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 of dsp.LowpassFilter to 'FIR' or 'IIR' to
implement the object as a FIR or IIR lowpass filter.
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?.
Creation
Description
LPF = dsp.LowpassFilterLPF, with the default filter settings.
Calling the object with the default property settings filters the input data with 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.
LPF = dsp.LowpassFilter(Name,Value)Name,Value pair arguments. Name is the
property name and Value is the corresponding value.
Name must appear inside single quotes (' '). You can specify
several name-value pair arguments in any order as
Name1,Value1,...,NameN,ValueN.
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
Impulse and Frequency Response of FIR and IIR Lowpass Filters
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.
fvtool(FIRLPF,'Analysis','impulse')
Plot the impulse response of the IIR lowpass filter.
fvtool(IIRLPF,'Analysis','impulse')
Plot the magnitude and phase response of the FIR lowpass filter.
fvtool(FIRLPF,'Analysis','freq')
Plot the magnitude and phase response of the IIR lowpass filter.
fvtool(IIRLPF,'Analysis','freq')
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)
Filter White Gaussian Noise Signal With Lowpass Filter
Create a lowpass filter with default properties.
LPF = dsp.LowpassFilter;
Create a spectrum analyzer object.
hSA = dsp.SpectrumAnalyzer('SampleRate',44.1e3,... 'PlotAsTwoSidedSpectrum',false,'ShowLegend',true,'YLimits',... [-150 30],... 'Title',... 'Input Signal and Output Signal of Lowpass Filter'); hSA.ChannelNames = {'Input','Output'};
Implement step on LPF 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 = step(LPF,Input); step(hSA,[Input,Output]); end
Filter White Gaussian Noise
Note: This example runs only in R2016b or later. If you are using an earlier release, replace each call to the function with the equivalent step syntax. For example, myObject(x) becomes step(myObject,x).
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.
fvtool(LPF)
Create a spectrum analyzer object.
hSA = dsp.SpectrumAnalyzer('SampleRate',44.1e3,... 'PlotAsTwoSidedSpectrum',false,'ShowLegend',true,'YLimits',... [-150 30],... 'Title',... 'Input Signal and Output Signal of IIR Lowpass Filter'); hSA.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); hSA([Input,Output]); end
Measure Frequency Response Characteristics of Lowpass Filter
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
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
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.