APU basics

This article covers basic usage of the APU's four waveform channels on an NTSC NES. It does not cover the DMC, or more advanced usage. Any registers unrelated to basic operation are not even mentioned here. A simplified though fully usable model of the APU is presented, one that will serve many programmers.

Channels

The APU has five channels: two pulse waves, triangle wave, noise, and DMC (sample playback). Only the first four are covered here.

The channel registers begin at $4000, and each channel has four registers devoted to it. All but the triangle wave have 4-bit volume control (the triangle just has an un-mute flag).

$4000-$4003	First pulse wave
$4004-$4007	Second pulse wave
$4008-$400B	Triangle wave
$400C-$400F	Noise

In register descriptions below, bits listed as - can have any value written to them, while bits listed as 1 must have a 1 written, otherwise other APU features will be enabled, causing the registers to behave differently than described here.

Register initialization

Before using the APU, first initialize all the registers to known values that silence all channels.

init_apu:
        lda #$0F
        sta $4015
        
        ldy #0
@loop:  lda @regs,y
        sta $4000,y
        iny
        cpy #$18
        bne @loop
        
        rts
@regs:
        .byte $30,$08,$00,$00
        .byte $30,$08,$00,$00
        .byte $80,$00,$00,$00
        .byte $30,$00,$00,$00
        .byte $00,$00,$00,$00
        .byte $00,$0F,$00,$40

This writes values that are most useful for basic use of the channels. It's not important exactly what they do, just that they establish a known state.

Pulse wave channels

Main article: APU Pulse

There are two pulse wave channels, each with pitch, volume, and timbre controls.

$4000	$4004	%DD11VVVV	Duty cycle and volume DD: 00=12.5% 01=25% 10=50% 11=75% VVVV: 0000=silence 1111=maximum
$4002	$4006	%LLLLLLLL	Low 8 bits of raw period
$4003	$4007	%-----HHH	High 3 bits of raw period

To determine the raw period for a given frequency in Hz, use this formula (round the result to a whole number)::

raw period = 111860.8/frequency - 1

The following code plays a 400 Hz square wave (50% duty) at maximum volume:

jsr init_apu

lda #<279
sta $4002

lda #>279
sta $4003

lda #%10111111
sta $4000

All parameters can be changed while the tone is playing. To fade a note out, for example, write to $4000 or $4004 with the lower 4 bits decreasing every few frames.

Note that writing to $4003 and $4007 resets the phase, which causes a slight pop. This is an issue when doing vibrato, for example, and beyond the scope of this article.

Triangle wave channel

Main article: APU Triangle

The triangle channel allows control over frequency and muting.

$4008	%1U------	Un-mute
$400A	%LLLLLLLL	Low 8 bits of raw period
$400B	%-----HHH	High 3 bits of raw period
$4017	%1-------	Apply un-muting immediately

For any given period, the triangle channel's frequency is half that of the pulse channel, or a pitch one octave lower. To determine the raw period for a given frequency in Hz, use this formula (round the result to a whole number):

raw period = 55930.4/frequency - 1

The following code plays a 400 Hz triangle wave:

jsr init_apu

lda #<139
sta $400A

lda #>139
sta $400B

lda #%11000000
sta $4008
sta $4017

The raw period can be changed while the channel is playing.

To silence the wave, write %10000000 to $4008 and then $4017. Writing a raw period of 0 also silences the wave, but produces a pop, so it's not the preferred method.

Noise channel

Main article: APU Noise

The noise channel allows control over frequency, volume, and timbre.

$400C	%--11VVVV	Volume VVVV: 0000=silence 1111=maximum
$400E	%T---PPPP	Tone mode enable, Period

The following code plays a tone-like noise at maximum volume:

jsr init_apu

lda #%10000101
sta $400E

lda #%00111111
sta $400C

All parameters can be changed while the noise is playing.

Playing a musical note

To easily play a musical note, use the APU period table. The following code sets the first pulse wave's frequency based on the note number in the X register:

; Set first pulse channel's frequency to note code in X register
lda periodTableHi,x
sta $4003

lda periodTableLo,x
sta $4002

...

; NTSC period table generated by mktables.py
periodTableLo:
  .byte $f1,$7f,$13,$ad,$4d,$f3,$9d,$4c,$00,$b8,$74,$34
  .byte $f8,$bf,$89,$56,$26,$f9,$ce,$a6,$80,$5c,$3a,$1a
  .byte $fb,$df,$c4,$ab,$93,$7c,$67,$52,$3f,$2d,$1c,$0c
  .byte $fd,$ef,$e1,$d5,$c9,$bd,$b3,$a9,$9f,$96,$8e,$86
  .byte $7e,$77,$70,$6a,$64,$5e,$59,$54,$4f,$4b,$46,$42
  .byte $3f,$3b,$38,$34,$31,$2f,$2c,$29,$27,$25,$23,$21
  .byte $1f,$1d,$1b,$1a,$18,$17,$15,$14

periodTableHi:
  .byte $07,$07,$07,$06,$06,$05,$05,$05,$05,$04,$04,$04
  .byte $03,$03,$03,$03,$03,$02,$02,$02,$02,$02,$02,$02
  .byte $01,$01,$01,$01,$01,$01,$01,$01,$01,$01,$01,$01
  .byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
  .byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
  .byte $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00
  .byte $00,$00,$00,$00,$00,$00,$00,$00

The triangle plays an octave lower for the same raw period, so halve the value from the above table to get the desired note:

; Set triangle frequency to note code in X register
lda periodTableHi,x
lsr a
sta $400B

lda periodTableLo,x
ror a
sta $400A

The following full program plays pulse and triangle scales:

apu_scale.s