diff --git a/crap/tube.h b/crap/tube.h
index ff4e20b..5515c9d 100644
--- a/crap/tube.h
+++ b/crap/tube.h
@@ -3,6 +3,7 @@
 
 #include "util.h"
 #include "param.h"
+#include "os6iir.h"
 
 #define ID 0x50F7BA11
 #define LABEL "crap_tube"
@@ -52,37 +53,6 @@ distort(double x)
 	return (27*x + 9) / (9*x*x + 6*x + 19) - 9/19.;
 }
 
-#define BQSHIFT(i) \
-	h[i*2 + 1] = h[i*2 + 0]; \
-	h[i*2 + 0] = x;
-
-// b0 and b2 are equivalent and factored out
-// b0 and a0 are factored out as overall gain
-#define LOWPASS(i, b1, a1, a2) \
-	y = x + b1*h[i*2 + 0] +    h[i*2 + 1] \
-	      - a1*h[i*2 + 2] - a2*h[i*2 + 3]; \
-	BQSHIFT(i); \
-	x = y;
-
-static double
-oversample(double h[HIST_SIZE_2], double x)
-{
-	/* Fp = 20000.62
-	   os = 6; N = 8*2; Gp = 0.0135; Gs = 120;
-	*/
-	double y;
-	LOWPASS(0,-1.7310964991540,-1.7686201550064,+0.9924894080401);
-	LOWPASS(1,-1.7228987922703,-1.7591070571963,+0.9756664940293);
-	LOWPASS(2,-1.7030928294094,-1.7508784553758,+0.9530819894740);
-	LOWPASS(3,-1.6623628189757,-1.7420481292930,+0.9206849371458);
-	LOWPASS(4,-1.5767803723821,-1.7314029454409,+0.8755898544307);
-	LOWPASS(5,-1.3754954625889,-1.7189289035735,+0.8193999703446);
-	LOWPASS(6,-0.7934216412437,-1.7066722500921,+0.7627918423024);
-	LOWPASS(7,+1.1176227011686,-1.6987049565355,+0.7256338727337);
-	BQSHIFT(8);
-	return y*0.000005162322938;
-}
-
 static double
 process_one(double x, double drive, double wet)
 {
diff --git a/include/os6iir.h b/include/os6iir.h
new file mode 100644
index 0000000..7b8992a
--- /dev/null
+++ b/include/os6iir.h
@@ -0,0 +1,37 @@
+/* IIR filter
+     Fp = 20000.62
+     os = 6; N = 8*2; Gp = 0.0135; Gs = 120;
+   overall delay: ~5 samples
+*/
+
+#define BQSHIFT(i) \
+	h[i*2 + 1] = h[i*2 + 0]; \
+	h[i*2 + 0] = x;
+
+// b0 and b2 are equivalent and factored out
+// b0 and a0 are factored out as overall gain
+#define LOWPASS(i, b1, a1, a2) \
+	y = x + b1*h[i*2 + 0] +    h[i*2 + 1] \
+	      - a1*h[i*2 + 2] - a2*h[i*2 + 3]; \
+	BQSHIFT(i); \
+	x = y;
+
+static double
+oversample(double *h, double x)
+{
+	// h[] should be at least 18 doubles long
+	double y;
+	LOWPASS(0,-1.7310964991540,-1.7686201550064,+0.9924894080401);
+	LOWPASS(1,-1.7228987922703,-1.7591070571963,+0.9756664940293);
+	LOWPASS(2,-1.7030928294094,-1.7508784553758,+0.9530819894740);
+	LOWPASS(3,-1.6623628189757,-1.7420481292930,+0.9206849371458);
+	LOWPASS(4,-1.5767803723821,-1.7314029454409,+0.8755898544307);
+	LOWPASS(5,-1.3754954625889,-1.7189289035735,+0.8193999703446);
+	LOWPASS(6,-0.7934216412437,-1.7066722500921,+0.7627918423024);
+	LOWPASS(7,+1.1176227011686,-1.6987049565355,+0.7256338727337);
+	BQSHIFT(8);
+	return y*0.000005162322938;
+}
+
+#undef BQSHIFT
+#undef LOWPASS