# Questions tagged [ifft]

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### Complex Conjugate [closed]

If I have a complex spectrum and I add it to its complex conjugate will I get a zero phase version of the original? Will this also reduce noise? It seems like this is simply phase cancellation.
286 views

### Division of complex signal by frequency response

I have a measured signal (time domain) which I take the FFT of. I have a measured frequency dependent sensitivity (magnitude and phase (radians) as a function of frequency) which I need to apply to ...
604 views

### How to shift a signal to the center of the frequency domain?

Let's assume I have a 1024 x 1024 matrix that represents the spatial domain. After I perform a FFT and center the frequencies, I get the a signal that looks like the following in the frequency domain....
2k views

### MRI Slice image reconstruction

I have an MRI K-Space data 320 x 320 x 256 x 8 (4D complex double) from < mridata.org > . The data represents 320 x 320 K-Space of 256 slices from 8 channels. I am trying to reconstruct images for ...
234 views

### IFFT-based OFDM system

Please explain, mathematically, how can a bank of modulators be replaced with the IFFT in an OFDM system?
338 views

### Signal signs are changing after IFFT reconstruction

I want to take FFT of my signal and reconstruct it back via IFFT. It seems the real part of odd numbered samples and image part of even numbered samples are multiplied by -1 which can be observed ...
220 views

### image type after an ifft reconstruction

I reconstruct images from MRI k-space using ifft and root-sum-of-squares method. ...
642 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 ...
418 views

### Finding a signals complimentary sound / frequency

I know about complimentary colors and how to get them, since colors are basically frequency / wavelengths. Is it possible to find a signals complimentary, additive or subtractive frequencies in an ...
498 views

### DFT Filter Banks

i have a doubt about DFT filter banks. For example I have $M=4$ channels and a filter with cut-off frequency of $0.25$ right. The prototype filter $H_0$ is a low-pass centered around $0$ ok. The ...
2k views

### FFT, padding, IFFT and plot in time domain

I performed an FFT with some zero padding and would like to transform it back to time domain. When I plot the signal it looks wrong. Is it because of the padding used in the FFT? How can I make it ...
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 ...
849 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, ...
266 views

### FFT on circularly symmetric function

I have an circularly symmetric image in Fourier space: where the 0-frequency is in the center. Now, I want to take the FFT of this image. Having numpy's FFT conventions in mind (i.e. 0 frequency at (...
562 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: ...
311 views

### Would subtracting the phases of two images be a superior difference metric than subtracting the images directly?

I'm hoping someone can sanity check this idea as I am admittedly a bit of a noob when it comes to working with FFT's. Say I have two (registered) images of two of the same object and want to use one ...
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 ...
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 ...
669 views

### How do you construct common waveforms (square, sawtooth, triangle, noise) from sine wave partials using an IFFT?

In the lovely image below we see the addition of sine waves making a square wave. I am coding a wave table synthesiser and thought it would be great to get the user to be able to fiddle with the ...
144 views

### Lost power while using FFT with apodizing mask

Previously I asked this question about FFT artifacts. The solution that is most easily implemented is using an apodizing mask on the original image, tapering the borders to zero making the image left-...
236 views

### Phase Issue in IFFT

Hi all, I've been trying to zero centre a waveform prior to IFFT. my recent attempts are based on the following code which is producing the waveform below. I don't understand why this isn't ...
92 views

### Zero Centering Audio for Forward FFT

I'm trying to work out the best way to zero centre an audio clip for a forward FFT. I've been trying rotate my audio so that a peak exists mid way (N/2). This seam a little naive and I thought I ...
624 views

### Subtracting one FFT from another for noise reduction

I am using an FFT/iFFT library in c#, and working with arrays of audio & complex data. What I am hoping to do is take a small amount of 'background' audio signal, and use it as a fingerprint to ...
288 views

### sound FFT data to PCM data

I have some FFT data, 257 dimensions, every 10 ms, with 121 frames, i.e. 1.21 secs. I think the first dimension is probably something else and the remaining are the FFT coefficients, I guess. From ...
83 views

### Do all Harmonics of a periodic sound start from a 0 base

I've been thinking about additive synthesis and I'm wondering if all sounds begin at (theoretically) phase 0?
450 views

### Zero phase in IFFT

I'm trying to synthesise a sine waveform and I'm noticing that if I perform the IFFT with the Sine aligned to a bin frequency the phase is zeroed, however if I tune the sine off of the bin frequency ...
3k views

### What is the meaning of imaginary values in the time domain?

Say I take a real valued function in the time domain, take the Fourier transform to get its frequency domain representation, do some work on it in the frequency domain, which leads to imaginary values,...
893 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 ...
270 views

### IFFT Synthesis from an empty spectrum

I'm trying to generate a sawtooth wave using the IFFT. I can do this successfully by first performing an FFT and then putting that analysed spectrum into the IFFT, however I would really like to be ...
69 views

### Conditions on $v \in \mathbb{C}^{n}$ such that the complex to real DFT is sensible

I'm using FFTW to compute some DFTs, and as a memory space optimization, you can give it a $v \in \mathbb{R}^{n}$ and it will return a $\hat{v} \in \mathbb{C}^{\lfloor n/2 \rfloor + 1}$ via the call <...
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, ...
859 views

### PSS convertion from Frequency to time domain

Hi I have a question regarding primary sync signal in LTE. It is generated in frequency domain and its length is 63. How to convert this signals frim frequency to time domain. Should I use IFFT? what ...
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 ...
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 ...
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 ...
184 views

### How does the OFDM system receiver decide the OFDM symbol length? [duplicate]

I am trying to understand how the receiver decides the length of OFDM symbol in time domain before it goes into DFT process at the receiver. The complete OFDM symbol (with Cyclic Prefix added) is ...
575 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?
118 views

### Audio output distorted with any FFT Windowing other than rectangular

For an experiment i have a simple application that takes a quadrature signal from a radio and then does the following: Complex FFT -> Inverse FFT -> Cartesian to Polar -> audio out If I tune the radio ...
655 views

### filtering in frequency domain

I am performing decorrelation of two channels of audio using all pass filtering. The two all pass filters for two channels are of very high order (>1000 but with not all coefficients non zero). I have ...
350 views

### How do I zero pad time samples if I only have frequency samples?

Related to this question: Convolution in frequency space with fft and ifft -> num output samples? When doing convolution in time domain by doing multiplication in frequency domain, you apparently ...
365 views

### What interesting audio can be synthesized with ifft

I'm working on a presentation on audio synthesis for some fellow software engineers. I want to include some "fourier synthesis" creating time domain samples from frequency domain samples using ifft. ...
659 views

### FMCW SAR chirp return from target modelling

I have to Model a chirp signal that was return from a Distance D from target and then multiply with the reference transmitted signal. Which will give me beat frequency signal and then fft to get the ...
947 views

### Real time fft/ifft processing

when manipulating a live stream of audio data in the frequency domain, I've heard you need to do fft / ifft in small chunks, say 256 samples at a time. I've also heard you need to somehow employ a ...
478 views

### Convolution in frequency space with fft and ifft -> num output samples?

Let's say you had two 1 second sound samples at 44.1khz, and you wanted to convolve them in frequency space. You FFT each one to get two arrays of complex numbers that each have 44100 items in them. ...
362 views

### Fourier Synthesis With Varying Frequencies Over Time

I'm doing some fourier synthesis by making bins of complex numbers, filling them out with the amplitude and phase information and using IFFT to turn that into time domain samples. I'm able to make ...
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 ...
455 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 ...
156 views

### Microphone Array null steering unexpected results after IFFT

I am writing an open source microphone array simulation program:SharpEar. When I give uniform weight to microphone array elements, the result in the angle is a sinc function as expected. There are 41 ...