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fdesign.highpass

Highpass filter specification object

Syntax

D = fdesign.highpass
D = fdesign.highpass(SPEC)
D = fdesign.highpass(SPEC,specvalue1,specvalue2,...)
D = fdesign.highpass(specvalue1,specvalue2,specvalue3,
specvalue4)
D = fdesign.highpass(...,Fs)
D = fdesign.highpass(...,MAGUNITS)

Description

D = fdesign.highpass constructs a highpass filter specification object D, applying default values for the specification string, 'Fst,Fp,Ast,Ap'.

D = fdesign.highpass(SPEC) constructs object D and sets the Specification property to SPEC. Entries in the SPEC string represent various filter response features, such as the filter order, that govern the filter design. Valid entries for SPEC are shown below. The strings are not case sensitive.

    Note:   Specifications strings marked with an asterisk require the DSP System Toolbox™ software.

  • 'Fst,Fp,Ast,Ap' (default spec)

  • 'N,F3db'

  • 'N,F3db,Ap' *

  • 'N,F3db,Ast' *

  • 'N,F3db,Ast,Ap' *

  • 'N,F3db,Fp *

  • 'N,Fc'

  • 'N,Fc,Ast,Ap'

  • 'N,Fp,Ap'

  • 'N,Fp,Ast,Ap'

  • 'N,Fst,Ast'

  • 'N,Fst,Ast,Ap'

  • 'N,Fst,F3db' *

  • 'N,Fst,Fp'

  • 'N,Fst,Fp,Ap' *

  • 'N,Fst,Fp,Ast' *

  • 'Nb,Na,Fst,Fp' *

The string entries are defined as follows:

  • Ap — amount of ripple allowed in the pass band in decibels (the default units). Also called Apass.

  • Ast — attenuation in the stop band in decibels (the default units). Also called Astop.

  • F3db — cutoff frequency for the point 3 dB point below the passband value. Specified in normalized frequency units.

  • Fc — cutoff frequency for the point 6 dB point below the passband value. Specified in normalized frequency units.

  • Fp — frequency at the start of the pass band. Specified in normalized frequency units. Also called Fpass.

  • Fst — frequency at the end of the stop band. Specified in normalized frequency units. Also called Fstop.

  • N — filter order.

  • Na and Nb are the order of the denominator and numerator.

Graphically, the filter specifications look similar to those shown in the following figure.

Regions between specification values like Fst and Fp are transition regions where the filter response is not explicitly defined.

The filter design methods that apply to a highpass filter specification object change depending on the Specification string. Use designmethods to determine which design method applies to an object and its specification string.

Use designopts to determine which design options are valid for a given design method. For detailed information on design options for a given design method, METHOD, enter help(D,METHOD) at the MATLAB® command line.

D = fdesign.highpass(SPEC,specvalue1,specvalue2,...) constructs an object d and sets its specification values at construction time.

D = fdesign.highpass(specvalue1,specvalue2,specvalue3,
specvalue4)
constructs an object D with the default Specification property and the values you enter for specvalue1,specvalue2,....

D = fdesign.highpass(...,Fs) provides the sampling frequency for the filter specification object. Fs is in Hz and must be specified as a scalar trailing the other numerical values provided. If you specify a sampling frequency, all other frequency specifications are in Hz.

D = fdesign.highpass(...,MAGUNITS) specifies the units for any magnitude specification you provide in the input arguments. MAGUNITS can be one of

  • 'linear' — specify the magnitude in linear units

  • 'dB' — specify the magnitude in dB (decibels)

  • 'squared' — specify the magnitude in power units

When you omit the MAGUNITS argument, fdesign assumes that all magnitudes are in decibels. Note that fdesign stores all magnitude specifications in decibels (converting to decibels when necessary) regardless of how you specify the magnitudes.

Examples

Highpass filter a discrete-time signal consisting of two sine waves.

Create a highpass filter specification object. Specify the passband frequency to be 0.25π radians/sample and the stopband frequency to be 0.15π radians/sample. Specify 1 dB of allowable passband ripple and a stopband attenuation of 60 dB.

d = fdesign.highpass('Fst,Fp,Ast,Ap',0.15,0.25,60,1);

Query the valid design methods for your filter specification object, d.

designmethods(d)

Create an FIR equiripple filter and view the filter magnitude response with fvtool.

Hd = design(d,'equiripple');
fvtool(Hd);

Create a signal consisting of the sum of two discrete-time sinusoids with frequencies of π/8 and π/4 radians/sample and amplitudes of 1 and 0.25 respectively. Filter the discrete-time signal with the FIR equiripple filter object, Hd

n = 0:159;
x = cos((pi/8)*n)+0.25*sin((pi/4)*n);
y = filter(Hd,x);
Domega = (2*pi)/160;
freq = 0:(2*pi)/160:pi;
xdft = fft(x);
ydft = fft(y);
plot(freq,abs(xdft(1:length(x)/2+1)));
hold on;
plot(freq,abs(ydft(1:length(y)/2+1)),'r','linewidth',2);
legend('Original Signal','Lowpass Signal', ...
'Location','NorthEast');
ylabel('Magnitude'); xlabel('Radians/Sample');

Create a filter of order 10 with a 6-dB frequency of 9.6 kHz and a sampling frequency of 48 kHz.

d=fdesign.highpass('N,Fc',10,9600,48000);
designmethods(d)
% only valid design method is FIR window method
Hd = design(d);
% Display filter magnitude response
fvtool(Hd);

If you have the DSP System Toolbox software, you can specify the shape of the stopband and the rate at which the stopband decays.

Create two FIR equiripple filters with different linear stopband slopes. Specify the passband frequency to be 0.3π radians/sample and the stopband frequency to be 0.35π radians/sample. Specify 1 dB of allowable passband ripple and a stopband attenuation of 60 dB. Design one filter with a 20 dB/rad/sample stopband slope and another filter with 40 dB/rad/sample.

D = fdesign.highpass('Fst,Fp,Ast,Ap',0.3,0.35,60,1);
Hd1 = design(D,'equiripple','StopBandShape','linear','StopBandDecay',20);
Hd2 = design(D,'equiripple','StopBandShape','linear','StopBandDecay',40);
hfvt = fvtool([Hd1 Hd2]);
legend(hfvt,'20 dB/rad/sample','40 dB/rad/sample');
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