 44099.88, 24.54, 66, -27.34
1.90, -36.8, 36.8, 42126, 0, 0, 0, 0, 8363, 218, 240, 44100.00, 44116.79, 44099.87, 24.55, 68, -27.48
1.89, 0.0, 0.0, 43129, 0, 0, 0, 0, 8487, 227, 241, 44100.00, 44117.57, 44099.88, 24.56, 69, -27.32
1.99, -22.7, 22.7, 44132, 0, 0, 0, 0, 8576, 227, 240, 44100.00, 44118.53, 44099.87, 24.57, 70, -27.44
1.98, -39.7, 39.7, 45135, 0, 0, 0, 0, 8593, 228, 240, 44100.00, 44113.88, 44099.87, 24.60, 73, -27.43
2.00, -34.0, 34.0, 46138, 0, 0, 0, 0, 8577, 227, 240, 44100.00, 44117.10, 44099.88, 24.65, 75, -27.46
1.98, -22.7, 22.7, 47141, 0, 0, 0, 0, 8591, 204, 240, 44100.00, 44102.78, 44099.87, 24.67, 77, -27.59
1.98, -17.0, 17.0, 48144, 0, 0, 0, 0, 8587, 216, 225, 44100.00, 44102.38, 44099.88, 24.67, 79, -27.42
1.99, -28.3, 28.3, 49147, 0, 0, 0, 0, 8590, 214, 225, 44100.00, 44102.70, 44099.87, 24.66, 80, -27.57
1.85, -62.3, 62.3, 50150, 0, 0, 0, 0, 8518, 217, 226, 44100.00, 44103.16, 44099.87, 24.70, 83, -27.55
1.90, -28.3, 28.3, 51153, 0, 0, 0, 0, 8617, 216, 225, 44100.00, 44102.60, 44099.88, 24.66, 85, -27.42
1.89, -5.7, 5.7, 52156, 0, 0, 0, 0, 8615, 218, 225, 44100.00, 44103.37, 44099.88, 24.65, 86, -27.39
1.96, -19.8, 19.8, 53159, 0, 0, 0, 0, 8600, 219, 225, 44100.00, 44103.77, 44099.87, 24.62, 89, -27.53
1.99, -22.7, 22.7, 54162, 0, 0, 0, 0, 8567, 216, 225, 44100.00, 44103.12, 44099.87, 24.57, 92, -27.45
"Sync error in milliseconds" is the average deviation from exact synchronisation. The first line of the example above indicates that the output is on average 0.15 milliseconds behind exact synchronisation. Sync is allowed to drift by the general drift_tolerance_in_seconds setting — (± 0.002 seconds) by default — before a correction will be made.
"Net correction in ppm" 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 drift — the divergence between the rate at which frames are generated at the source and the rate at which the output device consumes them. In this case, the drift is negligible, but it can routinely be up to 150 ppm, especially with older machines.
"Corrections in ppm" 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 net corrections, the fewer "unnecessary" corrections are being made. Third party programs tend to have much larger levels of corrections.
"Min DAC queue size" is the minimum size the queue of samples in the output device's hardware buffer was measured at. It is meant to stand at 0.2 seconds = 8,820 frames at 44,100 frames per second, and will go low if the processor is very busy. If it goes below about 2,000 then it's an indication that the processor can't really keep up.
"Source nominal frames per second" is the rate at which audio is being sent to Shairport Sync according to information supplied by the source itself.
"Source actual frames per second" is the rate at which frames of audio are actually received by Shairport Sync. This can vary a great deal due to network conditions, but over a long time (more than 30 minutes) it should settle down to an accurate value. It does not account for retransmitted packets.
"Output frames per second" is the actual rate at which frames of audio are taken by the output device. On a system with a well-conditioned ntp-based clock (and without output underruns) this figure should be very accurate after playing material continuously for a period.
"Source clock drift in ppm" is an estimate of the difference in timekeeping between the audio source and the Shairport Sync device. It is calculated from a linear regression of drift sample data. The number of samples the estimate is based on is given in the next column, "Source clock drift sample count".
"Rough calculated correction in ppm" is a very crude estimate of the amount of interpolation that needs to applied, on average, to keep sync. It is not really to be relied on at this time.
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. The above sample is from an iPhone XS Max to Shairport Sync running on a WiFi-connected Raspberry Pi 3 using an IQaudIO Pi-DigiAMP+.
It's not unusual to have resend requests, late packets and even missing packets if some part of the connection to the Shairport Sync device is over WiFi. Late packets can sometimes be asked for and received multiple times. Sometimes late packets are sent and arrive too late, but have already been sent and received in time, so weren't needed anyway...
WiFi Issues
If you are using WiFi, you should ensure that WiFi power management is off. See TROUBLESHOOTING for more details.
Troubleshooting
Please refer to TROUBLESHOOTING for a few hints, contributed by users.
MQTT
Please refer to the MQTT INFO page for additional info on building, configuring and using MQTT to interface shairport-sync with common home automation systems (contributed by users).
Packaging status
