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A sound server is software that manages the use of and access to audio devices (usually a sound card). It commonly runs as a background process.
In a Unix-like operating system, a sound server mixes different data streams and sends out a single unified audio to an output device. The mixing is usually done by software, or by hardware if there is a supported sound card.
The "sound stack" can be visualized as follows, with programs in the upper layers calling elements in the lower layers:
Sound servers appeared in Unix-like operating systems after limitations in Open Sound System were recognized. OSS is a basic sound interface that was incapable of playing multiple streams simultaneously, dealing with multiple sound cards, or streaming sound over the network.
A sound server can provide these features by running as a daemon. It receives calls from different programs and sound flows, mixes the streams, and sends raw audio out to the audio device.
With a sound server, users can also configure global and per-application sound preferences.
As of 2012 [update] there are multiple sound servers; some focus on providing very low latency, while others concentrate on features suitable for general desktop systems. While diversification allows a user to choose just the features that are important to a particular application, it also forces developers to accommodate these options by necessitating code that is compatible with the various sound servers available. Consequently, this variety has resulted in a desire for a standard API to unify efforts.
In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and other computer programs to access hardware functions without needing to know precise details about the hardware being used.
GNU Hurd is the multiserver microkernel written as part of GNU. It has been under development since 1990 by the GNU Project of the Free Software Foundation, designed as a replacement for the Unix kernel, and released as free software under the GNU General Public License. When the Linux kernel proved to be a viable solution, development of GNU Hurd slowed, at times having slipped intermittently between stasis and renewed activity and interest.
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Advanced Linux Sound Architecture (ALSA) is a software framework and part of the Linux kernel that provides an application programming interface (API) for sound card device drivers.
The Open Sound System (OSS) is an interface for making and capturing sound in Unix and Unix-like operating systems. It is based on standard Unix devices system calls. The term also sometimes refers to the software in a Unix kernel that provides the OSS interface; it can be thought of as a device driver for sound controller hardware. The goal of OSS is to allow the writing of sound-based applications that are agnostic of the underlying sound hardware.
JACK Audio Connection Kit is a professional sound server API and pair of daemon implementations to provide real-time, low-latency connections for both audio and MIDI data between applications. JACK was developed by a community of open-source developers led by Paul Davis and has been a key piece of infrastructure and the de facto standard for professional audio software on Linux since its inception in 2002. The server is free software, licensed under the GNU GPL, while the library is licensed under the more permissive GNU LGPL.
freedesktop.org (fd.o) is a project to work on interoperability and shared base technology for free-software desktop environments for the X Window System (X11) and Wayland on Linux and other Unix-like operating systems. It was founded by Havoc Pennington from Red Hat in March 2000. The project's servers are hosted by Portland State University, which in turn are sponsored by HP, Intel and Google.
A digital audio workstation (DAW) is an electronic device or application software used for recording, editing and producing audio files. DAWs come in a wide variety of configurations from a single software program on a laptop, to an integrated stand-alone unit, all the way to a highly complex configuration of numerous components controlled by a central computer. Regardless of configuration, modern DAWs have a central interface that allows the user to alter and mix multiple recordings and tracks into a final produced piece.
aRts is an audio framework that is no longer under development. It was best known for previously being used in K Desktop Environment 2 and 3 to simulate an analog synthesizer.
Audio Stream Input/Output (ASIO) is a computer sound card driver protocol for digital audio specified by Steinberg, providing a low-latency and high fidelity interface between a software application and a computer's sound card. Whereas Microsoft's DirectSound is commonly used as an intermediary signal path for non-professional users, ASIO allows musicians and sound engineers to access external hardware directly.
In computing, the Enlightened Sound Daemon was the sound server for Enlightenment and GNOME. Esound is a small sound daemon for both Linux and UNIX. ESD was created to provide a consistent and simple interface to the audio device, so applications do not need to have different driver support written per architecture. It was also designed to enhance capabilities of audio devices such as allowing more than one application to share an open device. ESD accomplishes these things while remaining transparent to the application, meaning that the application developer can simply provide ESD support and let it do the rest. On top of this, the API is designed to be very similar to the current audio device API, making it easy to port to ESD.
The Linux kernel provides several interfaces to user-space applications that are used for different purposes and that have different properties by design. There are two types of application programming interface (API) in the Linux kernel that are not to be confused: the "kernel–user space" API and the "kernel internal" API.
NetworkManager is a daemon that sits on top of libudev and other Linux kernel interfaces and provides a high-level interface for the configuration of the network interfaces.
PulseAudio is a network-capable sound server program distributed via the freedesktop.org project. It runs mainly on Linux, various BSD distributions such as FreeBSD and OpenBSD, macOS, as well as Illumos distributions and the Solaris operating system.
The XTS-400 is a multilevel secure computer operating system. It is multiuser and multitasking that uses multilevel scheduling in processing data and information. It works in networked environments and supports Gigabit Ethernet and both IPv4 and IPv6.
HAL is a software subsystem for UNIX-like operating systems providing hardware abstraction.
Jami is a SIP-compatible distributed peer-to-peer softphone and SIP-based instant messenger for Linux, Microsoft Windows, OS X, iOS, and Android. Jami was developed and maintained by the Canadian company Savoir-faire Linux, and with the help of a global community of users and contributors, Jami positions itself as a potential free Skype replacement.
Celerra is a discontinued NAS device produced by EMC Corporation, available either as an integrated unit or as a NAS header which can be added to an independent EMC storage array such as a Clariion or a Symmetrix. It supports the SMB, NFS, FTP, NDMP, TFTP and MPFS protocols. A Celerra Unified Storage device use Clariion storage array as its storage layer and also provides iSCSI and Fibre Channel block-level storage.
sndio is the software layer of the OpenBSD operating system that manages sound cards and MIDI ports. It provides an optional sound server and a documented application programming interface to access either the server or the audio and MIDI hardware in a uniform way. sndio is designed to work for desktop applications, but pays special attention to synchronization mechanisms and reliability required by music applications.