Commodore AmigaAudioBeginner

Playing a Sample

Point one of Paula's four audio channels at an 8-bit sample in chip RAM, set its pitch and volume, and let DMA play it — the foundation of all Amiga sound.

paulasamplesdmaaudio

Overview

Paula, the Amiga’s sound chip, has four audio channels, and each plays a stream of 8-bit signed samples straight from chip RAM by DMA — no CPU involvement once it’s running. That’s what made Amiga audio feel a generation ahead: real recorded sound, four voices, for free. Getting one channel going is four steps: point it at the sample, tell it how long, set the pitch and the volume, then switch its DMA on.

Code

; =============================================================================
; PLAYING A SAMPLE - AMIGA (Paula)
; Channel 0 loops an 8-bit sample from chip RAM via DMA
; CPU: None (DMA plays it) | Memory: the sample, which MUST be in chip RAM
; =============================================================================

CUSTOM   equ $dff000
DMACON   equ $096            ; DMA control
AUD0LC   equ $0a0            ; Channel 0 sample pointer (longword)
AUD0LEN  equ $0a4            ; Channel 0 length, in WORDS (bytes / 2)
AUD0PER  equ $0a6            ; Channel 0 period (pitch)
AUD0VOL  equ $0a8            ; Channel 0 volume (0-64)

            section code,code_c

start:
            lea     CUSTOM,a5

            ; --- 1. Point channel 0 at the sample, and give its length ---
            move.l  #sample,AUD0LC(a5)
            move.w  #(sample_end-sample)/2,AUD0LEN(a5)   ; length in words

            ; --- 2. Pitch, then 3. volume ---
            move.w  #428,AUD0PER(a5)     ; period 428 = the note C-3 (~8.3 kHz PAL)
            move.w  #64,AUD0VOL(a5)      ; 64 = full volume

            ; --- 4. Enable channel 0's DMA — Paula begins, and loops, at once ---
            move.w  #$8201,DMACON(a5)    ; SET | DMAEN | AUD0

mainloop:
            bra.s   mainloop

; --- 8-bit SIGNED sample, in CHIP RAM (one cycle of a sine; Paula loops it) ---
            section data,data_c
            even
sample:
            dc.b    0,48,90,117,127,117,90,48,0,-48,-90,-117,-127,-117,-90,-48
sample_end:

Trade-offs

Aspect Cost
CPU None — DMA streams the sample by itself
Memory The sample, in chip RAM (Paula can’t see fast RAM)
Limitation 8-bit samples; Paula loops by default — a one-shot needs a little more

When to use: Every Amiga sound — music, effects, speech. This is the base case.

When to avoid: Never for sound; it’s the only way in. (For a single beep you could poke AUDxDAT by hand, but DMA is how real audio is done.)

Paula’s four channels

Each channel is a 16-byte block of registers; channel n is at $DFF0A0 + n*$10:

Offset Register Purpose
+$0 AUDxLC Sample start (longword pointer)
+$4 AUDxLEN Length, in words
+$6 AUDxPER Period (pitch)
+$8 AUDxVOL Volume, 0–64

And the channels are wired to the stereo outputs: 0 and 3 go left, 1 and 2 go right. Music that wants a channel each side pairs (0,3) against (1,2).

Period and pitch

Pitch is set by the period — the number of bus cycles between output samples — so lower period means higher pitch. On PAL the playback rate is:

sample rate (Hz) = 3546895 / period

Trackers pick periods from a fixed table where each entry is a semitone; period 428 is C-3, halve it (214) and you’re an octave up. To play a recorded sample at the right pitch you set the period to match how it was sampled.

Chip RAM, always

The single most common Amiga audio bug: Paula’s DMA reads only chip RAM. A sample left in fast RAM produces silence or noise. Put sample data in a _c (chip) section, or copy it to chip RAM before playing — hence section data,data_c above.

Looping vs one-shot

Enable a channel and Paula plays AUD0LC/AUD0LEN and then loops it forever — perfect for a sustained instrument. To play a sound once, you catch Paula’s channel interrupt after the first pass and repoint it at a silent word; see One-Shot Samples.

Patterns: VBlank Game Loop

Vault: Paula | Commodore Amiga