Table of Contents
The sound controller has 16 channels with configurable replay frequencies, and volume envelope generators. Each channel has two 22-bit pointers to a sample in RAM or ROM, one used for the initial sound trigger and the second used as a loop point. Samples can be either 8-bit or 16-bit unsigned PCM, or IMA ADPCM.
Channel register map
| Address | Function |
|---|---|
| 30×0 | Channel X sample address |
| 30×1 | Channel X control (bit 14: 16M, 13-12: ToneMode, 11-6: Loop address segment, 5-0: sample address segment) |
| 30×2 | Channel X loop address |
| 30×3 | Channel X panning and volume (7 bits each) |
| 30×4 | Channel X envelope target volume and increment |
| 30×5 | Channel X envelope CNT and EDD (?) |
| 30×6 | Channel X repeat count (bits 15-9), repeat enable (8), Envelope load (7-0) |
| 30×7 | Channel X IRQ address (15-9), IRQ enable, envelope segment (6-0) |
| 30×8 | Channel X envelope address |
| 30×9 | Channel X wave data 0 |
| 30xA | Channel X ramp down offset (15-9) and envelope address offset (8-0) |
| 30xB | Channel X wave data |
| 32×0 | Phase high bits (2-0) |
| 32×1 | Phase accumulator high (2-0) |
| 32×2 | Target phase high (2-0) |
| 32×3 | Ramp down clock |
| 32×4 | Phase low bits |
| 32×5 | Phase accumulator low bits |
| 32×6 | Target phase low bits |
| 32×7 | Phase control: time step (15-13), sign (12), offset (11-0) |
Global register map
| Address | Function |
|---|---|
| 3400 | Channel enable (1 bit per channel) |
| 3401 | Main volume (7 bits) |
| 3402 | Channel FIQ enable |
| 3403 | Channel FIQ status |
| 3404 | Beat base counter |
| 3405 | Beat counter (13-0), IRQ enable (15) and status (14) |
| 3406 | Envelope interval, channels 0-3 |
| 3407 | … channels 4-7 |
| 3408 | … channels 8-11 |
| 3409 | … channels 12-15 |
| 340A | Envelope fast ramp-down |
| 340B | Channel stop status (1 bit per channel) |
| 340C | Zero cross enable (1 bit per channel) |
| 340D | Control flags: NoInt (9), LPEn (8), HVol(7-6), SOF (5), Init (3), PCM (2) |
| 340F | Channels status |
| 3410 | Left mixer input |
| 3411 | Right mixer input |
| 3412 | Left mixer output |
| 3413 | Right mixer output |
| 3414 | Channel repeat enable (1 bit per channel) |
| 3415 | Channel envelope mode |
| 3416 | Channel tone release control |
| 3417 | Channel envelope IRQ status |
| 3418 | Channel pitch band enable |
Sound formats and tone modes
The formats supported by the SPU is IMA ADPCM, 8-bit unsigned PCM and 16-bit unsigned PCM.
The sound format and tone mode used for each channel are controlled by the channel mode register (30×1), which contains the following fields:
- IMA ADPCM enabled (bit 15)
- Sample bit depth (bit 14)
- 0 - 8-bit PCM
- 1 - 16-bit PCM (should also be used for ADPCM)
- Tone mode (bits 12-13)
- 00 - software mode (software writes samples manually to wave data register)
- 01 - auto-end mode
- 10 - auto-repeat mode
- 11 - reserved
- High bits (16-21) of loop address (bits 6-11)
- High bits (16-21) of wave address (bits 0-5)
The low 16 bits of the wave address and loop address are stored in 30×0 and 30×2 respectively.
The auto-end and auto-repeat modes automatically fetch samples during play from the value address pointed at by the wave address register, which is automatically incremented by hardware during playing. When an end marker value is fetched (FF in any of the word halves for 8-bit PCM and FFFF for 16-bit PCM and ADPCM), the hardware will either stop the channel when in auto-end mode or reset the wave address to the value of the loop address when in auto-repeat mode.
Channel phase logic
The Phase (32×0, 32×4) and Phase Accumulator (32×1, 32×5) registers are 19-bit registers used for linearly interpolating between samples and controlling the sample rate of each channel when in auto-end or auto-repeat mode.
For each 281250 Hz tick (27 MHz/96), the Phase Accumulator register for each active channel is increased by the value of the Phase register. If the Phase Accumulator register wraps around, the next sample is loaded from the wave address into the Wave Data register of the channel and the previous one is moved into the Wave Data 0 register.
If interpolation is turned on in the control flag register (340D), the channel output is generated
by the linear interpolation formula ((WaveData0 * (0x80000 - PhaseAcc)) >> 19) + ((WaveData * PhaseAcc) >> 19).
The exact precision used in the interpolation is currently unknown.
The phase value to use for a sample can be calculated from the audio sample rate with the formula Phase = SampleRate * (0x80000 / 281250).
When initializing a channel, Wave Data and Wave Data 0 should be set to 0x8000 if 16-bit or ADPCM, or 0x8080 if 8-bit to avoid buggy interpolation in the initial sample.
Document the optional phase pitch bend feature
