Paula Makes a Sound

Every unit so far has been about the screen. The Amiga's fourth custom chip, Paula, is about the speakers — and she works on the same idea as everything else: hand a chip some data in memory and switch on its DMA.

Paula plays samples. A sample is a list of numbers, each one describing the height of a sound wave at one instant. Feed those numbers to a speaker fast enough and you hear the waveform they trace. Paula reads them out of memory by herself, on a DMA channel — she has four, one per voice — looping back to the start when she reaches the end. The CPU sets her going and then does nothing; the sound keeps playing.

To start a voice you give Paula four things: where the sample is, how long it is, how fast to play it (the period — the pitch), and how loud (the volume). Then you switch on her DMA. This unit plays one steady tone.

What you'll hear by the end

A steady tone, held for as long as the machine runs. It's a square wave — the sample is half high, half low — which is why it has that flat, buzzy, early-computer voice. The waveform strip above is the real captured output: a sample whose two halves are plainly visible.

Hand Paula the sample

;──────────────────────────────────────────────────────────────
; Meet the Machine (Amiga) - Unit 18: Paula Makes a Sound
;
; Paula is the sound chip. You hand her a sample - a list of numbers describing
; a waveform - tell her how loud, how fast and how long, and switch on her DMA.
; She then plays it on her own, looping, while the CPU does nothing. Here: a tone.
;──────────────────────────────────────────────────────────────

CUSTOM      equ $dff000
DMACON      equ $096
INTENA      equ $09a
INTREQ      equ $09c
COP1LC      equ $080
COPJMP1     equ $088
BPLCON0     equ $100
COLOR00     equ $180
AUD0LC      equ $0a0            ; channel 0 sample address (long)
AUD0LEN     equ $0a4            ; length in words
AUD0PER     equ $0a6            ; period (pitch)
AUD0VOL     equ $0a8            ; volume (0-64)

SAMPLELEN   equ 32              ; bytes in the sample (one square-wave cycle)

            section code,code_c

start:
            lea     CUSTOM,a5
            move.w  #$7fff,INTENA(a5)
            move.w  #$7fff,INTREQ(a5)
            move.w  #$7fff,DMACON(a5)

            ; --- a background colour, so the screen shows it's running ---
            lea     copperlist,a0
            move.l  a0,COP1LC(a5)
            move.w  d0,COPJMP1(a5)

            ; ----------------------------------------------- YOUR CODE START
            ; --- hand Paula the sample and start it on channel 0 ---
            lea     sample,a0
            move.l  a0,AUD0LC(a5)       ; where the waveform lives
            move.w  #SAMPLELEN/2,AUD0LEN(a5)  ; its length in words
            move.w  #320,AUD0PER(a5)    ; period - lower is higher-pitched
            move.w  #64,AUD0VOL(a5)     ; full volume

            move.w  #$8281,DMACON(a5)   ; DMA on: master + Copper + audio ch0
            ; ------------------------------------------------- YOUR CODE END

forever:
            bra.s   forever

;──────────────────────────────────────────────────────────────
; The sample: one cycle of a square wave - half high, half low.
; Paula reads signed 8-bit numbers; $40 is +64, $c0 is -64.
;──────────────────────────────────────────────────────────────
            even
sample:
            dc.b    $40,$40,$40,$40,$40,$40,$40,$40
            dc.b    $40,$40,$40,$40,$40,$40,$40,$40
            dc.b    $c0,$c0,$c0,$c0,$c0,$c0,$c0,$c0
            dc.b    $c0,$c0,$c0,$c0,$c0,$c0,$c0,$c0

            even
copperlist:
            dc.w    BPLCON0,$0200       ; no bitplanes - just a flat colour
            dc.w    COLOR00,$008f       ; background blue, "sound is playing"
            dc.w    $ffff,$fffe

The sound is five lines:

lea     sample,a0
move.l  a0,AUD0LC(a5)       ; where the waveform lives
move.w  #SAMPLELEN/2,AUD0LEN(a5)  ; its length in words
move.w  #320,AUD0PER(a5)    ; period - lower is higher-pitched
move.w  #64,AUD0VOL(a5)     ; full volume

move.w  #$8281,DMACON(a5)   ; DMA on: master + Copper + audio ch0

AUD0LC is the sample's address — channel 0's "where." AUD0LEN is its length in words. AUD0PER is the period: it sets how fast Paula steps through the sample, and so the pitch — a smaller period plays faster and sounds higher. AUD0VOL is the volume, 0 to 64. With those four set, switching on the audio-0 DMA bit in DMACON starts the voice, and Paula loops the sample for as long as the channel stays on.

The sample itself is a single square-wave cycle — sixteen high numbers, then sixteen low:

sample:
        dc.b    $40,$40,...     ; +64, the top of the wave
        dc.b    $c0,$c0,...     ; -64, the bottom

Paula reads these as signed 8-bit numbers, so $40 is +64 and $c0 is −64. Played in a loop, that step from high to low and back is the square wave you hear. A richer sample — a recorded instrument, a voice — is the same idea with more numbers.

Assemble, master, and run

make

A steady tone from the speakers; a blue screen that does nothing.

Try this: change the pitch

AUD0PER is the period — #320 here. Halve it to #160 and the tone jumps an octave; double it to #640 and it drops one. The sample doesn't change; you're telling Paula to read it faster or slower.

Try this: shape the wave

Edit the sample. Make it eight high and twenty-four low and the tone goes thin and reedy; ramp the numbers up and down instead of stepping ($00, $20, $40, $60…) for a softer, rounder sound. The numbers are the waveform — change their shape and you change the timbre.

If it doesn't work

• Silence. The audio-0 DMA bit isn't set — DMACON needs it ($8281 here switches on audio channel 0). Volume at 0 is silent too.
• A click, then nothing. AUD0LEN is wrong. It's the length in words (bytes ÷ 2); too short and Paula loops a sliver, too long and she reads past the sample.
• The pitch is wildly off. A tiny AUD0PER plays faster than Paula can sustain. Keep it in the hundreds until the tone is steady.

What you've learnt

Paula plays sound by reading a sample — a list of numbers tracing a waveform — out of memory on a DMA channel. You set four registers per voice: location (AUD0LC), length (AUD0LEN), period (AUD0PER, the pitch), and volume (AUD0VOL), then switch on the channel's DMA. She loops it on her own while the CPU is free. The numbers in the sample are the waveform itself; their shape is the sound.

What's next

That's all four custom chips — Copper, Blitter, sprites, and now Paula — and the whole of the 68000 underneath them. One unit remains, and it isn't about a new instruction. Next — The Machine Trusts You — what it means to program a computer that hands you the hardware and gets out of the way.