Struna-14 is a 14-voice polyphonic synthesizer built around an ESP32, designed with a focus on musicality, low CPU usage, and a clean yet expressive timbre inspired by plucked and resonant string instruments.
The synthesis engine is intentionally simple and efficient, relying on a sine-based oscillator subtly enriched to achieve brightness and presence without harshness.
Link to video sample on YouTube (with a MIDI sequencer)
- 14-voice polyphony
- ESP32-based (Arduino framework)
- MIDI input (DIN via TRS) at 31,250 baud
- Audio output via PCM5102 I2S DAC
- Sine oscillator core with controlled harmonic enrichment
- Plucked-string inspired excitation (noise burst + decay)
- Subtle inharmonic component for realism
- Per-voice envelope (attack / exponential decay)
- Static per-voice detune for width and stability
- Very low CPU footprint
Each voice is composed of:
- A primary sine oscillator (fundamental)
- A small secondary inharmonic sawtooth oscillator (to introduce high-frequency energy and improve clarity)
- A short noise-based pluck excitation
- A simple one-pole low-pass filter on the pluck
- An exponential decay envelope
The overall architecture favors clarity, articulation, and musical balance over brute-force spectral density.
Struna-14 is controlled via standard MIDI input:
- MIDI baud rate: 31,250
- UART:
Serial2 - MIDI IN implemented using an optocoupler (tested with 6N137)
Supported MIDI functionality:
- Note On / Note Off
- Polyphonic note handling up to 14 voices
The MIDI RX pin is configurable and mapped to a GPIO assigned to Serial2.
Audio is generated digitally at 44.1 kHz and output via an external PCM5102 DAC using the ESP32 I2S peripheral.
- DAC: PCM5102 / PCM5102A
- Interface: I2S
- Output: mono (summed internally)
- Internal processing: floating-point
The PCM5102 provides significantly better audio quality than the internal ESP32 DAC and is the recommended output method for Struna-14.
An external analog stage (RC filter / buffer / amplifier) is recommended after the DAC.
Typical setup:
- ESP32 (e.g. WROOM-32 DevKit)
- MIDI IN (DIN 5) via optocoupler (6N137)
- PCM5102 I2S DAC module
- Optional analog output stage (filter, buffer, amplifier)
- Language: C++
- Framework: Arduino (ESP32 core)
- Sample rate: 44.1 kHz
The code is structured for clarity and easy experimentation with synthesis parameters (pluck behavior, detune, inharmonicity, envelopes).
Struna-14 is a working and playable synthesizer engine.
Current focus:
- sound voicing and timbral balance
- articulation and dynamic response
Possible future additions:
- optional delay / echo
- simple timbre macro control
- velocity or MIDI CC modulation
Struna-14 is intended as a musical instrument, not a DSP showcase. The goal is expressiveness, efficiency, and a coherent sonic identity.