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Questions tagged [ifft]

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19
votes
4answers
22k views

Reconstruction of audio signal from Spectrogram

I have a set of songs for which I extracted the magnitude spectrogram using a Hamming Window with 50% overlap. After extracting the spectrogram, I did some dimensionality reduction using Principal ...
13
votes
2answers
24k views

Intuitive explanation of cross-correlation in frequency domain

According to the cross-correlation theorem : the cross-correlation between two signals is equal to the product of fourier transform of one signal multiplied by complex conjugate of fourier transform ...
10
votes
3answers
2k views

MATLAB: $\tt fft$ and $\tt ifft$ scaling

In MATLAB, the outputs of the fft and/or ifft functions often require additional processing before being considered for analysis....
8
votes
3answers
23k views

How to make a signal conjugate symmetric?

Take the simple frequency-domain band-pass filtering operation below . . . ...
7
votes
1answer
889 views

Does our brain constantly ifft to hear?

From my understanding our ears have hairs/cilia in the cochlea that resonate at frequencies within our hearing range. To me this means we are hearing in the frequency domain as opposed to the time ...
6
votes
2answers
446 views

Efficient calculation of correlation function every $N^{\rm th}$ point

I would like to calculate a long correlation function of length say, 1e6 points. I have a prior knowledge that the correlation peak will be in point k*1000. Is there an efficient way to apply this ...
6
votes
2answers
3k views

Filter size vs. FFT size and Overlap add

First of all I would like to say that : I'm new to DSP and in a "learning curve" hence the "basicness" of my questions. I've read as many posts/articles as possible on the internet but still need "...
4
votes
1answer
17k views

Cross correlation with FFT and fftshift

When I calculate the cross correlation between two vectors with the following code ...
4
votes
2answers
844 views

Alamouti code after or before IFFT block in OFDM

We are trying to implement Alamouti STBC on underlying ofdm signal. We were thinking as to where to write alamouti coder block. I thought we could write it after the cyclic prefixer block as we get a ...
4
votes
1answer
262 views

Zero padding effect on a FFT of gaussian noise

I have a gaussian noise $\nu(t)$ with variance $\sigma^2$. After a FFT I get $X(\omega)$. If now I do the IFFT on the $X(\omega)$ can I say that the result is still a gaussian noise of variance $\...
4
votes
1answer
70 views

Multidimensional FFT with mixed directions

Let's say I have three-dimensional complex array $X[m,n,p]$, $m \in \{0, 1, \ldots, M-1\}$, $n \in \{0, 1, \ldots, N-1\}$, $p \in \{0, 1, \ldots, P-1\}$; i.e. $X \in \mathbb C^{M \times N \times P}$. ...
4
votes
1answer
2k views

Inverse Fourier transform - porting from MATLAB to Python

I am porting a script from MATLAB to Python, but I am failing when it comes to the inverse Fourier transform. The function in MATLAB (ifft) includes a 'symflag', which treats the data as conjugate ...
4
votes
3answers
1k views

Phase measurement

I have a problem with phase measuring. I'm acquiring two signal with a USRP (complex signals) with a coherent generator and I want to measure the phase different between them.. One is at 150Mhz and ...
4
votes
2answers
662 views

What assumptions should be used to invert spectrum into time domain data?

This question comes from a realistic problem that how to obtain a time domain wind pressure from known wind spectrum. Or similarly, gets rail surface curve from spatial rail spectrum. Obviously, ...
3
votes
4answers
9k views

Is it possible to apply inverse Fourier transform (I-FFT) to the following image?

Suppose, I have this image in my hand and nothing else. Clearly, this is a Magnitude-plot of some unknown image. Is it possible to apply an Inverse Fast Fourier Transform (I-FFT) operation to recover ...
3
votes
1answer
1k views

How does the number of frequency-domain sampling points influence the outcome of an inverse FFT?

Given some frequency-domain representation of an impulse response. How does the number of frequency-domain sampling points influence the outcome of an inverse FFT, if I keep the sampling frequency ...
3
votes
3answers
724 views

Calculate ifft using only REAL forward FFT

There are already questions on this topic here and here. I am trying to calculate the ifft using a DSP that only has a library function for a forward REAL (assumes conjugate symmetry) fft. For a given ...
3
votes
1answer
414 views

Is it possible to reconstruct an image using its magnitude only?

Let us take any image like Lena or cameraman. I know that when Fourier transform is applied on image,spatially global features like amplitude and phase are obtained locally. My question is whether it ...
3
votes
2answers
474 views

What is the effect of the natural logarithm in the frequency domain?

I have read that taking the natural logarithm of a signal $x$ in the time domain will introduce a spectral broadening and hence "up-sampling" process before the logarithm is required. I tried to test ...
3
votes
1answer
558 views

Matlab - Constructing a Cosine wave with IFFT

My apology if following question is too simplistic to be asked but I have the following code which generates a nice sine wave for me: ...
3
votes
1answer
2k views

Implementing Frequency Domain Convolution in MATLAB (Convolution Theorem)

I'm trying to prove convolution in time domain is same as multiplication in frequency domain but I'm not getting the same answer in matlab. Here is the code:
3
votes
1answer
691 views

Spectral centroid manipulations

So, I've created a simple sound analysis application and one of the features I've implemented is the spectral centroid (as explained here https://en.wikipedia.org/wiki/Spectral_centroid). In order to ...
3
votes
2answers
2k views

How to transmit the real+imaginary output of an IFFT over a transmission channel?

I'm working on a modem system using a 16-QAM scheme. The transmitter runs an IFFT to modulate the signal, and has a 12-bit DAC. The receiver runs an FFT to demodulate the signal, and has a 12-bit ADC. ...
3
votes
1answer
118 views

Is it possible to approximate given skewness and kurtosis values using an IFFT?

I have time domain data for a signal that behaves randomly but has non-random (non-Gaussian?) skewness and kurtosis values of ~0.9 and ~7.4, respectively. The FFT of the signal shows that it's not ...
3
votes
1answer
174 views

Accuracy of IFFT(FFT(X)) unit transformation

I want to use FFTs to do seriously accurate interpolation on band-limited data. To do that, I need to get a handle on the fundamental accuracy of the FFT() and IFFT() algorithms available. My idea ...
3
votes
0answers
182 views

Down sampling in the frequency domain

I would like to down sample a signal from it's original sampling rate of 2,000,000 samples per second to 48,000 (or 44100) starting from the output of an FFT. This FFT delivers 16384 data points (...
2
votes
2answers
1k views

Number of samples in one period of subcarrier after IFFT

I have a quick question about the number of samples in one period of a sub carrier in OFDM systems. The IFFT process generates N-parallel sub carriers in discrete time domain which are sent ...
2
votes
3answers
2k views

ifft of signal without phase information

I have a chemical spectrum with amplitude on the Y axis and wavenumbers on the X axis, sampling interval I think this is $1.5^{...
2
votes
2answers
866 views

OFDM modulator in LTE

My understanding of what an OFDM modulator does, is that it takes a parallel sequence of QAM symbols of size M, pads it with zeroes corresponding to the zero subcarriers, then passes them through an ...
2
votes
2answers
41 views

Why there is no $dxdy$ and $\Delta k_x \Delta k_y$ in MATLAB's $\tt fft2()$ & $\tt ifft2()$ functions?

In analytical 2D Fourier Transform, integrand is multiplied by $dxdy$. In algorithm of MATLAB function fft2(), this is not done. In discrete case, $\Delta x = Lx/...
2
votes
2answers
5k views

Implementing inverse FFT using forward FFT

I am trying to implement an inverse FFT using the forward FFT. For clarity: Let S[t] be a signal in time, and S[w] the transformed signal. As per this site, it seems one can reverse S[w], use the ...
2
votes
1answer
64 views

What is the proper way to filter out unwanted noise from accelerometers?

I'm a beginner in signal processing and have a hard time to filter out environmental noise from the floor vibration data collected using accelerometers. I have tried using butterworth filter (IIR) ...
2
votes
2answers
220 views

image type after an ifft reconstruction

I reconstruct images from MRI k-space using ifft and root-sum-of-squares method. ...
2
votes
3answers
1k views

Reflecting negative frequency to positive frequency

I am trying to synthesize sinusoids by using window function in the frequency domain. It involves: In frequency domain, shift the window to center around the peak frequency To generate a DFT frame, ...
2
votes
1answer
2k views

Upsampling and Downsampling using IFFT and FFT

Assume in OFDM , there are N=64 subcarriers. I would like to upsample my signal by Factor 8 after IFFT and pass it through channel then downsample it by factor 8 before FFT. I can use upsample, ...
2
votes
1answer
1k views

Filter - Spatial Domain Versus Frequency Domain

Note: This is a followup question of How can I create a custom band-pass filter?. Can you explain the following python source code?. Bandpass filter bank. Suppose, I have an equation ...
2
votes
1answer
8k views

Delete frequency components with FFT and IFFT [duplicate]

I have a signal in Matlab defined by signal and t. As it is a noisy signal, I want to delete every component below a certain ...
2
votes
2answers
839 views

Determining channel frequency response from measured data via IFFT

I've been working on a small system that transmits a linear chirp over the 35Hz-20kHz frequency range in a room and simultaneously records the transmitted signal with echoes (from the floor, walls, ...
2
votes
1answer
2k views

Fourier transform of an image

I am doing a project where I need to take the FFT and IFFT of a photo in MATLAB. There is a principle in optics where the intensity distribution from a lens is equal to the Fourier transform of the ...
2
votes
2answers
927 views

Frequency Domain Filtering with big kernel size

I use FFT to do filtering in frequency domain, but when I use big kernel I got shift on border, I think it's due to nature of cyclical convolution and I need to do more zero padding. But how to ...
2
votes
2answers
910 views

Is magnitude information enough to reconstruct an audio signal

I have used an MMSE STSA estimator to obtain the magnitude of an audio signal. The original signal is combined with white noise and I used an algorithm given in an old research paper by Ephraim and ...
2
votes
0answers
81 views

Correct way of derivating in frequency domain with FFT

I believe I am very close to the answer and only need a small nudge to get to the answer. What I want: I want to take a signal, use FFT to transform it to the frequency domain (FD), multiply it by $...
2
votes
0answers
199 views

How to iplement fast DCT-III (for inverse of DCT-II) using FFT or iFFT

I have to implement DCT-II and its inverse which is equivalently given by a forward DCT-III. All from real to real. I have been checking some paper where it is specified how to do it. However, I ...
2
votes
3answers
1k views

How do you save/convert an image to be in the frequency domain?

Quick Note: I don't believe this is a subjective question... however if it is I will gladly modify to be more objective In ImageJ, if I take the FFT of an image, how can I save this new frequency ...
2
votes
0answers
91 views

Signal Relationships and Conversion: Transforming one Signal into another?

I'm trying to relate a near shore tidal signal (point A) to 3 points along a long model boundary (points B C D). I want to possibly have a relationship between B C D with which we can convert A ...
1
vote
3answers
639 views

Extrapolation after taking inverse Fourier transform

Suppose I have 2000 point(time series) signal and I take 512 point FFT, after this some operation is done in frequency domain. Now if I take IFFT to get 512 points in time domain, Is there a way to ...
1
vote
1answer
401 views

How the increasing of sampling frequency in OFDM didn't cause increase on the required channel bandwidth

In OFDM, how the increasing of sampling frequency didn't cause increase on the required channel bandwidth (fs>>BW), as i know sampling frequency means number of samples per second, if this number of ...
1
vote
1answer
572 views

Why remove energy at Nyquist before ifft?

I've heard that you should remove the energy at Nyquist before performing the ifft. Why is it that you can't leave the Nyquist energy there, is there danger of aliasing somehow?
1
vote
1answer
1k views

How does MATLAB recover picture from magnitude spectrum alone?

This is the transformation I did. The code fft2() the Lena picture than ifft2() it back to the original. Add some ...
1
vote
1answer
4k views

Difference between frequency sampling and windowing method

These two filter design methods seem fairly similar. Both ask to determine a frequency response and then take a inverse DFT to get the filter coefficients. So what's the difference?