The amp connects via USB, and exposes two devices: an audio device, and a HID device.
The HID device is used for communication between the computer and amp over interrupt transfers.
Each packet is 64 bytes, with 1 byte of padding, and encoded as TLV in the following format:
Host -> Device
| 1 | 2 | 3-63 | 64 |
|---|---|---|---|
| tag | length | value | 0x00 |
Device -> Host
| 1 | 2 | 3 | 4-64 |
|---|---|---|---|
| 0x00 | tag | length | value |
The tags I've identified are as follows:
| Tag | Meaning |
|---|---|
| 0x33 | Start of multi-packet data |
| 0x34 | Continuation of multi-packet data |
| 0x35 | Final packet (or only packet) |
For multi-packet messages, it's necessary to drop the 0x00 padding and the tag and length bytes of each packet and concatenate them to get the message.
The value in each message is a protobuf encoded message (FenderMessageLT) that contains a responseType and a submessage containing the data for the message (documentation).
The protobuf definition files were extracted from the Fender Mustang LT Desktop executable.
The Fender Tone LT Desktop app initializes the connection by sending SYNC_BEGIN modalStatusMessage, and a firmwareVersionRequest, followed by a retrievePreset message for all 60 presets:
(>>> is host to amp, <<< is amp to host)
>>> { "responseType": "UNSOLICITED", "modalStatusMessage": { "context": "SYNC_BEGIN", "state": "OK" } }
<<< { "responseType": "IS_LAST_ACK", "modalStatusMessage": { "context": "SYNC_BEGIN", "state": "OK" } }
>>> { "responseType": "UNSOLICITED", "firmwareVersionRequest": { "request": true } }
<<< { "responseType": "IS_LAST_ACK", "firmwareVersionStatus": { "version": "2.1.4" } }
>>> { "responseType": "UNSOLICITED", "retrievePreset": { "slot": 1 } }
<<< { "responseType": "IS_LAST_ACK", "presetJSONMessage": { "data": "{...}", "slotIndex": 1 } }
>>> { "responseType": "UNSOLICITED", "retrievePreset": { "slot": 2 } }
<<< { "responseType": "IS_LAST_ACK", "presetJSONMessage": { "data": "{...}", "slotIndex": 2 } }
...
>>> { "responseType": "UNSOLICITED", "retrievePreset": { "slot": 60 } }
<<< { "responseType": "IS_LAST_ACK", "presetJSONMessage": { "data": "{...}", "slotIndex": 60 } }
In my testing, initializing the connection to the amp only required a SYNC_BEGIN modalStatusMessage, followed by a SYNC_END modalStatusMessage, in order to clear the synchronization message from the screen.
>>> { "responseType": "UNSOLICITED", "modalStatusMessage": { "context": "SYNC_BEGIN", "state": "OK" } }
<<< { "responseType": "IS_LAST_ACK", "modalStatusMessage": { "context": "SYNC_BEGIN", "state": "OK" } }
>>> { "responseType": "UNSOLICITED", "modalStatusMessage": { "context": "SYNC_END", "state": "OK" } }
<<< { "responseType": "IS_LAST_ACK", "modalStatusMessage": { "context": "SYNC_END", "state": "OK" } }
It's also necessary to send a heartbeat message every second to keep the connection open.
>>> { "responseType": "UNSOLICITED", "heartbeat": { "dummyField": true } }
The presets are sent and received as JSON (schema).