reffilter

Reference filter for fixed-point or single-precision filter

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Syntax

href = reffilter(hd)

Description

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.

Examples

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Open Live Script

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);


H = freqz(h);
H1 = freqz(h1);
[HRef,WRef] = freqz(href);
H2 = freqz(h2);

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

plot(WRef/pi,[db(HRef) db(H) db(H1) db(H2)])
legend("Reference filter","16-bits","12-bits","8-bits")

Figure contains an axes object. The axes object contains 4 objects of type line. These objects represent Reference filter, 16-bits, 12-bits, 8-bits.

Version History

Introduced in R2011a