Kraftor SPO vox MIDI
Media
User Manual
Version 1.0
Kraftor SPOvox MIDI is a phoneme-level speech synthesis firmware for Kraftor hardware. It drives a SP0256-AL2 allophone chip and accepts control over USB MIDI or the hardware serial MIDI input.
Each MIDI Note On triggers one SP0256-AL2 phoneme directly, so any DAW or MIDI controller
can sequence speech phoneme by phoneme.
Version History
- v1.0 : SP0256-AL2 phoneme playback via USB/serial MIDI, per-phoneme volume from velocity,
LTC6903 clock control via pitch bend, channel volume via CC7, and OLED status display support.
MIDI Channel Configuration
The firmware responds to a single active channel. The default active channel is channel 1. Messages on
all other channels are ignored.
| MIDI Input | Function | Channel |
|---|---|---|
| USB MIDI | Trigger SP0256-AL2 phonemes and control volume / pitch |
Active channel (default 1) |
| Serial MIDI | Trigger SP0256-AL2 phonemes and control volume / pitch |
Active channel (default 1) |
MIDI Messages
Note On — Phoneme Triggering
Each MIDI Note On message triggers one SP0256-AL2 allophone. The phoneme index is note − 36, giving
access to all 64 phonemes of the SP0256-AL2 ROM.
- Notes 36–99: trigger phoneme 0 (note 36) through phoneme 63 (note 99).
- Notes outside 36–99: ignored.
- Velocity: sets the output volume for that phoneme (velocity × 2, range 0–254).
- Note Off: silences the SP0256-AL2 immediately.
| MIDI Note | Phoneme Index | SP0256-AL2 Token |
|---|---|---|
| 36 | 0 | PA1 (pause / silence) |
| 37 | 1 | PA2 |
| 38 | 2 | PA3 |
| … | … | … |
| 99 | 63 | last SP0256-AL2 allophone |
Pitch Bend — Clock Control
Pitch Bend controls the programmable oscillator that clocks the SP0256-AL2, changing the speech rate and
perceived pitch simultaneously. The full 14-bit MIDI pitch bend range is mapped to clock control values
1023–0, so lower MIDI bend values produce higher clock control values (faster clock, higher pitch).
Pitch bend updates take effect immediately on the clock and can be sent while a phoneme is still playing (Note On held),
allowing real-time pitch or speed sweeps within a single phoneme.
| Pitch Bend Value | LTC6903 Clock Control | Effect |
|---|---|---|
| +8191 | 0 | Lowest clock |
| 0 | about 512 | Center |
| -8192 | 1023 | Highest clock |
Default clock control value on startup is 651.
CC7 and Aftertouch — Volume
Both CC7 (Channel Volume) and Channel Aftertouch set the output level via the onboard op amp using the same mapping: value × 2, range 0–254.
Both messages take effect immediately and can be sent while a phoneme is still playing (Note On held), allowing real-time amplitude shaping.
Velocity only sets the level at the instant the Note On fires.
| Value | Output Level |
|---|---|
| 0 | 0 (silent) |
| 127 | 254 (maximum) |
Default startup level is 200.
Ignored MIDI Messages
- Notes outside the range 36–99
- Control Change messages other than CC7
- All other MIDI messages
Quick Reference Table
| Message | Function | Range |
|---|---|---|
| Note On | Trigger SP0256-AL2 phoneme; velocity sets volume | Notes 36–99, phonemes 0–63 |
| Note Off | Silence SP0256-AL2 | Any note on active channel |
| Pitch Bend | Clock control (pitch / speed) — continuous, effective while Note On is held |
-8192 to +8191, mapped to clock 1023–0 |
| CC7 / Aftertouch | Volume — continuous, effective while Note On is held | 0–127 mapped to level 0–254 |
Connections
The Kraftor SPOvox MIDI firmware accepts control from:
- USB MIDI for notes, pitch bend, and control changes
- Serial MIDI on the hardware serial MIDI input
OLED Display
If an OLED display is detected at I2C address 0x3C, it shows:
- SPO vox MIDI and the firmware version on startup
Tools
The best way to compose phoneme sequences for Kraftor SPOvox MIDI is the companion SPOvox translator tool:
https://deladriere.github.io/Spo_vox_translator/
SPOvox translator runs in the browser and converts English text to SP0256-AL2 allophone sequences. It sends the
sequence over Web MIDI (Note On per phoneme, pitch bend before each note) or exports a SPOvox.mid
Standard MIDI File for your DAW. Use a Chromium-based browser for Web MIDI support.
Firmware
Do not use this firmware for Emy, Emy Terminal, or Kraftor SAM.
How to install the firmware on
Kraftor
