Pseudo Allpass Filter Implementation

Questions
1

Hi everyone!

I’m trying to make a list processing Allpass Filter for my educational purposes.
As you can see in the code below there is already the code for IIR & FIR Comb that works with a Circular Buffer and a List as Input.
What I’m trying to achieve is a Schroeder Allpass Filter.
I’ve read that the Allpass Filter is a series combination of a feedforward and feedback comb filter (having equal delays) which creates this allpass filter when the feedforward coefficient is the negative of the feedback coefficient.
I find it hard to wrap my head around how to achieve this combination here with a function.

// A list processing IIR & FIR

a = [1, 0 ,0 ,0 ,0 ,0 ,0 ,0 ,0 ,0 ,0]; // Impulse

// Circular Buffer
(
~circGen = {|len = 4| var mem, pointer=0;
	mem = Array.fill(len,0);
	{|in| var out;
		mem[pointer%len] = in;
		out=mem[pointer-(len-1)%len];
		pointer=pointer+1;
		out;
	};
	
};
);

// IIR Comb
(
~iir_comb = {arg cof = 0.9; var circ, prev=0, out;
	circ = ~circGen.(2);
	{|in|
		prev = circ.(prev)+in*cof;
		
	};
};
);

f = ~iir_comb.(0.9);

z = a.collect(f);

z.plot; // Impulse Response of IIR Filter


// FIR COMB
(
~fir_comb ={|cof= 0.9| var circ,prev=0,out;
	circ = ~circGen.(3);
	{|in|
		out=circ.(prev)+in*cof;
		prev=in;
		out;
	};
};
);

f = ~fir_comb.(0.9);
z = a.collect(f);
z.plot;

Furthermore I would like to do it with a real signal combining FIR Comb and IIR Comb by means of DelayN.ar and thru a LocalIn/LocalOut Feedback. How could you combine both in a single function to achieve an Allpass?

//FIR Comb
(
{
	a=WhiteNoise.ar;
	b=DelayN.ar(a,0.1,0.0002);
	a-b;

}.freqscope
);

//IIR Comb
Server.killAll;
s.options.blockSize_(64);
s.boot;

(
{
	a=WhiteNoise.ar;
	a=a+LocalIn.ar*0.9.neg;
	LocalOut.ar(a);
	a;
}.freqscope
);

Hope I was clear enough :slight_smile: Any help/hints/tips are appreciated! Thank you!