Re: [OctDev] m-file error?

[Paul K. <pki...@ja...>]

On Wed, Jun 19, 2002 at 11:13:02AM -0500, Hammett, Rich (Contractor - SimTech) wrote:
> To whom should I direct questions about "toolkit" m-files?  My specific
> question is about the signal processing fir2.m file.  There is a line
> toward the bottom where the output is being centered and truncated,
> as so:
> 
> b = real ([ b([2*grid_n-floor(mid)+1:2*grid_n]) ; b(1:ceil(mid)) ]);
> 
> However, the left half of that vector only goes to 2*grid_n, while
> the input data goes to 2*grid_n + 1.  And since we're wrapping the
> output data, the point being left out is from the middle of the
> data (just left of DC), not an outlier.

	grid = interp1(f,m,linspace(0,1,grid_n+1)');
				 	^^^^^^^^
grid will have grid_n+1 points.

  	b = ifft([grid ; grid(grid_n:-1:2)]);
			      ^^^^^^^^^^^
length(grid_n:-1:2) is grid_n-1, so b will have 
grid_n+1 + grid_n-1 = 2*grid_n points, not 2*grid_n + 1.  The
reflection contains neither the 0 nor the pi frequency components.

Paul Kienzle
pki...@us...