extract

Extract time-domain, frequency-domain, or time-frequency-domain features

Since R2021b

Syntax

features = extract(sFE,x)

[features,info] = extract(sFE,x)

[features,info,framelimits] = extract(sFE,x)

Description

features = extract(sFE,x) returns a matrix or a table containing features extracted from input x. The output depends on the settings of the feature extractor object sFE.

[features,info] = extract(sFE,x) returns a structure info that maps a specific feature to its column location in the output feature matrix features. This syntax is valid only when you set the FeatureFormat property of the feature extractor object to "matrix".

[features,info,framelimits] = extract(sFE,x) returns a matrix framelimits whose i-th row contains the beginning and end limits of the i-th frame. This syntax is valid only when you set the FeatureFormat property of the feature extractor object to "matrix".

Examples

Extract Daily Peaks from Temperature Data

Load a set of temperature readings in Celsius taken every hour at Logan Airport in Boston for 31 days. Plot the data.

load bostemp
days = (1:31*24)/24;
plot(days, tempC)
axis tight
ylabel('Temp (\circC)')
xlabel('Time elapsed from Jan 1, 2011 (days)')
title('Logan Airport Dry Bulb Temperature (source: NOAA)')

Create a signalTimeFeatureExtractor object and enable the PeakValue feature. To obtain the maximum absolute temperature reading per day, set the frame size to 24 samples and the frame overlap to 0 samples.

sFE = signalTimeFeatureExtractor(FrameSize=24,FrameOverlapLength=0,PeakValue=true);

Call the extract function on the object to extract the daily absolute maximum temperatures in the data set.

peaktemps = extract(sFE,tempC)

peaktemps = 31×1

    2.8000
    6.1000
    8.3000
    3.3000
    2.8000
    4.4000
    4.4000
    6.1000
   10.6000
   11.7000
      ⋮

Confirm the extracted peak values. Divide the signal into 24-sample segments representing temperature readings per day and compute the maximum absolute value of each segment. Compare the resulting vector to peaktemps.

y = buffer(tempC,24);
[mx,idx] = max(abs(y));

tf = isequal(peaktemps,mx')

tf = logical
   1

Extract Mean Values from Timetable Data

Load a set of temperature readings in Celsius taken every hour at Logan Airport in Boston for 31 days. Plot the data.

load bostemp
days = (1:31*24)/24;
plot(days,tempC)
axis tight
ylabel("Temp (\circC)")
xlabel("Time elapsed from Jan 1, 2011 (days)")
title("Logan Airport Dry Bulb Temperature (source: NOAA)")

Convert the array of temperature readings to a timetable. The first temperature reading was taken on January 1, 2011.

tt = array2timetable(tempC,TimeStep=hours(1),StartTime=datetime(2011,01,01));

Create a signalTimeFeatureExtractor object and enable the Mean feature to obtain the weekly average temperatures. Specify the sample rate and frame size (1 week = 168 hours) of the extractor. Set the output format of the extracted values to a table.

fs = 1/3600;
sFE = signalTimeFeatureExtractor(SampleRate=fs,FrameSize=168,Mean=true,FeatureFormat="table");

Call the extract function to extract the weekly average temperatures from the data set. The function does not include the readings taken in the last three days (72 hours) since they do not span a full week.

meantemps = extract(sFE,tt)

meantemps=4×3 table
    FrameStartTime    FrameEndTime     Mean  
    ______________    ____________    _______

           1              168         -1.8357
         169              336         -4.3095
         337              504          1.7976
         505              672          2.0911

Extract Features from Signal

Consider a quadratic chirp sampled at 1 kHz for 2 seconds. The chirp has an initial frequency of 100 Hz that increases to 200 Hz at t = 1 second. Compute and display the spectrogram.

fs = 1e3;
t = 0:1/fs:2;
y = chirp(t,100,1,200,'quadratic');
pspectrum(y,fs,'spectrogram')

Create a signalFrequencyFeatureExtractor object to obtain the mean and median frequencies from the signal. Specify the sample rate.

sFE = signalFrequencyFeatureExtractor(SampleRate=fs,MeanFrequency=true,MedianFrequency=true);

Extract the features. info returns the column index in features of each extracted feature.

[features,info] = extract(sFE,y)

features = 1×2

  226.0160  199.7034

info = struct with fields:
      MeanFrequency: 1
    MedianFrequency: 2

Set the FrameSize and FrameRate properties of the feature extractor object to divide the signal into two frames. The first frame represents the chirp oscillating at the initial frequency of 100 Hz and the second frame represents the chirp oscillating at 200 Hz. Extract the mean and median frequencies for each frame and include the frame limits in the output.

sFE.FrameSize = round(length(y)/2);
sFE.FrameRate = 1000;
[features,info,framelimits] = extract(sFE,y)

features = 2×2

  131.4921  124.9820
  331.2664  324.6992

info = struct with fields:
      MeanFrequency: 1
    MedianFrequency: 2

framelimits = 2×2

           1        1001
        1001        2001

Input Arguments

sFE — Feature extractor object

signalFrequencyFeatureExtractor object | signalTimeFeatureExtractor object | signalTimeFrequencyFeatureExtractor object

Feature extractor object, specified as a signalFrequencyFeatureExtractor object, a signalTimeFeatureExtractor object, or a signalTimeFrequencyFeatureExtractor object.

x — Input signal

vector | matrix | timetable

Input signal, specified as a vector, matrix, or timetable.

If x is a timetable, you must specify the SampleRate property of the feature extractor object sFE. The sample rate in timetable x must equal the sample rate specified in sFE.

Data Types: single | double

Output Arguments

features — Extracted features

3-D array

Extracted features, returned as an L-by-M-by-N array:

• L — Number of frames

• M — Number of features extracted per frame

• N — Number of channels

When you set the FeatureFormat property of the input feature extractor object to "table", the function returns the extracted features in a table with the frame limits listed in the first two table variables. When you set the FeatureFormat property of the input feature extractor object to "matrix", the function returns the extracted features in a matrix.

info — Feature information

structure

Feature information, returned as a structure. The function maps each feature to its column location in the output matrix features. This argument applies only when you set the FeatureFormat property of the input feature extractor object to "matrix".

framelimits — Frame limits

matrix

Frame limits, returned as a matrix. The i-th row in framelimits contains the beginning and end limits of the i-th frame. This argument applies only when you set the FeatureFormat property of the input feature extractor object to "matrix".

Version History

Introduced in R2021b