Something I've been playing around with, on and off over the past few months has been Software Defined Radio, or SDR for short. A few months ago, I ordered a kit receiver from Tony, KB9YIG. He has been continuing the "Softrock" series originally sold by the American QRP Club. The latest "Softrock 40's" currently are synthesized, and are controlled totally by computer. The theory is a bit deep as to how these work, but for simplicity, I'll say that the sound card in a computer does all of the demodulation of the signals.
This opens up a whole new world of radio. Here, a small pc board, which costs about $50 will tune the HF bands thru 6 meters, and do it so well, it rivals almost any radio on the market. The features of the radio is only limited by the software used in conjunction with it. Using this radio, and a program called "PowerSDR", the receiver will tune, general coverage from 1.6 MHz thru 30 MHz, or from 3.5 MHz thru 52 MHz. It will also receive all modes, USB, LSB, CW, AM, FM and DRM. Using external software, the receiver will receive all of the amateur digital modes (PSK, SSTV, Olivia, Hellscreiber, etc), and with Dream software, will also decode shortwave DRM Broadcasts.
Tonight, I listened to Radio Canada International for about an hour, running Dream, listening to their digital broadcast. They did a commentary, News, etc, which is what you would normally hear on a government operated shortwave station, but they also played some music - Gospel, Jazz, etc. I will say, the quality was almost as good as FM broadcast quality. Absolutely no noise whatsoever, and the frequency response and distortion were exceptionally great!
Of course, the receiver will listen to the analog AM broadcasts, as well as SSB and CW with no additional software.
The display shows a panoramic view of the frequency domain (It looks like a spectrum analyser), and by using different sound card sample rates, you can see anywhere between about 50 KHz to about 200 KHz of spectrum using 48K thru 192K sample rates respectively, provided your sound card is capable of the faster rates. I usually run the 48K rate, as the 96K rate my sound card sounds choppy.
Originally, I purchased this device as an aid to finding rain scatter signals on 10 GHz (with the appropriate converters), however, this is just so cool, that I have been using it as a general coverage receiver.
I just purchased a electronically switched bandpass filter kit for the input of the receiver. The way I have been using this so far has been by changing out little monoband filter boards that cover sections of the HF spectrum. The kits, including the bandpass filter are mostly surface mount devices, and the parts are extremely tiny. There are special techniques for building with these components. I'm finding my eyesight isnt as good as it used to be when I was younger, as I've been having trouble seeing what I'm doing. I can only work on it for about half an hour at a time till my eyes get "buggy".
The link to this really cool project can be found at http://www.wb5rvz.com/sdr/
Hope you try one of these, you'll have a blast with it! They also make a version that transmits also.