APU
The MCPX contains an APU (Audio Processing Unit).
- SSL = Stream Segment List
- SGE = Scatter Gather Entry
- PRD = Physical Resource Descriptor (Same thing as SGE?!)
Contents
Voice Processor (VP)
A powerful voice processor. There can be up to 256 voices[citation needed] and 64[citation needed] of those can be 3D.
Per-voice settings:
- Input type (8bit, 16bit, 24bit, ADPCM)
- Head-related transfer function (HRTF)
- Low-frequency oscillation (LFO)
- Pitch
- 2x Pitch (?) envelope
- 2x LFO (?) envelope
- 8 target bins, each with a custom volume for this voice
There are 32 bins which these voices will be mixed into.
Related APU memory
- VPV = VP Voices
- VPHT = VP HRTF Target
- VPHC = VP HRTF Current
- VPSGE = VP SGEs
- VPSSL = VP SSLs
Voice lists
The voices are kept in a single-linked list. There are 3 voice lists:
- 2D
- 3D
- MP (Multipass?)
Voice structure
This is 0x80 bytes
Pitch calculation
The 16 bit signed pitch value (p) can be converted to and from a unsigned frequency in Hz (f) using the following formulas:
p = 4096 * log2(f / 48000) f = pow2(p / 4096) * 48000
Operation
Voices are stored in VPV. Input data (from the CPU) is loaded using VPSGE. Voices are then processed and written to the GP MIXBUF.
Global Processor (GP)
The GP runs all enabled sound effects on the voice bins.
The GP DSP seems to run at 160 MHz
MIXBUF
The MIXBUF is a 0x400 word (24-Bit, stored as 32-Bit) section. It is split into 32 * 0x20 words. Each 0x20 word block represents one of the 32 voice bins of the VP. The 0x20 words are 24-Bit PCM mono samples to be played back at 48kHz. The duration of each frame is hence 0.6ms.
Memory map
Related APU memory
- GPS = GP Scratch (?)
- GPF = GP FIFO
Encode Processor (EP)
The EP encodes the final AC3 stream for SPDIF. [FIXME] It is not used during the Boot Animation.
Memory map
Related APU memory
- EPS = EP Scratch (?)
- EPF = EP FIFO
Usage in DirectSound
This topic deserves it's own article[FIXME]
The bins are used [FIXME] DirectSound allows to load custom GP DSP code for filter / effects. [FIXME] The GP waits for the frame interrupt which signals that MIXBUF data is available. It then goes through a filter chain. At the end of the chain, the GP DSP will verify that the execution didn't take longer than the frame duration.
The GP will then issue 6 DMA requests to output the processed frames to a ringbuffer in scratch space. The frameformat will be the same format as the GP MIXBUF format (also 0x20 words per channel). Each ringbuffer is 0x200 words and therefore holds the last 16 frames. Therefore, the ringbuffer region is 6 * 0x800 Bytes = 0x3000 Bytes in physical memory.
The order of the channels in the ringbuffer is (also DMA order):
- 0: Front Left
- 1: Center[citation needed]
- 2: Front Right
- 3: Rear Left[citation needed]
- 4: Rear Right[citation needed]
- 5: Low-frequency effects (LFE)[citation needed]
The EP maps the same data to its own scratch space. It is assumed that it will DMA this region to its own internal memory. The EP then AC3 encodes the audio data[citation needed] and writes it to the EP FIFO memory[FIXME]. [FIXME] The data is then send to the ACI AC97 using EP FIFO channels 0 (PCM) and 1 (SPDIF)[citation needed].
Modifications for Boot Animation
During the Boot Animation a different version of DirectSound is used. The EP is disabled in this case. The data is send to the ACI AC97 using GP FIFO channel 0 (PCM). There is no AC3 / SPDIF during the boot animation[1][citation needed].
