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73 lines
1.7 KiB
Markdown
73 lines
1.7 KiB
Markdown
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[← Home](../README.md) · [Devices](README.md)
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# audio.device — DMA Audio Channels
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## Overview
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`audio.device` provides access to the Amiga's 4 DMA audio channels. Each channel plays 8-bit PCM samples from Chip RAM at programmable rates.
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---
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## Channel Allocation
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```c
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UBYTE allocationMap[] = { 1, 2, 4, 8 }; /* channel masks */
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struct IOAudio *aio = (struct IOAudio *)
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CreateIORequest(port, sizeof(struct IOAudio));
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aio->ioa_Request.io_Message.mn_Node.ln_Pri = 0;
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aio->ioa_Data = allocationMap;
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aio->ioa_Length = sizeof(allocationMap);
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OpenDevice("audio.device", 0, (struct IORequest *)aio, 0);
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/* aio->ioa_AllocKey = allocation key for this channel */
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```
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---
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## Playing a Sample
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```c
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aio->ioa_Request.io_Command = CMD_WRITE;
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aio->ioa_Request.io_Flags = ADIOF_PERVOL;
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aio->ioa_Data = sampleData; /* MUST be in Chip RAM */
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aio->ioa_Length = sampleLength; /* in bytes */
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aio->ioa_Period = 428; /* ~8287 Hz (PAL) */
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aio->ioa_Volume = 64; /* 0–64 */
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aio->ioa_Cycles = 1; /* 0 = loop forever */
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BeginIO((struct IORequest *)aio);
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```
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### Period Calculation
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```
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Period = clock_constant / desired_frequency
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PAL: clock = 3546895 Hz → Period = 3546895 / freq
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NTSC: clock = 3579545 Hz → Period = 3579545 / freq
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```
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| Frequency | Period (PAL) |
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|---|---|
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| 8287 Hz | 428 |
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| 11025 Hz | 322 |
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| 22050 Hz | 161 |
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| 28867 Hz | 124 (minimum safe) |
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---
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## Channel Registers
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| Channel | Address | Description |
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|---|---|---|
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| 0 | `$DFF0A0` | AUD0 (left) |
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| 1 | `$DFF0B0` | AUD1 (right) |
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| 2 | `$DFF0C0` | AUD2 (right) |
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| 3 | `$DFF0D0` | AUD3 (left) |
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Each channel: pointer (PTH/PTL), length (LEN), period (PER), volume (VOL).
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---
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## References
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- NDK39: `devices/audio.h`
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- HRM: audio DMA chapter
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