Nintendo Entertainment SystemAudioBeginner

A Square Wave

Enable one of the NES APU's pulse channels, set its duty, volume and 11-bit timer, and it holds a tone on its own — the foundation of every NES sound.

apupulsesquare-waveaudio

Overview

The NES doesn’t play samples — its APU synthesises five fixed voices: two pulse (square) channels, a triangle, noise, and a one-off sample channel. The two pulse channels are where you start. Unlike Paula’s sample DMA, you don’t hand the NES a waveform; you switch a channel on, pick its duty (the shape of the square), its volume, and an 11-bit timer that sets the pitch — and the hardware oscillates by itself, forever, until you change it. Getting a tone out is three registers and an enable bit.

Code

; =============================================================================
; A SQUARE WAVE - NES (APU)
; Hold a continuous tone on pulse channel 1
; =============================================================================

APUSTATUS    = $4015    ; channel enable (write) / status (read)
PULSE1_VOL   = $4000    ; DDLC VVVV - duty, length-halt, const-vol, volume
PULSE1_SWEEP = $4001    ; frequency sweep unit
PULSE1_LO    = $4002    ; timer, low 8 bits
PULSE1_HI    = $4003    ; timer high 3 bits + length-counter load

start:
    lda #%00000001         ; bit 0 = enable pulse channel 1
    sta APUSTATUS

    lda #%10111111         ; duty 10 (50%), length halted, constant vol 15
    sta PULSE1_VOL
    lda #%00000000
    sta PULSE1_SWEEP       ; sweep off — a steady pitch

    lda #$FD               ; timer low  \  253 -> ~440 Hz (A-4)
    sta PULSE1_LO          ;             /
    lda #$00               ; timer high bits 0 (length load ignored — halted)
    sta PULSE1_HI

    ; the note now plays on its own — nothing more to do

Trade-offs

Aspect Cost
CPU None — the channel oscillates in hardware
Memory A handful of register writes
Limitation Fixed square shape; volume is a coarse 0–15; no sample playback here

When to use: Every NES tone — leads, bleeps, the start of any music routine.

When to avoid: Nothing simpler exists on the NES; this is the base case.

The five voices

The APU is a fixed synthesiser, not a sampler. Each channel has its own character and its own register block:

Channel Registers Role
Pulse 1 $4000–$4003 Square wave, four duties, lead
Pulse 2 $4004–$4007 A second square, harmony
Triangle $4008–$400B Fixed-volume triangle, bass
Noise $400C–$400F Pseudo-random, percussion & effects
DMC $4010–$4013 1-bit delta samples, drums & speech

Nothing plays until you set its bit in $4015: bit 0 pulse 1, bit 1 pulse 2, bit 2 triangle, bit 3 noise, bit 4 DMC. Write %00001111 to arm all but the DMC.

Timer into pitch

Pitch is the timer — an 11-bit value split across $4002 (low 8 bits) and the bottom 3 bits of $4003. A pulse channel’s frequency on NTSC is:

frequency (Hz) = 1789773 / (16 * (timer + 1))

So a bigger timer means a lower note. Timer 253 gives ≈440 Hz (A-4). Music routines keep a table of timer values, one per semitone; halving a timer raises the note an octave. The triangle uses the same idea but divides by 32, so it sounds an octave lower for the same value — handy for bass.

Why a channel goes silent

Three things can mute a pulse channel, and each is a classic first-tone bug:

  • The length counter. Every channel has a counter that ticks down and silences it. Set bit 5 of $4000 (length-counter halt) to freeze it, as above, and the tone sustains. Leave it clear and the note fades out on its own — which is what you want for effects.
  • Constant vs envelope volume. Bit 4 set means “the low nibble is the volume”. Clear it and that nibble becomes an envelope rate instead, and the channel decays from 15 to 0.
  • The sweep unit. If $4001 is misconfigured it can drive the pitch out of range and mute the channel. Write $00 to leave it alone.

Patterns: NMI Game Loop

Vault: APU | Nintendo Entertainment System