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+# Introduction
+Many Linux systems have [PulseAudio](https://www.freedesktop.org/wiki/Software/PulseAudio/) or [PipeWire](https://pipewire.org) installed as [sound servers](https://en.wikipedia.org/wiki/Sound_server), typically providing
+audio facilities for the system's GUI. Unfortunately, they cause problems for Shairport Sync. As you'll see, these problems can be worked around.
+
+The following remarks apply to PulseAudio and PipeWire, so, for simplicity, let's refer to PulseAudio only.
+
+To understand the problems, first consider a  Linux system that _does not_ have PulseAudio installed.
+1. Sound is managed by the Advanced Linux Sound Architecture ("[ALSA](https://www.alsa-project.org/wiki/Main_Page)") subsystem.
+2. The ALSA subsystem is loaded and becomes available at system startup.
+3. The ALSA `"default"` output device is mapped to the system's audio output DAC.
+
+Shairport Sync loads when system startup is complete and provides its service, routing audio to the ALSA default device.
+
+Now, consider a Linux system with a Graphical User Interface (GUI) that has PulseAudio installed.
+1. As before, sound is managed by the ALSA subsystem.
+2. As before, The ALSA subsystem is loaded and becomes available at system startup.
+3. PulseAudio loads and becomes a client of ALSA.
+4. The PulseAudio service becomes available after a user has logged in through the GUI. Importantly, if you don't log in through the GUI, you won't have a PulseAudio service.
+5. The ALSA `"default"` device no longer connects to a real output device. Instead audio sent to the `"default"` device is routed into PulseAudio. Importantly, if you have no PulseAudio service, then the ALSA default device either doesn't exist at all or goes nowhere.
+
+# The Problem
+When Shairport Sync is installed as a system service, it starts after the system has booted up.
+It runs under a low-priviliged user called `shairport-sync`. 
+Unfortunately, per (4) above, since it was not logged in through the GUI, it won't have a PulseAudio service.
+Therefore, per (5) above, the ALSA `"default"` device either won't exist or -- if it does exist -- won't work. Hence, Shairport Sync will either terminate
+or remain silent.
+
+# The Fixes
+There are three ways to address this problem:
+1. Stop using Shairport Sync as a system service. Instead, launch it from a terminal window after log-in through the GUI.
+Alternatively, create a user service startup script so that it will be launched automatically after the user has logged in. This means that the system can not run without user intervention.
+2. Stop using the ALSA `"default"` device. Instead, set the output device to a real ALSA hardware output device. Use `$ shairport-sync -h` (or, better, `sps-alsa-explore`) to get a list of output devices, and use the configuration file to set the device. Now this might prevent PulseAudio from working properly. 
+# The Best Fix
+3. The best of all fixes is, if possible, to avoid using a system containing PulseAudio altogether. Linux operating systems typically have a "server", "lite" or "headless" version that has no GUI and no PulseAudio.