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136 lines
4.4 KiB
Markdown
136 lines
4.4 KiB
Markdown
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[← Home](../../README.md) · [Hardware](../README.md) · [OCS](README.md)
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# Paula — Audio DMA
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## Overview
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**Paula** (MOS 8364) provides four independent audio DMA channels (0–3), corresponding to the four hardware voice outputs:
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| Channel | Stereo Output |
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|---|---|
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| 0 | Left |
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| 1 | Right |
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| 2 | Right |
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| 3 | Left |
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Each channel has its own DMA pointer, length, period (pitch), and volume registers. Paula fetches sample data from **Chip RAM** automatically at the rate set by the period register.
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## Audio Registers Per Channel
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Channel `n` (n = 0–3) registers at `$DFF0A0 + n×$10`:
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| Offset | Name | Description |
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|---|---|---|
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| +$00/$01 | AUDnLCH/AUDnLCL | Sample pointer high/low (Chip RAM address) |
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| +$04 | AUDnLEN | Sample length in words (not bytes) |
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| +$06 | AUDnPER | Period — clock divider for playback rate |
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| +$08 | AUDnVOL | Volume: 0–64 (0 = silent, 64 = full) |
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| +$0A | AUDnDAT | Current sample word (direct, for non-DMA mode) |
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## Sample Playback Rate
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Paula derives playback rate from the system clock divided by the period register:
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```
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Sample rate (Hz) = System clock / Period
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```
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| System | Clock | Period for 8363 Hz | Period for 22050 Hz |
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|---|---|---|---|
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| PAL | 3,546,895 Hz | 428 | ~161 |
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| NTSC | 3,579,545 Hz | 428 | ~162 |
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> **8363 Hz** is the standard MOD tracker reference — period 428 on PAL plays middle-A at exactly 8363 Hz.
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Period range: 124–65535 (minimum period = maximum frequency ≈ 28 kHz PAL).
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## Starting Audio DMA
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```asm
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; Load channel 0 with sample at SampleData, length 256 words
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move.l #SampleData, d0
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move.w d0, AUD0LCL+custom
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swap d0
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move.w d0, AUD0LCH+custom
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move.w #256, AUD0LEN+custom ; 256 words = 512 bytes
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move.w #428, AUD0PER+custom ; period 428 = 8287 Hz PAL
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move.w #64, AUD0VOL+custom ; full volume
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; Enable audio DMA for channel 0
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move.w #$8201, DMACON+custom ; SET + MASTER + AUD0EN
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; Enable audio interrupt (optional)
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move.w #$A080, INTENA+custom ; SET + INTEN + AUD0
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```
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## ADKCON — Audio/Disk Control
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| Bit | Name | Description |
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|---|---|---|
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| 15 | SET/CLR | Write: 1=set, 0=clear bits |
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| 11 | PRECOMP1 | Disk precompensation |
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| 10 | PRECOMP0 | Disk precompensation |
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| 9 | MFMPREC | MFM precompensation |
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| 7 | MSBSYNC | Audio sync from MSB |
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| 6 | WORDSYNC | Disk word sync enable |
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| 4 | ATPER | Channel 3 period from channel 2 data |
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| 3 | ATVOL | Channel 3 volume from channel 2 data |
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| 2 | ATPER2 | Channel 1 period from channel 0 data |
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| 1 | ATVOL2 | Channel 1 volume from channel 0 data |
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| 0 | (unused) | |
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`ATPER`/`ATVOL` bits enable **audio modulation** — channel N's period/volume is modulated by the sample data of channel N-1. Used for AM synthesis effects (e.g., in many MOD players for 8-channel soft mixing).
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## Interrupt Handling
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Audio channels raise interrupts when their DMA pointer wraps to the beginning of the next buffer:
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```
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INTENA/INTREQ bits:
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AUD0 = bit 7 (IPL 4)
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AUD1 = bit 8 (IPL 4)
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AUD2 = bit 9 (IPL 4)
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AUD3 = bit 10 (IPL 4)
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```
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The interrupt fires when the channel has consumed its buffer and reloaded the pointer from the DMA registers. At this point, software can update AUDnLCH/L and AUDnLEN with the next buffer.
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## Double-Buffering Pattern
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```c
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/* In interrupt handler (IPL 4 server for AUD0): */
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void audio_interrupt(void)
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{
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/* Swap buffers */
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UWORD *next = (active_buf == buf_a) ? buf_b : buf_a;
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active_buf = next;
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/* Load next buffer address */
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custom.aud[0].ac_ptr = (UWORD *)next;
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custom.aud[0].ac_len = BUF_WORDS;
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}
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```
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## Direct (Non-DMA) Audio
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For simple sound effects without DMA overhead, write directly to `AUDnDAT`:
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```asm
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move.w #$0080, AUD0VOL+custom ; volume 64 (approx)
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move.w #$7FFF, AUD0DAT+custom ; max positive sample
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```
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This only produces a single sample word — not practical for continuous audio but useful for one-shot clicks/beeps.
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## audio.device
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AmigaOS provides `audio.device` for arbitrated, multi-application audio access:
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- Allocates channels (bitmask: `{1,2,4,8}`)
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- Creates `IOAudio` request, uses standard device I/O (`BeginIO`/`WaitIO`)
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- Handles period, volume, sample pointer, cycle count
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- See [10_devices/audio_device.md](../../10_devices/audio_device.md) for full API reference
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## References
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- ADCD 2.1 Hardware Manual — Paula audio chapter
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- NDK39: `hardware/custom.h` (struct AudChannel), `devices/audio.h`
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- Autodocs: audio.device — http://amigadev.elowar.com/read/ADCD_2.1/Includes_and_Autodocs_3._guide/node0081.html
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