Design
PulseAudio is a sound server, a background process accepting sound input from one or more sources (processes or capture devices) and redirecting it to one or more sinks (sound cards, remote network PulseAudio servers, or other processes).
One of the goals of PulseAudio is to reroute all sound streams through it, including those from processes that attempt to directly access the hardware (like legacy OSS applications). PulseAudio achieves this by providing adapters to applications using other audio systems, like aRts and ESD.
In a typical installation scenario under Linux, the user configures ALSA to use a virtual device provided by PulseAudio. Thus, applications using ALSA will output sound to PulseAudio, which then uses ALSA itself to access the real sound card. PulseAudio also provides its own native interface to applications that want to support PulseAudio directly, as well as a legacy interface for ESD applications, making it suitable as a drop-in replacement for ESD.
For OSS applications, PulseAudio provides the padsp utility, which replaces device files such as /dev/dsp, tricking the applications into believing that they have exclusive control over the sound card. In reality, their output is rerouted through PulseAudio.
Few programs cannot communicate with PulseAudio:
Sound source ⟾ ALSA soundserver ⟾ PulseAudio ⟾ ALSA driver ⟾ hardware
Most can:
Sound source ⟾ PulseAudio ⟾ ALSA driver ⟾ Hardware
PulseAudio is network capable:
Sound source ⟾ PulseAudio ⟾ network ⟾ PulseAudio ⟾ ALSA driver ⟾ hardware
A program can circumvent PulseAudio and communicate directly with the soundcard driver:
Sound source ⟾ ALSA driver ⟾ Hardware
A program can circumvent PulseAudio and communicate with the ALSA soundserver
Sound source ⟾ ALSA soundserver ⟾ ALSA driver ⟾ hardware
Read more about this topic: Pulse Audio
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