[sdiy] Starting a Four Pole Filter in an FPGA.....

ASSI Stromeko at nexgo.de
Sun Nov 2 09:17:27 CET 2008


On Sonntag 02 November 2008, Jim Patchell wrote:
> I am just now starting to implement the four pole filters that I am
> going to attempt to get into my FPGA based synth...
>
> I am still trying to figure out the pipe-lining...which is probably
> the most difficult part of the whole thing.

The difficult part is coming up with a suitable filter topology.  I 
guess you'll be doing bit-parallel integer arithmetic.  Then it is 
problematic to use biquads - they are going unstable at low cutoffs 
(unless you put in extra precision) due to quantization errors.  
State variable wacks out at high frequency depending on Q and 
therefore requires oversampling.  A few pointers:

http://www.earlevel.com/Digital%20Audio/index.html
http://www.musicdsp.org/archive.php?classid=3 (mostly FP code)

Other than that I'm afraid I won't be of much help, it simply has been 
too many years since I've been doing real DSP myself...


Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk]>+

SD adaptation for Waldorf rackAttack V1.04R1:
http://Synth.Stromeko.net/Downloads.html#WaldorfSDada




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