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  • DevynCJohnson
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    • @devyncjohnson

    Many Linux users have probably heard of PulseAudio, but many users may not completely understand why it is need or specifically how it works. PulseAudio is a part of the sound system and helps to make all of the sound components work together as you will see.

    NOTE: This article will explain how the sound system works. This article does not explain installation or command-line parameters.

    There are many ways to send sound from one location to another. Sound may come from a process/application (like Clementine, VLC, or a video game), the network (like a meeting with a friend over the Internet), from a microphone, or other source. Then, the sound may be sent to a file, over the network, to a speaker, to headphones, etc. How can Linux properly and accurately ensure sound is sent from point A to point B? The answer is PulseAudio (PulseAudio.org). PulseAudio is a daemon that acts as a proxy server for sound. All sound goes through PulseAudio. PulseAudio will then determine where the sound is meant to go. When a process makes a sound (like your music or video game), the application does not send sound straight to the speaker. Rather, it is sent to PulseAudio. If PulseAudio were to be killed with a kill command, then you would lose your sound. However, PulseAudio tends to restart itself on many systems, or users may have an alternative to PulseAudio.

    Once PulseAudio receives the sound, what happens next? Well, once PulseAudio has a sound destined for the speaker or headphones, PulseAudio will send the sound to Advanced Linux Sound Architecture (ALSA) (www.alsa-project.org). ALSA is a sound API for the Linux kernel. ALSA runs in kernel-space and contains many sound drivers and features. Users can install various applications that allow them to control the audio system for various reasons (like JACK {http://jackaudio.org/}). AlsaMixer is an interface to ALSA used to configure sound and volume (http://alsa.opensrc.org/Alsamixer).

    After ALSA makes any modifications to the sound (due to mixers and such), the drivers perform their low-level operations with the hardware. The sound is then sent to the desired destination. If PulseAudio sends sound over the network, then the client's ALSA process will perform such operations.

    Some users have the Open Sound System (OSS) instead of ALSA. However, ALSA is the newest and most recommended sound-interface/sound-architecture for the kernel.

    Sound can be sent via Bluetooth (like for headphones) thanks to the "bluetooth-alsa" software (http://bluetooth-alsa.sourceforge.net/).

    As with anything, there are exceptions. Processes can send sound directly to ALSA, thus bypassing PulseAudio. Alternately, processes can send data to ALSA's own sound server. However, most systems use a "PulseAudio to ALSA to hardware" approach.

    This is a little off topic, but I am sure some of you have thought of this while reading this article. So, I will briefly inform you on some related software. GStreamer is a multimedia-framework that provides many codecs and helps link various sound components. Phonon is like a Qt/KDE alternative to Gstreamer (both of which have entirely different code and teams of coders).

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