Add delay tempo sync, rack faceplate, README, docs

- Add host-tempo-synced delay: FX screen gains a 4th SYNC row for
  rhythmic divisions (1/16 .. 1/1) that lock delay time to DAW tempo
- Embed rack faceplate background (b2.png) via juce binary data
- Add offline dev probe (rmx_probe) for GUI-less MIDI testing
- Allow overriding JUCE_ROOT when configuring the build
- Write comprehensive README describing the instrument, UI, MIDI, DSP,
  and build/install steps

Source/SynthEngine.cpp

@@ -64,8 +64,9 @@ SynthEngine::Voice* SynthEngine::acquireVoice()
 
 void SynthEngine::noteOn (int midiNote, float velocity)
 {
-    const float noteFreq = (float) (440.0 * std::pow (2.0, (midiNote - 69.0) / 12.0));
+    const float noteFreq = (float) (440.0 * std::exp2 ((midiNote - 69.0) / 12.0));
     Voice* v = acquireVoice();
+    const bool fresh = ! v->active;
     v->active = true;
     v->note = midiNote;
     v->vel = 1.0f - std::pow (1.0f - velocity, 1.35f);
@@ -77,6 +78,7 @@ void SynthEngine::noteOn (int midiNote, float velocity)
     v->lfoIdx = 0;
     v->lfoSH = rng.nextFloat() * 2.0f - 1.0f;
     v->lp = v->bp = 0.0f;
+    v->fcSm = 0.0f;
     v->outSmooth = 0.0f;
 
     const float p = juce::jlimit (-1.0f, 1.0f, lastPan + (rng.nextFloat() - 0.5f) * 0.1f);
@@ -88,7 +90,7 @@ void SynthEngine::noteOn (int midiNote, float velocity)
     v->amp.reset(); v->flt.reset();
     v->amp.noteOn(); v->flt.noteOn();
     lastNoteFreq = noteFreq;
-    ++active;
+    if (fresh) ++active;
 }
 
 void SynthEngine::noteOff (int midiNote)
@@ -106,9 +108,6 @@ void SynthEngine::noteOff (int midiNote)
 
 void SynthEngine::render (const VParams& p, float* left, float* right, int num)
 {
-    float peakL = metL, peakR = metR;
-    metL = metR = 0.0f;
-
     for (auto& v : voices)
     {
         v.amp.setRates (p.ampA, p.ampD, p.ampS, p.ampR, sr);
@@ -120,8 +119,8 @@ void SynthEngine::render (const VParams& p, float* left, float* right, int num)
 
     const float glideRate = p.glide <= 0.001f ? 1.0f
                          : 1.0f - std::exp (-1.0f / (p.glide * (float) sr));
-    const float detuneRatio = std::pow (2.0f, p.detune / 1200.0f);
-    const float tuneRatio   = std::pow (2.0f, p.tune / 12.0f);
+    const float detuneRatio = std::exp2f (p.detune / 1200.0f);
+    const float tuneRatio   = std::exp2f (p.tune / 12.0f);
     const float maxFc = 18000.0f, minFc = 30.0f;
 
     for (int s = 0; s < num; ++s)
@@ -160,10 +159,10 @@ void SynthEngine::render (const VParams& p, float* left, float* right, int num)
             // frequency, vibrato, bend, tune
             float freq = v.curFreq * tuneRatio;
             if ((int) p.lfoTarget == 1)
-                freq *= std::pow (2.0f, lfo * p.lfoDepth * 1.6f / 12.0f);
+                freq *= std::exp2f (lfo * p.lfoDepth * 1.6f / 12.0f);
             if (bendSmooth != bendCents)
                 bendSmooth += (bendCents - bendSmooth) * 0.08f;
-            freq *= std::pow (2.0f, bendSmooth / 1200.0f);
+            freq *= std::exp2f (bendSmooth / 1200.0f);
 
             const float d1 = (float) (freq / sr);
             const float d2 = d1 * detuneRatio;
@@ -180,16 +179,19 @@ void SynthEngine::render (const VParams& p, float* left, float* right, int num)
 
             // cutoff with keytrack + filter envelope + LFO + mod wheel
             float fc = p.fltCut;
-            fc *= std::pow (freq / 261.6256f, p.fltKey * 0.01f);
-            fc *= std::pow (maxFc / fc, p.fltEnv * 0.01f * fe);
+            fc *= std::exp2f (p.fltKey * 0.01f * std::log2f (freq / 261.6256f));
+            fc *= std::exp2f (p.fltEnv * 0.01f * fe * std::log2f (maxFc / fc));
             if ((int) p.lfoTarget == 0)
-                fc *= std::pow (2.0f, lfo * p.lfoDepth * 10.0f / 12.0f);
-            fc *= std::pow (2.0f, mod);
+                fc *= std::exp2f (lfo * p.lfoDepth * 10.0f / 12.0f);
+            fc *= std::exp2f (mod);
             fc = std::min (maxFc, std::max (minFc, fc));
+            v.fcSm += (fc - v.fcSm) * 0.25f;
+            fc = v.fcSm;
 
-            // SVF
-            const float f1 = 2.0f * std::sin (juce::MathConstants<float>::pi * fc / (float) sr);
-            const float q1 = 1.0f / (0.5f + p.fltRes * 3.5f);
+            // SVF (cap f1 to keep the Chamberlin structure stable; past ~1.5 the
+            // resonance feedback can self-oscillate and sputter at maxed cutoff+res)
+            const float f1 = std::min (1.50f, 2.0f * std::sin (juce::MathConstants<float>::pi * fc / (float) sr));
+            const float q1 = 1.0f / (1.0f + p.fltRes * 3.0f);
             v.f1c = f1;
             const float hp = sig - v.lp - q1 * v.bp;
             v.bp += f1 * hp;
@@ -211,7 +213,8 @@ void SynthEngine::render (const VParams& p, float* left, float* right, int num)
                 ampMod = 1.0f - p.lfoDepth * 0.5f * (1.0f + lfo);
 
             v.outSmooth += (filtered - v.outSmooth) * 0.12f;
-            const float o = v.outSmooth * (ae * ampMod) * v.vel;
+            float o = v.outSmooth * (ae * ampMod) * v.vel;
+            if (! std::isfinite (o)) o = 0.0f;
 
             mixL += o * v.panL;
             mixR += o * v.panR;
@@ -223,13 +226,12 @@ void SynthEngine::render (const VParams& p, float* left, float* right, int num)
             }
         }
 
-        peakL = std::max (peakL, std::abs (mixL));
-        peakR = std::max (peakR, std::abs (mixR));
+        if (! std::isfinite (mixL)) mixL = 0.0f;
+        if (! std::isfinite (mixR)) mixR = 0.0f;
+        mixL = std::tanh (mixL);
+        mixR = std::tanh (mixR);
 
         left[s]  += mixL;
         right[s] += mixR;
     }
-
-    metL = peakL;
-    metR = peakR;
 }