Phonon is a new KDE technology that offers a consistent API to use audio or video within multimedia applications. The API is designed to be Qt-like, and as such, it offers KDE developers a familiar style of functionality (If you are interested in the Phonon API, have a look at the online docs, which may or may not be up to date at any given moment).

Firstly, it is important to state what Phonon is not: it is not a new sound server, and will not compete with xine, GStreamer, ESD, aRts, etc. Rather, due to the ever-shifting nature of multimedia programming, it offers a consistent API that wraps around these other multimedia technologies. Then, for example, if GStreamer decided to alter its API, only Phonon needs to be adjusted, instead of each KDE application individually.

Phonon is powered by what the developers call "engines" and there is one engine for each supported backend. Currently there are four engines in development: xine, NMM, GStreamer and avKode (the successor to aKode). You may rest comfortably in the knowledge that aRts is now pretty much dead as a future sound server, and no aRts engine is likely to be developed. However, aRts itself may live on in another form outside of KDE. The goal for KDE 4.0 is to have one 'certified to work' engine, and a few additional optional engines.

Other engines that have been suggested include MPlayer, DirectShow (for the Windows platform), and QuickTime (for the Mac OS X platform). Development on these additional engines has not yet started, as the Phonon core developers are more concerned with making sure that the API is feature-complete before worrying about additional engines. If the Phonon developers attempt to maintain too many engines at once while the API is still in flux, the situation could become quite messy (If you would like to contribute by writing an engine, jump into the #phonon channel at irc.freenode.org).

When an engine is selected by the user or application, Phonon will use the selected engine to determine what file formats and codecs each backend supports, and will then dynamically allow the KDE application to play your media. As it currently exists in the KDE 3 series, the user would have to manually change engines in each application (Kaffeine, Amarok, JuK, etc.) rather than being able to select engines for use across KDE.

Once an engine is selected for Phonon, it allows the programs to do the standard multimedia operations for that engine. This includes the usual actions performed in a media player, like Play, Stop, Pause, Seek, etc. Support also exists in Phonon for higher-level functions, like defining how tracks fade into one another, so that applications can share this functionality instead of re-implementing it each time. Of course, some applications will want more control over their cross-fading, and so are still free to design their own implementation.

The engine with the greatest progress so far is xine, which I was able to set up and run on my system. I was unable to get the NMM (notoriously hard to compile/setup) or GStreamer engines to compile on my system, whilst avKode is currently disabled by default. I would show you a screenshot of Juk or Noatun playing audio with Phonon, but right now these applications look just like their KDE 3.x versions (only with a somewhat ugly/broken interface!). When they are getting polished for release, I will show them off in a later article.

Matthias Kretz offers a short video which, if you turn your speakers on while watching, demonstrates device switching. Phonon lets you switch audio devices on the fly, and you can hear the specific moment when the music switches from his various outputs (headphones, speakers, etc.).

Matthias also submits the following screenshot of output device selection using Phonon's configuration module. This is also a work-in-progress, and so take it with a grain of usability salt.

There are not many things that I can take a screenshot of which show Phonon in use (screenshots of an audio framework are notoriously difficult to compose!), but I can describe one of the neat side effects of using Phonon: network transparency. KDE has long used KIOSlaves to access files over the network as easily as if they were stored on your local computer. Multimedia apps like JuK or Amarok should be able to add files transparently over the network to their collections without having to be concerned about whether or not the back-end engine is aware of how to deal with ioslaves. This support is already partially implemented in KDE 4, and is most visible through audio thumbnails, which are working for many people over any KIO protocol, including sftp:// and fish:// - two popular protocols among KDE power users. They do not yet work for me due to some instability in the fish:// KIOSlave of my current compilation, but the developers in the #phonon IRC channel claim that it this functionality will be ready and working when fish:// is more stable.

So, Phonon, while still in development, is going to be a great pillar technology for KDE application programmers, making their job easier and removing the redundancy and instability caused by constantly-shifting back-end technologies, and (eventually) making support for other platforms a piece of cake. This means that those developers can spend more time working on other parts of their applications to ensure KDE Multimedia applications shine even more brightly than they currently do.

A couple of quickies here to note: Mark Kretschmann, lead developer for Amarok has officially opened up Amarok 2.0 development this week, and seems to be quite interested in what Phonon can do for Amarok 2.0. He doesn't rule out keeping their own engine implementations, like they currently do in the Amarok 1.4 series. However, given its early stage of development, Phonon can likely be adjusted to ensure that it will do everything Amarok asks of it.

If you're looking for a way to help out with KDE and are not a programmer, Matthias Kretz, lead developer of Phonon (Vir on IRC) has requested some help in keeping the Phonon website up-to-date.

And lastly, a few translations of these articles have been popping up around the world in various languages. Sometimes more than one translation is happening for a specific language. If you are translating or plan to translate these articles, send me a message so that we can save everyone some work and avoid redundancy (lets keep the redundancy-reduction spirit of Phonon alive!).

Until next week...

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