Guitar Tuner ECE 5330 Embedded Systems Design Final Project Results Mike Heiny December 8, 2008

Proposal Implement hardware to allow connecting an electric guitar to the board. User selects which string is to be tuned. User plays the string. AVR “listens” to the note and communicates to the user, via LEDs, how close to perfect the note is and whether it is high or low.

Guitar Output I wrote a sampling program. Getting a usable signal from the guitar was the hardest part. I tried to use an opamp and a buffer to generate a digital signal, but my power supply wouldn’t work to power the opamp.

Usable Output Ended up using unamplified signal straight from guitar. Breakthrough was soldering leads to connector. Did some crude filtering by dividing signal by 4 or 8 and by throwing out signals that weren’t close to expected values.

Software My sampling routine checked the ADC every 250 µs and located leading edges of pulses. (Had to go this slow to allow it to execute between readings.) Calculated period by counting the number of 250 µs intervals since the last leading edge. Compared the rolling sum of 20 periods to the target value. Changed output accordingly.

Output User selects string to be tuned. LED’s light up indicating how close to target you are and whether you’re high or low.

Results Actually works!!! (Not that I’m surprised.) I had to use a lot of filtering, though. Theoretical target values resulted in wrong tones, at least according to the commercial tuner I used as a reference. I instead calibrated with the commercial tuner and used empirical values for my targets.

Questions?