Cepstral Feature Extractor

(To be removed) Extract cepstral features from audio segment

The Cepstral Feature Extractor block will be removed in a future release. For more information, see Version History.

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Libraries:
Audio Toolbox / Measurements

Description

The Cepstral Feature Extractor block extracts cepstral features from an audio segment. Cepstral features are commonly used to characterize speech and music signals.

Examples

Extract Cepstral Coefficients

Use the Cepstral Feature Extractor block to extract and visualize cepstral coefficients from an audio file.

• Open Model

Ports

Input

Port_1 — Audio input to cepstral feature extractor

column 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

Output

coeffs — Cepstral coefficients

column 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 the Number of coefficients to return and Log energy usage parameters. M equals the number of input audio channels.

When the Log energy usage parameter is set to:

• Append –– The block prepends the log energy value to the coefficients vector. The length of the coefficients vector is 1 + NumCoeffs, where NumCoeffs is the value specified in the Number of coefficients to return parameter.

• Replace –– The block replaces the first coefficient with the log energy of the signal. The length of the coefficients vector is NumCoeffs.

• Ignore –– The block does not calculate or return the log energy.

This port is unnamed until you select Output delta parameter, the Output delta-delta parameter, or both.

Data Types: single | double

delta — Change in coefficients

column vector | matrix

Change in coefficients over consecutive calls to the algorithm, returned as a column vector or a matrix. The delta array is of the same size and data type as the coeffs array.

Dependencies

To enable this port, select the Output delta parameter.

Data Types: single | double

deltaDelta — Change in delta values

column vector | matrix

Change in delta values over consecutive calls to the algorithm, returned as a column vector or a matrix. The deltaDelta array is the same size and data type as the coeffs and delta arrays.

Dependencies

To enable this port, select the Output delta-delta parameter.

Data Types: single | double

Parameters

If a parameter is listed as tunable, then you can change its value during simulation.

Filter bank type — Type of filter bank

Mel (default) | Gammatone

Type of filter bank, specified as either Mel or Gammatone:

• Mel –– The block computes the mel frequency cepstral coefficients (MFCC).

• Gammatone –– The block computes the gammatone cepstral coefficients (GTCC).

Tunable: No

Domain of the input signal — Input signal domain

Time (default) | Frequency

Input signal domain, specified as either Time or Frequency.

Tunable: No

Number of coefficients to return — Number of coefficients to return

13 (default) | positive integer

Number 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(κ <= floor(fs/2))-2, where κ is the number of band edges in the mel filter bank and fs is the sample rate.

• Gammatone –– The number of valid passbands is defined as ceil(hz2erb(R(2))-hz2erb(R(1))), where R is the frequency range of the gammatone filter bank.

Tunable: No

Data Types: single | double

Nonlinear rectification — Type of nonlinear rectification

Log (default) | Cubic-Root

Type of nonlinear rectification applied prior to the discrete cosine transform.

Tunable: No

Inherit FFT length from input dimensions — Inherit FFT length from input

on (default) | off

When you select this parameter, the FFT length is equal to the number of rows in the input signal.

Tunable: No

Dependencies

To enable this parameter, set Domain of the input signal to Time.

FFTLength — FFT length

[] (default) | positive integer

FFT length, specified as a positive integer. The default, [], means that the FFT length is equal to the number of rows in the input signal.

Tunable: No

Dependencies

To enable this parameter, set Domain of the input signal to Time and select the Inherit FFT length from input dimensions parameter.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64

Log energy usage — 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 block prepends the log energy to the coefficients vector. The length of the coefficients vector is 1 + NumCoeffs, where NumCoeffs is the value specified in the Number of coefficients to return parameter.

• Replace –– The block replaces the first coefficient with the log energy of the signal. The length of the coefficients vector is NumCoeffs.

• Ignore –– The block does not calculate or return the log energy.

Tunable: No

Output delta — Output delta values

off (default) | on

When you select this parameter, an additional output port, delta, is added to the block. This port outputs the change in coefficients over consecutive calls to the algorithm.

Tunable: No

Output delta-delta — Output delta-delta values

off (default) | on

When you select this parameter, an additional output port, deltaDelta, is added to the block. This port outputs the change in delta values over consecutive calls to the algorithm.

Tunable: No

Inherit sample rate from input — Specify source of input sample rate

off (default) | on

When you select this parameter, the block inherits its sample rate from the input signal. When you clear this parameter, you specify the sample rate in Input sample rate (Hz) parameter.

Tunable: No

Input sample rate (Hz) — Sample rate of input

16000 (default) | positive scalar

Input sample rate in Hz, specified as a real positive scalar.

Dependencies

To enable this parameter, clear the Inherit sample rate from input parameter.

Simulate using — Specify type of simulation to run

Code generation (default) | Interpreted execution

• Code generation –– Simulate model using generated C code. The first time you run a simulation, Simulink^® generates C code for the block. The C code is reused for subsequent simulations, as long as the model does not change. This option requires additional startup time, but the speed of the subsequent simulations is comparable to Interpreted execution.

• Interpreted execution –– Simulate model using the MATLAB^® interpreter. This option shortens startup time but has a slower simulation speed than Code generation. In this mode, you can debug the source code of the block.

Tunable: No

Advanced Tab

Gammatone frequency range (Hz) — Frequency range of gammatone filter bank (Hz)

[50 8000] (default) | two-element row vector

Frequency range of the gammatone filter bank in Hz, specified as a positive, monotonically increasing two-element row vector. The maximum frequency range can be any finite number. The center frequencies of the filter bank are equally spaced across the frequency range on the ERB scale.

Tunable: No

Dependencies

To enable this parameter, set Filter bank type to Gammatone.

Band edges of Mel filter bank (Hz) — Band edges of mel filter bank

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 thereafter. The default band edges are set as recommended by [1].

Tunable: No

Dependencies

To enable this parameter, set Filter bank type to Mel.

Domain for Mel filter bank design — Mel filter bank design domain

Hz (default) | Bin

Mel filter bank design domain, specified as either Hz or Bin. The filter bank is designed as overlapped triangles with band edges specified by the Band edges of filter bank (Hz) parameter.

The band edges are specified in Hz. When you set the design domain to:

• Hz –– Filter bank triangles are drawn in Hz and are mapped onto bins. For details, see [1].

• Bin –– The band edge frequencies in Hz are converted to bins. The filter bank triangles are drawn symmetrically in bins. For details, see [2].

Tunable: No

Dependencies

To enable this parameter, set Filter bank type to Mel.

Filter bank normalization — Normalize filter bank

Bandwidth (default) | Area | None

Normalization technique used to normalize 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.

Tunable: No

Block Characteristics

Data Types	double | single
Direct Feedthrough	no
Multidimensional Signals	no
Variable-Size Signals	no
Zero-Crossing Detection	no

Algorithms

Auditory Cepstrum Coefficients

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.

Mel Filter Bank

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

Gammatone Filter Bank

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.

Log Energy

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

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

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

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

References

[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

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2018a

R2022b: To be removed

The Cepstral Feature Extractor block will be removed in a future release. Use the MFCC block or a combination of the Auditory Spectrogram, Cepstral Coefficients, and Audio Delta blocks instead.

Cepstral Feature Extractor Configuration	Recommended Replacement
Filter bank type parameter set to Mel	Use the MFCC block.
Filter bank type parameter set to Gammatone	Use the Auditory Spectrogram block combined with the Cepstral Coefficients block. See Extract GTCC from Audio in Simulink for an example.
Output delta or Output delta-delta parameters selected	If using the MFCC block, select the Append delta or Append delta-delta parameters. If using the Cepstral Coefficients block instead, use the Audio Delta block to extract delta features.
Log energy usage parameter set to Append or Replace	No replacement
Band edges of Mel filter bank (Hz) parameter specified	No replacement
Domain for Mel filter bank design parameter set to Bin	No replacement