Paula: The Amiga's Voice

Overview

Paula was years ahead of its time. While other computers synthesised sound, the Amiga played back digital samples directly from memory. Four independent channels, stereo output, and variable sample rates enabled music and sound effects that rivalled dedicated audio hardware.

Fast facts

Chip: 8364 (unchanged across OCS/ECS/AGA).
Channels: 4 independent voices.
Sample depth: 8-bit signed.
Sample rate: variable, up to ~28 kHz per channel.
Output: stereo (channels 0+3 left, 1+2 right).

Audio architecture

Channel	Stereo	Typical use
0	Left	Melody/effects
1	Right	Melody/effects
2	Right	Bass/drums
3	Left	Bass/drums

DMA playback

Paula plays samples without CPU intervention:

Set sample start address and length.
Set period (sample rate).
Set volume.
Enable DMA—Paula handles the rest.

Sample rate calculation

The period register determines playback rate:

Sample rate = Clock / (Period × 2)

PAL clock:  3,546,895 Hz
NTSC clock: 3,579,545 Hz

Period	PAL rate	Note equivalent
124	14,306 Hz	—
214	8,287 Hz	—
428	4,143 Hz	C-2 base
856	2,072 Hz	C-1

Channel registers

Each channel has dedicated registers:

Register	Offset	Purpose
AUDxLCH	+$00	Sample address high
AUDxLCL	+$02	Sample address low
AUDxLEN	+$04	Sample length (words)
AUDxPER	+$06	Period (sample rate)
AUDxVOL	+$08	Volume (0-64)
AUDxDAT	+$0A	Sample data (direct write)

Base addresses: $DFF0A0 (ch0), $DFF0B0 (ch1), $DFF0C0 (ch2), $DFF0D0 (ch3).

Volume control

Value	Level
0	Silent
64	Maximum
>64	Wraps (avoid)

Audio modulation

Paula supports hardware modulation between channel pairs:

Mode	Effect
AM	Channel N modulates volume of N+1
FM	Channel N modulates period of N+1

Rarely used in games but powerful for synthesis.

Interrupt generation

Paula generates interrupts when:

A sample finishes playing.
A new sample buffer is needed.

This enables double-buffering for seamless playback.

MOD format connection

The ProTracker MOD format maps directly to Paula:

4 channels match Paula’s hardware.
Period values in MOD = Paula period registers.
Volume 0-64 matches Paula’s range.
Sample data plays without conversion.

Additional functions

Paula also handles:

Function	Purpose
Disk I/O	Floppy drive data transfer
Serial	RS-232 communication
Interrupts	CIA and internal interrupt routing

Programming example

; Play a sample on channel 0
    lea     $dff0a0,a0          ; Channel 0 base
    move.l  #sample,AUD0LCH(a0) ; Sample address
    move.w  #length/2,AUD0LEN(a0) ; Length in words
    move.w  #428,AUD0PER(a0)    ; Period (C-2)
    move.w  #64,AUD0VOL(a0)     ; Full volume
    move.w  #$8201,$dff096      ; Enable DMA

Four channels of sample-based audio