The Fast Fourier Transform (FFT) calculated using blocking. There are two forms.

Syntax

y = BlockFft(vec, block_size, overlap, length)

y = BlockFft(vec, window, overlap, length)

Arguments

Name

Description

vec

A vector of input signal.

block_size

The number of elements to be used for each FFT.  Must be a positive integer and not greater than the length of vec.

window

A vector of window weights to apply to each block.  It's length is used as the block_size.

overlap

The number of elements shared between consecutive blocks. Must be a non-negative integer and less than block_size.

length (optional)

The length of the output vectors if specified as a positive integer.  If omitted or zero, the length will be the block size or window length.  If the length is specified as ‘pad’, the length will be rounded up, if necessary, to obtain a power of 2.  See Comments.

Output

Name

Description

y

A vector of the complex valued FFT. Its length is equal to that of block_size or window unless argument length is specified.

Example 1

Find the FFT of a signal stored in vector data, using a block size of 256 and an overlap of 128:

Syntax

output = BlockFft(data, 256, 128)

Result

output is a vector of the complex valued FFT.

Example 2

Repeat the above example with a Hanning window instead:

Syntax

output = BlockFft(data, HannWin(256), 128)

Results

output is a vector of the complex valued FFT.

Comments

The BlockFft function uses blocking to calculate the Fast Fourier Transform (FFT).  The FFT is complex-valued and used to map time-domain data into the frequency domain. vec is assumed to be evenly sampled.

The BlockFft function is different from a normal FFT in that it introduces blocking.  The input vector is subdivided into blocks.  An FFT is then performed on each individual block.  The results of these FFTs are then averaged to give the final result.

When length is set to ‘pad’, the block size is padded with zeros (if needed) to round it up to power of 2 before performing a FFT on the block.

The output can be used to extract magnitude and phase.

See Also:

Abs

BlockFftMag

BlockFftPhase

BlockFftReal

Fold

Freq

Phase