You already knew you could use a Linux box to create your own "Internet radio" station that streams MP3s onto the 'net, right? But did you know you could turn your Linux system into a real radio transmitter, using nothing but the system's parallel port as the wireless media interface?
Well, don't get too excited... because this is really just a very clever way to demonstrate the real-time capabilities of a popular real-time Linux add-on, called RTAI (the Real-Time Application Interface).
How it works
First you need to know a bit about RTAI. Please bear with me, for I'm about to attempt a highly simplified two-paragraph explanation of RTAI.
Basically, RTAI is a tiny kernel that assumes ultimate control of system resources and runs Linux as a low-priority task beneath itself. Thereafter, RTAI has dominion over system Interrupts, a situation which allows it to respond in a near-instantaneous manner to certain real-world events when they occur. The term "near-instantaneous" is, of course, relative. At the risk of setting myself up for an email deluge, I'll oversimplify it like this: Linux itself is capable of handling response times, depending on who you ask, in the range of a few milliseconds to a few dozen milliseconds. RTAI, according to Lineo's specs on Embedix RealTime, can respond to interrupts within approximately 15 microseconds--making it around a thousand times as responsive as the Linux kernel.One further comment, before moving on, is that although RTAI can obviously greatly improve a system's responsiveness to real-world events in comparison to normal Linux, that improvement comes at a price--which is that the techniques needed to take advantage of RTAI fall outside the normal Linux programming model. You can't, for instance, simply install RTAI and instantly see improvements in applications such as streaming multimedia--unless they were designed to take advantage of RTAI.
In any case, it is this thousand-fold improvement in event responsiveness provided by RTAI that forms the essence of the software radio demo.
The demo software creates a radio carrier in the "long wave" radio spectrum, using a task running under RTAI to turn parallel port data lines on and off at a fast rate. The carrier is further modulated with a lower frequency, in the audio range, to produce music. Now for more detail.
