diff --git a/fdzp.hh b/fdzp.hh
index 68e0d75..09d71e4 100644
--- a/fdzp.hh
+++ b/fdzp.hh
@@ -10,28 +10,29 @@ Copyright 2019 Ahmet Inan <inan@aicodix.de>
 
 namespace DSP {
 
-template <int BINS, int FACT, typename CMPLX>
+template <int OUTPUT, int INPUT, typename CMPLX>
 class FDZP
 {
 	typedef typename CMPLX::value_type VALUE;
-	FastFourierTransform<BINS, CMPLX, -1> fwd;
-	FastFourierTransform<BINS * FACT, CMPLX, 1> bwd;
-	CMPLX tmp[BINS * FACT];
-	static constexpr VALUE SCALE = VALUE(1) / VALUE(BINS);
+	FastFourierTransform<INPUT, CMPLX, -1> fwd;
+	FastFourierTransform<OUTPUT, CMPLX, 1> bwd;
+	CMPLX tmp[OUTPUT];
+	static constexpr VALUE SCALE = VALUE(1) / VALUE(INPUT);
+	static_assert(INPUT < OUTPUT, "OUTPUT must be larger than INPUT");
 public:
 	void operator ()(CMPLX *output, const CMPLX *input)
 	{
 		fwd(tmp, input);
-		if (!(BINS&1)) {
-			tmp[BINS/2] *= VALUE(0.5);
-			tmp[BINS*FACT-BINS/2] = tmp[BINS/2];
+		if (!(INPUT&1)) {
+			tmp[INPUT/2] *= VALUE(0.5);
+			tmp[OUTPUT-INPUT/2] = tmp[INPUT/2];
 		}
-		for (int i = (BINS+1)/2+1; i < BINS; ++i)
-			tmp[BINS*(FACT-1)+i] = tmp[i];
-		for (int i = (BINS+1)/2+1; i < BINS; ++i)
+		for (int i = (INPUT+1)/2+1; i < INPUT; ++i)
+			tmp[OUTPUT-INPUT+i] = tmp[i];
+		for (int i = (INPUT+1)/2+1; i < INPUT; ++i)
 			tmp[i] = 0;
 		bwd(output, tmp);
-		for (int i = 0; i < BINS * FACT; ++i)
+		for (int i = 0; i < OUTPUT; ++i)
 			output[i] *= SCALE;
 	}
 };