VGA Music Sequencer — DE1-SoC

Co-Authors: Tannaz Chowdhury & Dareen Nasreldin

An embedded step sequencer implemented in C on the DE1-SoC FPGA board. Compose music on a sheet-music-style grid rendered over VGA, play it back in real time through the onboard audio output, and edit notes live using a PS/2 keyboard.

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Features

• Sheet-music grid with 4 staves, up to 8 pages, and 16 columns per staff
• Multiple note durations: whole, half, quarter, 8th, 16th, and rest
• Sharps, flats, and naturals per note
• Three instrument modes: Beep (square-wave oscillator), Piano (PCM samples with sustain loop), Xylophone (PCM samples, one-shot)
• Adjustable BPM (40–999) with live tempo control
• Animated green playhead tracking playback column by column
• Preloaded song templates to load and edit
• Pause, stop, and restart during playback without losing your composition

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Hardware Requirements

• Altera DE1-SoC board
• PS/2 keyboard
• VGA display
• Audio output (headphones or speakers via 3.5 mm jack)

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Building and Running

Important: Follow these steps exactly every time. The GDB server must be restarted when switching between programs — skipping this causes the server to cache the old .elf and the board will not update.

First time (or after switching from another program)

Tab 1:

cd path\to\main
./gmake DE1-SoC
./gmake DETECT_DEVICES
./gmake GDB_SERVER        ← leave this tab open

Tab 2 (new PowerShell window):

cd path\to\main
./gmake
./gmake GDB_CLIENT

Switching from another program

1. Close the GDB_CLIENT tab (Ctrl+C or close the window)
2. Close the GDB_SERVER tab (Ctrl+C or close the window) — this step is mandatory
3. In Tab 1: cd into main, then ./gmake GDB_SERVER
4. In Tab 2: cd into main, then ./gmake and ./gmake GDB_CLIENT

If the keyboard does not respond after loading, the GDB server was not restarted. The server caches the old .elf in memory. Always restart it when changing programs.

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Keyboard Controls

Cursor movement

Key	Action
W A S D	Move cursor up / left / down / right

Note placement and deletion

Key	Action
Space	Place note at cursor
Delete	Remove note at cursor
↑ / ↓	Shift placed note up or down one pitch slot

Note duration

Key	Duration
1	Whole note
2	Half note
3	Quarter note
4	Beamed pair of 8th notes
5	Beamed group of 4 sixteenth notes
6	Beamed pair of 16th notes
7	Single 16th note
8	Rest

Accidentals

Key	Accidental
Z	None
X	Sharp (toggle)
C	Flat (toggle)
V	Natural (toggle)

Playback

Key	Action
Q	Play from page 1
R	Restart playback
E (during playback)	Pause / resume
T (during playback)	Stop

Tempo

Key	Action
-	Decrease BPM by 5
+	Increase BPM by 5

Pages

Key	Action
← / →	Navigate to previous / next page
K	Add a page (max 8)
L	Remove the last page

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How It Works

Audio engine

Playback is driven by a polling loop. The sequencer iterates staff by staff, column by column. For each column it calls play_column(), which pushes a fixed number of samples directly into the WM8731 audio FIFO via memory-mapped I/O. The number of samples is determined by:

quarter_samples = (8000 Hz × 60) / BPM
note_samples    = duration_64 × (quarter_samples / 16)

where duration_64 is 64 for a whole note, 32 for a half, 16 for a quarter, and so on down to 4 for a single 16th. The CPU busy-waits on the FIFO empty-slot registers (wsrc/wslc) before writing each sample, so timing is locked to the hardware sample rate of 8 kHz with no jitter.

Pitch and instruments

• Beep — a fixed-point phase accumulator generates a square wave at the target frequency in real time. Changing pitch changes the phase increment; no samples are involved.
• Piano / Xylophone — each pitch slot maps to a pre-recorded 16-bit PCM array stored in flash. Piano samples sustain by looping the final 256 samples of the recording. Xylophone samples play once and then output silence (natural percussive decay). Sharps and flats select entirely separate pre-recorded arrays rather than doing any pitch shifting at runtime.

Note struct

Every placed note is stored in a flat Note array (max 512 entries). Each Note records its page, staff, column, pitch slot (0–10 mapping G5 down to D4), duration, accidental, and the pixel coordinates of every note head. Multi-head notes (beamed groups) store up to 4 head positions within a single struct entry. Erasing a note restores the background pixels under its bounding box, then swaps the entry with the last element in the array (notes[i] = notes[num_notes - 1]; num_notes--) for O(1) deletion.

Pages

All notes across all pages share one array, tagged by their page field. Switching pages redraws the background and replays only the notes matching the new cur_page value. During playback, main() pre-scans the array to find the last occupied page, staff, and column so playback stops immediately after the final note rather than running empty columns to the end.