diff --git a/filter.hh b/filter.hh
index cb7d8ec..006aacd 100644
--- a/filter.hh
+++ b/filter.hh
@@ -18,7 +18,7 @@ class LowPass
 	TYPE f;
 public:
 	LowPass(TYPE cutoff) : f(TYPE(2) * cutoff) {}
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		TYPE x = TYPE(n) - TYPE(0.5) * TYPE(N - 1);
 		return f * sinc(f * x);
@@ -32,7 +32,7 @@ class LowPass2
 	TYPE fac;
 public:
 	LowPass2(int num, int den) : num(num), den(den), fac(TYPE(2*num)/TYPE(den)) {}
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		int twox = 2 * n - (N - 1);
 		return !twox ? fac : fac *
@@ -47,7 +47,7 @@ class HighPass
 	TYPE f;
 public:
 	HighPass(TYPE cutoff) : f(TYPE(2) * cutoff) {}
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		TYPE x = TYPE(n) - TYPE(0.5) * TYPE(N - 1);
 		// if (N%1) return delta(x) - f * sinc(f * x);
@@ -63,7 +63,7 @@ class HighPass2
 	TYPE fac;
 public:
 	HighPass2(int num, int den) : num(num), den(den), fac(TYPE(2*num)/TYPE(den)) {}
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		int twox = 2 * n - (N - 1);
 		return !twox ? TYPE(1) - fac :
@@ -80,7 +80,7 @@ class BandPass
 public:
 	BandPass(TYPE cutoff0, TYPE cutoff1) :
 		f0(TYPE(2) * cutoff0), f1(TYPE(2) * cutoff1) {}
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		TYPE x = TYPE(n) - TYPE(0.5) * TYPE(N - 1);
 		return f1 * sinc(f1 * x) - f0 * sinc(f0 * x);
@@ -90,7 +90,7 @@ public:
 template <typename TYPE>
 struct HilbertTransform
 {
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		if (N&1) {
 			int x = n - (N - 1) / 2;
diff --git a/window.hh b/window.hh
index 17fd00c..8eac7ee 100644
--- a/window.hh
+++ b/window.hh
@@ -16,13 +16,13 @@ namespace DSP {
 template <typename TYPE>
 struct Rect
 {
-	TYPE operator () (int n, int N) { return n >= 0 && n < N ? 1 : 0; }
+	TYPE operator () (int n, int N) const { return n >= 0 && n < N ? 1 : 0; }
 };
 
 template <typename TYPE>
 struct Hann
 {
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		return TYPE(0.5) * (TYPE(1) - UnitCircle<TYPE>::cos(n, N - 1));
 	}
@@ -31,7 +31,7 @@ struct Hann
 template <typename TYPE>
 struct Hamming
 {
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		return TYPE(0.54) - TYPE(0.46) * UnitCircle<TYPE>::cos(n, N - 1);
 	}
@@ -40,7 +40,7 @@ struct Hamming
 template <typename TYPE>
 struct Lanczos
 {
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 #if 0
 		return sinc(TYPE(2 * n) / TYPE(N - 1) - TYPE(1));
@@ -60,7 +60,7 @@ public:
 	Blackman(TYPE a) : Blackman((TYPE(1) - a) / TYPE(2), TYPE(0.5), a / TYPE(2)) {}
 	// "exact Blackman"
 	Blackman() : Blackman(TYPE(7938) / TYPE(18608), TYPE(9240) / TYPE(18608), TYPE(1430) / TYPE(18608)) {}
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		return a0 - a1 * UnitCircle<TYPE>::cos(n, N-1) + a2 * UnitCircle<TYPE>::cos((2*n)%(N-1), N-1);
 	}
@@ -72,7 +72,7 @@ class Gauss
 	TYPE o;
 public:
 	Gauss(TYPE o) : o(o) {}
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		return exp(- TYPE(0.5) * pow((TYPE(n) - TYPE(N - 1) / TYPE(2)) / (o * TYPE(N - 1) / TYPE(2)), TYPE(2)));
 	}
@@ -110,7 +110,7 @@ class Kaiser
 	}
 public:
 	Kaiser(TYPE a) : a(a) {}
-	TYPE operator () (int n, int N)
+	TYPE operator () (int n, int N) const
 	{
 		return i0(Const<TYPE>::Pi() * a * sqrt(TYPE(1) - sqr(TYPE(2 * n) / TYPE(N - 1) - TYPE(1)))) / i0(Const<TYPE>::Pi() * a);
 	}