[PD] biquad~ based riaa filter

[katja]

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


On Wed, Oct 15, 2014 at 12:14 AM, me.grimm <megrimm at gmail.com> wrote:

> all the biquad talk had me remember trying to figure out the riaa filter
> for vinyl playback/production a couple years back which i never quite
> figured out. attached is what I had attempted.
>
> the playback filter seems to be correct but when I switch to the
> production filter. which should be the inverse of the playback filter, i
> get nothing but I think im swapping the coefficients incorrectly...
>
> anyway, here is the thread this is based off of:
>
> http://www.musicdsp.org/showone.php?id=122
>
> and here is some help on the inverse:
>
> http://music.columbia.edu/pipermail/music-dsp/2003-February/055408.html
>
> maybe someone can make heads or tails of this and give me a help?
>
> basically I am attempting to 3d print a record cutting head so I mostly
> need the production (iRIAA) filter as I will be going straight from PD to a
> record lathe.
>
> thanks!
> m
>
> ps. this was helpful too http://jiiteepee.tripod.com/riaa.html
>
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