123 lines
7.8 KiB
C++
123 lines
7.8 KiB
C++
#pragma once
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// pitch_shift — a PURE, per-voice, duration-preserving pitch shifter: the S16 "Preserve"
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// engine's DSP core. Time-domain delay-line shifter with CORRELATION-ALIGNED SPLICES
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// (SOLA-style): one active read tap chases the write head at the shift ratio; when it drifts
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// out of its safe delay band it is relocated by a nominal window jump REFINED BY A
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// CROSS-CORRELATION SEARCH so the new read point is waveform-aligned, then the old and new
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// taps are crossfaded (raised-cosine, amplitude-complementary). Source is consumed 1:1 and
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// output produced 1:1 (duration held); only the PITCH changes — an octave up plays the same
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// wall-clock length as the root note, unlike the Varispeed `readPos_ += ratio_` resample path.
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//
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// WHY CORRELATED SPLICES (GA-Preserve fix, 2026-07). The first S16 implementation was the
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// naive two-tap OLA: taps hard-locked half a window apart, Hann-crossfaded by write-head
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// distance. Its taps read the same stream at delays differing by exactly w/2, so their outputs
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// carried a FIXED relative phase of 2*pi*f_src*(w/2) — arbitrary and source-frequency-
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// dependent. Near anti-phase (roughly half of all frequencies) every crossfade midpoint
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// nearly CANCELLED: deep periodic AM + phase slew = strong sidebands. A repitched pure sine
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// came out mangled ("multiple partials" on a spectrogram) while the root stayed clean (unity
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// freezes the crossfade). The fix is structural: splices must be PHASE-ALIGNED, so each jump
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// is snapped to the best waveform match within a bounded lag search — a pure sine's jump
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// lands on an integer period count and the output stays a single shifted tone.
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//
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// WHY A HAND-ROLLED PURE MODULE, NOT WDL (S16-F2, decided at build). The spec's lean was
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// route (a) `WDL_SimplePitchShifter`. But its include chain
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// (simple_pitchshift.h -> queue.h -> heapbuf.h -> wdltypes.h) does `#ifdef _WIN32 ->
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// #include <windows.h>` unconditionally, which CANNOT enter the pure sampler_core module
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// (CLAUDE.md load-bearing split: NO vendor/host/SDK types; sampler_core_tests links neither
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// SDK and compiles outside the DAW). So the Preserve DSP lands as route (b): a house-native
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// pure module alongside peaks / wav_trim, CTest-testable, RT-disciplined. Same
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// PitchEngine::Preserve contract behind the seam — if WDL is ever preferred it swaps in at
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// the SHELL, never in the pure core.
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//
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// PURE MODULE: NO VST3, NO REAPER, NO SWELL, NO vendor/ includes. Standard library only.
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// Shares the `AudioSample` float alias from peaks (the one house precedent — sampler_core /
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// wav_trim do the same).
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//
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// RT DISCIPLINE (S16 hard constraint). `configure()` sizes the ring ONCE (off the audio
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// thread, at voice allocation). `warm()` pre-fills the ring with silence so steady-state
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// latency is reached before the first real sample (no cold-start click). `process()` does
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// NO allocation and NO locks — it reads/writes the pre-sized ring only. The splice-time
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// correlation search is a bounded burst of multiply-adds (coarse+refine over a fixed lag
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// range) that fires once per splice cadence (window / |ratio-1| frames), never per frame.
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#include <cstddef>
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#include <cstdint>
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#include <vector>
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#include "peaks.h" // AudioSample (float)
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namespace reasampler {
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// A per-channel time-domain splice-aligned pitch shifter. One instance transposes ONE channel;
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// a stereo voice owns two — the algorithm is per-sample and channel-count agnostic, matching
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// the S7 "one read head, per-channel value" idiom of the core.
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//
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// The default-constructed shifter is INERT: with no configure() it passes input through
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// unchanged (shift ratio 1.0, empty ring), so a Varispeed voice that never touches it is
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// byte-identical to the pre-S16 engine.
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class PitchShifter {
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public:
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// Size the delay ring for `windowFrames` (the nominal splice-jump length; the ring is 2x
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// that for splice/search headroom) and derive the fade/search geometry. `windowFrames`
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// <= 1 degrades to pass-through (no ring), so a degenerate configure never divides by
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// zero or wraps a zero span. Called OFF the audio thread (allocates). Resets all running
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// state. A larger window = fewer splices and a deeper alignment search but more latency
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// (steady-state latency stays window/2); the shell picks it from kPreserveWindowMs.
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void configure(std::int64_t windowFrames);
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// Pre-fill the ring with silence (one full window of zero writes) so the read tap reaches
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// steady state before the first real sample. Removes the cold-start seam (the S16 "onset
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// click absent" requirement) — call once at voice allocation after configure(). No-op when
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// unconfigured (pass-through needs no warm-up).
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void warm();
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// The pitch shift ratio: 2^((note - root)/12) plus any per-frame pitch-envelope bias.
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// 1.0 = no shift (pass-through-equivalent output, no splices ever fire). Set per frame is
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// fine (cheap); the tap advance simply uses the current value. Values <= 0 are ignored
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// (kept at the last valid ratio) so a bad input never runs the tap backward or stalls it.
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void setShiftRatio(double ratio);
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// Transform ONE input frame into ONE output frame (duration-preserving: 1 in, 1 out).
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// RT-safe: reads/writes the pre-sized ring only, no allocation, no lock. When unconfigured
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// (window <= 1) returns `in` unchanged (pass-through). Otherwise writes `in` at the write
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// head, reads the active tap (crossfading against the outgoing tap while a splice fade is
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// live), then advances the write head by one and the tap(s) by the shift ratio. When the
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// active tap leaves its safe delay band, a correlation-aligned splice is scheduled.
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AudioSample process(AudioSample in);
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// Reset running state to a freshly-warmed-equivalent silence (ring zeroed, heads re-seeded)
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// WITHOUT reallocating — for voice reuse without a re-configure. Keeps the current window.
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void reset();
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// True once configure() sized a real ring (window > 1). A pass-through shifter is false.
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bool configured() const { return window_ > 1; }
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std::int64_t window() const { return window_; }
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private:
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double readTap(double pos) const; // fractional ring read, linear interp
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void splice(std::int64_t nominalJump); // relocate the active tap, start the fade
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std::vector<AudioSample> ring_; // delay line, length `ringLen_` == 2 * window_
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std::int64_t window_ = 0; // nominal splice jump in frames; <= 1 = pass-through
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std::int64_t ringLen_ = 0; // ring length (2 * window_): splice + search headroom
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std::int64_t writePos_ = 0; // integer write head into the ring (source rate)
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double posA_ = 0.0; // active read tap (advances at the shift ratio)
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double posB_ = 0.0; // outgoing tap during a splice crossfade
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bool fading_ = false; // a splice crossfade is in flight
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std::int64_t fadePos_ = 0; // crossfade progress, [0, fadeLen_)
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std::int64_t fadeFrames_ = 0; // NOMINAL crossfade length (window_/4)
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std::int64_t fadeLen_ = 0; // LIVE crossfade length for the in-flight splice —
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// ratio-scaled at splice time so an up-shift's outgoing
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// tap can never drain into the writer mid-fade
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std::int64_t maxLag_ = 0; // correlation search half-range (window_/4)
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std::int64_t corrFrames_ = 0; // correlation segment length (dLow_-1, capped at 512, so
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// the reference read forward from the tap stays behind
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// the writer BY CONSTRUCTION at an up-splice)
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std::int64_t dLow_ = 0; // splice trigger: active-tap delay below this (up-shift)
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std::int64_t dHigh_ = 0; // splice trigger: active-tap delay above this (down-shift)
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double ratio_ = 1.0; // current shift ratio (>0)
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};
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} // namespace reasampler
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