I'm using sox to emulate RIAA pre-emphasis and de-emphasis effects on audio files. For pre-emphasis I call it like this:
sox --plot octave in.wav out_biquad.wav biquad 1 -1.700724 0.7029382 0.2380952 -0.1718791 -0.0442981
The biquad coefficients are taken from http://jiiteepee.tripod.com/de-pre-emphasis.html. I wonder if there is any easy way to calculate the biquad coefficients if I want to apply pre-emphasis only partially - in the range from 0% (no pre-emphasis) to 100% (full pre-emphasis).
The case for 0% seems to be easy, coefficients
1 0 0 1 0 0 would work just fine. But I'm not sure how to approach calculation of biquad coefficients for the combination of zero_filter + float_fraction * full_filter.