The simple answer is: because I couldn't find this information anywhere else. Every time I picked up a reference on the subject, it was full of poles and zeros, not sensible frequency responses in Hertz and dBs. Most of these references also made my head hurt - it complains when it sees six pages of calculus followed by a phrase 'from this it is obvious...'

The was also a problem in that there seemed to be at least half a dozen ways of working out what went into the filter, with no great preference as to which should be used in a given circumstance - and some of them were much more complicated than others. This wasn't helped by the fact that different references seemed to use different notations and methods...all very confusing!

So, I sat down and wrote some code to design digital filters. Then, I had to write some more code to test the filters with. After that, it made sense to write some more code to implement the filters I'd designed...and so it goes.

I hope you find the information and the code samples here useful. Please let me know what you think!