Hi, I was doing some research on filter programming and was trying to implement a butterworth lowpass filter in Csound based on the instructions given on page 484 of the Audio Programming Book in order to better understand the concept. I am running into some trouble in my implementation. 

The feedback lines, which I have tried to implement in 2 different ways have given negative results. When I used the delay1 opcode it creates extreme uncontrolled feedback which almost blew up my speakers, and when I use afeedback2 = afeedback 1, afeedback1 = aout, it creates a lot of resonance. I tried leaving out the feedback, and it seems that changing the cutoff no longer has anything to do with frequency, but instead changes output level. 

I double checked the formula, so perhaps it is something with my implementation in Csound? Is there a problem with my method of setting aout via an equation like I do?

Here is the instrument: 

instr 3

icutoff = 5000

ilambda = 1 / (tan(3.14159 * icutoff / sr))

icoef = 1 / (1 + (2 * ilambda) + ilambda^2)

ifb1coef = 2 * icoef * (1 - (ilambda^2))

ifb2coef = icoef * (1 + (-2 * ilambda) + (ilambda^2))

aout init 0

afeedback1 init 0

afeedback2 init 0

ain, ain2 diskin "purple.wav", 1

adelay1 delay1 ain

adelay2 delay1 adelay1

;afeedback1 delay1 aout

;afeedback2 delay1 afeedback1

aout = (ain * icoef) + (2 * icoef * adelay1) + (icoef * adelay2) - (ifb1coef * afeedback1) - (ifb2coef * afeedback2)

afeedback2 = afeedback1

afeedback1 = aout

outs aout, aout

endin

Csound mailing list Csound@listserv.heanet.ie https://listserv.heanet.ie/cgi-bin/wa?A0=CSOUND Send bugs reports to https://github.com/csound/csound/issues Discussions of bugs and features can be posted here

No, I had not. That fixed the problem. Is it the case that = statements will run at krate in Csound? Very good to know.

I also fixed the delay problem by switching the order of the delay1 opcodes, the way I had it set up before they were both being set to aout. I also think the way it things were ordered in that method delayed the output by an extra sample anyway, leading to a steeper cutoff and a duller sound. If I use

afeedback2 delay1 aout

aout = (ain * icoef) + (2 * icoef * adelay1) + (icoef * adelay2) - (ifb1coef * aout) - (ifb2coef * afeedback2)

it fixes the cutoff. 

Thanks for your help!

On Sat, Jul 9, 2016 at 3:35 AM, Victor Lazzarini <Victor.Lazzarini@nuim.ie> wrote:

did you set ksmps=1?

Victor Lazzarini

Dean of Arts, Celtic Studies, and Philosophy

Maynooth University

Ireland

On 9 Jul 2016, at 06:47, Emmett Palaima <epalaima@BERKLEE.EDU> wrote:

Hi, I was doing some research on filter programming and was trying to implement a butterworth lowpass filter in Csound based on the instructions given on page 484 of the Audio Programming Book in order to better understand the concept. I am running into some trouble in my implementation. 

The feedback lines, which I have tried to implement in 2 different ways have given negative results. When I used the delay1 opcode it creates extreme uncontrolled feedback which almost blew up my speakers, and when I use afeedback2 = afeedback 1, afeedback1 = aout, it creates a lot of resonance. I tried leaving out the feedback, and it seems that changing the cutoff no longer has anything to do with frequency, but instead changes output level. 

I double checked the formula, so perhaps it is something with my implementation in Csound? Is there a problem with my method of setting aout via an equation like I do?

Here is the instrument: 

instr 3

icutoff = 5000

ilambda = 1 / (tan(3.14159 * icutoff / sr))

icoef = 1 / (1 + (2 * ilambda) + ilambda^2)

ifb1coef = 2 * icoef * (1 - (ilambda^2))

ifb2coef = icoef * (1 + (-2 * ilambda) + (ilambda^2))

aout init 0

afeedback1 init 0

afeedback2 init 0

ain, ain2 diskin "purple.wav", 1

adelay1 delay1 ain

adelay2 delay1 adelay1

;afeedback1 delay1 aout

;afeedback2 delay1 afeedback1

aout = (ain * icoef) + (2 * icoef * adelay1) + (icoef * adelay2) - (ifb1coef * afeedback1) - (ifb2coef * afeedback2)

afeedback2 = afeedback1

afeedback1 = aout

outs aout, aout

endin

Csound mailing list Csound@listserv.heanet.ie https://listserv.heanet.ie/cgi-bin/wa?A0=CSOUND Send bugs reports to https://github.com/csound/csound/issues Discussions of bugs and features can be posted here

Csound mailing list Csound@listserv.heanet.ie https://listserv.heanet.ie/cgi-bin/wa?A0=CSOUND Send bugs reports to https://github.com/csound/csound/issues Discussions of bugs and features can be posted here

Csound mailing list Csound@listserv.heanet.ie https://listserv.heanet.ie/cgi-bin/wa?A0=CSOUND Send bugs reports to https://github.com/csound/csound/issues Discussions of bugs and features can be posted here