Guitar Multi-FX and Recording Device

Matthew Calligaro and Giselle Serate
Fall 2018
Final Project for Microprocessors Course (E155) at Harvey Mudd College

Summary

An electric guitarist may require over a thousand dollars of equipment to apply and modulate effects while recording and playing audio through speakers. This device simplifies this process into a single, affordable design. It uses an FPGA to digitally apply overdrive, delay, chorus, and distortion effects. These effects can be modulated by a distance sensor which attaches to the user's guitar. The processed audio is sent to the microcontroller which can record, play, and loop the signal. This signal is outputted through a 3.5 mm audio jack, and recordings can be uploaded to the internet as WAV audio files via an Apache webserver.

User Interface

The user interface consists of a series of pushbuttons, DIP switches, and an LED.

FPGA (Audio Effects)

The FPGA receives user input from 5 switches and 1 pushbutton. Any number of effects can be applied at one time.

Input	Effect
Switch 0	Applies overdrive effect
Switch 1	Applies delay effect
Switch 2	Applies chorus effect
Switch 3	Applies distortion effect
Switch 4	Turns on distance sensor
Reset button	Recalibrates device and sets noise gate

To calibrate the device, turn on all electronics and plug in the guitar. Without playing any noise, press the Reset button to calibrate the default input voltage. To configure the noise gate, orient the 5 switches with the desired binary value (with switch 4 as the most significant bit) and press the Reset button. A higher value should result in less noise.

Microcontroller (Recording and Looping)

The Microcontroller receives user input from 2 switches and 3 pushbuttons and displays state through an LED. The two switches are used to set the device's mode.

Loop Switch	Record Switch	Mode
Off	Off	Linear playback
Off	On	Linear record
On	Off	Loop
On	On	Loop settings

In each mode, the pushbuttons have the following effects.

Mode	Play button	Reset button	Save button
Linear playback	Play/pause the recording	Restart at the beginning of the recording	Save the recording to the internet
Linear record	Start/stop recording	Delete the current recording	Save the recording to the internet
Loop	Play/pause the current loop	Begin recording a new loop	Save the loop to the internet
Loop settings	Tap to set tempo	Increase number of measures	Save the loop to the internet

In each mode, the LED indicates the following information. In all modes, 3 flashes indicates that the current recording has been successfully saved to the internet.

1. Linear playback
   • On: currently playing
   • 1 flash: returned to the beginning of the recording
2. Linear record
   • On: currently recording
   • 1 flash: current recording deleted
3. Loop
   • Flashes to indicate tempo
4. Loop settings
   • Flashes to indicate tempo
   • Upon changing the number of measures, flashes log2(measures) + 1 times

Instructions for Recording

1. Place the device in Linear record mode with the switches.
2. Clear the current recording by pressing the Reset button.
3. Begin recording by pressing the Play button. The LED will turn on.
4. Stop recording by pressing the Play button. The LED will turn off.
5. Place the device in Linear playback made with the switches.
6. Play the recording by pressing the Play button. The LED will turn on.
7. To save the recording to the internet, press the Save button. The LED will flash 3 times.

Instructions for Looping

1. Place the device in Loop settings mode with the switches.
2. Tap out the desired tempo on the Play button. The LED will flash the current tempo.
3. By default, a loop consists of 4 measures. To increase this, press the Reset button, which will allow you to select 1, 2, 4, 8, or 16 measures. The LED will flash log2(measures) + 1 times.
4. Place the device in Loop mode with the switches.
5. The device will click 4 times and then begin recording for the set number of measures. Afterwards, the recorded loop will play indefinitely. The LED will continue to flash to indicate the tempo.
6. To pause the current loop, press the Play button.
7. To record a new loop, press the Restart button.
8. To save the loop to the internet, press the Save button. The LED will flash 3 times.

Hardware

Circuit Diagram

Parts list

Part	Quantity	Purpose	Manufacturer
Raspberry Pi 3 Model B+	1	Microcontroller	Raspberry Pi Foundation
Cyclone IV EP4CE6E22C8N	1	FPGA	Altera
LM386N-4	1	Audio Amplifier	National Semiconductor
MCP3002	1	10-bit ADC	Microchip Technology
HC-SR04	1	Ultrasonic distance sensor	SainSmart
1/4" instrument cable jack	1	Receive guitar input	N/A
pushbutton	4	FGPA and Microcontroller input	N/A
5 DIP switch	1	FPGA input	N/A
2 DIP switch	1	Microcontroller input	N/A
10 uF capacitor	1	Bypass capacitor	N/A
1 uF capacitor	1	Bypass capacitor	N/A
10 kΩ resistor	11	Pulldown resistor	N/A
220 Ω resistor	1	LED resistor	N/A

Steps for Setting Up

1. Wire the hardware according to the circuit diagram shown above.
2. Use the .sv files in the FPGA directory to configure the FPGA, with FPGA.sv as the top-level module.
3. Set up an Apache webserver on the Raspberry Pi.
4. Load the files in the Pi directory onto the Raspberry Pi.
5. Navigate to these files and make.
6. Connect your guitar to the device with a 1/4" instrument cable.
7. Connect a speaker or headphones to the 3.5 mm audio jack on the Raspberry Pi. Keep the speaker turned off.
8. On the Raspberry Pi, make run.
9. Turn on the speaker.