diff --git a/xm.js b/xm.js
index 49c00da..e29bfb5 100644
--- a/xm.js
+++ b/xm.js
@@ -90,8 +90,8 @@ function filterCoeffs(f_c) {
   var e = Math.exp(-wct);
   var c = e * Math.cos(wct);
   var s = e * Math.sin(wct);
-  var gain = (1 - 2*c + c*c + s*s) / 2;
-  return [gain, 2*c, -c*c - s*s];
+  var gain = (1 - 2*c + e*e) / 2;
+  return [gain, 2*c, -e*e];
 }
 
 function updateChannelPeriod(ch, period) {