[sdiy] Digital 4 Pole Multimode Filter

[paula at synth.net]

Just for the record, I'm loving this thread :)


On 2018-05-03 08:37, Richie Burnett wrote:
> If you already know how to implement the 2-pole digital (chamberlain)
> state variable filter in code, then the easiest route to a 4-pole
> filter is to cascade two discrete digital SVFs. For the purpose of
> clarity: pass the audio samples through one digital SVF then pass the
> output of that filter through another digital SVF.
> 
> You get some flexibility in the choice of overall response as each SVF
> can be LP, BP, HP or notch, and you can independently set the fc and q
> for each. You can also cascade even more stages for 6-pole, 8-pole,
> etc, if the desire is there and resources permit.
> 
> From a programming point of view, each SVF has two internal integrator
> states, and most DSPs have dual accumulators, so it fits nicely to
> process blocks of audio through one 2-pole SVF at a time keeping the
> integrator states at maximum-precision in the MAC accumulators. This
> is harder to do with a moog 4-pole cascade that has 4 internal states.
> 
> -Richie,
> 
> Sent from my Xperia SP on O2
> 
> ---- Scott Gravenhorst wrote ----
> 
>> 
>> Hi List,
>> 
>> I'm looking for information on digital 4 pole filters.  I need to be 
>> able
>> to control Q and Fc.  Among the many documents I found with google was 
>> this
>> document:
>> 
>> http://www.rane.com/pdf/linriley.pdf
>> 
>> which describes an analog version of a 4th order state variable 
>> filter, but
>> my math lessons were so many years ago, that it's not entirely 
>> helpful.  On
>> page 4 the document shows how the resistors are to be given values 
>> (right
>> side) for a Linkwitz-Riley crossover.  I have a feeling this is close 
>> to
>> what I want, but
>> 
>> 1) I'm unable to determine how to change the filter so that I can 
>> control Q
>> and Fc.
>> 2) I'm unable to convert this to a block diagram representation of the 
>> same
>> circuit as a digital model.
>> 3) Information on how to tap the structure for modes other than 
>> lowpass
>> would be very helpful.
>> 
>> I'm familiar with the 2nd order state variable filter, but expanding 
>> that
>> by adding two more integrators is not all that simple to my eyes.  I 
>> can
>> see from the Linkwitz-Riley circuit that each integrator has a 
>> negative
>> feedback path.
>> 
>> I would also be interested in other 4 pole filter topolgies where 
>> access is
>> given for Q and Fc control parameters.
>> 
>> I would be extremely grateful for pointers to readings and information
>> helpful in this quest.
>> 
>> -- ScottG
>> ________________________________________________________________________
>> -- Scott Gravenhorst
>> -- http://scott.joviansynth.com/
>> -- When the going gets tough, the tough use the command line.
>> -- Matt 21:22
>> 
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