Monophonic Digital Bass Synthesizer

I have a YouTube video demonstrating this project here: https://www.youtube.com/watch?v=EuSdQnB61mw

Overview

This project implements a monophonic digital bass synthesizer on the TI TM4C129 series microcontroller. It combines real-time waveform generation, an ADS (attack/decay/sustain) envelope, and an SPI-driven TFT display to create an embedded bass instrument tailored for an Embedded Systems (student-defined) project by me. The firmware drives an external UDA1334A I2S DAC for audio output, samples three analog potentiometers for envelope control, and accepts note input from a 4x3 matrix keypad.

The codebase is organised into small modules responsible for waveform generation, hardware interfaces (I2S, SPI, DMA, ADC, GPIO), and user interaction. DMA-powered ping-pong buffering keeps the audio pipeline running while the CPU services input scanning and display updates.

Features

• Four waveform modes (sine, saw, triangle, square) with precomputed lookup tables for low-jitter playback.
• Monophonic keyboard driven by a 4x3 matrix keypad mapped to two octaves of chromatic notes (C2–B3).
• ADS envelope with three potentiometers controlling attack, decay, and sustain in real time.
• Visual feedback on a 320×240 SPI TFT display showing the selected waveform and live ADS values.
• I2S audio output using SSI3 with DMA ping-pong buffers to stream 16-bit samples to a UDA1334A DAC breakout.
• Interrupt-driven control loop (Timer0A) for keypad scanning, waveform cycling button debouncing, and ADC refresh.
• UART debug console for runtime logging and system verification.

Hardware Requirements

• TI TM4C1294NCPDT (Tiva-C) microcontroller or equivalent LaunchPad.
• UDA1334A (or compatible) I2S DAC module with line-level output.
• 4×3 membrane matrix keypad for note entry.
• Three analog potentiometers wired to ADC channels AIN6, AIN7, and AIN8.
• Momentary push button with two indicator LEDs to select waveforms.
• ILI9341-based SPI TFT display (used via LCD_Display driver library).
• D Latch as T Flip Flop connected to the SSI line of the uC and the FSS line of the DAC (converts SPI to I2S protocol)
• Optional: Audio amplifier, oscilloscope, headphones for evaluation.

Note: Audio amplification is outside the firmware scope; feed the DAC output into an external amp or powered speakers.

Firmware Architecture

Signal Path

1. Wave Tables – waveforms.c fills 512-sample lookup tables for sine, saw, triangle, and square waves, plus a chromatic frequency table (CHROMATIC_LEN = 24).
2. Function Generator – function_gen.c maintains the phase accumulator, waveform selection state, and ADS envelope. next_sample() mixes the current waveform sample with the envelope level before handing it to the audio buffers.
3. DMA Ping-Pong – dma.c initialises uDMA channel 15 to alternately fill pingBuffer and pongBuffer. Each buffer holds 1024 frames and is replenished in the SSI3_Handler interrupt when the DMA mode stops.
4. I2S Transport – SSI.c configures SSI3 for 16-bit I2S-like transfers. When the transmit FIFO signals half-empty (DMATXMIS), the interrupt handler refills buffers and re-enables DMA.
5. DAC Output – Samples leave SSI3, clocked by BCLK at roughly 16 bits × 2 channels × SAMPLE_FREQ (default 48 kHz) and latched by a T flip-flop generated WSEL.

Control Loop & Inputs

• Timer0A ISR (TIMER0A_Handler in input.c) runs every 4 ms to scan the keypad, cycle waveforms on button press (handle_waveform_state), trigger ADC sampling, and feed the new ADS values into the envelope.
• Keypad Mapping – Key presses map to chromatic semitone steps, updating the phase increment and optionally resetting the phase for clean attacks.
• Envelope Engine – Attack, decay, and sustain values (0–1000 ms/level) come from the potentiometer ADC readings.
• Display Updates – display_utils.c redraws the selected waveform and renders ADS values on the TFT. The main loop continually refreshes the display when waveform_changed is set by the button handler.

Support Peripherals

• ADC0 Sequencer 0 samples three channels back-to-back under Timer0A trigger.
• UART0 provides serial logging at 115200 baud for diagnostics.
• GPIO LEDs on PG1/PK4 mirror the currently selected waveform.

Building the Firmware

1. Toolchain: The project targets the Keil µVision environment (see .uvprojx and .uvoptx files) with TI’s TM4C device packs. Alternatively, you can port the build to GCC/Make by replicating the startup code and linker script used by the TM4C129.
2. Dependencies:
   • TI TM4C129 device headers (TM4C129.h) and ES_Lib utilities.
   • LCD_Display driver library for the ILI9341 TFT.
3. Steps:
   • Open embedded-assignment.uvprojx in Keil µVision.
   • Select the appropriate device (TM4C1294NCPDT) and ensure the system clock is set to 120 MHz (the firmware uses ES_setSystemClk(120)).
   • Build the project and flash it to the LaunchPad via the ICDI debugger.

Configuration Tweaks

• Adjust SAMPLE_FREQ, BASE_FREQ, or SSI prescalers in config.h to experiment with different sample rates.
• Change FRAME_COUNT to trade audio latency for buffer refresh frequency.

Usage

1. Power the LaunchPad and peripherals.
2. Connect the I2S lines (BCLK, WSEL, DIN) to the UDA1334A DAC; route the DAC output to speakers or headphones (through an amplifier).
3. Use the keypad to trigger notes. Keys map from 1 (C2) to # (B3).
4. Turn the attack, decay, and sustain potentiometers to sculpt the envelope. Values update every 4 ms.
5. Press the waveform button to cycle Sine → Saw → Triangle → Square; LEDs and the TFT label indicate the active mode.

Testing & Validation

• Aural Evaluation: Waveforms were validated by ear in each mode. Clean tones emerge once the envelope reaches steady state; high sustain can introduce distortion due to quantisation and headroom limits.
• Oscilloscope Capture: DAC output (yellow) vs. digital data (green) confirms each waveform and the I2S timing (BCLK ≈ 1.54 MHz, WSEL ≈ 44–48 kHz).
• Display Check: Visual inspection confirmed waveform previews and ADS values, though minor rendering glitches remain.

Known Issues & Future Work

• Sustain set to zero can lock the envelope, requiring a reset.
• Occasional display artefacts should be investigated in the SPI driver or drawing routines.
• Output headroom is limited; reducing waveform table amplitude or migrating to a higher-resolution DAC could lower distortion and hum.
• Planned enhancements include MIDI-over-USB input, polyphony, sequencer/arpeggiator features, preset storage (EEPROM), dedicated octave buttons, standalone ADC interrupt triggering, improved shielding, and live waveform visualisation.

Acknowledgements

Developed as part of the Embedded Systems course project (September 2025). The audio demos, oscilloscope traces, and further documentation referenced in the project brief complement this firmware.