transform

Transform signal datastore

Syntax

transformDatastore = transform(sds,@fcn)

transformDatastore = transform(sds,@fcn,'IncludeInfo',infoIn)

Description

transformDatastore = transform(sds,@fcn) creates a new datastore that transforms output from the read function.

transformDatastore = transform(sds,@fcn,'IncludeInfo',infoIn) includes the info returned by the read of sds.

Examples

Find Spectrogram of Chirps

The sample_chirps folder contains sample files included with Signal Processing Toolbox™. Each file contains a chirp and a random sample rate, fs, ranging from 100 to 150 Hz. Create a signal datastore that points to four of the files in sample_chirps and specify the name of the sample rate variable.

addpath("sample_chirps")
files = ["chirp_1.mat","chirp_4.mat","chirp_9.mat","chirp_10.mat"];
sds = signalDatastore(files,SampleRateVariableName="fs");

Define a function that takes the output of the read function and computes and returns:

• The spectrograms of the chirps.

• The vector of time instants corresponding to the centers of the windowed segments.

• The frequencies corresponding to the estimates.

function [dataOut,infoOut] = extractSpectrogram(dataIn,info)
    [dataOut,F,T] = pspectrum(dataIn,info.SampleRate,"spectrogram",...
                              TimeResolution=0.25,...
                              OverlapPercent=40,Leakage=0.8);
    infoOut = info;
    infoOut.CenterFrequencies = F;
    infoOut.TimeInstants = T;
end

Call the transform function to create a datastore that computes the spectrogram of each chirp using the function you defined.

sdsNew = transform(sds,@extractSpectrogram,IncludeInfo=true);

While the transformed datastore has unread files, read from the new datastore and visualize the spectrograms in three-dimensional space.

t = tiledlayout("flow");
while hasdata(sdsNew)
    nexttile
    [sig,infoOut] = read(sdsNew);
    waterfall(infoOut.TimeInstants,infoOut.CenterFrequencies,sig)
    xlabel("Frequency (Hz)")
    ylabel("Time (S)")
    view([30 70])
end

Compute Envelopes of Signals

Specify the path to four signals included with MATLAB®. The signals are recordings of a bird chirp, a gong, a train, and a splat. All signals are sampled at 8192 Hz.

folder = fullfile(matlabroot,'toolbox','matlab','audiovideo', ...
         ["chirp.mat","gong.mat","train.mat","splat.mat"]);

Create a signal datastore that points to the specified files. Each file contains the variable Fs that denotes the sample rate.

sds1 = signalDatastore(folder,'SampleRateVariableName','Fs');

Define a function that takes the output of the read function and calculates the upper and lower envelopes of the signals using spline interpolation over local maxima separated by at least 80 samples. The function also returns the sample times for each signal.

function [dataOut,infoOut] = signalEnvelope(dataIn,info)
    [dataOut(:,1),dataOut(:,2)] = envelope(dataIn,80,'peak');
    infoOut = info;
    infoOut.TimeInstants = (0:length(dataOut)-1)/info.SampleRate;
end

Call the transform function to create a second datastore, sds2, that computes the envelopes of the signals using the function you defined.

sds2 = transform(sds1,@signalEnvelope,"IncludeInfo",true);

Combine sds1 and sds2 create a third datastore. Each call to the read function from the combined datastore returns a matrix with three columns:

• The first column corresponds to the original signal.

• The second and third columns correspond to the upper and lower envelopes, respectively.

sdsCombined = combine(sds1,sds2);

Read and display the original data and the upper and lower envelopes from the combined datastore. Use the extractBetween function to extract the file name from the file path.

tiledlayout('flow')
while hasdata(sdsCombined)
    [dataOut,infoOut] = read(sdsCombined);
    ts = infoOut{2}.TimeInstants;
    nexttile
    hold on
    plot(ts,dataOut(:,1),'Color','#DCDCDC','LineStyle',':')
    plot(ts,dataOut(:,2:3),'Linewidth',1.5)
    hold off
    xlabel('Time (s)')
    ylabel('Signal')
    title(extractBetween(infoOut{:,2}.FileName,'audiovideo\','.mat'))
end

Input Arguments

sds — Signal datastore

signalDatastore object

Specify sds as a signalDatastore object.

@fcn — Function that transforms data

function handle

Function that transforms data, specified as a function handle. The signature of the function depends on the IncludeInfo parameter.

• If IncludeInfo is set to false (default), the function transforms the signal output from read. The info output from read is unaltered.

  The transform function must have this signature:

  function dataOut = fcn(dataIn)
  ...
  end

• If IncludeInfo is set to true, the function transforms the signal output from read, and can use or modify the information returned from read.

  The transform function must have this signature:

  function [dataOut,infoOut] = fcn(signal,infoIn)
  ...
  end

infoIn — Pass info through customized read function

false (default) | true

Pass info through the customized read function, specified as true or false. If true, the transform function can use or modify the information it gets from read. If unspecified, IncludeInfo defaults to false.

Data Types: logical

Output Arguments

transformDatastore — New datastore with customized read

TransformedDatastore

New datastore with customized read, returned as a TransformedDatastore with UnderlyingDatastores set to sds, Transforms set to fcn, and IncludeInfo set to true or false.

Version History

Introduced in R2020a