Commodore AmigaAudioIntermediate

One-Shot Samples

Play a sample once instead of looping, using Paula's channel interrupt to stop it after a single pass — the way sound effects fire, and the seed of tracker playback.

paulasamplesinterruptaudio

Overview

Enable a channel and Paula loops the sample forever — right for a held note, wrong for an explosion. To play a sound once, you need to know the moment it finishes. Paula tells you: at the end of every pass it raises that channel’s audio interrupt. Catch it, silence the channel, and you have a one-shot. That same interrupt, instead of stopping the sound, is how a tracker hands the channel its next note — so this pattern is the doorway to music too.

Code

; =============================================================================
; ONE-SHOT SAMPLES - AMIGA (Paula)
; Fire a sample on channel 0; its interrupt stops it after one pass
; Assumes master interrupts on, and the level-4 autovector ($70) -> audio_irq
; =============================================================================

CUSTOM   equ $dff000
DMACON   equ $096
INTENA   equ $09a            ; Interrupt enable
INTREQ   equ $09c            ; Interrupt request (acknowledge here)
AUD0LC   equ $0a0
AUD0LEN  equ $0a4
AUD0PER  equ $0a6
AUD0VOL  equ $0a8

AUD0INT  equ $0080           ; Audio channel 0 interrupt bit (bit 7)

            section code,code_c

play_shot:
            lea     CUSTOM,a5

            ; --- Set the sample up, exactly as for a looping one ---
            move.l  #sample,AUD0LC(a5)
            move.w  #(sample_end-sample)/2,AUD0LEN(a5)
            move.w  #428,AUD0PER(a5)
            move.w  #64,AUD0VOL(a5)

            ; --- Arm channel 0's interrupt, then start the DMA ---
            move.w  #$8080,INTENA(a5)           ; SET | AUD0INT ($8000|$0080) — arm ch 0
            move.w  #$8201,DMACON(a5)           ; SET | DMAEN | AUD0 — Paula plays

            rts

; --- Level-4 interrupt handler: fires when the pass ends ---
audio_irq:
            move.w  #AUD0INT,INTREQ(a5)     ; acknowledge (bit 15 = 0 -> CLEAR)
            move.w  #$0001,DMACON(a5)       ; CLR | AUD0 — stop the channel
            rte

            section data,data_c
            even
sample:
            dc.b    0,60,110,127,100,50,-10,-70,-120,-127,-90,-40,10,55,30,0
sample_end:

Trade-offs

Aspect Cost
CPU A tiny interrupt per sound
Memory Sample in chip RAM, plus a handler
Limitation The interrupt fires after the pass — a stopped channel can replay a sliver

When to use: Sound effects — shots, jumps, hits — and as the tick that drives a tracker’s sequencing.

When to avoid: Sustained instrument tones that are meant to loop; leave those looping.

The glitch-free variant

Because the interrupt arrives at the end of the pass, Paula may already be a moment into the next loop before the handler stops it — a faint replay. The classic cure needs no interrupt at all: right after enabling DMA (once Paula has latched the real sample), point the channel at a one-word silent loop:

            move.l  #silence,AUD0LC(a5)     ; queue silence as the loop...
            move.w  #1,AUD0LEN(a5)          ; ...one word of zero
; ...
silence:    dc.w    0                       ; in chip RAM

The real sample plays once, then Paula loops the silence — inaudibly. The catch is timing: you must write the silence pointer after Paula has copied the real LC/LEN into its internal registers (a scanline’s wait is ample), or the sample never plays.

From one-shot to music

Don’t silence the channel in the handler — hand it the next sample instead (new LC/LEN/PER/VOL) — and you’ve built the core of a sample sequencer. A four-channel MOD player is exactly this on all four voices: a tick, a pattern of notes, and the period table turning note numbers into pitches.

Patterns: Playing a Sample, Four-Channel Music

Vault: Paula | Commodore Amiga