Phase-Shifting at Specified Frequency?

Questions
1

I know that the Allpass uGens use delay time for its calculations - but I’m wondering what it would take to do phase shifting at specific frequencies? Is this something that would be better served with a different object? Is at as simple as just putting 1/freq in the delayTime argument of Allpass?

3

Sorry to ask a related question. Since SOS is being discussed:

  1. am I wrong in noticing that b and a are backwards between the cookbook and SOS?

  2. why do the cookbook formulas blow up below around 700hz (both HPF and LPF) with SOS? or am I messing something up?

//the the volume down. it isn't deadly, but it does blow up:
//LPF
({
	var sr = SampleRate.ir;
	var freq = MouseX.kr(20,10000).poll;
	var q = 5;

	var omega = 2pi*(freq/sr);

	var sine = omega.sin;
	var csine = omega.cos;

	var alpha = sine/(2*q);

	var b0 = (1-csine)/2;
	var b1 = 1-csine;
	var b2 = b0;
	var a0 = 1+alpha;
	var a1 = -2*csine;
	var a2 = 1-alpha;

	var sig = WhiteNoise.ar(1);

	sig = SOS.ar(sig, b0, b1, b2, a1.neg, a2.neg);
	sig.dup*0.1

}.play;
)

//HPF
({
	var sr = SampleRate.ir;
	var freq = MouseX.kr(20,10000).poll;
	var q = 5;

	var omega = 2pi*(freq/sr);

	var sine = omega.sin;
	var csine = omega.cos;

	var alpha = sine/(2*q);

	var b0 = (1+csine)/2;
	var b1 = (1+csine).neg;
	var b2 = (1+csine)/2;
	var a0 = 1+alpha;
	var a1 = -2*csine;
	var a2 = 1-alpha;

	var sig = WhiteNoise.ar(1);

	sig = SOS.ar(sig, b0, b1, b2, a1.neg, a2.neg);
	sig.dup*0.1

}.play;
)

The formula from the max gen help does not blow up:

({
	var sr = SampleRate.ir;
	var freq = MouseX.kr(20,10000);
	var q = 5;

	var omega = (freq*2pi/sr).poll;

	var sine = omega.sin;
	var csine = omega.cos;

	var alpha = sine/(2*q);

	var b0 = 1/(1+alpha);
	var a2 = ((1-csine)*0.5)*b0;
	var a0 =  a2;
	var a1 = (1-csine)*b0;
	var b1 = (-2*csine)*b0;
	var b2 = (1-alpha)*b0;

	var sig = WhiteNoise.ar(1);

	sig = SOS.ar(sig, a0, a1, a2, b1.neg,b2.neg);
	sig.dup*0.1

}.play;
)

Sam

4

Answering my own question. I did not divide by a0. But tulio, do notice that a and b are backwards from the cookbook.

({
	var sr = SampleRate.ir;
	var freq = MouseX.kr(20,10000).poll;
	var q = 5;

	var omega = 2pi*(freq/sr);

	var sine = omega.sin;
	var csine = omega.cos;

	var alpha = sine/(2*q);

	var b0 = (1-csine)/2;
	var b1 = 1-csine;
	var b2 = b0;
	var a0 = 1+alpha;
	var a1 = -2*csine;
	var a2 = 1-alpha;

	var sig = WhiteNoise.ar(1);

	sig = SOS.ar(sig, b0/a0, b1/a0, b2/a0, a1.neg/a0, a2.neg/a0);
	sig.dup*0.1

}.play;
)

Sam

6

Well, there’s no type of sound in particular, I’m experimenting - but these descriptions help, although I might need to find a maths teacher. There’s not a “transfer functions for aged computer musicians” online course that can get me squared away in two hours, is there?

I understand that I wouldn’t be changing one single frequency’s phase, but rather an area of frequencies would be impacted. I am interested in experimenting with some types of cancellation - and maybe looking into a Frequency Shifter that could shift with a complex secondary signal instead of a sine wave.

Thanks to both of you.

7

Sam,

Sorry to ask a related question. Since SOS is being discussed:

  1. am I wrong in noticing that b and a are backwards between the cookbook and SOS?
    I discovered that a and b are not consistent in the literature, so
    always double-check. I started to refer to them as “forward” and
    “backwards” coefficients for myself.