Amiga Digital Sampler

S/PDIF to parallel-port sampler interface for Amiga.

Overview

This project provides fully digital audio sampling for the Amiga using:

• Raspberry Pi for S/PDIF capture and DSP
• Raspberry Pi Pico for cycle-accurate parallel output
• Amiga sampling software (e.g., ProTracker)

The result is a modern, clean, high-quality digital sampler that plugs directly into the Amiga's parallel port.

Status

1.3 – New DSP engine, real-time UI, presets, shaping, filtering, compression, saturation, and dither

The sampler has a terminal UI with:

• real-time VU / peak meters
• clip detection
• DSP load meter
• quantizer noise meter
• DC offset monitor
• runtime DSP toggles
• eight presets

The sampler supports multiple DSP modes suitable for different content types:

• Raw / punchy: filter off, shaping off
• Clean / pads: shaping on (auto-filter on)
• Bright transients / snares: filter off, shaping off
• Lo-fi / crunchy: shaping on, filter off (intentional aliasing)

Motivation

Traditional analog Amiga parallel-port samplers are noisy, have DC offsets, and dynamics issues. Rather than fight the hardware, this project implements a fully digital sampler:

S/PDIF → Raspberry Pi → DSP → Pico → Amiga DB25

No noise, no DC offsets, and precise sample rate control.

UI Display

The UI includes:

• VU meter (grey → green → yellow → red)
• Peak meter
• Clip indicator & event counter
• DSP load (real-time load)
• Quantizer noise (oversampled quantizer error energy)
• DC offset

Hardware

Required Components

• Raspberry Pi 3A+ or 4
• HiFiBerry Digi+ I/O
• Raspberry Pi Pico
• 74HCT245 octal buffer
• 74LVX14 Schmitt trigger inverter
• DB25 male connector
• 0.1µF ceramic capacitors (3×)
  • Rails
  • 74HCT245 decoupling
  • 74LVX14 decoupling

Connections

SPI (Pi → Pico)

Pi GPIO 10 (MOSI)  →  Pico GP16
Pi GPIO 11 (SCLK)  →  Pico GP18
Pi GPIO 8  (CE0)   →  Pico GP17
GND                →  GND

STROBE (from Amiga → Pico via 74LVX14)

74LVX14 Pin 1  IN   →  DB25 Pin 1 (STROBE)
74LVX14 Pin 2  OUT  →  Pico GP10
Pin 14 → 3.3V
Pin 7  → GND
0.1µF across VCC/GND

Sampling Activity (Pico → Pi)

Pico GP20  →  Pi GPIO 5

The Pico drives this pin HIGH while actively receiving STROBE pulses from the Amiga, LOW when idle. The Pi monitors this to run hook scripts (sampler_active.sh / sampler_inactive.sh) for automation (e.g., muting DAW returns to prevent feedback).

Data Buffer (Pico → Amiga via 74HCT245)

74HCT245 Pins 2–9   ←  Pico GP2–9
74HCT245 Pins 11–18 →  DB25 Pins 2–9
DIR → 5V
OE  → Pico GP11
VCC → 5V, GND → GND
0.1µF across VCC/GND

IMPORTANT: All devices including the Amiga must share ground.

Software

Raspberry Pi

System Setup (first time only)

Install build dependencies:

sudo apt update
sudo apt install build-essential libasound2-dev libgpiod-dev

Enable SPI (Pi → Pico link):

sudo raspi-config nonint do_spi 0   # 0 = enable

Enable the HiFiBerry Digi+ I/O HAT. Edit the boot config (/boot/firmware/config.txt on current Pi OS; older releases use /boot/config.txt):

dtparam=audio=off
dtoverlay=hifiberry-digi

Make sure no dtparam=audio=on line is still active, or the onboard audio will take card 0. Reboot, then verify both interfaces:

ls /dev/spidev*      # expect /dev/spidev0.0
arecord -l           # expect the hifiberry digi capture card

The code opens the capture device as hw:0,0 (see ALSA_DEVICE in audio.c). If arecord -l shows the card at a different number, update that constant to match (e.g. hw:1,0).

Build & Run

cd pi
make
./sampler

Use the keyboard to enable DSP sections or change presets (1-8).

Runtime Controls (Keyboard)

Presets (1–8):

• Raw
• Raw + Saturation
• Filter Only
• Shaper
• Shaper + Dither
• Dirty LoFi
• Comp + Sat
• Clean + Compressor

DSP Toggles:

f  = toggle filter
s  = toggle shaper
d  = toggle dither
c  = toggle compressor
t  = toggle saturator
x  = reset peak + clip counters
q  = quit

Command-line Options

--gain X       Input gain (default 1.0)
--rate Hz      Target sample rate (default 28149.96)
--test-tone    Generate sine wave
--tone-freq Hz Frequency of sine (default 1000 Hz)
--test-ramp    Generate test ramp

Pico

cd pico
mkdir build && cd build
cmake ..
make
# Copy pico_amiga_sampler.uf2 to the Pico in BOOTSEL mode

ProTracker Setup

1. Set sampling note to A-3
2. After sampling, set Finetune +1 for exact pitch

This matches the Amiga's 28149.96 Hz sampling mode for PT A-3.

Technical Details

DSP Signal Path (with options)

48kHz S/PDIF input
↓
DC-block (always)
↓
Pre-FIR LPF (optional: filter)
↓
Compressor (optional)
↓
Saturator (optional)
↓
Oversample quantizer at 48kHz (always)
  • 3rd-order shaping (optional)
  • HP-TPDF dither (optional)
↓
Post-FIR LPF (optional: filter)
↓
Decimation to ~28.15kHz (always)
↓
Final 8-bit quantizer (always)
  • 2nd-order shaping (optional)
↓
SPI burst output → Pico
↓
PIO-driven parallel bus → Amiga

Notes on DSP Behavior

• Oversampling is always active, ensuring stable, artifact-free decimation
• Shaping automatically enables filtering when enabled via preset
• Disabling filters is ideal for snares/kicks
• Enabling shaping + filtering is ideal for pads/melodic sounds
• Shaping without filtering produces aliasing (intentional LoFi mode)
• Toggle DSP sections on/off in UI and monitor DSP state

Timing

• Amiga STROBE ≈ 28149.96 Hz
• Pico latches the sample exactly on each STROBE edge
• Pi → Pico SPI transfer uses small bursts to minimize latency
• 8KB ringbuffer smooths jitter

Warning

Be careful, don't break your Amiga! Check wiring twice. Ensure common ground. Only drive DB25 D0–D7 pins — nothing else.

Acknowledgements

• Thanks to echolevel for Open Amiga Sampler
• Thanks to 8bitbubsy for help debugging ProTracker timing

License

MIT