Linux.com Article DB: Linux Gaming APIs: Chapter 3 of PROGRAMMING LINUX GAMES

Audio APIs

Linux supports most of today's sound cards. There are two competing standards for kernel-level sound support -- OSS and ALSA -- but fortunately neither is difficult to work with, and games commonly support both.

OSS

The Open Sound System (OSS) is the original sound-programming interface for Linux. Maintained by 4Front Technologies, OSS provides a consistent kernel-based interface to sound hardware. Its API is not especially pretty, but if you close your eyes and pretend you're doing something fun you can almost forget about it.

OSS supports most of today's sound cards, but some of the newer drivers are not free and require a commercial OSS license. The free portions of OSS (OSS/Free) are included in the Linux kernel (and are no longer directly maintained by 4Front).

There are two types of OSS programs: "nice" and "rude." Nice OSS programs are likely to work on just about anything that remotely claims to be OSS compatible, including vendor-supplied drivers, FreeBSD's sound system, and ALSA's OSS emulation module. In fact, most OSS programs are basically nice. Rude OSS programs do unusual things with the driver, such as memory-mapping the driver's DMA buffer. While the maintainers of OSS discourage this, some people do it anyway (Quake 3 is a notable example). We'll discuss a variety of OSS programming techniques in Chapter 5.

More information on OSS is available from 4Front Technologies athttp://www.4front.com.

ALSA

Advanced Linux Sound Architecture (ALSA) is a community project that seeks to surpass OSS in all areas. The ALSA team has created a complete set of kernel-level sound card drivers, an easy-to-use programming interface, and a facility for emulating OSS. ALSA is not without its fair share of quirks, but it is a viable alternative to OSS for sound support and, with few exceptions, games that support OSS are also compatible with ALSA. It would be good to see ALSA grow in popularity since it has a lot of functionality and a lot of promise. The only serious problem with ALSA is that it is somewhat of a moving target; its API changes frequently. For more information on ALSA, visit http://www.alsa-project.org. We'll address ALSA programming in Chapter 5.

ESD

The Enlightened Sound Daemon (ESD, also called EsounD) is a sound server that allows multiple applications to share a single sound card. ESD-aware applications send their sound streams to ESD, and ESD mixes them internally into a single output stream. Some people love ESD, and some hate it; it has its fair share of technical problems, but results are acceptable in most cases. The main problem with ESD (other than its bugginess and lack of documentation) is the basic fact that it takes time for audio data to travel over a network, and this results in a significant delay before sound actually gets to the soundcard. ESD currently uses a fixed-sized buffer, regardless of the type of network or sound card. This latency can be rather disruptive for gameplay, but it's usually not a problem for music playback and other things that don't need to be precisely timed.

Recently some sound card drivers have started to support multiple device opens; that is, the driver allows multiple programs to use the sound card at once. This renders ESD more or less obsolete, but these drivers are in the minority right now.

ESD is an excellent software package, but programming information is very sparse, other than a few spare comments in the header file and various ESD-enabled projects that users have written. We will cover the basics of ESD programming in Chapter 5.

OpenAL

The Open Audio Library (OpenAL) is an environmental 3D audio library that supports just about every major platform. It aims to provide an open replacement for proprietary (and generally incompatible) 3D audio systems such as EAX and A3D. OpenAL can add realism to a game by simulating attenuation (degradation of sound over distance), the Doppler effect (change in frequency as a result of motion), and material densities. OpenAL has been used in several Linux game ports, including Heavy Gear II and Sid Meier's Alpha Centauri. The OpenAL web site is http://www.openal.org, and we will cover its API in Chapter 5.

Originally Published: Tuesday, 26 June 2001	Author: John Hall and Loki Software
Published to: develop_articles/Development Articles	Page: 4/6 - [Printable]
Linux Gaming APIs: Chapter 3 of PROGRAMMING LINUX GAMES Today's Linux.com article, derived from Chapter 3 of the book Programming Linux Games by John Hall, takes a look at the various Linux gaming APIs you can use to construct your game. Don't reinvent the wheel, and don't compromise your open source roots, and check out this excerpt from No Starch Press.

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Audio APIs Linux supports most of today's sound cards. There are two competing standards for kernel-level sound support -- OSS and ALSA -- but fortunately neither is difficult to work with, and games commonly support both. OSS The Open Sound System (OSS) is the original sound-programming interface for Linux. Maintained by 4Front Technologies, OSS provides a consistent kernel-based interface to sound hardware. Its API is not especially pretty, but if you close your eyes and pretend you're doing something fun you can almost forget about it. OSS supports most of today's sound cards, but some of the newer drivers are not free and require a commercial OSS license. The free portions of OSS (OSS/Free) are included in the Linux kernel (and are no longer directly maintained by 4Front). There are two types of OSS programs: "nice" and "rude." Nice OSS programs are likely to work on just about anything that remotely claims to be OSS compatible, including vendor-supplied drivers, FreeBSD's sound system, and ALSA's OSS emulation module. In fact, most OSS programs are basically nice. Rude OSS programs do unusual things with the driver, such as memory-mapping the driver's DMA buffer. While the maintainers of OSS discourage this, some people do it anyway (Quake 3 is a notable example). We'll discuss a variety of OSS programming techniques in Chapter 5. More information on OSS is available from 4Front Technologies athttp://www.4front.com. ALSA Advanced Linux Sound Architecture (ALSA) is a community project that seeks to surpass OSS in all areas. The ALSA team has created a complete set of kernel-level sound card drivers, an easy-to-use programming interface, and a facility for emulating OSS. ALSA is not without its fair share of quirks, but it is a viable alternative to OSS for sound support and, with few exceptions, games that support OSS are also compatible with ALSA. It would be good to see ALSA grow in popularity since it has a lot of functionality and a lot of promise. The only serious problem with ALSA is that it is somewhat of a moving target; its API changes frequently. For more information on ALSA, visit http://www.alsa-project.org. We'll address ALSA programming in Chapter 5. ESD The Enlightened Sound Daemon (ESD, also called EsounD) is a sound server that allows multiple applications to share a single sound card. ESD-aware applications send their sound streams to ESD, and ESD mixes them internally into a single output stream. Some people love ESD, and some hate it; it has its fair share of technical problems, but results are acceptable in most cases. The main problem with ESD (other than its bugginess and lack of documentation) is the basic fact that it takes time for audio data to travel over a network, and this results in a significant delay before sound actually gets to the soundcard. ESD currently uses a fixed-sized buffer, regardless of the type of network or sound card. This latency can be rather disruptive for gameplay, but it's usually not a problem for music playback and other things that don't need to be precisely timed. Recently some sound card drivers have started to support multiple device opens; that is, the driver allows multiple programs to use the sound card at once. This renders ESD more or less obsolete, but these drivers are in the minority right now. ESD is an excellent software package, but programming information is very sparse, other than a few spare comments in the header file and various ESD-enabled projects that users have written. We will cover the basics of ESD programming in Chapter 5. OpenAL The Open Audio Library (OpenAL) is an environmental 3D audio library that supports just about every major platform. It aims to provide an open replacement for proprietary (and generally incompatible) 3D audio systems such as EAX and A3D. OpenAL can add realism to a game by simulating attenuation (degradation of sound over distance), the Doppler effect (change in frequency as a result of motion), and material densities. OpenAL has been used in several Linux game ports, including Heavy Gear II and Sid Meier's Alpha Centauri. The OpenAL web site is http://www.openal.org, and we will cover its API in Chapter 5.
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