LOWPASS Construct a lowpass filter designer.
D = FDESIGN.LOWPASS(SPECSTRING,VALUE1,VALUE2,...) constructs a lowpass
filter designer D. Note that D is not the design itself, it only
contains the design specifications. In order to design the filter, one
needs to invoke the DESIGN method on D.
For example (more examples below):
D = fdesign.lowpass('Fp,Fst,Ap,Ast',0.4,0.5,1,80);
H = design(D,'equiripple'); % H is a DFILT
SPECSTRING is a string that determines what design specifications will
be used. There are several possible specifications, a complete list is
given below.
Different specification types may have different design methods
available. Use DESIGNMETHODS to get a list of design methods
available for a given SPEC: designmethods(D).
VALUE1, VALUE2, etc. are scalars that provide the value of the
corresponding specification. In the example above, this means that Fp =
0.4, Fst = 0.5, Ap = 1, and Ast = 80. Use get(D, 'description') for a
description of VALUE1, VALUE2, etc.
By default, all frequency specifications are assumed to be in
normalized frequency units. Moreover, all magnitude specifications are
assumed to be in dB.
D = FDESIGN.LOWPASS(...,Fs) provides the sampling frequency of the
signal to be filtered. Fs must be specified as a scalar trailing the
other numerical values provided. For this case, Fs is assumed to be in
Hz as are all other frequency values provided. Note that you don't
change the specification string in this case. In the example above, if
the input signal is sampled at 8 kHz, we can obtain the same filter by
specifying the frequencies in Hz as:
D = fdesign.lowpass('Fp,Fst,Ap,Ast',1600,2000,1,80,8000);
H = design(D,'equiripple');
D = FDESIGN.LOWPASS(...,MAGUNITS) specifies the units for any magnitude
specification given. MAGUNITS can be one of the following: 'linear',
'dB', or 'squared'. If this argument is omitted, 'dB' is assumed. Note
that the magnitude specifications are always converted and stored in dB
regardless of how they were specified. If Fs is provided, MAGUNITS must
be provided after Fs in the input argument list.
The full list of possible values for SPECSTRING (not case sensitive)
is:
'Fp,Fst,Ap,Ast' (minimum order; default)
'N,F3dB'
'Nb,Na,F3dB'
'N,F3dB,Fst' (*)
'N,F3dB,Ap' (*)
'N,F3dB,Ast' (*)
'N,F3dB,Ap,Ast' (*)
'N,Fc'
'N,Fc,Ap,Ast'
'N,Fp,Ap'
'N,Fp,Ap,Ast'
'N,Fp,F3dB' (*)
'N,Fp,Fst'
'N,Fp,Fst,Ap' (*)
'N,Fp,Fst,Ast' (*)
'N,Fst,Ast'
'N,Fst,Ap,Ast' (*)
'Nb,Na,Fp,Fst' (*)
where
Ap - Passband Ripple (dB)
Ast - Stopband Attenuation (dB)
F3dB - 3dB Frequency
Fc - Cutoff Frequency
Fp - Passband Frequency
Fst - Stopband Frequency
N - Filter Order
Nb - Numerator Order
Na - Denominator Order
D = FDESIGN.LOWPASS(Fpass, Fstop, Apass, Astop) uses the default
SPECSTRING ('Fp,Fst,Ap,Ast') and sets the passband-edge frequency,
stopband-edge frequency, passband ripple, and stopband attenuation.
% Example #1 - Design a minimum-order equiripple lowpass filter.
% Specify ripples in linear units.
d = fdesign.lowpass('Fp,Fst,Ap,Ast',0.1,0.12,0.1,1e-4,'linear');
Hd = design(d, 'equiripple');
freqz(Hd)
% Example #2 - Design a minimum-phase FIR lowpass filter with
% equiripple passband and a stopband that decays as (1/f)^2. (*)
d = fdesign.lowpass('N,Fp,Fst',40,0.3,0.35);
designopts(d,'equiripple') % List equiripple design options
Hd = design(d,'equiripple','StopbandShape','1/f','StopbandDecay',2,...
'minphase',true);
freqz(Hd)
% Example #3 - Design a Chebyshev Type I IIR filter with a passband
% ripple of 0.5 dB and a 3 dB cutoff frequency at 9600 Hz. (*)
Fs = 48000; % Sampling frequency of input signal
d = fdesign.lowpass('N,F3dB,Ap', 10, 9600, .5, Fs);
Hd = design(d, 'cheby1');
freqz(Hd)
%(*) DSP System Toolbox required
See also FDESIGN, FDESIGN/SETSPECS, FDESIGN/DESIGN, FDESIGN/DESIGNOPTS.
Documentation for fdesign.lowpass
doc fdesign.lowpass
