Short-Time FFT

Nonparametric estimate of spectrum using short-time fast Fourier transform (STFT) method

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Libraries:
DSP System Toolbox / Transforms

Description

The Short-Time FFT block computes a nonparametric estimate of the spectrum. The block buffers, applies a window, and zero pads the input signal. The block then takes the FFT of the signal, transforming it into the frequency domain.

Examples

Speech Enhancement with Gain in Frequency Domain

Denoise a signal by applying selective gain on the signal in the frequency domain. Convert the denoised signal back into time domain and play the signal using Audio Device Writer.

• Open Model

Ports

Input

x[n] — Input signal

vector | matrix

Specify the time-domain input signal as a vector or a matrix. If the input is a matrix, the block treats each column as an independent channel. The input can be a variable-size signal. That is, the frame size of the signal can change while the simulation is running. The number of channels must remain the same.

If the input is fixed point, it must be a signed integer or a signed fixed point value with a power-of-two slope and zero bias.

Data Types: single | double | int8 | int16 | int32 | fixed point
Complex Number Support: Yes

w[n] — Analysis window vector

vector

Connect your single-channel analysis window to the w[n] port. For the Analysis window length parameter, enter the length of the analysis window W. The block buffers the input signal such that it has a frame length of W.

If the window vector is fixed point, it must be a signed integer or signed fixed point with a power-of-two slope and zero bias.

Data Types: single | double | int8 | int16 | int32 | fixed point

Output

X[nL,k] — Short-time FFT output

vector | matrix

The block outputs the complex-valued single-channel or multichannel short-time FFT at this port.

Data Types: single | double | int8 | int16 | int32 | fixed point
Complex Number Support: Yes

Parameters

Analysis window length — Length of analysis window

512 (default) | positive integer greater than 1

Specify the frame length of the analysis window as a positive integer greater than 1. The block buffers the input into overlapping window segments using the specified window length and overlap length, and then multiplies each overlapped segment by the window.

Overlap between consecutive windows (in samples) — Overlap length

384 (default) | nonnegative integer

Specify the number of samples by which consecutive windows overlap in each frame of the input signal as a nonnegative integer. The window overlap reduces artifacts at the data boundaries.

FFT length — FFT length

1024 (default) | positive integer greater than or equal to 2

Enter the length up to which the block pads the input signal as a positive integer greater than or equal to 2. After the block buffers and windows the signal, it zero-pads the signal before computing the FFT. This parameter determines the length of the short-time fast Fourier transform output (number of rows). The FFT length must be greater than or equal to the window length.

Block Characteristics

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

More About

Fixed-Point Data Types

The following diagram shows the data types used within the Short-Time FFT subsystem block for fixed-point signals.

The values of the fixed-point parameters of the Array-Vector Multiply block in this diagram are:

• Rounding Mode — Floor

• Saturate on integer overflow — Wrap

• Product output — Inherit via internal rule

• Accumulator — Inherit via internal rule

• Output — Same as first input

The values of the fixed-point parameters of the FFT block in this diagram are:

• Rounding Mode — Floor

• Saturate on integer overflow — Wrap

• Sine table — Same word length as input

• Product output — Inherit via internal rule

• Accumulator — Inherit via internal rule

• Output — Inherit via internal rule

See the FFT and Array-Vector Multiply block reference pages for more information.

References

[1] Quatieri, Thomas E. Discrete-Time Speech Signal Processing. Englewood Cliffs, NJ: Prentice-Hall, 2001.

Extended Capabilities

C/C++ Code Generation

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

Usage notes and limitations:

• Generated code relies on the memcpy or memset function (string.h) under certain conditions.

• When the FFT length is not a power of two, the executable generated from this block relies on prebuilt dynamic library files (.dll files) included with MATLAB^®. Use the packNGo function to package the code generated from this block and all the relevant files in a compressed zip file. Using this zip file, you can relocate, unpack, and rebuild your project in another development environment where MATLAB is not installed. For more details, see How To Run a Generated Executable Outside MATLAB.

  When the FFT length is a power of two, you can generate standalone C and C++ code from this block.

Version History

Introduced before R2006a