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Reference filter for fixed-point or single-precision filter


href = reffilter(hd)


href = reffilter(hd) returns a new filter href that has the same structure as hd, but uses the reference coefficients and has its arithmetic property set to double. Note that hd can be either a fixed-point filter (arithmetic property set to 'fixed', or a single-precision floating-point filter whose arithmetic property is 'single').

reffilter(hd) differs from double(hd) in that

  • the filter href returned by reffilter has the reference coefficients of hd.

  • double(hd) returns the quantized coefficients of hd represented in double-precision.

To check the performance of your fixed-point filter, use href = reffilter(hd) to quickly have the floating-point, double-precision version of hd available for comparison.


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Compare several fixed-point quantizations of a filter with the same double-precision floating-point version of the filter.

h = dfilt.dffir(firceqrip(87,.5,[1e-3,1e-6])); % Lowpass filter.
h1 = copy(h); h2 = copy(h); % Create copies of h.
h.arithmetic = 'fixed';   % Set h to filter using fixed-point...
                          % arithmetic.
h1.arithmetic = 'fixed';  % Same for h1.
h2.arithmetic = 'fixed';  % Same for h2.
h.CoeffWordLength  = 16;  % Use 16 bits to represent the...
                          % coefficients.
h1.CoeffWordLength = 12;  % Use 12 bits to represent the...
                          % coefficients.
h2.CoeffWordLength =  8;  % Use 8 bits to represent the...
                          % coefficients.
href = reffilter(h);
hfvt = fvtool(href,h,h1,h2);
set(hfvt,'ShowReference','off'); % Reference displayed once
                                 % already.
legend(hfvt,'Reference filter','16-bits','12-bits','8-bits');

The fvtool shows href, the reference filter, and the effects of using three different word lengths to represent the coefficients.

See Also

Introduced in R2011a