diff --git a/libavcodec/mdct.c b/libavcodec/mdct.c
index fe19f1ff3d..0447c64124 100644
--- a/libavcodec/mdct.c
+++ b/libavcodec/mdct.c
@@ -59,9 +59,10 @@ float *ff_sine_windows[5] = {
 
 // Generate a sine window.
 void ff_sine_window_init(float *window, int n) {
+    float alpha = M_PI / (2.0 * n);
     int i;
     for(i = 0; i < n; i++)
-        window[i] = sin((i + 0.5) / (2 * n) * M_PI);
+        window[i] = sin((i + 0.5) * alpha);
 }
 
 /**