MATLAB: How to use a filter in a parfor with the more numerically stable z,p,k

Question

Hello,

I would like to use a filter in a parfor loop. Using [b,a] coefficients there is no problem. Because of a better stability I have to use the [z,p,k] coefficients. It perfectly works in a for loop. However, in a parfor loop the error massage "Not enough input arguments." appears. Does anyone know a workaround?

Here is a little example: Anything works but the last loop!

% Signal
x = sin(0:pi/1000:10*pi);
% Filter properties
N = 1;
fc = 1/(2*pi);
fs = 1000/pi;
Wn = fc/(fs/2);
ftype = 'low';
% Transfer Function design
[b,a] = butter(N,Wn,ftype); 
% Zero-Pole-Gain design
[z,p,k] = butter(N,Wn,ftype);
[sos,g] = zp2sos(z,p,k);
hd = dfilt.df2tsos(sos, g); 
%%Loops
for n=1:2
    y = filter(b,a,x);
end
parfor n=1:2
    y = filter(b,a,x);
end
for n=1:2
    y = filter(hd, x);
end
parfor n=1:2
    y = filter(hd, x);
end

Answer 1

Modifying the parfor loop as follows seems to resolve the error. The filter object was causing the error to occur for some reason, but creating the filter object on each worker separately fixes it.

parfor n=1:2
    hd = dfilt.df2tsos(sos, g); 
    y(n,:) = filter(hd, x);
end