# 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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36 views

### Getting non smooth spectrum after performing fft on numerically obtained acceleration data

I am trying to calculate the spectrum of Bremmstrahlung, which involves calculating the Fourier transformed acceleration. I am solving a non-linear ODE to numerically calculate the acceleration in the ...
142 views

### Square wave FFT results show frequency that seems too low?

I have created an open source plotting package for a low cost oscilloscope - see link here: GitHub Plotting Software But during my testing I have one result that may be wrong: a noisy 7 kHz square ...
80 views

### Recovering a signal after filtering

A signal $x[n]$ is to be transmitted over a communication channel. The communication channel is described by an FIR filter of length $M$ such that the received signal is given by r[n]=\sum_{k=0}^...
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### Plotting curved peaks on PSD graph

I am a PhD student with little engineering experience, currently attempting to recreate the signal processing techniques used in this paper: Monitoring Vital Signs and Postures During Sleep Using WiFi ...
53 views

### Dominant Frequency Peak Decreases with Increasing Window Size

I have a signal that looks like this. I analyse it using fast Fourier transforms to identify the frequency with the largest peak, which is always close to zero. (There are no other clear peaks.) If I ...
42 views

### Non-physical peaks from the Fourier transform process

I was doing some reading on transient signals. When transient signals is analysed by DFT, there are some non-physical peaks from the Fourier transform process (on the figure below). I understand this ...
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### Using a Butterworth high-pass filter together with Fast Fourier transform

I have a signal and I'm interested in identifying the dominant frequencies greater than 1/3600 Hz. To do this, I use Fast Fourier Transforms and examine the locations of the peaks in the frequency ...
62 views

### Is RMS of a sound wave good measure of loudness?

I'm not talking about FFT, just the raw sound wave and the RMS of it. Thanks
40 views

### Using a Multirate FFT system to analyse audio

I am trying to implement a multirate fft system to try and achieve frequency windowed analysis of audio. Say I have managed to get an impulse response of a system. This impulse may be say one ...
88 views

### PyAudio recording is taking much longer than planned

I'm trying to record sounds in chunks using PyAudio for 15 seconds, chunk size being 1024 and sampling rate is 1024 *40. I understand the number of chunks would be (time of recording) * (sampling rate)...
62 views

### Partitioned overlap-add convolution - strange behavior at buffer boundaries

I've implemented a convolution reverb that operates in real-time, one audio buffer at a time (using FFTS for the fft bits). However, there's some strange behavior at the start of every buffer. ...
28 views

### Audio Spectrogram / FFT - Amplitudes and Missing Frequencies

I have an input wave which I want to transform to a spectrogram. In the example code in a previous question , I get the result, shown below, for a pure sine wave 440Hz. From this video, if I do an FFT ...
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### Could kernel density map be recovered with known kernel function?

I'm confused whether a KDE map could be recovered with known kernel function. The KDE map (with no noise) generated with fast fourier trasnformation (FFT) could be recovered on very high accuracy (<...
160 views

### What is the meaning of the length of diagonal lines in FFT

I am relatively new to FFT analysis of images and have a question about them. I like to think I understand the basics but many of the properties shown in a FFT image are beyond me. My main question ...
201 views

### Methods for aligning FFT bins with an FSK signal

Say I want to demodulate some FSK modulation. My (novice) understanding is a pretty good way to do this is by taking the FFT, with the length chosen such that the bin spacing is equal to the tone ...
45 views

### Understanding convolution of Chirp z algorithm

I dont´t understand how works the convolution part of the Chirp z. I understand how the DFT is transformed \begin{align*} x(k) = \sum_{n=0}^{N-1} x(n) W_N^{kn} \end{align*} to this expresion: \...
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### Why are low frequency peaks not sharp in scipy fft?

I am using numpy/scipy to plot graphs of sine waves. frequencies at 15 Hz or higher give nice, sharp peaks, but at lower frequencies, the peaks are smeared and the actual peak frequency can't be seen ...
158 views

### What determines peaks in FFT?

I ran FFT on three audio files and found that the results for some have more peaks than the other. Could anyone give me any conceptual explanation as to what determines these peaks? Below are plots of ...
100 views

### Why the spectral coherence is unity for all frequencies between single-frequency time series and itself

In the example below, I am plotting the coherence between time series and itself. The time series do has one frequency.The coherence magnitude was one for all frequencies. I wonder why it is not zero ...
28 views

### Skew measurement with phase deviation

I am calculating the standard deviation of skew (= phase/2*pi*fin) from the 1000 monte carlo runs of the following code. In the y0, there is no input skew., but in y1 there is a skew present. Its ...
74 views

### What is the difference between the V-OFDM and OSDM

I am trying to understand the difference(s) between V-OFDM (Vector Orthogonal Frequency Division Multiplexing) and OSDM (Orthogonal Signal Division Multiplexing) which are used as a multicarrier ...
790 views

### FFT processing of IQ data

My time-domain sampling rate of IQ data is Fs. Matlab FFT assumes spectral bandwidth is Fs/2, per the Nyquist criterion. I understand that IQ data has bandwidth of Fs. How do I do FFT of such IQ data ...
342 views

### Signal processing using numpy python

To process a .wav audio file with numpy (using fast Fourier transform algorithm). I want to process an audio signal at a particular interval with a sampling frequency 44100hz and sampling rate of 20ms ...
312 views

### Prefactors computing PSD of a signal with numpy.fft VS. scipy.signal.welch

The power spectral density St of a signal u may be computed as the product of the FFT of the signal, ...
9 views

### Noise evaluation using piezo sensor

I'm not familiar with piezo (contact) sensors for measuring vibrations/noise. Suppose an object produces sound up to 5khz in frequency, then in order to capture this information my piezo sensor would ...
74 views

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### Obtain points in signals which cause correlation

I've got two large .wav files (tens or hundreds of minutes each). I know that at some points these files contain very similar areas - actually, these areas are the ...
60 views

### Why there is error between My DFT and built in Matlab FFT?

I'm trying to implement DFT/IDFT and FFT/IFFT in Matlab and C. I got wrong values while I implement IDFT/IFFT. So I'm trying to find a mistake. Just for DFT, I used this algorithm to implement it in ...
61 views

### FFT Amplitude Resolution

What's the relationship between sample rate, sample length, ADC quantization, and FFT output amplitude resolution? Suppose I have N samples, sampled at Fs such that I can use a radix-2 FFT. My ADC ...
16 views

### What is wrong with band passing by inverse Fourier transforming a section of a signal's frequency spectrum? [duplicate]

If the question is unclear, here is what I mean in Python using scipy.fftpack: ifft(fft(arr)[x:y], len(arr)) What is wrong with this method of band passing?
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### improving performance of KNN classifier on FFT

I need to classify whether a product is passing or failing based on a noise check. I have 100 labeled "good" product and 100 "bad" product. For each, I recorded the sound for 3 seconds and each chunk ...
205 views

### Reverse engineering a digital filter

I have 2 simultaneous signals that both are designed to measure eye movements. They are sampled at 250 Hz. We have 12 subject recordings. For 3 stable periods in each subject, we choose 256 points and ...
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### Is spectral leakage normal for FFT?

I record a sample of 2000 data points, with a sampling rate of 40khz, so freq. resolution is 20hz. I multiplied the chunk of data with a Hanning window prior to doing the FFT. I recorded a 1khz test ...
43 views

### The frequency-domain spectrum obtained using MATLAB's FFT function seems inaccurate

I am using MATLAB R2018b on a Windows 10 PC. In order to obtain the frequency domain information of a signal, I'm applying MATLAB's FFT function. The signal of interest is obtained from an experiment ...
49 views

### Is the whole data needed for the fast fourier transform with an ongoing signal?

I sample an signal with around 840k values a second, to check the spectrum around <400kHz. But i can't save the whole data as it is just to much for my microcontroller. I know that for the ...
102 views

### Intermittent Harmonics, Is there a physical explanation?

I am doing some frequency analysis of a vibration signal from a spinning rotor, I am expecting to see peaks corresponding to the rotational frequency of the rotor. So the fundamental frequency ...
2k views

### How do I find the peak frequency of a wav file in python?

I am fairly new to python and signal processing and I was given a task to record audio for 'x' seconds and then find the peak frequency in the audio file. So far I have successfully implemented the ...
178 views

### Multi Resolution STFT for audio

I am looking at trying to achieve a multi resolution STFT analysis on a log sine sweep audio measurement. I have an impulse, and I would like to perform two different windowed length FFTs on the ...