Extract cepstral features from audio segment

The `cepstralFeatureExtractor`

System
object™ extracts cepstral features from an audio segment. Cepstral features are commonly
used to characterize speech and music signals.

To extract cepstral features:

Create the

`cepstralFeatureExtractor`

object 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? (MATLAB).

`cepFeatures = cepstralFeatureExtractor`

`cepFeatures = cepstralFeatureExtractor(Name,Value)`

`cepFeatures = cepstralFeatureExtractor`

creates a System
object, `cepFeatures`

, that calculates cepstral features
independently across each input channel. Columns of the input are treated as individual
channels.

`cepFeatures = cepstralFeatureExtractor(`

sets each property `Name,Value`

)`Name`

to the specified `Value`

.
Unspecified properties have default values.

```
cepFeatures =
cepstralFeatureExtractor('InputDomain','Frequency','SampleRate',fs,'LogEnergy','Replace')
```

accepts a signal in the frequency domain, sampled at `fs`

Hz. The first
element of the coefficients vector is replaced by the log energy value.Unless otherwise indicated, properties are *nontunable*, which means you cannot change their
values after calling the object. Objects lock when you call them, and the
`release`

function unlocks them.

If a property is *tunable*, you can change its value at
any time.

For more information on changing property values, see System Design in MATLAB Using System Objects (MATLAB).

`FilterBank`

— Type of filter bank`'Mel'`

(default) | `'Gammatone'`

Type of filter bank, specified as either `'Mel'`

or
`'Gammatone'`

. When `FilterBank`

is set to
`Mel`

, the object computes the mel frequency cepstral coefficients
(MFCC). When `FilterBank`

is set to `Gammatone`

, the
object computes the gammatone cepstral coefficients (GTCC).

**Data Types: **`char`

| `string`

`InputDomain`

— Domain of input signal`'Time'`

(default) | `'Frequency'`

Domain of the input signal, specified as either `'Time'`

or
`'Frequency'`

.

**Data Types: **`char`

| `string`

`NumCoeffs`

— Number of coefficients to return`13`

(default) | positive integerNumber of coefficients to return, specified as an integer in the range [2,
*v*], where *v* is the number of valid passbands.
The number of valid passbands depends on the type of filter bank:

`Mel`

–– The number of valid passbands is defined as`sum(`

.`BandEdges`

<= floor(`SampleRate`

/2))-2`Gammatone`

–– The number of valid passbands is defined as`ceil(`

.`hz2erb`

(`FrequencyRange`

(2))-`hz2erb`

(`FrequencyRange`

(1)))

**Data Types: **`single`

| `double`

`FFTLength`

— FFT length`[]`

(default) | positive integerFFT length, specified as a positive integer. The default, `[]`

,
means that the FFT length is equal to the number of rows in the input signal.

To enable this property, set `InputDomain`

to
`'Time'`

.

**Data Types: **`single`

| `double`

| `int8`

| `int16`

| `int32`

| `int64`

| `uint8`

| `uint16`

| `uint32`

| `uint64`

`LogEnergy`

— Specify how the log energy is shown`'Append'`

(default) | `'Replace'`

| `'Ignore'`

Specify how the log energy is shown in the coefficients vector output, specified as:

`'Append'`

–– The object prepends the log energy to the coefficients vector. The length of the coefficients vector is 1 +`NumCoeffs`

.`'Replace'`

–– The object replaces the first coefficient with the log energy of the signal. The length of the coefficients vector is`NumCoeffs`

.`'Ignore'`

–– The object does not calculate or return the log energy.

**Data Types: **`char`

| `string`

`SampleRate`

— Input sample rate (Hz)`16000`

(default) | positive scalarInput sample rate in Hz, specified as a real positive scalar.

**Tunable: **Yes

**Data Types: **`single`

| `double`

`BandEdges`

— Band edges of mel filter bank (Hz)row vector

Band edges of the filter bank in Hz, specified as a nonnegative monotonically increasing row vector in the range [0, ∞). The maximum bandedge frequency can be any finite number. The number of bandedges must be in the range [4, 80].

The default band edges are spaced linearly for the first ten and then logarithmically after. The default band edges are set as recommended by [1].

To enable this property, set `FilterBank`

to
`Mel`

.

**Data Types: **`single`

| `double`

`FrequencyRange`

— Frequency range of gammatone filter bank (Hz)`[50 8000]`

(default) | two-element row vectorFrequency range of the filter bank in Hz, specified as a positive, monotonically
increasing two-element row vector. The maximum frequency can be any finite number. The
center frequencies of the filter bank are equally spaced between

and
`hz2erb`

(`FrequencyRange`

(1))

on the ERB scale.`hz2erb`

(`FrequencyRange`

(2))

To enable this property, set `FilterBank`

to
`Gammatone`

.

**Data Types: **`single`

| `double`

`FilterBankDesignDomain`

— Domain for mel filter bank design`'Hz'`

(default) | `'Bin'`

Domain for filter bank design, specified as either `'Hz'`

or
`'Bin'`

. The filter bank is designed as overlapped triangles with
band edges specified by the `BandEdges`

property.

The `BandEdges`

property is specified in Hz. When you set the
design domain to:

`'Hz'`

–– Filter bank triangles are drawn in Hz and are mapped onto bins.Here is an example that plots the filter bank in bins when the

`FilterBankDesignDomain`

is set to`'Hz'`

:[audioFile, fs] = audioread('NoisySpeech-16-22p5-mono-5secs.wav'); duration = round(0.02*fs); % 20 ms audio segment audioSegment = audioFile(5500:5500+duration-1); cepFeatures = cepstralFeatureExtractor('SampleRate',fs)

Pass the audio segment as an input to the cepstral feature extractor algorithm to lock the object.cepFeatures = cepstralFeatureExtractor with properties: Properties InputDomain: 'Time' NumCoeffs: 13 FFTLength: [] LogEnergy: 'Append' SampleRate: 22500 Advanced Properties BandEdges: [1×42 double] FilterBankDesignDomain: 'Hz' FilterBankNormalization: 'Bandwidth'

Using the[coeffs,delta,deltaDelta] = cepFeatures(audioSegment);

`getFilters`

function, get the filter bank. Plot the filter bank.[filterbank, freq] = getFilters(cepFeatures); plot(freq(1:150),filterbank(1:150,:))

For details, see [1].

`'Bin'`

–– The bandedge frequencies in`'Hz'`

are converted to bins. The filter bank triangles are drawn symmetrically in bins.Change the

`FilterBankDesignDomain`

property to`'Bin'`

:`release(cepFeatures); cepFeatures.FilterBankDesignDomain = 'Bin'; [coeffs,delta,deltaDelta] = cepFeatures(audioSegment); [filterbank, freq] = getFilters(cepFeatures); plot(freq(1:150),filterbank(1:150,:))`

For details, see [2].

To enable this property, set `FilterBank`

to
`Mel`

.

**Data Types: **`char`

| `string`

`FilterBankNormalization`

— Normalize filter bank`'Bandwidth'`

(default) | `'Area'`

| `'None'`

Normalization technique used on the weights of the filter bank, specified as:

`'Bandwidth'`

–– The weights of each bandpass filter are normalized by the corresponding bandwidth of the filter.`'Area'`

–– The weights of each bandpass filter are normalized by the corresponding area of the bandpass filter.`'None'`

–– The weights of the filter are not normalized.

**Data Types: **`char`

| `string`

```
[coeffs,delta,deltaDelta]
= cepFeatures(audioIn)
```

`[`

returns the cepstral coefficients, the log energy, the delta, and the delta-delta.`coeffs`

,`delta`

,`deltaDelta`

]
= cepFeatures(`audioIn`

)

The log energy value prepends the coefficient vector or replaces the first element of
the coefficients vector based on whether you set the `LogEnergy`

property to `'Append'`

or `'Replace'`

. For details, see
coeffs.

`audioIn`

— Audio input to cepstral feature extractorcolumn vector | matrix

Audio input to the cepstral feature extractor, specified as a column vector or a matrix. If specified as a matrix, the columns are treated as independent audio channels.

**Data Types: **`single`

| `double`

`coeffs`

— Cepstral coefficientscolumn vector | matrix

Cepstral coefficients, returned as a column vector or a matrix. If the
coefficients matrix is an *N*-by-*M* matrix,
*N* is determined by the values you specify in
`NumCoeffs`

and `LogEnergy`

properties.
*M* equals the number of input audio channels.

When the `LogEnergy`

property is set to:

`'Append'`

–– The object prepends the log energy value to the coefficients vector. The length of the coefficients vector is 1 +`NumCoeffs`

. This is the default setting of the`LogEnergy`

property.`'Replace'`

–– The object replaces the first coefficient with the log energy of the signal. The length of the coefficients vector is`NumCoeffs`

.`'Ignore'`

–– The object does not calculate or return the log energy.

**Data Types: **`single`

| `double`

`delta`

— Change in coefficientscolumn vector | matrix

Change in coefficients over consecutive calls to the algorithm, returned as a
vector or a matrix. The `delta`

array is of the same size and data
type as the `coeffs`

array.

In this example, `cepFeatures`

is the cepstral feature extractor
that accepts audio input signal sampled at 12 kHz. Stream in three segments of audio
signal on three consecutive calls to the object algorithm. Return the cepstral
coefficients of the filter bank and the corresponding `delta`

values.

```
cepFeatures = cepstralFeatureExtractor('SampleRate',12000);
[coeff1,delta1] = cepFeatures(audioIn);
[coeff2,delta2] = cepFeatures(audioIn);
[coeff3,delta3] = cepFeatures(audioIn);
```

`delta2`

is computed as `coeff2-coeff1`

,
while `delta3`

is computed as `coeff3-coeff2`

.
The initial array, `delta1`

, is an array of zeros.

**Data Types: **`single`

| `double`

`deltaDelta`

— Change in delta valuescolumn vector | matrix

Change in `delta`

values over consecutive calls to the
algorithm, returned as a vector or a matrix. The `deltaDelta`

array
is the same size and data type as the `coeffs`

and
`delta`

arrays.

In this example, consecutive calls to the cepstral feature extractor algorithm
return the `deltaDelta`

values in addition to the coefficients and
the `delta`

values.

```
cepFeatures = cepstralFeatureExtractor('SampleRate',12000);
[coeff1,delta1,deltaDelta1] = cepFeatures(audioIn);
[coeff2,delta2,deltaDelta2] = cepFeatures(audioIn);
[coeff3,delta3,deltaDelta3] = cepFeatures(audioIn);
```

`deltaDelta2`

is computed as
`delta2-delta1`

, while `deltaDelta3`

is computed
as `delta3-delta2`

. The initial array,
`deltaDelta1`

, is an array of zeros.

**Data Types: **`single`

| `double`

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)

Specific to cepstralFeatureExtractor

`getFilters` | Get auditory filter bank |

Extract the mel frequency cepstral coefficients and the log energy values of segments in a speech file. Return `delta`

, the difference between current and the previous cepstral coefficients, and `deltaDelta`

, the difference between the current and the previous `delta`

values. The log energy value the object computes can prepend the coefficients vector or replace the first element of the coefficients vector. This is done based on whether you set the `LogEnergy`

property of the `cepstralFeatureExtractor`

object to `'Replace'`

or `'Append'`

.

Read an audio signal from `'SpeechDFT-16-8-mono-5secs.wav'`

file. Extract a 40 ms segment from the audio data. Create a `cepstralFeatureExtractor`

object. The cepstral coefficients computed by the default object are the mel frequency coefficients. In addition, the object computes the log energy, delta, and delta-delta values of the audio segment.

[audioFile, fs] = audioread('SpeechDFT-16-8-mono-5secs.wav'); duration = round(0.04*fs); % 40 ms audioSegment = audioFile(5500:5500+duration-1); cepFeatures = cepstralFeatureExtractor('SampleRate',fs)

cepFeatures = cepstralFeatureExtractor with properties: Properties FilterBank: 'Mel' InputDomain: 'Time' NumCoeffs: 13 FFTLength: [] LogEnergy: 'Append' SampleRate: 8000 Show all properties

The `LogEnergy`

property is set to `'Append'`

. The first element in the coefficients vector is the log energy value and the remaining elements are the 13 cepstral coefficients computed by the object. The number of cepstral coefficients is determined by the value you specify in the `NumCoeffs`

property.

[coeffs,delta,deltaDelta] = cepFeatures(audioSegment)

`coeffs = `*14×1*
3.8281
-19.4827
11.7649
-6.2989
5.8894
-0.3366
0.9583
0.8768
-2.0384
2.3678
⋮

`delta = `*14×1*
0
0
0
0
0
0
0
0
0
0
⋮

`deltaDelta = `*14×1*
0
0
0
0
0
0
0
0
0
0
⋮

The initial values for the `delta`

and `deltaDelta`

arrays are always zero. Consider another 40 ms audio segment in the file and extract the cepstral features from this segment.

audioSegmentTwo = audioFile(5820:5820+duration-1); [coeffsTwo,deltaTwo,deltaDeltaTwo] = cepFeatures(audioSegmentTwo)

`coeffsTwo = `*14×1*
3.0899
-20.4756
10.4455
-5.8759
7.2215
-1.2027
-0.0236
1.9183
-1.2127
2.0669
⋮

`deltaTwo = `*14×1*
-0.7382
-0.9928
-1.3194
0.4230
1.3321
-0.8661
-0.9819
1.0415
0.8257
-0.3009
⋮

`deltaDeltaTwo = `*14×1*
-0.7382
-0.9928
-1.3194
0.4230
1.3321
-0.8661
-0.9819
1.0415
0.8257
-0.3009
⋮

Verify that the difference between `coeffsTwo`

and `coeffs`

vectors equals `deltaTwo`

.

isequal(coeffsTwo-coeffs,deltaTwo)

`ans = `*logical*
1

Verify that the difference between `deltaTwo`

and `delta`

vectors equals `deltaDeltaTwo`

.

isequal(deltaTwo-delta,deltaDeltaTwo)

`ans = `*logical*
1

Many feature extraction techniques operate on the frequency domain. Converting an audio signal to the frequency domain only once is efficient. In this example, you convert a streaming audio signal to the frequency domain and feed that signal into a voice activity detector. If speech is present, mel-frequency cepstral coefficients (MFCC) features are extracted from the frequency-domain signal using the `cepstralFeatureExtractor System object™`

.

Create a `dsp.AudioFileReader`

System object to read from an audio file.

```
fileReader = dsp.AudioFileReader('Counting-16-44p1-mono-15secs.wav');
fs = fileReader.SampleRate;
```

Process the audio in 30 ms frames with a 10 ms hop. Create a default `dsp.AsyncBuffer`

object to manage overlap between audio frames.

samplesPerFrame = ceil(0.03*fs); samplesPerHop = ceil(0.01*fs); samplesPerOverlap = samplesPerFrame - samplesPerHop; fileReader.SamplesPerFrame = samplesPerHop; buffer = dsp.AsyncBuffer;

Create a `voiceActivityDetector`

System object and a `cepstralFeatureExtractor`

System object. Specify that they operate in the frequency domain. Create a `dsp.SignalSink`

to log the extracted cepstral features.

VAD = voiceActivityDetector('InputDomain','Frequency'); cepFeatures = cepstralFeatureExtractor('InputDomain','Frequency','SampleRate',fs,'LogEnergy','Replace'); sink = dsp.SignalSink;

In an audio stream loop:

Read one hop's of samples from the audio file and save the samples into the buffer.

Read a frame from the

`buffer`

with specified overlap from the previous frame.Call the voice activity detector to get the probability of speech for the frame under analysis.

If the frame under analysis has a probability of speech greater than 0.75, extract cepstral features and log the features using the signal sink. If the frame under analysis has a probability of speech less than 0.75, write a vector of NaNs to the sink.

threshold = 0.75; nanVector = nan(1,13); while ~isDone(fileReader) audioIn = fileReader(); write(buffer,audioIn); overlappedAudio = read(buffer,samplesPerFrame,samplesPerOverlap); X = fft(overlappedAudio,2048); probabilityOfSpeech = VAD(X); if probabilityOfSpeech > threshold xFeatures = cepFeatures(X); sink(xFeatures') else sink(nanVector) end end

Visualize the cepstral coefficients over time.

timeVector = linspace(0,15,size(sink.Buffer,1)); plot(timeVector,sink.Buffer) xlabel('Time (s)') ylabel('MFCC Amplitude') legend('Log-Energy','c1','c2','c3','c4','c5','c6','c7','c8','c9','c10','c11','c12')

Create a `dsp.AudioFileReader`

object to read in audio data frame-by-frame. Create an `audioDeviceWriter`

to write the audio to your sound card. Create a `dsp.ArrayPlot`

to visualize the GTCC over time.

```
fileReader = dsp.AudioFileReader('RandomOscThree-24-96-stereo-13secs.aif');
deviceWriter = audioDeviceWriter(fileReader.SampleRate);
scope = dsp.ArrayPlot;
```

Create a `cepstralFeatureExtractor`

that extracts GTCC.

cepFeatures = cepstralFeatureExtractor('FilterBank','Gammatone', ... 'SampleRate',fileReader.SampleRate);

In an audio stream loop:

Read in a frame of audio data.

Extract the GTCC from the frame of audio.

Visualize the GTCC.

Write the audio frame to your device.

while ~isDone(fileReader) audioIn = fileReader(); coeffs = cepFeatures(audioIn); scope(coeffs) deviceWriter(audioIn); end release(cepFeatures) release(scope) release(fileReader)

Auditory cepstrum coefficients are popular features extracted from speech signals for use in recognition tasks. In the source-filter model of speech, cepstral coefficients are understood to represent the filter (vocal tract). The vocal tract frequency response is relatively smooth, whereas the source of voiced speech can be modeled as an impulse train. As a result, the vocal tract can be estimated by the spectral envelope of a speech segment.

The motivating idea of cepstral coefficients is to compress information about the vocal tract (smoothed spectrum) into a small number of coefficients based on an understanding of the cochlea. Although there is no hard standard for calculating the coefficients, the basic steps are outlined by the diagram.

Two popular implementations of the filter bank are the mel filter bank and the gammatone filter bank.

The default mel filter bank linearly spaces the first 10 triangular filters and logarithmically spaces the remaining filters.

The default gammatone filter bank is composed of gammatone filters spaced linearly
on the ERB scale between 50 and 8000 Hz. The filter bank is designed by `gammatoneFilterBank`

.

If the input (*x*) is a time-domain signal, the log energy is
computed using the following equation:

$$\mathrm{log}E=\mathrm{log}(\text{sum}({x}^{2}))$$

If the input (*x*) is a frequency-domain signal, the log energy is
computed using the following equation:

$$\mathrm{log}E=\mathrm{log}\left(\text{sum}\left({\left|x\right|}^{2}\right)/FFTLength\right)$$

[1] Auditory Toolbox. https://engineering.purdue.edu/~malcolm/interval/1998-010/AuditoryToolboxTechReport.pdf

[2] ETSI ES 201 108 V1.1.3 (2003-09). https://www.etsi.org/deliver/etsi_es/201100_201199/201108/01.01.03_60/es_201108v010103p.pdf

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

System Objects in MATLAB Code Generation (MATLAB Coder)

Cepstral Feature
Extractor | Voice Activity
Detector | `gammatoneFilterBank`

| `gtcc`

| `mfcc`

| `pitch`

| `voiceActivityDetector`

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