Help needed with DSP filter problems

Hi I have been experimenting writing DSP software, using librtlsdr and a NESDR dongle from NooElec. After much searching the Internet for code to design and implement DSP filters (with little success), I remembered that years ago I bought the "Digital Signal Processing" book by Steven W. Smith. I used the BASIC software included to produce C code to calculate the a and b coefficients for Low Pass or High Pass Chebyshev filters, from 2-pole to 20-pole if needed. Unfortunately, the cut-off frequency for a 6-pole Low Pass filter I implemented turns out to be 2x the required value, even though the design function that calculates the a and b coefficients produces values that match exactly values tabled in the book. This leaves me I think with the following possibilities: 1. That the calculated a and b coefficients are correct but my own code implementing a recursive filter is incorrect. 2. That I have incorporated the LPF filter function into my librtlsdr-based application incorrectly, perhaps misunderstanding parameters like the sampling rate or the required bandwidth. 3. That both the above implementations are correct but the BASIC code included in the book has a bug somewhere, so that both my own results for a and b and those tabled in the book are incorrect. Rather unlikely I thought.... So I am asking for help here from more knowledgeable DSP'ers hoping to resolve this matter, as the above mentioned book is very good, with very clear and simplified teaching of DSP for engineers _and_ it includes source code in BASIC for most of the algorithms it teaches. And it is available openly on the Internet with source code here!: http://dspguide.com/ My own GPL'ed application is available for inspection here: www.5b4az.org/pkg/sdrx/sdrx-0.3-beta.tar.bz2 The filter functions are in src/filters.c and SDR functions in src/rtlsdr.c My thanks here in advance! -- Best Regards Neoklis - Ham Radio Call:5B4AZ http://www.5b4az.org/