Questions tagged [ifft]
The ifft tag has no usage guidance.
226
questions
1
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1answer
37 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, [...
0
votes
2answers
51 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^{...
1
vote
3answers
95 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 ...
0
votes
0answers
13 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 ...
0
votes
1answer
21 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 ...
0
votes
1answer
56 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 ...
3
votes
1answer
128 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 ...
0
votes
1answer
46 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 ...
0
votes
1answer
55 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.
...
0
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0answers
34 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 ...
2
votes
1answer
116 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 ...
0
votes
0answers
56 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 ...
1
vote
0answers
51 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 ...
0
votes
0answers
16 views
Noise and harmonic elements w/in a discrete time signal
So I have a discrete-time signal with around 300 samples. It is part of an environmental science application, so is not-strictly periodic although there clearly are periodic components. I have applied ...
0
votes
1answer
59 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?
1
vote
0answers
93 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 ...
1
vote
1answer
182 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 ...
1
vote
2answers
213 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 ...
-1
votes
1answer
99 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 $...
0
votes
0answers
32 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:
...
1
vote
1answer
80 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 ...
0
votes
1answer
29 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 ...
0
votes
1answer
45 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 ...
2
votes
0answers
201 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 ...
2
votes
2answers
3k 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 ...
1
vote
0answers
45 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 ...
0
votes
1answer
53 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 ...
1
vote
2answers
2k 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. ...
1
vote
2answers
112 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, ...
0
votes
1answer
71 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 (...
1
vote
3answers
130 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 ...
0
votes
1answer
45 views
Hamming window in frequency domain and back
I can't figure out the scaling and slight artifact in fourth plot.
...
0
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3answers
131 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 "...
0
votes
2answers
512 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 ...
0
votes
2answers
61 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
0
votes
0answers
62 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?
0
votes
1answer
94 views
Can we use discrete cosine transform with imaginary part
I have been reading about the difference between the use of iFFT and iDCT with OFDM.
What I understood that iDCT can only be used with one phase modulated symbol which is PAM/BPSK modulation.
It ...
0
votes
2answers
450 views
What is Magnitude and Phase actually represents in Fast Fourier Transform?
When we reconstruct the image from its magnitude only using Inverse Fast Fourier Transform, then why the resulting image looks nothing like but the original image? Can someone please explain me in ...
0
votes
1answer
44 views
bandpass waveform detector?
I am hoping that somebody might have a good idea for detecting low-frequency pulses in some rather noisy data (I'm working on improving the snr). I have repurposed a "Raspberry Shake 1D" to record the ...
0
votes
1answer
154 views
MATLAB: Applying Inverse Fast Fourier Transform to an array of real numbers
I'm trying to understand the difference between applying the Discrete Cosine Transform (DCT) and the Inverse Discrete Fourier Transform (IDFT) to the log Mel-filterbank energies as explained in the ...
2
votes
2answers
144 views
how I can use the FFT data to compute the convolution?
First,I have a vector A of length N,through process
...
0
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0answers
204 views
Stolt interpolation and ifft in range migration algorithm
I am using range migration algorithm for focusing stripmap synthetic aperture radar data. I have successfully tested my algorithm using the following steps after range compression (similar to this ...
0
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0answers
46 views
How to applly Hanning window on S parameter results?
I am working on Time domain computation. I have S parameter results simulated, using these results I do the Inverse FFT to get results in time domain.
I want to filter/window results using Hanning ...
0
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0answers
83 views
Cepstrum calculation with 5-smoothing zero padding
I am trying to compute cepstrum of a signal x
An implementation of this in python is:
...
0
votes
1answer
309 views
FFT of square wave: maths and actual numbers do not coincide
this fact is driving me insane... Theory states that the Fourier transform of a square wave is a sinc function, but if I compute the fft of a synthetic (perfect) square wave of amplitude 1 and ...
0
votes
2answers
249 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 ...
2
votes
0answers
137 views
White noise generation in frequency domain (stfft) produces a periodic pattern
I have created a set of spectral processing modules for SynthEdit next to be released, implenting short time fft, so I am now quite experienced with the matter, but recently encounteted an unexpected ...
1
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0answers
115 views
Scaling issue after doing manipulations in FFT
I have two filters which must apply (in series) to same input data $x[n]$ e.g of length 256 (real time asio input buffers). One filter is e.g. an impulse response $h_1[n]$ of a speaker and other one ...
0
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1answer
61 views
0
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1answer
154 views
IFFT of a complex, conjugate-symmetric transfer function - Value at Nyquist frequency
Using the MATLAB 'symmetric' flag to compute the IFFT of a complex-valued, length-N (with N even), transfer function, the imaginary part of the complex value at the index ((N/2)+1) corresponding to ...
