diff --git a/libavcodec/ac3dec_fixed.c b/libavcodec/ac3dec_fixed.c
index 682fe935b0..5a868e4abf 100644
--- a/libavcodec/ac3dec_fixed.c
+++ b/libavcodec/ac3dec_fixed.c
@@ -69,7 +69,7 @@ static void scale_coefs (
     int temp, temp1, temp2, temp3, temp4, temp5, temp6, temp7;
 
     mul = (dynrng & 0x1f) + 0x20;
-    shift = 4 - ((dynrng << 23) >> 28);
+    shift = 4 - (sign_extend(dynrng, 9) >> 5);
     if (shift > 0 ) {
       round = 1 << (shift-1);
       for (i=0; i<len; i+=8) {