The syntax for biquad~'s arguments, according to the help patch:
fb1 fb2 ff1 ff2 ff3
In the patch you swap fb2<>ff2 and ff1<>ff3. The syntax in the help file is confusing. For better consistency it could be called:
fb1 fb2 ff0 ff1 ff2
Fill the coefficients for the 44.1 reproduction filter as mentioned in the musicdsp topic:
-0.721892 -0.186052 1 -1.70072 0.702938
But this doesn't seem to produce a minimum phase filter, and it can't be inverted. I guess the signs of feedback coefficients must be flipped to match Pd's biquad implementation:
0.721892 0.186052 1 -1.70072 0.702938
This gives a frequency and phase response much resembling the plots on
http://jiiteepee.tripod.com/riaa.html. This filter can be inverted by swapping the feedback and feed forward coefficients and also flipping the signs:
1.70072 -0.702938 1 -0.721892 -0.186052
This gives the exact inverse frequency and phase response.
Katja