From 1aab0b117fe0288091490b264970b1b22c1a1b18 Mon Sep 17 00:00:00 2001
From: Damien Zammit <damien@zamaudio.com>
Date: Sat, 18 Aug 2018 20:58:24 +1000
Subject: ZamTube: Fix Taylor expansion as per original zamvalve optimization
 by fundamental

---
 plugins/ZamTube/triode.cpp | 23 ++++++++++++++++-------
 plugins/ZamTube/triode.h   |  6 ++++++
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/plugins/ZamTube/triode.cpp b/plugins/ZamTube/triode.cpp
index f725fd0..b40220f 100644
--- a/plugins/ZamTube/triode.cpp
+++ b/plugins/ZamTube/triode.cpp
@@ -89,21 +89,28 @@ T Triode::fgdash(T VG) {
 
 T Triode::ffp(T VP) {
 	static bool prepared = false;
-	static double coeff[4];
 	if(!prepared) {
 		//go go series expansion
 		const double L2 = log(2.0);
 
 		const double scale = pow(L2,gamma-2)/(8.0*pow(c,gamma));
-		coeff[0] = 8.0*L2*L2*scale;
-		coeff[1] = gamma*c*L2*4*scale;
-		coeff[2] = (c*c*gamma*gamma+L2*c*c*gamma-c*c*gamma)*scale;
-		coeff[3] = 0.0;
+		ffp_raw[0] = 8.0*L2*L2*scale;
+		ffp_raw[1] = gamma*c*L2*4*scale;
+		ffp_raw[2] = (c*c*gamma*gamma+L2*c*c*gamma-c*c*gamma)*scale;
 		prepared = true;
 	}
 
-	double A = VP/mu+vg;
-	return (Pb+Pr*((g*(coeff[0]+coeff[1]*A+coeff[2]*A*A))+(Gb-vg)/Gr)-VP);
+	return ffp_coeff[0]+ffp_coeff[1]*VP+ffp_coeff[2]*VP*VP;
+}
+
+void Triode::prepare(void)
+{
+    const double c0 = ffp_raw[0],
+                 c1 = ffp_raw[1],
+                 c2 = ffp_raw[2];
+    ffp_coeff[0] = Pb + Pr*Gb/Gr + c2*Pr*vg*vg - Pr*vg/Gr + c1*Pr*vg + c0*Pr;
+    ffp_coeff[1] = 2.*c2*Pr*vg/mu + c1*Pr/mu - 1.;
+    ffp_coeff[2] = c2*Pr/(mu*mu);
 }
 
 T Triode::ffp_insane(T VP) {
@@ -405,6 +412,8 @@ T Triode::zeroffp ( T a, T b, T t )
 	T sa;
 	T sb;
 	T tol;
+
+	prepare();
 //
 //	Make local copies of A and B.
 //
diff --git a/plugins/ZamTube/triode.h b/plugins/ZamTube/triode.h
index e7a70fb..a3b50ba 100644
--- a/plugins/ZamTube/triode.h
+++ b/plugins/ZamTube/triode.h
@@ -67,6 +67,7 @@ public:
 	
 	Triode();
 	void compute(T Kbb, T Gbb, T Pbb);
+	void prepare(void);
 	T getC(void);
 	T getG(void);
 	T getP(void);
@@ -79,6 +80,11 @@ public:
 	T zeroffp ( T a, T b, T t );
 	T zeroffp_insane ( T a, T b, T t );
 	T zeroffg ( T a, T b, T t );
+
+private:
+	//Taylor series coefficients for fast calculations
+	double ffp_raw[3];
+	double ffp_coeff[3];
 };
 
 #endif
-- 
cgit v1.2.3