GNU DSP Filter library?

[Frederick (Rick) A Niles]

Suggestion: GSL should include functions to implement functions of 
standard DSP filtering such as Butterworth, Chebyshev, and elliptic 
filters.

GNU Octave does this but, not with an easily accessable library for 
external programs to call.  Octave is more for filter design, not 
implementation.

Is there already another GNU library to do this?

    Thanks,
    Rick Niles.