Arduino frequency input help for tone detector

I could use some help setting the arduino's sampling frequency.

Here's what I'm trying to do. I want to use the arduino to detect a tone at 18 kHz, and output a tone at 8 kHz when it detects the 18 kHz tone. The trick is that I want to do this using aliasing. I need to set the sampling frequency of the arduino to be approximately 26 kHz so that when it tries to detect the 18 kHz tone, the arduino will think the tone is at 8 kHz due to aliasing.

My plan is to use a fft to get the frequency of the tone (should be 8 kHz), and then use that frequency to generate a tone. I haven't picked out a specific microphone or speaker yet, so if you had any suggestions for those, that would be great.

So, to reiterate, could you help me set the sampling frequency of an arduino duemilanove? Thanks a lot, gang!

I'm doing something similar -- needing both to create and detect tones with the Arduino.

One thing I've considered is detecting "beat" frequencies or anti-aliasing on a signal. However, I need to detect the tone amid noise -- and that would likely make that approach a challenge.

Have either of you gotten any further with a tone detector? I have an idea for a simple morse code decoder based on an old MBASIC program I found ages ago, which would automatically shift to the WPM speed, and output the decoded message to the serial port (the computer connected to the USB) I did something similar, converting the program from PC to a TRS-80 Model 100 (using the data pin of the Barcode reader, as a "Signal' pin, using a 576 tone detector as the receiver. What I want to do, is use the Arduino as both detector and decoder.

Thanks, Stephen Griswold (gelfling6)