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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
42 lines
1.5 KiB
Text
42 lines
1.5 KiB
Text
// Per-frame pitch envelope on a pulse SFX. The user authors the
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// `pitch:` array as one byte per frame (matching the per-frame
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// `volume:` array) and the runtime audio tick walks both pointers
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// in lockstep, writing pitch to `$4002` and volume to `$4000` on
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// every NMI. The result is a frequency-sweeping "siren" tone — the
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// classic latch-once pulse SFX driver couldn't model this at all.
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//
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// Per-frame pitch is opt-in: if the `pitch:` array has a single
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// value (or repeats one byte) the compiler emits the byte-identical
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// pre-pitch-envelope sequence and no extra blob, so existing
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// programs that just want a static pitch keep working unchanged.
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// See `runtime/gen_audio_tick` for the gated extension.
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//
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// Build: cargo run -- build examples/sfx_pitch_envelope.ne
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game "SFX Pitch Envelope" {
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mapper: NROM
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}
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// 16-frame pitch sweep from $40 down to $20 paired with a slow
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// volume ramp. Both arrays are the same length so the runtime's
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// lockstep walker handles the simplest possible case.
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sfx Siren {
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duty: 2
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pitch: [0x40, 0x3D, 0x3A, 0x37, 0x34, 0x31, 0x2E, 0x2B, 0x28, 0x26, 0x24, 0x22, 0x21, 0x20, 0x20, 0x20]
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volume: [15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
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}
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var tick: u8 = 0
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on frame {
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// Re-trigger the sfx every 60 frames so the siren restarts
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// whenever the previous pass mutes itself, giving the
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// emulator harness a stable pattern to capture.
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tick += 1
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if tick == 60 {
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tick = 0
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play Siren
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}
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}
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start Main
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