rtl_fm -p 30 -d 1 -N -o 4 -f 424.175M -s 16k -C|sox -r 16000 -t .raw -e signed-integer -b 16 -c 1 -V1 - -d

everything is playing fine.  I can hear 424.175MHz.  The issue is that I'm hearing another station 50KHz higher on 424.225Mhz in addition to the frequency I want.  So it looks as if it's not using a 16Khz filter.  Am I doing something wrong or is there an issue with rtl_fm.exe?  Thanks 

Chris