Signal Processing

Questions tagged [ifft]

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18 views

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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15 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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1answer
49 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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1answer
40 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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1answer
39 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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1answer
120 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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30 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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39 views

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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2answers
59 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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2answers
114 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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2answers
76 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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1answer
47 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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1answer
277 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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1answer
30 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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43 views

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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1answer
44 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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2answers
168 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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3answers
608 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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20 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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1answer
57 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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1answer
195 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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1answer
173 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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1answer
82 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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1answer
176 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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38 views

Extrapolation of band-limited frequency data

I'm now studying "How to do accurate transform from band-limited frequency data specifically S-parameter data of high speed interconnect to the causal impulse response in time domain. There are ...
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1answer
259 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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106 views

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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91 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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1answer
63 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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188 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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1answer
224 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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2answers
455 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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1answer
147 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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36 views

Does this FFT algorithm give correct IFFT when the input does not contain symmetry?

This FFT function is from Numerical Recipies in C and I am using it in my java projects. The source code is given below: ...
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1answer
126 views

calculate fft convolution for an LTI system given its input (in time domain) and its poles and zeros

I have considered taking the fft (MatLab) of the input (x) and create frequency bins for the first half (problems with an odd number of samples, tried solution zero paddings) and evaluate the transfer ...
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1answer
33 views

how twiddles are calculated in the code snippet below from ti.com?

I am trying to port a C code for fft/ifft to a DSP (TMDSEVM6678LE) from ti.com. In one part of the code it tries to calculate the twiddle coefficients as you can see below; however, I can't understand ...
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1answer
106 views

Construct IFFT from only the major frequencies of an input signal?

I have data which when plotted looks like the green line in the below image. The data is from the walking-motion of a reinforcement-learning-model in a simulation and therefore the values do not ...
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0answers
451 views

How to take the inverse FFT of Welch's power spectral density in Python?

I have calculated the power spectral density of a weakly-stationary signal using Welch's method (as implemented by scipy.signal.welch in Python), and am looking to take the inverse Fourier transform ...
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2answers
4k views

FFT showing peaks at 0Hz?

so I've been attempting to do a FFT on some data and I'm seeing a peak at 0Hz, which I can't really comprehend why since I'm quite new to signal processing. I read in my file using pandas, saved them ...
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47 views

Pitch correction using Fast Foruier Transform [duplicate]

I am very new to digital signal processing and I have a project about audio programming. For the project, I wanted to try to make an Auto-tune like effect. I started by experimenting on the signal ...
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1answer
74 views

Circular convolution of real signal N by N/2 sized complex IFFT

There are two related questions with good answers; for calculating the forward FFT in N/2 values, and calculating inverse FFT from N real values, now I'm in search for the missing link of packing the ...
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2answers
5k views

Am I supposed to normalize FFT in Python?

I'm sorry if this question is posed often, I can't seem to understand the answers already out there. I'm working with several forms of the Fourier transform, including the FFT, PSD, and spectrograms. ...
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2answers
126 views

Fourier Transform of sampled reflectance measurements

Goal: Calculate the complex index of refraction ($\hat n = n +jk)$ from reflectance measurements. Data: Reflectance measurements for various materials were taken with a FTIR (Nicolet™ iS50 FTIR, ...
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1answer
81 views

On the use of Permutation matrix to perform iFFT

I have a question about using the permutation matrix for performing $iFFT$ for such matrix and then reshape it row-wise and column-wise way. Let's say that we have a random matrix $x$ whose size is (...
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3answers
173 views

Frequency domain samples to time domain without knowing the sampling rate?

I have some frequency response data from 802.11a OFDM communication in channel 8 of the 5 GHz band, and I would like to inverse transform this to produce the corresponding time domain response. That ...
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1answer
61 views

Hamming window in frequency domain and back

I can't figure out the scaling and slight artifact in fourth plot. ...
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3answers
164 views

Windowed N-point FFT

In the previous question, Smoothing in frequency domain and impulse reponse of the filter in time-domain I guess I have made a mistake in-terms for applying windowing to FFT. After reading "...
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2answers
1k views

Converting PSD to Time Domain Signal

I am currently using MATLAB and I'm thinking if I can reconstruct PSD back to time domain signal using MATLAB's ifft. I found a link in solving this problem but it's in Phyton and when I tried it in ...
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2answers
70 views

Is there any other method to get the DFT of a signal without actually computing it using normal calculation

Is there any other method to get the DFT of a signal without actually computing it using a normal calculation. Please help me out in this query
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66 views

If you are given an inverse DFT, then how do you convert it back to the DFT without actually computing any DFT?

If you are given an inverse DFT, then how do you convert it back to the DFT without actually computing any DFT?

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