This DIY Audio Spectrum Monitor by a Japanese engineer looks and works great, there are full construction details and code available on his site. You have to watch the video, it’s very impressive.

Video after the jump.

“This is an experimental work to monitor a spectrum pattern in radio band, and is a continuous project from Audio Spectrum Monitor. To analyze the spectrum of an input signal, I chose an Atmel AVR microcontroller that used in the Audio Spectrum Monitor to process FFT. When think it easy, it can be thought that sample an input RF signal directly and analyze it will do. However, you will able to recoginize that there are some techinical difficulties from following reasons.

In this project, an intermediate frequency signal from mixer output of an AM radio (fC=455kHz) is used as an input signal. It is converted again down to zero hertz in complex signal, so that the signal path, mixer and local oscillator, must be composed for complex signal processing. The complex signal is expressed in two wire IQ signal, the real part corresponds to I signal and the imaginary part corresponds to Q signal. The arithmetic circuits for IQ signal are realized in method of complex arithmetic. For example, a mixing circuit for IQ signals requires four multiplyers and two adders from the formula:
(a1+jb1)(a2+jb2) = (a1a2-b1b2)+j(a1b2+a2b1)”

Thanks for the tip Dick