dsp.ParameterSmoother

Design and implement a second-order section filter using the designBandpassIIR function and the dsp.SOSFilter object. Vary the lower and higher 3-dB cutoff frequencies and the stopband attenuation of the filter. Smooth these parameters using the dsp.ParameterSmoother object. Vary the smoothing factor and note the change in how the object smooths these parameter values.

Set the target values of the three frequency specification parameters.

F3dB1 = 0.8; 
F3dB2 = 0.9;
Astop = 120;

Initialize the dsp.ParameterSmoother object with a smoothing factor of 0.995. As the smoothing factor approaches 1, smoothing increases and the parameter values change more gradually.

smooth = dsp.ParameterSmoother(3,SmoothingFactor=0.995)

smooth = 
  dsp.ParameterSmoother with properties:

      NumParameters: 3
      SmoothingMode: 'Smoothing factor'
    SmoothingFactor: 0.9950

Initialize the dsp.SOSFilter object.

sosFilter = dsp.SOSFilter(CoefficientSource="Input port")

sosFilter = 
  dsp.SOSFilter with properties:

            Structure: 'Direct form II transposed'
    CoefficientSource: 'Input port'
       HasScaleValues: false

  Use get to show all properties

Initialize the spectrumAnalyzer object to plot the spectrum of the filter output. The two timescope objects plot the variation in the parameters with time and the target values of the parameters.

sa = spectrumAnalyzer(PlotAsTwoSidedSpectrum=false,...
    AveragingMethod="exponential",ForgettingFactor=0.7,...
    SampleRate=2);
tsFreq = timescope(SampleRate=2,...
    TimeSpanSource="property",TimeSpan=2999,...
    YLimits=[0 1],...
    ShowLegend=1,ChannelNames=["TargetF3dB1","TargetF3dB2","F3dB1","F3dB2"], ...
    Title="Smoothing Factor = 0.995");
tsAstop = timescope(SampleRate=2,...
    TimeSpanSource="property",TimeSpan=2999,...
    YLimits=[0 130],...
    ShowLegend=1,ChannelNames=["TargetAstop","Astop"],...
    Title="Smoothing Factor = 0.995");

Change the frequency specification parameters for a brief period during simulation. The dsp.ParameterSmoother object smooths the values when they change. The target frequency specification parameters changes abruptly, but the smoothed values change more gradually. Design the second-order section filter using the smoothed values. Pass a random signal to the filter and plot the output spectrum in the spectrum analyzer. Since the input is a random signal, the output spectrum shows the frequency response of the filter.

With each iteration, the time scope plots update with the changing values of the parameters. The frequency response of the filter updates accordingly in the spectrum analyzer.

for idx = 1:6000
    if idx == 2000
        F3dB1 = 0.5;
        F3dB2 = 0.7;
        Astop = 80;
    elseif idx == 4000
        F3dB1 = 0.8;
        F3dB2 = 0.9;
        Astop = 120;
    end
    [F3dB1S,F3dB2S,AstopS] = smooth(F3dB1,F3dB2,Astop);
    [b,a] = designBandpassIIR(HalfPowerFrequency1=F3dB1S,HalfPowerFrequency2=F3dB2S,StopbandAttenuation=AstopS,CascadeSectionsForm="sos");
    y = sosFilter(randn(1024,1),b,a);
    sa(y);
    tsFreq(F3dB1,F3dB2,F3dB1S,F3dB2S)
    tsAstop(Astop,AstopS)
end

Change the gain of a speech signal. Smooth the gain parameter by specifying the smoothing time. Smoothing time and smoothing factor are equivalent ways of specifying the smoothing behavior of the parameters. This example shows the effect of changing the smoothing time on the gain of the speech signal.

Initialize the dsp.ParameterSmoother object with a smoothing time of 2 seconds.

smooth = dsp.ParameterSmoother(1,SmoothingMode="Smoothing time",...
    SmoothingTime=2,SampleTime=1)

smooth = 
  dsp.ParameterSmoother with properties:

    NumParameters: 1
    SmoothingMode: 'Smoothing time'
       SampleTime: 1
    SmoothingTime: 2

Create an audio file reader to read a speech signal containing 1024 samples per frame.

afr = dsp.AudioFileReader

afr = 
  dsp.AudioFileReader with properties:

           Filename: '/mathworks/devel/bat/filer/batfs2566-0/Bdoc24b.2725827/build/runnable/matlab/toolbox/dsp/dsp/samples/speech_dft.mp3'
          PlayCount: 1
          ReadRange: [1 Inf]
    SamplesPerFrame: 1024
     OutputDataType: 'double'

  Use get to show all properties

Play the signal using the audioDeviceWriter object.

adw = audioDeviceWriter(SampleRate=afr.SampleRate)

adw = 
  audioDeviceWriter with properties:

        Device: 'Default'
    SampleRate: 22050

  Use get to show all properties

Visualize the variation in the gain parameter using the timescope object.

ts = timescope(ChannelNames=["Target Gain","Gain"],...
    TimeSpanSource="property",TimeSpan=100,...
    YLimits=[0 1.2],Title="Smoothing Time = 2 seconds");

Initialize the target gain to 1. Change the target gain to 0.1 for a brief period of the signal duration. The dsp.ParameterSmoother object smooths the gain value when it changes. The time scope shows the smoothing behavior. The target gain value changes abruptly, but the smoothed gain changes more gradually.

targetgain = 1;
for i = 1:100
    gainS = smooth(targetgain);
    y = gainS*afr();
    if i == 20
        targetgain = 0.1;
    elseif i == 70
        targetgain = 1;
    end
    adw(y);
    ts(targetgain,gainS)
end