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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75 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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1answer
54 views

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 ...
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2answers
41 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 ...
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1answer
41 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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74 views

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 ...
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2answers
51 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
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22 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 ...
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1answer
44 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)...
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1answer
42 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. ...
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1answer
25 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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13 views

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 (<...
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2answers
81 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 ...
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1answer
70 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 ...
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41 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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40 views

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 ...
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1answer
46 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 ...
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1answer
62 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 ...
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26 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 ...
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1answer
40 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 ...
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1answer
78 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 ...
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1answer
106 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 ...
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1answer
61 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, ...
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6 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 ...
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1answer
66 views

Frequency estimation of circularly shifted single tone signal

I have a discrete signal $y[n] = <e^{j ~ 2 \pi f ~ n}>_J + ~w[n]$ with $n \in [0, N[$ and $w[n]$ AWGN, $<x[n]>_K$ denotes the signal $x[n]$ circularly shifted by $K$ samples. Let's define $...
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2answers
102 views

How to do FFT fractional time delay (SOLVED)

I am trying to time-shift a signal using the FFT, however I have encountered some strange effects that depend on the size of the time-shift. I need to be able to shift the time by an arbitrary amount -...
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2answers
82 views

Evaluate performance of FFT Windows

lately i've been interested in FFT windowing, i've experimented various windows in Matlab, that made me more curious: There are normalized parameters (CG, Scalloping loss etc...) for every window ...
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18 views

Detection time of a spur using FFT?

Say I have the following parameters, NFFT=2^14 Fs= 50 MHz main signal @ 22 MHz and its alias at 28 MHz at signal power -40 dbfs and a spur @ 2 MHz and its alias at 48 MHz Leave aside the alias's. ...
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411 views

Python tool for time-frequency analysis

I am trying to perform time-frequency analyses using the PyWavelets (pywt) toolkit for python. My ultimate goal is to perform time-frequency analyses for EEG signals but I am starting with something ...
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2answers
250 views

FFT of triangular wave

I am new in dsp.. so please bear with me. I want to plot FFT of a triangular wave. I am using the code below to first generate the triangular wave and then take its FFT ...
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2answers
82 views

Matlab and R functions for fft(): N-point FFT and dimension issue [closed]

In MATLAB, Y = fft(X,n,dim) returns the Fourier transform along the dimension dim. For example, if ...
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2answers
155 views

Explain Auto-Tune in a simple way

I have to do a presentation about Auto-Tune and its relation to the Fourier transform. What is a good explanation on how does Auto-Tune work?
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1answer
23 views

Matlab applying an FIR filter to a signal, generated in Matlab

I want to create an FIR in Matlab and apply it to a signal (also generated in Matlab). You will find the code below. I will put my questions in order from here. 1) The filter coefficients vector "b" ...
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5k views

Is there any practical application for performing a double Fourier transform? …or an inverse Fourier transform on a time-domain input?

In mathematics you can take the double derivative, or double integral of a function. There are many cases where performing a double derivative models a practical real-world situation, like finding the ...
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1answer
28 views

Aliasing correspondance with respect to DFT

Given a sampling frequency Fs lets say we plot the magnitude of the fft of a temporal signal $x$, for different frequencies above the Nyquist frequency, to show the effect of aliasing: ...
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8 views

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 ...
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2answers
40 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 ...
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37 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 ...
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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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22 views

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 ...
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3answers
117 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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2answers
66 views

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 ...
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1answer
36 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 ...
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35 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 ...
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2answers
97 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 ...
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0answers
280 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 ...
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2answers
89 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 ...
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1answer
65 views

Why would FFT interpolation using zero-padding undershoot the true frequency of a single tone sinusoid?

Why would FFT interpolation by zero-padding or using the Chirp Z-Transform produce a maximum at a bin that corresponds to a frequency less than the input frequency of a single tone sinusoid? I am ...
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52 views

Normalizing FFT after recording with Pyaudio

I have a USB microphone that has a 24 bit ADC, however, Pyaudio doesn't allow for 24 bit encoding so I used 16 bit. I hope this is not an issue. Having said that, I hope to make sure I'm applying the ...
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1answer
52 views

What measurement errors occur in an inteferogram?

This question is bordering on physics, but it is the signal side that I am interested in so I hope this is the correct forum. I am very familiar with instrumental noise sources in CCD based ...
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2answers
70 views

Is the value of a frequency bin of a DFT-output the average of the 'real' frequency values within that bin's range?

I am wondering whether the value of a frequency bin with a certain resolution is the average of the fourier transform values of the 'real' frequencies within that bin's range.