dsp.LowpassFilter
FIR or IIR lowpass filter
Description
The dsp.LowpassFilter object independently filters each channel of the
input over time using the given design specifications. You can set the
FilterType property to 'FIR' or
'IIR' to implement the object as an FIR or an IIR lowpass filter.
When the FilterType property is set to 'FIR',
using this object is an alternative to using the firceqrip and
firgr functions with dsp.FIRFilter. The
dsp.LowpassFilter object condenses the two-step process into one. You can
use measure to verify that the design meets the prescribed specifications.
To filter each channel of your input:
Create the
dsp.LowpassFilterobject and set its properties.Call the object with arguments, as if it were a function.
To learn more about how System objects work, see What Are System Objects?
This object supports C/C++ code generation and SIMD code generation under certain conditions. This object also supports code generation for ARM® Cortex®-M and ARM Cortex-A processors. For more information, see Code Generation.
Creation
Description
LPF = dsp.LowpassFilter8 kHz, a stopband frequency of 12 kHz, a passband
ripple of 0.1 dB, and a stopband attenuation of 80
dB.
LPF = dsp.LowpassFilter(Name=Value)Name-Value pair arguments. Name is the
property name and Value is the corresponding value. For example,
PassbandFrequency=8000 sets the passband frequency specification of
the filter to 8000 Hz.
Properties
Usage
Syntax
Description
Input Arguments
Output Arguments
Object Functions
To use an object function, specify the
System object™ as the first input argument. For
example, to release system resources of a System object named obj, use
this syntax:
release(obj)
Examples
Create a minimum-order FIR lowpass filter for data sampled at 44.1 kHz. Specify a passband frequency of 8 kHz, a stopband frequency of 12 kHz, a passband ripple of 0.1 dB, and a stopband attenuation of 80 dB.
Fs = 44.1e3; filtertype = 'FIR'; Fpass = 8e3; Fstop = 12e3; Rp = 0.1; Astop = 80; FIRLPF = dsp.LowpassFilter(SampleRate=Fs,... FilterType=filtertype,... PassbandFrequency=Fpass,... StopbandFrequency=Fstop,... PassbandRipple=Rp,... StopbandAttenuation=Astop);
Design a minimum-order IIR lowpass filter with the same properties as the FIR lowpass filter. Change the FilterType property of the cloned filter to IIR.
IIRLPF = clone(FIRLPF);
IIRLPF.FilterType = 'IIR';Plot the impulse response of the FIR lowpass filter. The zeroth-order coefficient is delayed by 19 samples, which is equal to the group delay of the filter. The FIR lowpass filter is a causal FIR filter.
impz(FIRLPF)
Plot the impulse response of the IIR lowpass filter.
impz(IIRLPF)
Plot the magnitude and phase response of the FIR lowpass filter.
freqz(FIRLPF)
Plot the magnitude and phase response of the IIR lowpass filter.
freqz(IIRLPF)
Calculate the cost of implementing the FIR lowpass filter.
cost(FIRLPF)
ans = struct with fields:
NumCoefficients: 39
NumStates: 38
MultiplicationsPerInputSample: 39
AdditionsPerInputSample: 38
Calculate the cost of implementing the IIR lowpass filter. The IIR filter is more efficient to implement than the FIR filter.
cost(IIRLPF)
ans = struct with fields:
NumCoefficients: 18
NumStates: 14
MultiplicationsPerInputSample: 18
AdditionsPerInputSample: 14
Calculate the group delay of the FIR lowpass filter.
grpdelay(FIRLPF)
Calculate the group delay of the IIR lowpass filter. The FIR filter has a constant group delay (linear phase), while its IIR counterpart does not.
grpdelay(IIRLPF)
Create a lowpass filter using the dsp.LowpassFilter System object™. Setting the NormalizedFrequency property to true designs the filter with frequency specifications in normalized frequency units.
LPF = dsp.LowpassFilter(NormalizedFrequency=true)
LPF =
dsp.LowpassFilter with properties:
FilterType: 'FIR'
DesignForMinimumOrder: true
PassbandFrequency: 0.3628
StopbandFrequency: 0.5442
PassbandRipple: 0.1000
StopbandAttenuation: 80
NormalizedFrequency: true
Show all properties
Create a spectrumAnalyzer object to visualize the input and output signal spectra. With a sample rate of 44.1e3 Hz, the passband frequency and the stopband frequency of the filter translate to 8000 Hz and 12000 Hz, respectively.
SA = spectrumAnalyzer(SampleRate=44.1e3,... PlotAsTwoSidedSpectrum=false,ShowLegend=true,... YLimits=[-150 30],... Title='Input Signal and Output Signal of Lowpass Filter'); SA.ChannelNames = {'Input','Output'};
Run the lowpass filter algorithm to filter the white Gaussian noisy input signal. View the input and output signals using the spectrum analyzer.
for k = 1:100 Input = randn(1024,1); Output = LPF(Input); SA([Input,Output]); end
Set up the IIR lowpass filter. The sampling rate of the white Gaussian noise is 44,100 Hz. The passband frequency of the filter is 8 kHz, the stopband frequency is 12 kHz, the passband ripple is 0.1 dB, and the stopband attenuation is 80 dB.
Fs = 44.1e3; filtertype = 'IIR'; Fpass = 8e3; Fstop = 12e3; Rp = 0.1; Astop = 80; LPF = dsp.LowpassFilter(SampleRate=Fs,... FilterType=filtertype,... PassbandFrequency=Fpass,... StopbandFrequency=Fstop,... PassbandRipple=Rp,... StopbandAttenuation=Astop);
View the magnitude response of the lowpass filter.
filterAnalyzer(LPF)
Create a spectrum analyzer object.
SA = spectrumAnalyzer(SampleRate=Fs,... PlotAsTwoSidedSpectrum=false,... ShowLegend=true,YLimits=[-150 30],... Title='Input Signal and Output Signal of IIR Lowpass Filter'); SA.ChannelNames = {'Input','Output'};
Filter the white Gaussian noisy input signal. View the input and output signals using the spectrum analyzer.
for k = 1:100 Input = randn(1024,1); Output = LPF(Input); SA([Input,Output]); end
Measure the frequency response characteristics of a lowpass filter. Create a dsp.LowpassFilter System object with default properties. Measure the frequency response characteristics of the filter.
LPF = dsp.LowpassFilter
LPF =
dsp.LowpassFilter with properties:
FilterType: 'FIR'
DesignForMinimumOrder: true
PassbandFrequency: 8000
StopbandFrequency: 12000
PassbandRipple: 0.1000
StopbandAttenuation: 80
NormalizedFrequency: false
SampleRate: 44100
Show all properties
LPFMeas = measure(LPF)
LPFMeas = Sample Rate : 44.1 kHz Passband Edge : 8 kHz 3-dB Point : 9.1311 kHz 6-dB Point : 9.5723 kHz Stopband Edge : 12 kHz Passband Ripple : 0.08289 dB Stopband Atten. : 81.6141 dB Transition Width : 4 kHz
More About
Algorithms
References
[1] Shpak, D.J., and A. Antoniou. "A generalized Remez method for the design of FIR digital filters." IEEE® Transactions on Circuits and Systems. Vol. 37, Issue 2, Feb. 1990, pp. 161–174.
[2] Selesnick, I.W., and C. S. Burrus. "Exchange algorithms that complement the Parks-McClellan algorithm for linear-phase FIR filter design." IEEE Transactions on Circuits and Systems. Vol. 44, Issue 2, Feb. 1997, pp. 137–143.
