Voice: Remove remove expapprox mode; use approximation for exp VCA

Remove the experimental exp vs expapprox VCA modes, and use x**5
for the exponential approximation, calling the mode 'LinExp'.

There are now three modes: exp env + lin vca (classic analog),
lin env + lin VCA (MiMi-a linear VCA mode), lin env + exp VCA
(MiMi-a exponential (default) VCA mode).

plugins/mimid/Engine/ParamDefs.h

@@ -74,7 +74,7 @@
 	PARAMPOINTS(SP_KEYSYNC, " Free ", "KeySync")
 	PARAMPOINTS(SP_OSC3WAVE, " Off ", "-1 Squ", "-2 Squ", "-2 Pul", "Noise")
 	PARAMPOINTS(SP_FILTERTYPE, " 24 ", "SVF", "SVF+", "SVFN") /* not used */
-	PARAMPOINTS(SP_ENVMODE, " Exp ", " Lin ", "LinExp", "LinX5")
+	PARAMPOINTS(SP_ENVMODE, " Exp ", " Lin ", "LinExp")
 
 	// Parameters
 

plugins/mimid/Engine/SynthEngine.h

@@ -456,9 +456,8 @@ class SynthEngine
 	void procEnvMode(float param)
 	{
 		// Exp env / Lin VCA - Lin env / Lin VCA - Lin env / Exp VCA
-		ForEachVoice(env.setLinear(param>0.25));
-		ForEachVoice(expvca = (param>0.50));
-		ForEachVoice(expapprox = (param>0.75));
+		ForEachVoice(env.setLinear(param>0.33));
+		ForEachVoice(expvca = (param>0.67));
 	}
 	void processOsc2Xmod(float param)
 	{

plugins/mimid/Engine/Voice.h

@@ -131,7 +131,6 @@ class Voice
 	bool oscmodEnable; // Oscillator modulation output enabled
 
 	bool expvca;
-	bool expapprox;
 
 	int voiceNumber; // Handy to have in the voice itself
 
@@ -179,7 +178,6 @@ class Voice
 	//	fenvd=new DelayLine(Samples*2);
 		oscmodEnable = false;
 		expvca = false;
-		expapprox = false;
 		lfo2.waveForm = 1; // Triangle
 		voiceNumber = 0; // Until someone else says something else
 		unused1=unused2=0; // TODO: Remove
@@ -312,11 +310,12 @@ class Voice
 
 		// VCA
 		if (expvca) {
-			if (expapprox)
-				envVal *= envVal * envVal * envVal * envVal; // x5
-			else
-				//envVal *= envVal * envVal; // cube
-				envVal = expf(7.5*(envVal-1)); // 1..0 -> 0..-65 dB
+			// Approximate exponential curve with x**5:
+			// - Faster to calculate yet reasonable approximation
+			// - Actually goes down to zero when envelope goes t0 0
+			// Empirically, MiMi-a exponential VCA would be:
+			// envVal = expf(7.5*(envVal-1)); // 1..0 -> 0..-65 dB
+				envVal *= envVal * envVal * envVal * envVal;
 		}
 		x1 *= envVal;
 		return x1;