How to specify which version of a function to use?

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I am trying to run the grpdelay.m function which calculates the group delay in samples of a filter given certain inputs.
There are two similar input methods for the function. It is calling the wrong one for me but I'm not sure how to tell it to call the right one.
Here are the two similar input methods:
## [g, f] = grpdelay(b,a,n,Fs)
## evaluates the group delay at n frequencies between 0 and Fs/2.
## [g, f] = grpdelay(b,a,f,Fs)
## evaluates the group delay at frequencies f (in Hz).
I would like the second option. Ie. I want to specify a frequency "f" and calculate the group delay at that frequency. However, it keeps running the first option instead, giving me the group delay at "n" frequencies.
How do I force it to take the third input variable as "f" rather than "n"?
Here is my sample code:
g = 0.903228;
a1 = -1.88142;
a2 = 0.903228;
B1 = -2.08299;
B2 = 1.10714;
b = g * [1, B1, B2];
a = [1,a1,a2];
f = 82.2;
Fs = 44100;
[g, f] = grpdelay(b,a,f,Fs);
[g, f]
Rather than outputing just one group delay at 82.2 Hz, it is outputting 82 different group delays across the frequency spectrum.
What am I doing wrong? Thanks.

Accepted Answer

Stephen23 on 14 May 2020
Edited: Stephen23 on 14 May 2020
The grpdelay documentation states "Frequencies, specified as a vector. fin must have at least two elements, because otherwise the function interprets it as n".
So the simple solution would be to provide two frequencies and remove the corresponding element from the outputs. You might even find use NaN as the extra input frequencies will work.
  1 Comment
Steven Lord
Steven Lord on 14 May 2020
The "two" frequencies you provide could be [f f].

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