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audio: per-frame pitch envelopes for pulse SFX

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Pulse-channel sfx with a multi-byte `pitch:` array used to silently
ignore everything past the first byte — the runtime audio tick
latched the period at trigger time and never updated it. Programs
that wanted a frequency sweep had no way to express it.

The compiler now compiles a per-frame pitch envelope blob alongside
the existing volume envelope when `decl.pitch` has more than one
distinct value. The blob is padded (or truncated) to the volume
envelope's length and ends in a zero sentinel so the runtime
walker stops both pointers on the same NMI. Sfx with a single
scalar pitch (or an array where every byte is the same) keep their
historical "no pitch blob, latch once" path and emit byte-identical
ROM bytes.

The runtime gains two new pieces, both gated on a new
`__sfx_pitch_used` codegen marker so programs without varying-pitch
sfx pay zero bytes:

1. `gen_audio_tick` emits a per-frame pitch update block inside
   the SFX tick: read a byte through `(AUDIO_SFX_PITCH_PTR),Y`,
   write it to `$4002` (pulse-1 period low), advance the pointer.
   The block bails on a zero high-byte pointer so a single
   program can mix scalar-pitch and varying-pitch sfx without
   one clobbering the other.

2. `emit_play_pulse` seeds `AUDIO_SFX_PITCH_PTR_LO/HI` with the
   pitch-blob label for varying-pitch sfx and zeros it for
   scalar-pitch sfx. The per-call branch is skipped entirely
   when the program has no varying-pitch sfx anywhere.

The new `examples/sfx_pitch_envelope.ne` exercises the path with
a 16-frame siren sweep. Triangle and noise per-frame pitch are
deferred — they share the same data shape but the runtime ticks
for those channels still write only their volume registers, see
docs/future-work.md for the gap.

https://claude.ai/code/session_01KEczoNUX3WmcFLfq6iAQxB
This commit is contained in:
Claude 2026-04-15 02:54:56 +00:00
parent db3a4adc57
commit 9878b7d87d
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12 changed files with 449 additions and 31 deletions
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19
src/linker/mod.rs
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@ -489,8 +489,13 @@ impl Linker {
let has_audio = has_label(user_code, "__audio_used");
let has_noise = has_label(user_code, "__noise_used");
let has_triangle = has_label(user_code, "__triangle_used");
let has_sfx_pitch = has_label(user_code, "__sfx_pitch_used");
if has_audio {
all_instructions.extend(runtime::gen_audio_tick(has_noise, has_triangle));
all_instructions.extend(runtime::gen_audio_tick(
has_noise,
has_triangle,
has_sfx_pitch,
));
all_instructions.extend(runtime::gen_period_table());
// Emit one data block per sfx blob: a label followed by
// the envelope bytes. `play Name` codegen emits a
@ -500,6 +505,18 @@ impl Linker {
&blob.label(),
blob.envelope.clone(),
));
// Optional pitch envelope blob. Only emitted for
// sfx the compiler decided actually need per-frame
// pitch updates — the pitch_envelope is empty for
// single-pitch sfx and the `gen_data_block` call
// is skipped, keeping ROM bytes identical to the
// pre-pitch-envelope behaviour.
if blob.has_pitch_envelope() {
all_instructions.extend(runtime::gen_data_block(
&blob.pitch_label(),
blob.pitch_envelope.clone(),
));
}
}
// Same for music: label + note stream.
for blob in music {