Questions tagged [ifft]

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How to properly filter offline data by zeroing the unwanted frequencies after FFT?

I am new to the field of DSP. I have a measured signal, and I wanted to apply a low pass filter on the data. I just simply applied FFT, wrote zeroes at the unwanted frequencies then applied inverse ...
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1 vote
1 answer
48 views

How to convert discrete-time QAM pulses to a continuous-time signal using the IFFT (for OFDM)?

So I've run into some trouble in trying to plot the PSD of an OFDM signal. Specifically, I cant see how we go from the discrete domain to the continuous in time domain. I have a time vector, which is ...
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1 answer
109 views

My summed sin waves seems to only work if my wavelength is divisible with the time steps. A problem with phase likely

I have created a program in python that can create any generic signal and a time range to display it as shown by the red '--' line below. I then created a way to calculate the phase, amplitude, and ...
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2 votes
3 answers
93 views

Delay/Lag calculation using cross correlation in frequency domain

I need to compute cross correlation of two signals using frequency domain. Hence, I am using the below equation for calculation. max(IFFT(FFT(a)*conj(FFT(b)))) ...
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3 votes
1 answer
123 views

Why signal add noise cause signal undiscoverable after `fft` and `ifft`?

As we all know, Fourier transform of the sinc(t) is rect(f). Simple python script to discover signal after fft and ifft with ...
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3 answers
473 views

Are there any reasons to use overlap algorithm when one could do ifft(X * Y) from the complete signal as well in similiar time complexity

We learned that overlap add and overlap save are used as fast convolution methods, because they can be applied with fft due to the formula $x[n] * y[n] = \text{IFFT}(X[k] Y[k])$ So now I was wondering ...
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1 answer
45 views

How to zero pad and do overlapping block convolution to reconstruct audio for frequency filtering in time-frequency domain

I am doing STFT (short-time Fourier transform) on real-time audio to filter frequencies by magnitudes (to reduce noise) which have certain drawbacks like noise spikes in reconstructed audio in the ...
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35 views

How to define u to solve greens equation

this is doing my nut in. I need to define u in the following equation (there is a lot more to it but this is a sample): $$G(x,x',z,z') = \int_{-\infty}^{\infty} \exp\left(-ju(x'-x)\right)exp(-jk|z-z'|)...
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1 answer
101 views

MATLAB: ifft after fft does not give the original result

I apply ifft after fft, but the result is not the same (x-fft(ifft(x)) is different from ...
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1 vote
2 answers
66 views

Inverse STFT causing unwanted noise spikes?

I have an (audio) signal. I am applying STFT continuously and after applying a certain magnitude threshold I am doing inverse STFT to reconstruct the signal. But I am getting some unwanted mini noise ...
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1 vote
1 answer
39 views

Plotting the real or absolute of correlation?

I had two vectors of data and I computed their correlation using corr(f,g) = ifft( fft(f) .* conj(fft(g)) ) / cov(f,g). I cannot figure out whether to use the real ...
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27 views

iFFT and extracting dw

I have a trivial fourier transform question. I have a correlation function, C(t), with complex components in the time-domain, and dt. I would like it in the frequency domain, C(w), like from ...
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47 views

What's wrong with a filter for subband OFDM?

I am confusing about why it is necessary to pack the QAM symbols onto the frequency bins, such that the allocated range of frequency bins is located in the middle of the subband (as in lines 26-29). ...
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Pressure signal

I have got an raw file which has pressure and time and temperature format and after coding I have got the plots but my main task is to analyse the data and understanding the influence of Bessel and ...
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1 answer
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Autocorrelation via real FFT and real IFFT

I am working on a fast autocorrelation function using the FFT (similar to scipy.signal.fftconvolve). The algorithm is pretty straight forward but I feel like I'm ...
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2 votes
0 answers
63 views

Frequency Interpolation of DFT to create Zero-Padded IFFT

As a response to this question I have proposed interpolating new samples in a DFT (meaning the frequency samples of an existing DFT result) sufficient to be the new samples that if we were to take the ...
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3 votes
2 answers
106 views

Adding multiple time-domain signals with different FFT sizes

I would like to add multiple sequences with different FFT/IFFT sizes. For instance, we have 3 vector signals, $\vec{x_1}$, $\vec{x_2}$, and $\vec{x_3}$, having FFT/IFFT sizes of 512, 1024, and 2048, ...
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-1 votes
1 answer
28 views

First element of the complex input array for the IFFT with OFDM

I am simulating OFDM by randomly selecting members of a constellation. That is, randomly selecting the I & Q from a 64QAM constellation. This gives me 64 possible phase & amplitude values. I ...
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FFT filtering by hand in python - iffy issue - cannot get filtering right

I am trying to filter an image by hand (with no using the image processing libraries) to understand how fft.shift works in python. No luck. I take a png an image from MNIST, e.g.. Here is the code: <...
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1 vote
0 answers
23 views

Properties of the Frequency Domain to Ensure a Periodic Inverse Discrete Fourier Transform

I've been working on a project to send data via sound waves. The way I've carried this out is by encoding the data in pulses in the frequency domain, and then creating a unique sound wave containing ...
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2 votes
1 answer
80 views

excluding the value of some frequency in a Fourier Transform

I have a variable in time and I calculated its mean which is 9.5. I have done the Fourier Transform of this variable, using the FFT in matlab. I would like to consider only a range of frequencies, so ...
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1 answer
46 views

Change spectrum and normalization

I have a question about magnitude of a spectrum. My task is to build the spectrum and make IFFT - the result waveform should have unity amplitude. For example: Create 2 arrays with Magnitudes and ...
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1 answer
51 views

OFDM IFFT/FFT processing

After the serial-to-parallel to break input bitstream into groupings we do the constellation mapping into values of phase/amplitude (or I, Q). For 64 sub-carriers this gives us 64 pairs of amplitude(n)...
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1 vote
1 answer
165 views

1ppm Sampling clock offset in Matlab

How to emulate a sampling clock offset (between the transmitter and the receiver) of 1 part per million (SCO=1ppm) in Matlab? Theoretically, I can upsample the signal by a factor of 1,000,001 (one ...
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36 views

implmentation of FFT using lower N point

I have random data which was generated using randi funtion of 1024 bits.. I will modulate this using QPSK and will pass through 512 point IFFT.. While doing reverse process, I want to use 256 point ...
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filtering large wiggles in signals

I am trying to filter out large wiggles showing in a signal. Those wiggles are happening due to many signal processing layers and a final sine Fourier transform (no phase) Here's the python code i am ...
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1 vote
2 answers
85 views

How an algorithm for measuring the analytic signal (Hilbert transform) gives us complex value at the end?

I have a question about the following codes for measuring analytic signal (Hilbert transform). Here,the time domain signal (x) is a real value signal. After making fft it becomes complex (X). Then it ...
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1 vote
2 answers
298 views

How to multiply two FFTs in MATLAB?

I want to find the output signal(y) from input signal(x) and impulse response(h). To do that first I take the fft of both x and h, then multiply them, and finally taking the ifft of the result. The ...
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0 votes
2 answers
186 views

Convolution Reverb Calculation

Here's my basic understanding of how to implement a convolution reverb: I measure the IR of a space using a sine sweep. The raw mic output is stored as a Voltage-Time data set. Let's assume that this ...
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1 answer
106 views

How to do sliding window multiplication in frequency domain

I want to perform the following computations with the impulse response of length L of a LTI system: Multiply the impulse response with a window w(n) of length M << L. Take the FFT of the result ...
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1 answer
732 views

What is imaginary part after IFFT and how to use it?

let's say we have signal in time domain. After we make FFT on it we will receive that signal in frequency domain. And as far as I know the real numbers mean magnitudes of each frequency bin, and ...
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37 views

Spectral efficiency of with OFDM vs without OFDM?

OFDM has advantage over frequency selective channel with addition of cyclic prefix after IFFT of MPSK or MQAM symbol. but I wonder that the spectral efficiency of with OFDM vs without OFDM.
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does ifft of higher order equal upsampling signal

I have a signal with length 2000, and am taking an fft by 2048, after I perform analysis and some operation on signal in frequency domain I tried to convert it back to time domain. I received a signal ...
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1 vote
1 answer
81 views

How do you map indices to twiddle factors in FFT

I am kinda lost on how to map my indices for each stage to my look up array of precomputed twiddle factors. For an $N=8$ point FFT i have $N/2$ twiddle factors in total: ${ [0] = W_8^0, [1] = W_8^1, [...
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2 answers
586 views

Understanding the twiddle factors

So I've been looking at this butterfly diagram to try to understand it better: And I am trying to get a good understanding of the twiddle factors. The definition is given as: FFT Twiddle Factor: ${e^{...
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1 vote
3 answers
1k views

Polyphase filter bank channelizer FFT or IFFT

I am referring to the Xilinx application note Polyphase Filter Bank Channelizer. The receiver portion should have FFT but the pdf mentions IFFT in both Tx and Rx. The paper by F. J. Harris, C. Dick ...
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69 views

How do I perform 2D Fourier domain multiplication if the filter mask doesn't match the image size?

Let's say I have an image that is 512 x 512 pixels. I've been tasked with creating two ideal half-band low-pass filters that will filter the image. The first filter is 8 x 8, and the second one is 16 ...
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1 answer
111 views

What is the algorithm to generate sine waves of arbitrary frequency in the STFT domain?

I'd like to write a DSP algorithm to do additive synthesis using arbitrary sine waves with inverse rectangular FFTs. This requires two things: The ability to generate phase/amplitude lists that cause ...
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0 votes
1 answer
303 views

FFT on a Raspberry Pi

I'm trying to determine the processing time required to perform a FFT of length 500,000 on a Raspberry Pi in C/C++. I have had difficulty finding any information that would be useful for determining ...
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3 votes
1 answer
273 views

Circular Convolution and FFT of power 2

I am currently trying to understand the Chirp Z Transform, and compute an FFT for a vector that has size which is not necessarily a power of 2. The key step in the Chirp Z transform to recognize that ...
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0 votes
1 answer
104 views

Generating a time domain signal using the fundamental frequencies

I currently have the power spectrum (in dB) of a signal through which I obtained the fundamental frequencies and amplitudes. My question is: how to generate a time domain signal using these amplitudes ...
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0 votes
1 answer
284 views

Swift vDSP rFFT not same as Python np.fft.rfft()

I am trying to implement real FFT in iOS, for that i am using Accelerate Framework. Here is my Code for Swift. ...
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2 votes
1 answer
377 views

Hilbert transform in the frequency domain

As I know, the Hilbert transform $$ H(x(t))=\frac{1}{\pi t}\star x(t) $$ in time domain is equal to $$ -j \operatorname{sgn}(f) \cdot X(f) $$ in frequency domain. so I tried simple example using ...
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FFTW audio artifacts when modifying magnitudes in frequency domain

I'm currently working with FFT (FFTW3) that I'm using to apply treatments on audio files frequencies. The Forward/Backward test is passed, since I can get the exact same soundfile when processing ...
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1 vote
0 answers
135 views

Sweep synthesis in frequency domain: How to correctly adjust magnitude?

I'm attempting to synthesize a logarithmic sweep signal to measure the IR of a system. For the most part, I'm following Section 5.2 of the paper Transfer Function Measurement with Sweeps. To generate ...
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1 answer
65 views

Can you use Bartlett method in reverse? [closed]

I'm wanting to do an inverse Fourier transform. Can I use the Welch method to generate this inverse, by replacing the FT used within with an IFT?
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1 vote
0 answers
302 views

Implementation of Cepstrum in Python

Actually I want to denoise a signal. I know how can I implement FFT in python to denoise it. This is the implementation which I use(From this Kaggle notebook): But I don't know : How can I use ...
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2 votes
1 answer
278 views

Sparse Inverse Fourier Transforms

I'm looking to compute an IDFT but my input is very sparse: Example: IDFT length: 7,408,800 (complex floats) Sparsity: 96.61% to 99.99% I found this Sparse FFT website but it looks like the library ...
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1 vote
2 answers
780 views

Inverse complex FFTW transform

I've been scanning all of the FFTW documentation, trying to figure out how to inverse FFT my FFT spectrum. The documentation only mentions how to inverse FFT real-to-complex transformations, using the ...
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-1 votes
1 answer
232 views

Basic upsampling of a signal

I've been learning about the (Fast) Fourier Transform and it's uses. Using the formula: $$FFT(concatenate(IFFT(signal), 0^\text{increase}))$$ I was able to go from this signal: to this: by settings $...
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