High geekdom - Software-defined radio

From this Wikipedia entry:

Software-defined radio
A software-defined radio system, or SDR, is a radio communication system where components that have been typically implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded computing devices. While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics render practical many processes which used to be only theoretically possible.

Although there had been a few proposals and sample systems earlier, the first big push was with the military in their SPEAKeasy project.The idea of this system is that you could have a single unit that would interoperate with over ten different legacy radio systems. There are now commercial units available but they are fairly expensive -- several thousand dollars for units like the WiNRADiO or the Flex. Well... It seems that a bunch of radio geeks have developed an open-source platform that blows these commercial units out of the water, is completely modular, runs on any computer platform and is about 1/3rd of the cost. Check this out:

High Performance Software Defined Radio
The HPSDR is an open source (GNU type) hardware and software project intended as a "next generation" Software Defined Radio (SDR) for use by Radio Amateurs ("hams") and Short Wave Listeners (SWLs). It is being designed and developed by a group of SDR enthusiasts with representation from interested experimenters worldwide.

The rationale behind the project is to break the overall design up into a number of modules. Each module is designed by an individual or group and connects to other modules using a pre-defined and common bus -- rather like plugging boards into a PC motherboard.

This modular approach enables prospective users to incorporate just the modules that interest them as well as designing their own variants if desired. The approach also enables new ideas and circuits to be tested by replacing an existing module. Since the majority of modules will be retained, such experimentation can be done with minimum disruption to an existing working system.

The modules vary in complexity from simple bandpass filters and input/output interfaces, to full blown DSP functions. Such variety enables experimenters with varying degrees of experience to contribute.

Thus far, the modules have each been named for easier identification when talking or writing about them. On this website, each module has its own web page, as noted by the tab selections near the top of the page. Some of the modules are being designed so that they can be either used in conjunction with others or stand-alone. Each module board size (except the backplane) will be 100 mm. by 120 to 220 mm. and use either a 96 pin or 64 pin DIN41612 type connector.

Some well-thought out design constraints. Power is supplied by a garden-variety ATX PC power supply. Cheap and available worldwide. The spacing of the modules on the bus is the same as the spacing for the plug-in cards for a PC so you can recycle an old computer case. The neat thing is that you can start off with a very simple receiver and add features (different reception modes, filters for improving your signal, a transmitter, etc...) as you continue to explore. Module prices are in the $30 to $300 range so you can develop quite the nice receiver for around $500.