# How does Overlap-Add work for IIR filter?

So let's say I'm trying to implement something like an LPC vocoder. I analyse a speech signal by breaking it up in small chunks and determining their LPC coefficients, which are by design, the coefficients of an all-pole (IIR) filter.

So far, I understand that for an FIR filter, the impulse response is a finite M samples, so when performing an overlap-add (OLA) block convolution, each signal of length N is extended into a sequence of N+M-1 length, and for a naive implementation, we can overlap and add these M-1 samples in the tail, with the next chunk.

If I want to process a buffer of audio with the IIR filter coming from the LPC analysis, the IR should be infinite, so the only way I can think of using a block convolution is by convolving a delta with the IIR to get a long FIR and truncating it (which I think is terrible).

I also see pages like this that say to use the Block Recursion method, which seems to establish the IIR filter coefficients as columns of a matrix (states?), and that if it is causal the upper-right triangle of this matrix is filled with zeros. I don't understand how this works or why such a matrix is needed?

According to wiki, it seems that overlap add technique usually works for FIR filters. I hasn't read the link you posted and the matrix form for IIR before. But when it comes to buffer for IIR, I have an alternative (hope simpler) method which might works in your case.

If I want to process a buffer of audio with the IIR filter coming from the LPC analysis, the IR should be infinite, so the only way I can think of using a block convolution is by convolving

The concept is filtering in sections and utilizing the memory in filters. Let's first look at the testing code and usage with following steps,

1. Dividing the x in to x1 and x2,
2. Filtering x1 and x2 seprately (with memories in filterszf)
3. Combining the ouput together.

The results confirm that filtering in section can be the same as x directly.

%% Filter in section
x = randn(10000,1); % psuido signal

x1 = x(1:5000);
x2 = x(5001:end);

b = [2,3];
a = [1,0.2]; % IIR filter coeffs

[y1,zf] = myFilter(b,a,x1); % The zf is the memory in IIR filter!
y2 = myFilter(b,a,x2,zf); % assign the zf when filter the next section.

y = myFilter(b,a,x); % filtering x directly.

isequal(y,[y1;y2]) % Verification


Here is the implementation of myFilter Function. In fact, myFilter imitates(implements) the built-in function fitler in matlab. The implementation architecture is Direct form II, which has advantages over least usage of delay operator and disadvantages over not free for numerical overflow for fixed points.

function [Y, z] = myFilter(b, a, X, z)
n = max(length(a),length(b));
a = [a zeros(1,n-length(a))];
b = [b zeros(1,n-length(b))];
z(n) = 0;
b = b / a(1);
a = a / a(1);
Y    = zeros(size(X));
for m = 1:length(Y)
Y(m) = b(1) * X(m) + z(1);
for i = 2:n
z(i - 1) = b(i) * X(m) + z(i) - a(i) * Y(m);
end
end
z = z(1:n - 1);
end

• Interesting! This is really cool concept - thanks a bunch for sharing! So I am curious about this for an all-pole filter, since zf depends on b(i), where for an all pole filter, b(i>1) doesn't exist. I guess I would just loose the b(i) * X(m) term entirely. – Aditya TB Apr 24 at 12:31