diff --git a/corrscope/corrscope.py b/corrscope/corrscope.py
index 6ec55d1..f25b23a 100644
--- a/corrscope/corrscope.py
+++ b/corrscope/corrscope.py
@@ -20,6 +20,7 @@ from corrscope.triggers import (
     CorrelationTriggerConfig,
     PerFrameCache,
     CorrelationTrigger,
+    SpectrumConfig,
 )
 from corrscope.util import pushd, coalesce
 from corrscope.wave import Wave, Flatten
@@ -118,6 +119,7 @@ def default_config(**kwargs) -> Config:
             responsiveness=0.5,
             buffer_falloff=0.5,
             use_edge_trigger=False,
+            pitch_invariance=SpectrumConfig()
             # Removed due to speed hit.
             # post=LocalPostTriggerConfig(strength=0.1),
         ),
diff --git a/corrscope/triggers.py b/corrscope/triggers.py
index ae5da83..75eea0c 100644
--- a/corrscope/triggers.py
+++ b/corrscope/triggers.py
@@ -1,6 +1,18 @@
 import warnings
 from abc import ABC, abstractmethod
-from typing import TYPE_CHECKING, Type, Tuple, Optional, ClassVar, Callable, Union
+from typing import (
+    TYPE_CHECKING,
+    Type,
+    Tuple,
+    Optional,
+    ClassVar,
+    Callable,
+    Union,
+    NewType,
+    Sequence,
+    List,
+    Any,
+)
 
 import attr
 import numpy as np
@@ -105,10 +117,184 @@ class PerFrameCache:
 # CorrelationTrigger
 
 
-class CorrelationTriggerConfig(ITriggerConfig):
+class SpectrumConfig(KeywordAttrs):
+    """
+    # Rationale:
+    If no basal frequency note-bands are to be truncated,
+    the spectrum must have freq resolution
+        `min_hz * (2 ** 1/notes_per_octave - 1)`.
+
+    At 20hz, 10 octaves, 12 notes/octave, this is 1.19Hz fft freqs.
+    Our highest band must be
+        `min_hz * 2**octaves`,
+    leading to nearly 20K freqs, which produces an somewhat slow FFT.
+
+    So increase min_hz and decrease octaves and notes_per_octave.
+    --------
+    Using a Constant-Q transform may eliminate performance concerns?
+    """
+
+    # Spectrum X density
+    min_hz: float = 20
+    octaves: int = 8
+    notes_per_octave: int = 6
+
+    # Spectrum Y power
+    exponent: float = 1
+    divide_by_freq: bool = True
+
+    # Spectral alignment and resampling
+    pitch_estimate_boost: float = 1.2
+    add_current_to_history: float = 0.1  # FIXME why does this exist?
+    max_octaves_to_resample: float = 1.0
+
+    @property
+    def max_notes_to_resample(self) -> int:
+        return round(self.notes_per_octave * self.max_octaves_to_resample)
+
+    # Time-domain history parameters
+    min_frames_between_recompute: int = 6
+    frames_to_lookbehind: int = 2
+
+
+class DummySpectrum:
+    # noinspection PyMethodMayBeStatic,PyUnusedLocal
+    def calc_spectrum(self, data: np.ndarray) -> np.ndarray:
+        return np.array([])
+
+
+# Indices are linearly spaced in FFT. Notes are exponentially spaced.
+# FFT is grouped into notes.
+FFTIndex = NewType("FFTIndex", int)
+# Very hacky and weird. Maybe it's not worth getting mypy to pass.
+if TYPE_CHECKING:
+    FFTIndexArray = Any  # mypy
+else:
+    FFTIndexArray = "np.ndarray[FFTIndex]"  # pycharm
+
+
+class LogFreqSpectrum(DummySpectrum):
+    """
+    Invariants:
+    - len(note_fenceposts) == n_fencepost
+
+    - rfft()[ : note_fenceposts[0]] is NOT used.
+    - rfft()[note_fenceposts[-1] : ] is NOT used.
+    - rfft()[note_fenceposts[0] : note_fenceposts[1]] becomes a note.
+    """
+
+    n_fftindex: FFTIndex  # Determines frequency resolution, not range.
+    note_fenceposts: FFTIndexArray
+    n_fencepost: int
+
+    def __init__(self, scfg: SpectrumConfig, subsmp_s: float, dummy_data: np.ndarray):
+        self.scfg = scfg
+
+        n_fftindex: FFTIndex = signal.next_fast_len(len(dummy_data))
+
+        # Increase n_fftindex until every note has nonzero width.
+        while True:
+            # Compute parameters
+            self.min_hz = scfg.min_hz
+            self.max_hz = self.min_hz * 2 ** scfg.octaves
+            n_fencepost = scfg.notes_per_octave * scfg.octaves + 1
+
+            note_fenceposts_hz = np.geomspace(
+                self.min_hz, self.max_hz, n_fencepost, dtype=FLOAT
+            )
+
+            # Convert fenceposts to FFTIndex
+            fft_from_hertz = n_fftindex / subsmp_s
+            note_fenceposts: FFTIndexArray = (
+                fft_from_hertz * note_fenceposts_hz
+            ).astype(np.int32)
+            note_widths = np.diff(note_fenceposts)
+
+            if np.any(note_widths == 0):
+                n_fftindex = signal.next_fast_len(n_fftindex + n_fftindex // 5 + 1)
+                continue
+            else:
+                break
+
+        self.n_fftindex = n_fftindex  # Passed to rfft() to automatically zero-pad data.
+        self.note_fenceposts = note_fenceposts
+        self.n_fencepost = len(note_fenceposts)
+
+    def calc_spectrum(self, data: np.ndarray) -> np.ndarray:
+        """ Unfortunately converting to FLOAT (single) adds too much overhead.
+
+        Input: Time-domain signal to be analyzed.
+        Output: Frequency-domain spectrum with exponentially-spaced notes.
+        - ret[note] = nonnegative float.
+        """
+        scfg = self.scfg
+
+        # Compute FFT spectrum[freq]
+        spectrum = np.fft.rfft(data, self.n_fftindex)
+        spectrum = abs(spectrum)
+        if scfg.exponent != 1:
+            spectrum **= scfg.exponent
+
+        # Compute energy of each note
+        # spectrum_per_note[note] = np.ndarray[float]
+        spectrum_per_note: List[np.ndarray] = split(spectrum, self.note_fenceposts)
+
+        # energy_per_note[note] = float
+        energy_per_note: np.ndarray
+
+        # np.add.reduce is much faster than np.sum/mean.
+        if scfg.divide_by_freq:
+            energy_per_note = np.array(
+                [np.add.reduce(region) / len(region) for region in spectrum_per_note]
+            )
+        else:
+            energy_per_note = np.array(
+                [np.add.reduce(region) for region in spectrum_per_note]
+            )
+
+        assert len(energy_per_note) == self.n_fencepost - 1
+        return energy_per_note
+
+
+def split(data: np.ndarray, fenceposts: Sequence[FFTIndex]) -> List[np.ndarray]:
+    """ Based off np.split(), but faster.
+    Unlike np.split, does not include data before fenceposts[0] or after fenceposts[-1].
+    """
+    sub_arys = []
+    ndata = len(data)
+    for i in range(len(fenceposts) - 1):
+        st = fenceposts[i]
+        end = fenceposts[i + 1]
+        if not st < ndata:
+            break
+        region = data[st:end]
+        sub_arys.append(region)
+
+    return sub_arys
+
+
+class CircularArray:
+    def __init__(self, size: int, *dims: int):
+        self.size = size
+        self.buf = np.zeros((size, *dims))
+        self.index = 0
+
+    def push(self, arr: np.ndarray) -> None:
+        if self.size == 0:
+            return
+        self.buf[self.index] = arr
+        self.index = (self.index + 1) % self.size
+
+    def peek(self) -> np.ndarray:
+        """Return is borrowed from self.buf.
+        Do NOT push to self while borrow is alive."""
+        return self.buf[self.index]
+
+
+class CorrelationTriggerConfig(ITriggerConfig, always_dump="pitch_invariance"):
     # get_trigger
     edge_strength: float
-    trigger_diameter: float = 0.5
+    trigger_diameter: Optional[float] = None
 
     trigger_falloff: Tuple[float, float] = (4.0, 1.0)
     recalc_semitones: float = 1.0
@@ -118,6 +304,9 @@ class CorrelationTriggerConfig(ITriggerConfig):
     responsiveness: float
     buffer_falloff: float  # Gaussian std = wave_period * buffer_falloff
 
+    # Pitch invariance = compute spectrum.
+    pitch_invariance: Optional["SpectrumConfig"] = None
+
     # region Legacy Aliases
     trigger_strength = Alias("edge_strength")
     falloff_width = Alias("buffer_falloff")
@@ -152,6 +341,10 @@ class CorrelationTriggerConfig(ITriggerConfig):
 class CorrelationTrigger(Trigger):
     cfg: CorrelationTriggerConfig
 
+    @property
+    def scfg(self) -> SpectrumConfig:
+        return self.cfg.pitch_invariance
+
     def __init__(self, *args, **kwargs):
         """
         Correlation-based trigger which looks at a window of `trigger_tsamp` samples.
@@ -181,6 +374,24 @@ class CorrelationTrigger(Trigger):
         self._prev_period: Optional[int] = None
         self._prev_window: Optional[np.ndarray] = None
 
+        # (mutable) Log-scaled spectrum
+        self.frames_since_spectrum = 0
+
+        if self.scfg:
+            self._spectrum_calc = LogFreqSpectrum(
+                scfg=self.scfg,
+                subsmp_s=self._wave.smp_s / self._stride,
+                dummy_data=self._buffer,
+            )
+            self._spectrum = self._spectrum_calc.calc_spectrum(self._buffer)
+            self.history = CircularArray(
+                self.scfg.frames_to_lookbehind, self._buffer_nsamp
+            )
+        else:
+            self._spectrum_calc = DummySpectrum()
+            self._spectrum = np.array([0])
+            self.history = CircularArray(0, self._buffer_nsamp)
+
     def _calc_data_taper(self) -> np.ndarray:
         """ Input data window. Zeroes out all data older than 1 frame old.
         See https://github.com/nyanpasu64/corrscope/wiki/Correlation-Trigger
@@ -242,6 +453,7 @@ class CorrelationTrigger(Trigger):
     # begin per-frame
     def get_trigger(self, index: int, cache: "PerFrameCache") -> int:
         N = self._buffer_nsamp
+        cfg = self.cfg
 
         # Get data
         stride = self._stride
@@ -253,50 +465,39 @@ class CorrelationTrigger(Trigger):
         period = get_period(data)
         cache.period = period * stride
 
-        if self._is_window_invalid(period):
-            diameter, falloff = [round(period * x) for x in self.cfg.trigger_falloff]
+        semitones = self._is_window_invalid(period)
+        # If pitch changed...
+        if semitones:
+            diameter, falloff = [round(period * x) for x in cfg.trigger_falloff]
             falloff_window = cosine_flat(N, diameter, falloff)
             window = np.minimum(falloff_window, self._data_taper)
 
+            # If pitch invariance enabled, rescale buffer to match data's pitch.
+            if self.scfg and (data != 0).any():
+                if isinstance(semitones, float):
+                    peak_semitones = semitones
+                else:
+                    peak_semitones = None
+                self.spectrum_rescale_buffer(data, peak_semitones)
+
             self._prev_period = period
             self._prev_window = window
         else:
             window = self._prev_window
 
+        self.history.push(data)
         data *= window
 
-        # prev_buffer
-        prev_buffer = self._windowed_step + self._buffer
+        prev_buffer: np.ndarray = self._buffer.copy()
+        prev_buffer += self._windowed_step
 
         # Calculate correlation
-        """
-        If offset < optimal, we need to `offset += positive`.
-        - The peak will appear near the right of `data`.
+        if self.cfg.trigger_diameter is not None:
+            radius = round(N * self.cfg.trigger_diameter / 2)
+        else:
+            radius = None
 
-        Either we must slide prev_buffer to the right:
-        - correlate(data, prev_buffer)
-        - trigger = offset + peak_offset
-
-        Or we must slide data to the left (by sliding offset to the right):
-        - correlate(prev_buffer, data)
-        - trigger = offset - peak_offset
-        """
-        corr = signal.correlate(data, prev_buffer)  # returns double, not single/FLOAT
-        assert len(corr) == 2 * N - 1
-
-        # Find optimal offset (within trigger_diameter, default=±N/4)
-        mid = N - 1
-        radius = round(N * self.cfg.trigger_diameter / 2)
-
-        left = mid - radius
-        right = mid + radius + 1
-
-        corr = corr[left:right]
-        mid = mid - left
-
-        # argmax(corr) == mid + peak_offset == (data >> peak_offset)
-        # peak_offset == argmax(corr) - mid
-        peak_offset = np.argmax(corr) - mid  # type: int
+        peak_offset = self.correlate_offset(data, prev_buffer, radius)
         trigger = index + (stride * peak_offset)
 
         # Apply post trigger (before updating correlation buffer)
@@ -306,11 +507,108 @@ class CorrelationTrigger(Trigger):
         # Update correlation buffer (distinct from visible area)
         aligned = self._wave.get_around(trigger, self._buffer_nsamp, stride)
         self._update_buffer(aligned, cache)
+        self.frames_since_spectrum += 1
 
         return trigger
 
-    def _is_window_invalid(self, period: int) -> bool:
-        """ Returns True if pitch has changed more than `recalc_semitones`. """
+    def spectrum_rescale_buffer(
+        self, data: np.ndarray, peak_semitones: Optional[float]
+    ) -> None:
+        """Rewrites self._spectrum, and possibly rescales self._buffer."""
+
+        scfg = self.scfg
+        N = self._buffer_nsamp
+
+        if self.frames_since_spectrum < self.scfg.min_frames_between_recompute:
+            return
+        self.frames_since_spectrum = 0
+
+        spectrum = self._spectrum_calc.calc_spectrum(data)
+        normalize_buffer(spectrum)
+
+        # Don't normalize self._spectrum. It was already normalized when being assigned.
+        prev_spectrum = self._spectrum_calc.calc_spectrum(self.history.peek())
+        prev_spectrum += scfg.add_current_to_history * spectrum
+
+        # rewrite spectrum
+        self._spectrum = spectrum
+
+        assert not np.any(np.isnan(spectrum))
+
+        # Find spectral correlation peak,
+        # but prioritize "changing pitch by ???".
+        if peak_semitones is not None:
+            boost_x = int(round(peak_semitones / 12 * scfg.notes_per_octave))
+            boost_y: float = scfg.pitch_estimate_boost
+        else:
+            boost_x = 0
+            boost_y = 1.0
+
+        # If we want to double pitch...
+        resample_notes = self.correlate_offset(
+            spectrum,
+            prev_spectrum,
+            scfg.max_notes_to_resample,
+            boost_x=boost_x,
+            boost_y=boost_y,
+        )
+        if resample_notes != 0:
+            # we must divide sampling rate by 2.
+            new_len = int(round(N / 2 ** (resample_notes / scfg.notes_per_octave)))
+
+            # Copy+resample self._buffer.
+            self._buffer = np.interp(
+                np.linspace(0, 1, new_len), np.linspace(0, 1, N), self._buffer
+            )
+            # assert len(self._buffer) == new_len
+            self._buffer = midpad(self._buffer, N)
+
+    @staticmethod
+    def correlate_offset(
+        data: np.ndarray,
+        prev_buffer: np.ndarray,
+        radius: Optional[int],
+        boost_x: int = 0,
+        boost_y: float = 1.0,
+    ) -> int:
+        """
+        This is confusing.
+
+        If data index < optimal, data will be too far to the right,
+        and we need to `index += positive`.
+        - The peak will appear near the right of `data`.
+
+        Either we must slide prev_buffer to the right,
+        or we must slide data to the left (by sliding index to the right):
+        - correlate(data, prev_buffer)
+        - trigger = index + peak_offset
+        """
+        N = len(data)
+        corr = signal.correlate(data, prev_buffer)  # returns double, not single/FLOAT
+        Ncorr = 2 * N - 1
+        assert len(corr) == Ncorr
+
+        # Find optimal offset
+        mid = N - 1
+
+        if radius is not None:
+            left = max(mid - radius, 0)
+            right = min(mid + radius + 1, Ncorr)
+
+            corr = corr[left:right]
+            mid = mid - left
+
+        # Prioritize part of it.
+        corr[mid + boost_x : mid + boost_x + 1] *= boost_y
+
+        # argmax(corr) == mid + peak_offset == (data >> peak_offset)
+        # peak_offset == argmax(corr) - mid
+        peak_offset = np.argmax(corr) - mid  # type: int
+        return peak_offset
+
+    def _is_window_invalid(self, period: int) -> Union[bool, float]:
+        """ Returns number of semitones,
+        if pitch has changed more than `recalc_semitones`. """
 
         prev = self._prev_period
 
@@ -319,12 +617,12 @@ class CorrelationTrigger(Trigger):
         elif prev * period == 0:
             return prev != period
         else:
-            semitones = abs(np.log(period / prev) / np.log(2) * 12)
-
+            # If period doubles, semitones are -12.
+            semitones = np.log(period / prev) / np.log(2) * -12
             # If semitones == recalc_semitones == 0, do NOT recalc.
-            if semitones <= self.cfg.recalc_semitones:
+            if abs(semitones) <= self.cfg.recalc_semitones:
                 return False
-            return True
+            return semitones
 
     def _update_buffer(self, data: np.ndarray, cache: PerFrameCache) -> None:
         """
diff --git a/tests/test_trigger.py b/tests/test_trigger.py
index 166dbe2..7595959 100644
--- a/tests/test_trigger.py
+++ b/tests/test_trigger.py
@@ -1,8 +1,10 @@
 import attr
 import matplotlib.pyplot as plt
+import numpy as np
 import pytest
 from matplotlib.axes import Axes
 from matplotlib.figure import Figure
+from pytest_cases import pytest_fixture_plus
 
 from corrscope import triggers
 from corrscope.triggers import (
@@ -11,6 +13,7 @@ from corrscope.triggers import (
     PerFrameCache,
     ZeroCrossingTriggerConfig,
     LocalPostTriggerConfig,
+    SpectrumConfig,
 )
 from corrscope.wave import Wave
 
@@ -25,10 +28,16 @@ def cfg_template(**kwargs) -> CorrelationTriggerConfig:
     return attr.evolve(cfg, **kwargs)
 
 
-@pytest.fixture(scope="session", params=[False, True])
-def cfg(request):
-    use_edge_trigger = request.param
-    return cfg_template(use_edge_trigger=use_edge_trigger)
+@pytest_fixture_plus
+@pytest.mark.parametrize("use_edge_trigger", [False, True])
+@pytest.mark.parametrize("trigger_diameter", [None, 0.5])
+@pytest.mark.parametrize("pitch_invariance", [None, SpectrumConfig()])
+def cfg(use_edge_trigger, trigger_diameter, pitch_invariance):
+    return cfg_template(
+        use_edge_trigger=use_edge_trigger,
+        trigger_diameter=trigger_diameter,
+        pitch_invariance=pitch_invariance,
+    )
 
 
 @pytest.fixture(
@@ -177,6 +186,43 @@ def test_trigger_should_recalc_window():
         assert trigger._is_window_invalid(x), x
 
 
+# Test pitch-invariant triggering using spectrum
+def test_correlate_offset():
+    """
+    Catches bug where writing N instead of Ncorr
+    prevented function from returning positive numbers.
+    """
+
+    np.random.seed(31337)
+    correlate_offset = CorrelationTrigger.correlate_offset
+
+    # Ensure autocorrelation on random data returns peak at 0.
+    N = 100
+    spectrum = np.random.random(N)
+    assert correlate_offset(spectrum, spectrum, 12) == 0
+
+    # Ensure cross-correlation of time-shifted impulses works.
+    # Assume wave where y=[i==99].
+    wave = np.eye(N)[::-1]
+    # Taking a slice beginning at index i will produce an impulse at 99-i.
+    left = wave[30]
+    right = wave[40]
+
+    # We need to slide `left` to the right by 10 samples, and vice versa.
+    for radius in [None, 12]:
+        assert correlate_offset(data=left, prev_buffer=right, radius=radius) == 10
+        assert correlate_offset(data=right, prev_buffer=left, radius=radius) == -10
+
+    # The correlation peak at zero-offset is small enough for boost_x to be returned.
+    boost_y = 1.5
+    ones = np.ones(N)
+    for boost_x in [6, -6]:
+        assert (
+            correlate_offset(ones, ones, radius=9, boost_x=boost_x, boost_y=boost_y)
+            == boost_x
+        )
+
+
 # Test the ability to load legacy TriggerConfig