Improve PianoSynth acoustic realism

- Add decaying noise burst to KS excitation, seeded by note for
  consistency. Models brief hammer-felt impact transient before
  the tonal string response settles.

- Double note-dependent brightness range (0.3 → 0.6) for more
  dramatic bass/treble contrast across the keyboard.

- Reduce default string detune from 0.3 (4.5 cents) to 0.1
  (1.5 cents); tighter unison is closer to a well-tuned acoustic
  piano rather than EP-style chorus.

Author: juzzlin

src/domain/devices/piano_synth_device.cpp

@@ -42,7 +42,7 @@ PianoSynthDevice::PianoSynthDevice(std::string name)
     addParameter(Parameter(Constants::NahdXml::xmlKeyReleaseTime().toStdString(), 0.3f, 0, 10000, 3000, 100));
     addParameter(Parameter(Constants::NahdXml::xmlKeyPanSpread().toStdString(), 0.7f, 0, 10000, 7000, 100));
     addParameter(Parameter(Constants::NahdXml::xmlKeyHardness().toStdString(), 0.5f, 0, 10000, 5000, 100));
-    addParameter(Parameter(Constants::NahdXml::xmlKeyStringDetune().toStdString(), 0.3f, 0, 10000, 3000, 100));
+    addParameter(Parameter(Constants::NahdXml::xmlKeyStringDetune().toStdString(), 0.1f, 0, 10000, 1000, 100));
 
     PianoSynthDevice::syncParameters();
 }
@@ -283,8 +283,8 @@ void PianoSynthDevice::handleNoteOn(uint8_t note, uint8_t velocity)
 {
     const float vel = static_cast<float>(velocity) / 127.0f;
     const float velBright = vel * m_hammerHardness;
-    // Higher notes are naturally brighter; shift brightness by ±0.15 across the keyboard.
-    const float noteBrightOffset = (static_cast<float>(note) - 60.0f) / 127.0f * 0.3f;
+    // Higher notes are naturally brighter; shift brightness by ±0.24 across the keyboard.
+    const float noteBrightOffset = (static_cast<float>(note) - 60.0f) / 127.0f * 0.6f;
     const float effectiveBright = std::clamp(m_brightness + velBright + noteBrightOffset, 0.0f, 1.0f);
     // Detuning in cents for the second string (0–15 cents).
     const double detuneCents = static_cast<double>(m_stringDetune) * 15.0;

src/domain/devices/piano_synth_device.hpp

@@ -104,7 +104,7 @@ class PianoSynthDevice : public Device
     float m_releaseTime { 0.3f };
     float m_stereoWidth { 0.7f };
     float m_hammerHardness { 0.5f };
-    float m_stringDetune { 0.3f };
+    float m_stringDetune { 0.1f };
 
     void handleNoteOn(uint8_t note, uint8_t velocity);
     void handleNoteOff(uint8_t note);

src/domain/dsp/waveguide_string.cpp

@@ -18,6 +18,7 @@
 #include <algorithm>
 #include <cmath>
 #include <numbers>
+#include <random>
 
 namespace noteahead {
 
@@ -73,6 +74,13 @@ void WaveguideString::trigger(uint8_t note, float velocity, float brightness, fl
     const size_t width = std::max(size_t { 1 }, N / std::max(size_t { 1 }, static_cast<size_t>(2.0 + static_cast<double>(velocity) * 6.0)));
     const double amplitude = static_cast<double>(velocity) * 0.5;
 
+    // Noise burst seeded by note so repeated strikes are consistent but each pitch differs.
+    // Decays in ~N/6 samples to model brief hammer-felt impact noise before the tone settles.
+    std::minstd_rand rng { static_cast<uint32_t>(note) + 17u };
+    std::uniform_real_distribution<double> noiseDist { -1.0, 1.0 };
+    const double noiseGain = static_cast<double>(velocity) * 0.08;
+    const double noiseDecay = 6.0 / static_cast<double>(N);
+
     // Pre-load excitation into the delay buffer so output starts immediately.
     m_delay.reset();
     for (size_t i = 0; i < N; i++) {
@@ -81,6 +89,7 @@ void WaveguideString::trigger(uint8_t note, float velocity, float brightness, fl
             const double t = static_cast<double>(i) / static_cast<double>(width);
             excite = amplitude * 0.5 * (1.0 - std::cos(2.0 * std::numbers::pi * t));
         }
+        excite += noiseDist(rng) * noiseGain * std::exp(-noiseDecay * static_cast<double>(i));
         m_delay.write(excite);
     }