Piano: Make decay more accurate

1. Make decay sample-granular rather than buffer-granular
You only have ~43 buffers per second which can make a jagged signal.

2. Calculate decay in milliseconds
Decay is supposed to be a time value.

Applications/Piano/AudioEngine.cpp

@@ -31,6 +31,7 @@
 
 AudioEngine::AudioEngine()
 {
+ set_decay(0);
 }
 
 AudioEngine::~AudioEngine()
@@ -45,6 +46,11 @@ void AudioEngine::fill_buffer(FixedArray<Sample>& buffer)
  for (size_t note = 0; note < note_count; ++note) {
  if (!m_note_on[note])
  continue;
+
+ m_power[note] -= m_decay_step;
+ if (m_power[note] < 0)
+ m_power[note] = 0;
+
  double val = 0;
  switch (m_wave) {
  case Wave::Sine:
@@ -70,16 +76,6 @@ void AudioEngine::fill_buffer(FixedArray<Sample>& buffer)
  buffer[i].right = buffer[i].left;
  }
 
- if (m_decay) {
- for (size_t note = 0; note < note_count; ++note) {
- if (m_note_on[note]) {
- m_power[note] -= m_decay / 100.0;
- if (m_power[note] < 0)
- m_power[note] = 0;
- }
- }
- }
-
  if (m_delay) {
  if (m_delay_buffers.size() >= m_delay) {
  auto to_blend = m_delay_buffers.dequeue();
@@ -201,10 +197,22 @@ void AudioEngine::set_wave(Direction direction)
  }
 }
 
+static inline double calculate_step(double distance, int milliseconds)
+{
+ if (milliseconds == 0)
+ return distance;
+
+ constexpr double samples_per_millisecond = sample_rate / 1000.0;
+ double samples = milliseconds * samples_per_millisecond;
+ double step = distance / samples;
+ return step;
+}
+
 void AudioEngine::set_decay(int decay)
 {
  ASSERT(decay >= 0);
  m_decay = decay;
+ m_decay_step = calculate_step(1, m_decay);
 }
 
 void AudioEngine::set_delay(int delay)

Applications/Piano/AudioEngine.h

@@ -77,7 +77,8 @@ class AudioEngine {
 
  int m_octave { 4 };
  int m_wave { first_wave };
- int m_decay { 0 };
+ int m_decay;
+ double m_decay_step;
  int m_delay { 0 };
 
  int m_time { 0 };

Applications/Piano/KnobsWidget.cpp

@@ -91,7 +91,7 @@ KnobsWidget::KnobsWidget(GUI::Widget* parent, AudioEngine& audio_engine, MainWid
  m_wave_value->set_text(wave_strings[new_wave]);
  };
 
- constexpr int max_decay = 20;
+ constexpr int max_decay = 1000;
  m_decay_knob = GUI::Slider::construct(Orientation::Vertical, m_knobs_container);
  m_decay_knob->set_range(0, max_decay);
  m_decay_knob->set_value(max_decay - m_audio_engine.decay());