Note that everything below may vary depending on the exact DSP firmware used and different variants have slightly different behaviours.
Summary
Some versions of the DSP firmware contain a decoder for the AAC audio codec. Such firmware may be dumped from specific games, like Pokemon X/Y or Rhythm Heaven Megamix. Interfacing with the AAC decoder (sending requests/receiving responses) happens via the DSP binary pipe (pipe 3)
Message Format
Below is the message format used for communicating with the AAC decoder. The same format is both when making requests and for responses, though the "request/response data" field changes appropriately.
Size : 0x20 bytes
| Offset in bytes | Type | Description |
|---|---|---|
| 0x0 | u16 | Mode (Seems to be 0 = None, 1 = Decode AAC, 2 = Encode AAC) |
| 0x2 | DecoderCommand | Command to be handled by the decoder (See information on DecoderCommand structure below) |
| 0x4 | u32 | Result Code |
| 0x8 | Request/Response data | A 24-byte chunk of data containing information about the request/response. See below for more info. |