DSP AAC Decoder
Note that everything below may vary depending on the exact DSP firmware used and different variants have slightly different behaviours. Also note that some things about the AAC firmware still need to be properly tested and/or reverse engineered.
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. |
enum DecoderCommand : u16
| Enum | Value |
|---|---|
| Initialize (Initialize AAC decoder) | 0 |
| DecodeEncode (Decode or encode an AAC stream) | 1 |
| Shutdown | 2 |
| LoadState | 3 |
| SaveState | 4 |
Request/Response Data
The 24-byte "request/response data" chunk can take on one of the formats listed below, depending on what sort of message it's found in.
AAC Decode request
Size : 24 bytes
| Offset in bytes | Type | Description |
|---|---|---|
| 0x0 | u32 | Physical address of input AAC stream |
| 0x4 | u32 | Byte size of input AAC stream |
| 0x8 | u32 | Physical address to output decoded PCM16 samples to (In the case of stereo audio, this corresponds to the left channel) |
| 0xC | u32 | Physical address to output decoded PCM16 samples for the right channel |
| 0x10 | u32 | Unknown? |
| 0x14 | u32 | Unknown? |
AAC Decode command response
Size : 24 bytes
| Offset in bytes | Type | Description |
|---|---|---|
| 0x0 | enum SampleRate | Sample rate of the decoded AAC stream (Note: This is an enum, NOT a numerical value for the sample rate) |
| 0x4 | u32 | Channel count (?) |
| 0x8 | u32 | Size |
| 0xC | u32 | Unknown? |
| 0x10 | u32 | Unknown? |
| 0x14 | u32 | Number of decoded samples (?) |
Sample rate enum
enum SampleRate : u32
| Enum | Value |
|---|---|
| 48000Hz (48KHz) | 0 |
| 44100Hz (44.1KHz) | 1 |
| 32000Hz (32KHz) | 2 |
| 24000Hz (24KHz) | 3 |
| 22050Hz (22.05KHz) | 4 |
| 16000Hz (16KHz) | 5 |
| 12000Hz (12KHz) | 6 |
| 11025Hz (11.025KHz) | 7 |
| 8000Hz (8KHz) | 8 |