APU Reference (NES)

Load-bearing facts about the Audio Processing Unit (integrated into the Ricoh 2A03) for reading NES sound code. Curated from nesdev.org/wiki.

Links: NES Research, PPU Reference, Game Annotation Series

Five channels

The APU has 5 fixed-function sound generators:

Plus control registers:

Compared to the TIA

The 2600’s TIA had 2 channels, 5-bit volume, 32 fixed pitches per channel — the iconic “2600 audio” sound. The NES APU has 2.5× the channels, real envelopes, sweeps, length counters, and sampled audio via DMC. Composers got actual instruments instead of bleeps. This is the audio side of the storytelling-vs-logic shift — NES games carry music that expresses something, where 2600 audio could only punctuate.

Pulse channel mechanics ($4000–$4003 / $4004–$4007)

Each pulse channel has 4 registers, all writable in any order during note setup.

Frequency formula: f = CPU_clock / (16 × (t + 1)) where t is the 11-bit timer (low byte from $4002, high 3 bits from $4003).

Silencing condition: if t < 8, channel is silenced. Sweep can also silence by driving frequency out of range.

Duty cycle sets the waveform shape: 12.5%, 25%, 50%, 75% — different timbres from the same frequency. Composers vary duty for melodic vs accompaniment voices.

Triangle channel ($4008–$400B)

No volume control — triangle plays at fixed volume when active. Frequency is one octave below the pulse channels at the same timer value.

The triangle has both a length counter (like other channels) AND a separate 7-bit linear counter — both must be non-zero for output. Useful for note durations independent of envelope.

Quirk: “Silencing” the triangle just halts its waveform position; it outputs the last value, not zero. Causes the famous “click” if not handled.

Noise channel ($400C–$400F)

16 noise periods via lookup table. The “buzzy” mode (bit 7 set) produces a metallic timbre — not pseudo-random — used for hi-hat and snare-like sounds.

DMC channel ($4010–$4013)

Plays back 7-bit delta-modulated PCM from a sample in CPU address space ($C000–$FFFF). Used for drum hits, voice samples, percussive accents that the synthesized channels can’t reproduce.

Trade-off: DMC samples eat ROM space (sample storage) and steal CPU cycles (DMA-style fetches that can stall the CPU mid-instruction). Heavy DMC use causes the famous “DMC glitch” with controller reads.

Frame counter ($4017)

Two modes, set by bit 7 of $4017:

Most games use 5-step mode to disable the frame IRQ (it conflicts with cleaner mapper-IRQ usage and DMC IRQ).

The frame counter is what advances envelopes and length counters — it’s the APU’s own internal clock independent of game frame timing.

Status / Enable ($4015)

Write: bits 0–4 enable Pulse 1 / Pulse 2 / Triangle / Noise / DMC respectively. A bit clear → that channel’s length counter is forced to 0 (silencing it). DMC bit (4) starts/stops sample playback.

Read: returns flags — bit per channel says “length counter > 0,” plus DMC sample-active and IRQ flags. Reading is the only way to query channel activity.

Length counter

A 5-bit value lookup-loaded into a length counter that decrements at the frame counter’s rate (~120 Hz NTSC in 4-step mode). When it reaches zero, the channel is silenced. Used to play notes of a specific duration without the CPU re-clocking each note.

The high 5 bits of $4003 / $4007 / $400B / $400F index into a 32-entry table of length values. Loading any of these triggers “key on” for the corresponding channel (also resets phase, etc.).

Envelope

Pulse and Noise channels have a shared envelope unit:

• Either constant volume (set bit 4 of $4000/$4004/$400C → use bottom 4 bits as volume)
• Or decay envelope (clear bit 4 → volume starts at 15 and decays at the rate in bottom 4 bits)
• Loop bit (bit 5) makes the envelope restart instead of staying at 0 — useful for sustained notes with vibrato.

Sweep (pulse only)

Each pulse channel has a sweep unit ($4001/$4005) that automatically modulates its frequency over time:

• Enable (bit 7), period (bits 6–4), negate (bit 3), shift (bits 2–0)
• Adds or subtracts (period » shift) from the timer value at each sweep clock.
• Used for sliding pitches, “pew” sounds, kick drum approximations.

Practical APU programming pattern

For a single note:

1. Write duty + envelope to $4000 (or equivalent).
2. Write sweep to $4001 (or disable with $08 = no shift, no negate, sweep off).
3. Write timer low to $4002.
4. Write timer high + length counter to $4003. The length counter write is the “key on.”

Continuous music driver:

• Maintain a “music engine” struct in CPU RAM with note, octave, duration, current pattern position per channel.
• Each frame (in NMI or via separate music timer): advance the pattern position, decode the next note, write the 4-byte sequence per channel.
• Length counters and envelopes do most of the per-note work; the engine just sets new notes at musical intervals.

Tags

6502 · reverse-engineering · assembly · nes