I'm designing a gadget which will light up LEDs using a microcontroller. I'd like for it to respond to an audio signal, so I was thinking there are a lot of ways I could do it: If I can accurately sample the voltage from a microphone or an analog signal (microphone out) fast enough I can run an FFT algorithm on the microcontroller and find out the gain at any arbitrary audible frequency, and flash the lights in tune, or other fancy fun stuff.
If it cannot sample at 44kHz, I can still get some meaningful data subject to aliasing. I'll need to flip through some textbooks in that case...
If my microcontroller is too slow to do a fourier transform, I figured I could still create an AC band-pass filter (can do it with two capacitors and two resitors) and amplify it so i could still give my microcontroller a couple of analog inputs representing certain frequency intensities.
So I think I am pretty sure how this plan works on a big-picture level, but I'm not sure where to start to figure out how to wire together my components. I have a couple of different op-amps, a few BJT's, some assorted resistors. My assorted capacitors are still in the mail. I have a few arduinos which will hopefully be sufficient for prototyping. I also have the requisite materials for printing my own circuit boards (copper plated laminate, etching acid, laser printer, iron) and stuff for soldering. However I do not have an oscilloscope or function generator, only a DMM and a current- and voltage-controlled DC power supply.
So an audio signal is carried over two wires. The difference in potential between the wires (and perhaps to some degree the current??) represents some function of the air pressure displacement due to sound at the point in space occupied by the microphone. This is due to the common design aspects of nearly all speakers and microphones. This would mean that this voltage will go both positive and negative (no DC bias, as I understand it) and therefore would require offsetting so the analog input read will not clip half of the waveform. How many volts would the amplitude of the signal be, from the headphone out on a sound card for example? What about an iPod? Could I play an mp3 of a sine wave on my computer, and examine the AC voltage on the audio jack using my DMM? How do I know what frequency range the DMM will make accurate AC voltage measurements?