# Questions tagged [fft]

The fast Fourier transform is an efficient algorithm to compute the discrete Fourier transform (DFT) and its inverse.

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120 views

### FFT analysis for series of different length

i have some data series in time domain which are in different length. i'm using FFT to transform them to frequency domain and now i want to calculate their average. Since their lengths are not equal i'...
51 views

### Detecting two changing inharmonic pitches

I have a recording of a newer type of siren that uses two pitches sliding back and forth in time. I want to be able to detect both of the pitches simultaneously or overlapping at short intervals. ...
4k views

### Fourier Transform of a Pulse Train (Frequency Comb Simulation)

I am trying to take a Fourier transform of a femtosecond pulse train and plot the frequency spectrum. I followed this question and am able to create a pulse train and plot both the time and frequency ...
343 views

### Discontinuity at the edge of Chebychev window

I am using Chebychev window for its narrow main lobe. The problem of chebychev window is that it has discontinuities at the edge, and it seems that Taylor window solves this issue. More detail: http:...
789 views

### Normalizing the magnitude of Goertzel Filter c#

I'm trying to detect some frequencies of 18Khz with sample rate of 44100 [samples/sec]. When I pass a buffer(~4500 samples) to the algorithm it returns a magnitude in a very strange range. Sometimes ...
1k views

### Drawing a Spectrogram off of FFT - what are the decibels relative to?

I am trying to draw a spectrogram from FFT data I get from audio files. The problem is that I can't figure out is what the yielded power (decibels) are relative to. I'm getting some arbitrary -100db-...
3k views

### Compare phase and magnitude spectrum results of 2 Images [duplicate]

I have read that from Fourier transform we obtain magnitude and phase spectrum. The magnitude spectrum tells you how strong are the harmonics in a image and the phase spectrum tells where this ...
119 views

### How do you find the position of when a signal changes frequency?

Suppose I have a signal which varies between two frequencies (which represent a stream of binary data). I think this is called BFSK. I have the entire signal already sampled. Let's say each 'bit' ...
570 views

### FFT of signal with starting value zeros

I have a signal, ...
88 views

### Length and spacing autocorrelation with FFT

I have calculated, with FFT and A-FFT, the autocorrelation of a discrete signal (coming from an integration) with spacing $\Delta T$ composed by $N$ samples padded to $2N$ with zeroes. What is the ...
440 views

### DTFT of beat signal with shift due to multiplication

I am simulating a FMCW radar (Ramp signal is used for transmission)and I have created the transmit signal from matlab FMCW model ...
99 views

### Orthogonality test on fixed point FFT

I have tried the following methods to test the orthogonality on Fixed point FFT implementation. FFT: Y = A*x, where A = FFT basis and x = input, Y= FFT output If the input (x) is set to A-1(A ...
496 views

### Implementing a Number Theoretic Transform (Fermat number transform--257). Should I consider alternatives to a radix-4 cooley-tukey?

This is kind of a comp sci question, but I figured I could use some input from FFT experts. I've already got a radix-4 cooley-tukey implementation of the NTT briefly described on page 9-10 of https://...
109 views

### Audio output distorted with any FFT Windowing other than rectangular

For an experiment i have a simple application that takes a quadrature signal from a radio and then does the following: Complex FFT -> Inverse FFT -> Cartesian to Polar -> audio out If I tune the radio ...
143 views

### What is the preferred method of aggregating audio FFT data given variable-size frequency bins?

I am coding a program which uses a FFT to process audio data. I need to aggregate the FFT output into three bins ("bass", "mid", and "treb") determined by two arbitrary frequency values. The end-goal ...
645 views

### FMCW SAR chirp return from target modelling

I have to Model a chirp signal that was return from a Distance D from target and then multiply with the reference transmitted signal. Which will give me beat frequency signal and then fft to get the ...
133 views

### Finding periodicity within sequence of time hacks

I have a series of time hacks for various events that have occurred. Some of these events occur at regular intervals (+/- some small variance) but I do not know what those intervals are and would ...
658 views

### Feature Extraction from Accelerometers for classification

I am looking into algorithms/techniques for feature extraction for accelerometer data - to then be used in classification. FFT is incredibly popular, however would this work on accelerometer data? I'...
1k views

### Amplitude of frequency in MATLAB FFT

I am trying to extract amplitude of specific frequency in Matlab FFT. Is it possible to use the abs(mag)... But I do not know in which sample to look for ...
250 views

### Finding amplitudes from FFT data of a 2D image

I'm working on experiments with 2D - wrinkled surfaces. Basically I am getting a bunch of images like this. This example is hand-made. Edit: The test image was made by sampling a cosine function ...
846 views

### Remove Checkered Noise (Pattern) from an Image

I'm trying to remove what looks like a checkered noise pattern that has been added to an image. Comparing the 2D FFTs The one on the left is the original, while the middle one is the corrupted one. ...
594 views

### matlab fft phase response square wave

I am new to phase analysis, recently I have been trying compare the phase between the input signal and output signal of a system. From the code below, I simulated a square wave and plotted the phase, ...
559 views

### why is the plot of the PSD of this autocorrelation function oscillating?

I was doing an exercise trying to compute the PSD of the following discrete autocorrelation function: \begin{equation} r_x(m) =(1/2)^{|m|} \end{equation} In this simple matlab code that I came up ...
170 views

### Most efficient phase linear x4 resampling algorithm

I'm working on a program in which I need to do the following about 20 times for each block of audio: - Upsample from 192 to 768 kHz (x4) - Clip - Downsample back to 192 kHz (/4) What I'm using now: ...
342 views

### What's the use case and explanation of frequency-domain subsampling?

In my solution to a certain problem I use subsampling in the frequency domain. Amazingly, it produces good results. Is it really that awkward to see subsampling in frequency domain? What the ...
134 views

### Does FFT magnitude spectrum graph consist of spectral lines or points

Hi I am using zedgraph to plot an FFT of a signal using c#.The FFT spectrum gives coefficients vs the frequency bins.I am confused whether the plot should consist of spectral lines(ie verticle lines) ...
82 views

### Signal FFT conversion problem

I got the time domain signal from experimental work as per below. The blue is x-direction,green is z-direction and red is y-direction. When i convert the signal into FFT using MATLAB, I got as per ...
1k views

### How to plot Full spectrum with negative frequencies

I'm trying to do frequency analysis on signal of a rotating shaft. The signal is generated in two measurement planes i,e X-position and Y-Position. I was able to use fft and transform into frequency ...
490 views

### Combine multiple FFT

I have been acquiring data through ADC and into memory, the input signal are square waves that step down in frequency as time goes by. Due to memory, I have a fixed maximum number of samples per FFT. ...
590 views

### Apply FIR filters to Extract Stego from WAV data

I appreciate anyone that takes a moment to help me with this problem. I've been banging my head against the keyboard for a while, searching forums and DSP tutorials and I can't figure this problem out....
602 views

### Matlab coding for bandwidth interval

I have time domain history data. Then, I convert it into FFT to get the frequency domain history data. The problem is, how to determine the bandwidth of the FFT data? Is it automatically synchronize ...
428 views

### How to narrow the frequency band of a wavelet

I have a ricker wavelet with a dominant frequency of 15 Hz. The fourier transform shows its frequency band is almost to 50 Hz. How can I narrow the frequency band of this 15 Hz ricker wavelet? I have ...
3k views

### How to determine cut-off frequency using power spectral density?

I am new to DSP and my question is a very simple one. It is about determining the right cut-off frequency. I calculate the power spectral density using FFT by Welch's algorithm. However I am confused ...
254 views

### Choosing the frequency for a comb filter

I have an audio sample. From doing FFT on it it looks as if I need to use a comb filter to remove resonant frequencies. This is what I generate: I honestly have no clue where to start unfortunately.
96 views

### How to simulate two OFDM Users with low complexity FFT/IFFT?

have two User one of them sending over freq (0-B] and the other transmit over (B-2B]. Both of them are using OFDM and each one has Nc subcarriers. I wanted to simulate the received signal using Matlab....
2k views

### Detect Frequency of Incoming Audio on Raspberry Pi

I've been lurking around these forums for a while and they've been a great help to me in the past. However, I've been researching this problem for the past 2 days or so and can't seem to find a ...
297 views

### Reducing latency due to FFT windowing for a real-time signal

I need to compute the amplitude of a certain frequency band as the input to a PID algorithm. My system looks like this: ...
148 views

### Best way to reduce data to its symmetric and antisymmetric components

I have some noisy data that looks like this: I want to find the components that are symmetric and antisymmetric with respect to magnetic field. The first thing I tried was just using: Symmetric part ...
99 views

### Merging pattern for FFT solutions

I have the generic problem of analysing a discretly sampled signal, lets say at 100Hz, and would like to compute features (ML application) of this signal at different scales: An instantiation of this ...
4k views

### Reconstructing time domain signal with Hanning window

I am currently working on a frequency domain real-time application on a digital signal processor. Currently for one time frame of my algorithm I read in time domain data into a buffer, perform a ...
4k views

### Cooley-Tukey Implementation of FFT in Matlab

For my course I need to implement a 30 point Cooley-Tukey DFT by transforming it into a 5x6 matrix. I have tried to implement using the following Matlab code: ...
60 views

### Frequency of the wave in frequency domain

If we have a 1-dimensional wave in time domain, it can be represented in frequency domain with x axis indicating the frequency of the wave and y axis indicating amplitude/magnitude of the wave. But ...
78 views

### Phase difference betwen 2 signals, 1 signal with unknown function

I understand this question may have been addressed however I have a major difference. I started out with a sinusoid of form A*sin(2 pi freq t + phase) and I know ...
43 views

### how can i do a 2^20 FFT if the hardware imposes a limit of 2^10 FFT size?

I need to implement an fft of size 2^20, but the hardware has imposed a limit of 2^10 fft size.Then how can i implement the <...
1k views

### direct Frequency domain FIR filtering vs Overlap-add method

I'm trying to do bandpass filtering of a EEG signal samples at 250Hz and benchmarking the following 4 methods of FIR filtering for different filter orders. The length of the signal is 15000 samples. ...
294 views

### Explain Shift property of DFT

Please check the link What effect does a delay in the time domain have in the frequency domain? Here it is said that if you delay your input signal by D samples, then each complex value in ...
358 views

### Welch's Overlapped Method

I am measuring wind speed using a number of different anemometers. These anemometers are placed on a structure which exhibits sinusoidal motion. Thus, the signal of the anemometers contains both the ...
892 views

### what is the significance of pilot carriers in ofdm [duplicate]

in an OFDM system having a radix two FFT (Say N=64) . all of these sub-carriers are not utilized. while some of the carriers are nulls to prevent leakage in to adjacent bands, say a four to five ...
Suppose I have a DFT matrix $F$ of dimensions $N \times N$ where $N$ is a power of $2$. I convert it into a unitary matrix $\displaystyle U = \frac{F}{\sqrt{N}}$ and compute $S = U^* U^*$. $S$ ...