diff --git a/tracker/spectrum.go b/tracker/spectrum.go
index d7f67e1..5114f7c 100644
--- a/tracker/spectrum.go
+++ b/tracker/spectrum.go
@@ -77,22 +77,27 @@ func (m *Model) BiquadCoeffs() (coeffs BiquadCoeffs, ok bool) {
 		// in state-space, the filter has the form:
 		// s(n+1) = A*s(n)+B*u, where A = [1 f;-f 1-f*r-f*f] and B = [0;f]
 		// y(n) = C*s(n)+D*u, where
-		//   C = [low band]
-		//
+		//   C_low = [1 0]*z, C_band = [0 1]*z, C_high = [-1 -f-r], D_high = [1]
 		// The transfer function is then H(z) = C*(zI-A)^-1*B + D
 		// z*I-A = [z-1 -f; f z+f*r+f*f-1]
 		// Invert it:
-		// (z*I-A)^-1 = 1/det * [z+f*r+f*f-1 f; -f z-1], where det = (z-1)*(z+f*r+f*f-1)+f^2 = z^2 + z * (f*r+f*f−2) + 1-f*r
-		// (z*I-A)^-1*B = 1/det * [-f*f; f*z-f]
+		// (z*I-A)^-1 = 1/det * [z+f*r+f*f-1 f; -f z-1], where det = (z-1)*(z+f*r+f*f-1)+f^2 = z*z+z*f*r+z*f*f-z-z-f*r-f*f+1+f^2 = z*z + z*(f*r+f*f-2)-f*r+1
+		// (z*I-A)^-1*B = 1/det * f * [f; z-1]
+		//   Low: z * [1,0] * f * [f;z-1] / det = f*f*z / det
+		//   Band: z * [0,1] * f * [f;z-1] / det = (f*z^2-f*z) / det
+		//   High: [-1,-f-r] * f * [f;z-1] / det + 1 = ((-f*f-r*f)*z+r*f)/det + 1 = ((-f*f-r*f)*z+r*f+det)/det = (z^2-2*z+1)/det
 		var a0 float32 = 1
 		var a1 float32 = r*f + f*f - 2
-		var a2 float32 = 1 - r*f
+		var a2 float32 = 1 - f*r
 		var b0, b1, b2 float32
 		if p["lowpass"] == 1 {
-			b2 = -f * f
+			b1 = f * f
 		}
-		b2 -= f * float32(p["bandpass"])
-		b1 += f * float32(p["bandpass"])
+		b0 += f * float32(p["bandpass"])
+		b1 -= f * float32(p["bandpass"])
+		b0 += float32(p["highpass"])
+		b1 += -2 * float32(p["highpass"])
+		b2 += float32(p["highpass"])
 		return BiquadCoeffs{a0: a0, a1: a1, a2: a2, b0: b0, b1: b1, b2: b2}, true
 	case "belleq":
 		f := float32(m.d.Song.Patch[i].Units[u].Parameters["frequency"]) / 128