Pitch tracker has a minimum amplitude threshold instead of SNR

Using SNR (as formulated) didn't work great because, surprise, it turns out
that the dumb approximation of mean level for noise level is correlated with
signal power.

Amplitude is simple and works as desired in practice, although it probably
requires tuning to any given environment for best results -.-

friture/pitch_tracker.py

@@ -27,7 +27,7 @@
 from PyQt5.QtQuick import QQuickWindow
 from PyQt5.QtQuickWidgets import QQuickWidget
 from PyQt5.QtQml import QQmlComponent, QQmlEngine
-from typing import Optional
+from typing import Any, no_type_check, Optional
 
 from friture.audiobackend import SAMPLING_RATE
 from friture.audioproc import audioproc
@@ -37,8 +37,8 @@
     DEFAULT_MAX_FREQ,
     DEFAULT_DURATION,
     DEFAULT_FFT_SIZE,
-    DEFAULT_MIN_SNR,
-    PitchTrackerSettingsDialog
+    DEFAULT_MIN_DB,
+    PitchTrackerSettingsDialog,
 )
 from friture.plotting.coordinateTransform import CoordinateTransform
 import friture.plotting.frequency_scales as fscales
@@ -127,15 +127,17 @@ def __init__(self, parent, engine):
 
         self.min_freq = DEFAULT_MIN_FREQ
         self.max_freq = DEFAULT_MAX_FREQ
-        self._pitch_tracker_data.vertical_axis.setRange(
+        self._pitch_tracker_data.vertical_axis.setRange( # type: ignore
             self.min_freq, self.max_freq)
-        self._pitch_tracker_data.vertical_axis.setScale(fscales.Octave)
+        self._pitch_tracker_data.vertical_axis.setScale( # type: ignore
+            fscales.Octave)
         self.vertical_transform = CoordinateTransform(
             self.min_freq, self.max_freq, 1, 0, 0)
         self.vertical_transform.setScale(fscales.Octave)
 
         self.duration = DEFAULT_DURATION
-        self._pitch_tracker_data.horizontal_axis.setRange(-self.duration, 0.)
+        self._pitch_tracker_data.horizontal_axis.setRange( # type: ignore
+            -self.duration, 0.)
 
         self.settings_dialog = PitchTrackerSettingsDialog(self)
 
@@ -185,8 +187,8 @@ def set_duration(self, value):
         self.duration = value
         self._pitch_tracker_data.horizontal_axis.setRange(-self.duration, 0.)
 
-    def set_min_snr(self, value: float):
-        self.tracker.min_snr = value
+    def set_min_db(self, value: float):
+        self.tracker.min_db = value
 
     # slot
     def settings_called(self, checked):
@@ -222,14 +224,14 @@ def pitch(self, pitch: float):
         self._pitch = pitch
         self.pitch_changed.emit(pitch)
 
-    @pyqtProperty(str, notify=pitch_changed)
+    @pyqtProperty(str, notify=pitch_changed) # type: ignore
     def pitch_unit(self) -> str:
         if self._pitch >= 1000.0:
             return "kHz"
         else:
             return "Hz"
 
-    @pyqtProperty(str, notify=pitch_changed)
+    @pyqtProperty(str, notify=pitch_changed) # type: ignore
     def note(self) -> str:
         if not self._pitch or np.isnan(self._pitch):
             return '--'
@@ -244,12 +246,12 @@ def __init__(
         fft_size: int = DEFAULT_FFT_SIZE,
         overlap: float = 0.75,
         sample_rate: int = SAMPLING_RATE,
-        min_snr: float = DEFAULT_MIN_SNR,
+        min_db: float = DEFAULT_MIN_DB,
     ):
         self.fft_size = fft_size
         self.overlap = overlap
         self.sample_rate = sample_rate
-        self.min_snr = min_snr
+        self.min_db = min_db
 
         self.input_buf = input_buf
         self.input_buf.grow_if_needed(fft_size)
@@ -307,11 +309,10 @@ def estimate_pitch(self, frame: np.ndarray) -> Optional[float]:
             # try to take the log of zero.
             return None
 
-        # Compute SNR for the detected pitch; if it's too low presume it's
+        # Compute dB for the detected fundamental; if it's too low presume it's
         # a false detection and return no result.
-        variance = np.mean(spectrum ** 2)
-        snr = 10 * np.log10((spectrum[pitch_idx] ** 2) / variance)
-        if snr < self.min_snr:
+        db = 10 * np.log10(spectrum[pitch_idx] ** 2 / self.fft_size ** 2)
+        if db < self.min_db:
             return None
         else:
             return self.proc.freq[pitch_idx]

friture/pitch_tracker_settings.py

@@ -25,7 +25,7 @@
 DEFAULT_MIN_FREQ = 80
 DEFAULT_MAX_FREQ = 1000
 DEFAULT_DURATION = 30
-DEFAULT_MIN_SNR = 3.0
+DEFAULT_MIN_DB = -70.0
 DEFAULT_FFT_SIZE = 16384
 
 class PitchTrackerSettingsDialog(QtWidgets.QDialog):
@@ -64,23 +64,23 @@ def __init__(self, parent):
         self.duration.valueChanged.connect(self.parent().set_duration)
         self.form_layout.addRow("Duration:", self.duration)
 
-        self.min_snr = QtWidgets.QDoubleSpinBox(self)
-        self.min_snr.setMinimum(0)
-        self.min_snr.setMaximum(50)
-        self.min_snr.setSingleStep(1)
-        self.min_snr.setValue(DEFAULT_MIN_SNR)
-        self.min_snr.setSuffix(" dB")
-        self.min_snr.setObjectName("min_snr")
-        self.min_snr.valueChanged.connect(self.parent().set_min_snr)
-        self.form_layout.addRow("Min SNR:", self.min_snr)
+        self.min_db = QtWidgets.QDoubleSpinBox(self)
+        self.min_db.setMinimum(-100)
+        self.min_db.setMaximum(0)
+        self.min_db.setSingleStep(1)
+        self.min_db.setValue(DEFAULT_MIN_DB)
+        self.min_db.setSuffix(" dB")
+        self.min_db.setObjectName("min_db")
+        self.min_db.valueChanged.connect(self.parent().set_min_db) # type: ignore
+        self.form_layout.addRow("Min Amplitude:", self.min_db)
 
         self.setLayout(self.form_layout)
 
     def save_state(self, settings):
         settings.setValue("min_freq", self.min_freq.value())
         settings.setValue("max_freq", self.max_freq.value())
         settings.setValue("duration", self.duration.value())
-        settings.setValue("min_snr", self.min_snr.value())
+        settings.setValue("min_db", self.min_db.value())
 
     def restore_state(self, settings):
         self.min_freq.setValue(
@@ -89,5 +89,6 @@ def restore_state(self, settings):
             settings.value("max_freq", DEFAULT_MAX_FREQ, type=int))
         self.duration.setValue(
             settings.value("duration", DEFAULT_DURATION, type=int))
-        self.min_snr.setValue(
-            settings.value("min_snr", DEFAULT_MIN_SNR, type=float))
+        self.min_db.setValue(
+            settings.value("min_db", DEFAULT_MIN_DB, type=float))
+