diff --git a/IO.cpp b/IO.cpp
index 779a3f7..60496bd 100644
--- a/IO.cpp
+++ b/IO.cpp
@@ -21,8 +21,8 @@
 #include "IO.h"
 
 // Generated using rcosdesign(0.2, 8, 5, 'sqrt') in MATLAB
-static q15_t   C4FSK_FILTER[] = {2667, 6995, 11237, 14331, 15464, 14331, 11237, 6995, 2667, 0};
-const uint16_t C4FSK_FILTER_LEN = 10U;
+static q15_t   C4FSK_FILTER[] = {-770, 2667, 6995, 11237, 14331, 15464, 14331, 11237, 6995, 2667, -770, 0};
+const uint16_t C4FSK_FILTER_LEN = 12U;
 
 // Generated using gaussfir(0.5, 4, 5) in MATLAB
 static q15_t   GMSK_FILTER[] = {8, 104, 760, 3158, 7421, 9866, 7421, 3158, 760, 104, 8, 0};