Communicating with the Play Acoustic

Auto-generated from param-defs.js — only verified, user-facing parameters are listed.

Introduction

The TC Helicon Play Acoustic is a vocal and guitar effects processor that connects to a computer via USB. While it appears as a standard USB MIDI device, it does not respond to conventional MIDI messages like Control Change (CC) or Program Change (PC). Instead, all device control happens through SysEx (System Exclusive) messages — a part of the MIDI specification designed for manufacturer-specific communication.

Standard MIDI vs SysEx

Standard MIDI messages are short (1–3 bytes) and well-defined: Note On, Note Off, Control Change, Program Change. Most MIDI devices respond to these — for example, sending a Program Change to a guitar pedal typically switches presets. The Play Acoustic sends these messages (CC#28 for HIT, CC#32 + PC for preset changes, CC#102–107 for footswitches) but ignores them when received.

SysEx messages are variable-length, manufacturer-specific data packets that can carry anything — preset data, configuration, firmware updates. They start with F0 (SysEx start) and end with F7 (SysEx end), with all data bytes between limited to 7-bit values (0x00–0x7F). This means regular 8-bit data must be encoded into 7-bit format.

How the Play Acoustic Communicates

All messages to and from the device use the TC Helicon SysEx header:

F0 00 01 38 00 70 [command] [data...] F7
│  │        │  │
│  │        │  └─ Play Acoustic device ID (0x70)
│  │        └──── Device group (0x00)
│  └───────────── Manufacturer ID (TC Helicon = 00 01 38)
└────────────────  SysEx start

What the Device Sends (outgoing standard MIDI)

• Program Change + Bank Select (CC#32) — when you switch presets on the device
• CC#28 — when HIT is toggled on or off
• CC#102–107 — when footswitches are pressed (requires Switch-3 or Switch-6 pedal configured to send MIDI)

What the Device Accepts (incoming SysEx only)

• Preset loading — the full preset (header + 5 data packets) must be transmitted. There is no "switch to preset N" command.
• Global settings — read and write the device's setup data (HIT state, guitar effects, pitch correction, looper input, etc.) via a read-modify-write cycle.

This is the same protocol that TC Helicon's VoiceSupport 2 application uses. Everything documented here was reverse-engineered by capturing and comparing data packets between VoiceSupport and the device.

Setup Parameter Map (Global Settings)

Global setup is stored in 7 packets of 112 bytes each (0x13 command). Each parameter has a logical index s<n> used by the editor; the underlying byte positions are handled by tch-format.js.

Read: BeginSync → RequestSetup (0x15 00) → receive 7 packets → EndSync

Write: BeginSync → RequestSetup → receive 7 packets → send 7 modified packets (30ms apart) → ACK → EndSync. Read and write must be in the same sync session.

Checksum: (sum of raw bytes 10–109) % 128, stored at raw byte 110.

SysEx Commands

All messages use header: F0 00 01 38 00 70 [command] [data] F7

Preset Parameter Map

Each preset consists of 5 data packets (0x21), each containing 25 parameters encoded as 4-byte 28-bit signed integers. Parameters are indexed 0–124 across the 5 packets.

Encoding: Each parameter uses 4 bytes in 7-bit MIDI format (28-bit signed). val = ((b0 & 0x7F) << 21) | ((b1 & 0x7F) << 14) | ((b2 & 0x7F) << 7) | (b3 & 0x7F). If val ≥ 0x08000000, subtract 0x10000000 for negative values.

Packet structure: F0 00 01 38 00 70 21 [chunk#] [100 bytes data] [checksum] F7 = 110 bytes total.

Packet 0 (Params 0–24)

Packet 1 (Params 25–49)

Packet 2 (Params 50–74)

Packet 3 (Params 75–99)

Packet 4 (Params 100–124)

Important Notes

• No MIDI CC input — the device only sends MIDI, never receives it.
• No preset switch command — full preset data must be sent each time.
• Guitar Boost on/off — the toggle is volatile (runtime only), not accessible via SysEx. Only the boost level (0–9 dB) can be read and written.
• Effect state on preset load — when a preset loads, it sets the guitar effect block enables in the global setup. These can be overridden by writing to the setup after the preset loads.
• Global Guitar FX — when enabled on the device, guitar effect states persist across preset changes. When disabled, each preset resets them.
• Global Guitar FX = Off gates the setup-side guitar FX bytes — they become read-only and writes are silently dropped. Toggling On→Off resets the BodyRez Custom values to factory defaults.
• Preset addressing — Bank 0: presets 1–127 (slots 1–127). Bank 1: presets 0–127 (slots 128–255). Full preset = bank × 128 + in-bank number.
• Verified mappings only — this page only lists parameters where the byte-to-meaning mapping has been verified on the device. Unmapped or speculative bytes (still tracked internally for round-trip integrity) are intentionally omitted.