# Questions tagged [ifft]

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### 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 ...
279 views

### How to find correlation between 3 signals

I have 3 signals, S1 S2 S3, with a same time domain and same time steps. I'm wondering if there is any correlation between these 3 signals. more specifically I want to know if it is possible to make ...
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 ...
2k 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 ...
644 views

### signal analysis and synthesis with different window length and FFT length

I'm coding an analysis/synthesis system on MATLAB. I was able to recover the original signal when the length of the window (winLen) is set equal to the length of ...
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 ...
279 views

### Confused about DFT symmetric behavior for real-valued signals?

I have a simple and might be a preliminary question about symmetric property of DFT of real valued signals. From definition we know, DFT coefficients of a real-valued signal is conjugate symmetric, so ...
887 views

### What would be the outcome of the convolution of two images? [closed]

Lena Mask I have written a C# code which produces the following output. Is my output correct? Am I padding in the correct way?
97 views

### Is it possible to Denormalize a matrix of complex numbers?

In this article, FFT and I-FFT are achieved as the following: FFT steps process starts with a Bitmap image. Bitmap-image to Integer-image Integer-image to Complex-image Apply Forward-FFT to Complex-...
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 ...
3k views

### Cross-correlation and auto-correlation of an image using $\tt fft$

I'm working on a project where I have to find the auto-correlation and cross- correlation of types of pixels in an image: I am using MATLAB and my code for the autocorrelation is: ...
3k 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....
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?
1k views

### MATLAB center frequency shift

I am wondering how to shift the center frequency of the below complex data from 0, to say 20 MHz (10 MHz to 30 MHz- 20 MHz bandwidth). I tried to change the indexing of the bins but I could not get ...
395 views

### Splitting a magnitude across multiple bins in an IFFT

By Georg-Johann own work, CC BY-SA 3.0. I’m trying to synthesise a sine wave using an IFFT at a frequency between bin centre frequencies. Here are the details ...
749 views

### Sine Wave Aliasing during IFFT

I’’m synthesising a simple sine wave using the IFFT. I’m controlling the pitch of the sine wave by controlling the bin number and the phase for that bin number. I’ve synthesised an F whose ...
2k views

### 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.
292 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 ...
620 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 ...
247 views

### IFFT-based OFDM system

Please explain, mathematically, how can a bank of modulators be replaced with the IFFT in an OFDM system?
346 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 ...
227 views

### image type after an ifft reconstruction

I reconstruct images from MRI k-space using ifft and root-sum-of-squares method. ...
678 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 ...
450 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 ...
527 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 ...
180 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 ...
889 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, ...
277 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 (...
606 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: ...
312 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 ...
690 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 ...
151 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-...
237 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 ...
651 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 ...
306 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 ...
84 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?
468 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,...
940 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 ...
276 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, ...
880 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 ...