The current major Linux sound servers are Enlightened Sound Daemon (ESD or EsounD) for Gnome, analog Real time synthesizer (aRts) for KDE2/3, and Advanced Linux Sound Architecture (ALSA), which works everywhere. Network Audio System (NAS) is a client/server networked sound system for thin clients. For applications that require OSS, ALSA, ESD, and aRts all include an OSS emulator. JACK is a popular professional-level low-latency audio server. One thing these all have in common is they require ALSA to provide the audio hardware drivers.
ESD and aRts both support networked sound, and manage sound streams from multiple sources. ESD is hard-coded into Gnome, but thanks to the PulseAudio developers it should soon be divorced from Gnome, as a proper modular Linux application should be. aRts was designed from the beginning as an independent, portable audio framework. ALSA provides device drivers, multi-device management, basic mixing and recording, and works in any Linux environment, including the console.
ESD and aRts both perform both low-level and higher-level functions. Both interface between sound hardware and applications, and also encode and decode your various file and streaming audio formats. aRts does everything; ESD handles sound server duties, and GStreamer handles the encoding and decoding. Both eventually pass everything down the pipeline to ALSA.
While describing the current state of Linux audio would require a book, this should give you an idea of its complexity, pitfalls, and boobytraps. The good news is we have a lot of great audio applications. The bad news is it's all rather a messy jumble. But that, perhaps, is changing.
PulseAudio is intended to be a drop-in replacement for ESD on Gnome. It is designed to be cross-platform, running on POSIX-compliant operating systems (like Linux), and on Win32. Before I discuss PulseAudio further, I must share an amusing true anecdote, which I promise is relevant. A good friend of mine has a number of health problems, so he spends a lot of time seeing doctors and taking a lot of medications. His favorite doctor is a Vietnamese woman with a bent sense of humor. He told her he didn't like how a certain drug was making him feel. She prescribed an additional medication. He asked why didn't she give him something to replace the nasty one, and she said "We never replace, we only add." She wasn't serious, but there was a grain of truth in it. And so it is with Linux applications and subsystems--it seems we never replace, only add.
However, PulseAudio has the potential to become the common Linux audio server, and actually replace some legacy servers like ESD and aRts. Why would we even want this? For one thing, it has a great advanced feature set:
- Individual volume controls for each playback stream
- Modular, extensible architecture
- Multiple backends for compatibility with other audio servers
- A consistent and common API
- Auto-discovery of other Pulse-enabled computers on a network
- Network sound server
- Mix-and-match multiple sound devices and playback streams
- USB hotplug support
- Both GUI and command-line controls
Next week we'll install PulseAudio on some random Linux and learn some useful and cool things to do with it, and some tips and tricks for getting past some of the bumpy parts.
This article was first published on LinuxPlanet.com.