Hardware and Software Communication

• When you tell your computer to do something, your instructions pass through several levels of software at lightning speed. Your keystroke tells an application program, like a word processor or Web browser, what you want to do. Your program passes instructions through the operating system, a sort of master program, which then delegates the work to a secondary program responsible for specific functions such as audio. That program in turn interacts with other programs that can work directly with the chips and boards in your computer. Segmenting the software this way makes each segment easier to write and troubleshoot.
  
  

ALSA

• ALSA is an acronym for the Advanced Linux Sound System. Essentially, ALSA takes responsibility for finding and communicating with the computer's underlying audio hardware, whatever it might be. For developers of audio software, this means they don't need to find ways for their program to work with hundreds of different sound cards. They simply write for ALSA, and ALSA takes care of the rest. ALSA replaced the older Open Sound System audio system (OSS) beginning with version 2.6 of the Linux kernel.
  
  

PulseAudio

• PulseAudio acts at a higher level, placing itself between ALSA and end-user applications such as movie or music players. It is referred to as a sound server, meaning that it can use its position between the hardware and application software to perform a wide variety of functions. For example, PulseAudio can route audio playback to a network or another computer rather than the one the sound is being played back on.
  
  

PulseAudio Functions

• One of the most common and most appreciated functions of PulseAudio is its ability to set and remember audio levels independently for various programs. For example, you might want your inbox to sound a gentle chime when you receive an email but have your movie player start at a relatively high volume. PulseAudio will do that. It also allows users to mix multiple audio streams into one recording device. For example, you can transfer a video from your camera, add narration through a microphone and add a quiet soundtrack of classical music.