Signal Processing Stack Exchange
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How to efficiently calculate only the low coefficients of a zero-padded FFT

I've got an algorithm that zero pads a sequence to 4N, does an FFT, and only uses the lowest frequency N points out of the generated 4N. This seems like a lot of wasted work, any ideas how this can be done faster?
Mark Borgerding
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Finding symmetric regions/patterns in image

I have a set of images representing the mean curvature of a human back surface. What I want to do is to "scan" the image for points which have similar, reflected "counterparts" in some other part of the image (most likely symmetrical to the midline,…
heltonbiker
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“The Fourier transform cannot measure two phases at the same frequency.” Why not?

I have read that the Fourier transform cannot distinguish components with the same frequency but different phase. For example, in Mathoverflow, or xrayphysics, where I got the title of my question from: "The Fourier transform cannot measure two…
Antoni Parellada
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Lower frequencies stronger in FFT?

I'm calculating FFT from microphone input. I notice that lower frequencies always seems to have more power (higher dB) than higher frequencies. I cut the data into frames of 24576 bytes (4096*6). Apply Hamming window: input[i] *= (0.54d -…
Tedd Hansen
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Why should an image be blurred using a Gaussian Kernel before downsampling?

I recently read that before downsampling an image, it should be blurred using a Gaussian Kernel. This way, the downsampled image is better than just picking a single pixel out of a NxN block or averaging over the block. After searching in this site…
Nagabhushan S N
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A basic question about the use of moving average vs low-pass filters in DSP

I'm newbie novice in digital signal processing. If one has a signal with some noise superimposed in time series, for which type of information/analysis one would use moving average or LP filter in practice. Imagine you have a 5 seconds of sampled…
user1245
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Discrete Wavelet Transform - Visualizing Relation between Decomposed Detail Coefficients and Signal

I'm trying to directly visualize the relation between discrete wavelet transform (DWT) detail coefficients and the original signal/its reconstruction. The goal is to show their relation in an intuitive way. I would like to ask (see questions below):…
geekoverdose
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what is better: up- or downsampling?

I want to compare two signals or curves. Unfortunately they have different sample rates. The first one is sampled at 30 Hz, the second at 2000 Hz. Matlab has the function 'resample' and I thought, this would make comparison much easier. My…
NelsonOrange
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How to detect corners in a binary images with OpenGL?

I have binary 160x120 images such as: I would like to detect corners of those white blobs. They are previously closed by mathematical morphology so there shouldn't have any inner corners. In this specific case, I would want 16 corners, like: My…
Stéphane Péchard
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Why does this Moiré pattern look like this?

I was making some gifs of Mobius transformations in Matlab, and some strange patterns began to appear. I'm not sure if a deeper knowledge of the filetype/algorithm is needed to understand this phenomenon, but I thought that there could perhaps be a…
B H
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Filter Order Rule of Thumb

Is there a rule of thumb for calculating the approximate filter order of a FIR/IIR based on passband vs sample rate? I seem to remember reading about it once in Undergrad. I'd think you could make a rough estimate like scaler * passband/stopband. I…
jeremy
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Image segmentation issue of different materials

Hi CV/Pattern Recognition Community, I've got a serious problem regarding the segmentation of an image. The scenario is an atmosphere within a furnace which makes my head go insane. And I need to detect object contours of different materials (glass,…
mchlfchr
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What are the relative merits of various upsampling schemes?

I recently encountered a DSP system which did some internal upsampling via zero padding. Expecting zero-order-hold, I was surprised to find that a DC signal did not produce a DC output; many harmonics of the internal (lower) sampling frequency were…
nibot
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What are the units of my data after an FFT?

Magnetometer measures the derivative of the magnetic field, or dB/dt, with an output in microvolts (mV). The Sampling rate is 128 Hz, so if we collect data for 2 minutes, $2 \times 60 \times 128=15360$ points (discrete case). When I perform an FFT…
MikiBelavista
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Most efficient way to extract frequency components of audio signal with near-human accuracy in real time

I'm trying to work out how (if possible) to extract the frequency components of an arbitrary audio sample (typically music) in an FFT-like manner, but in my research on the FFT algorithm, I'm learning that it suffers some severe restrictions for…
bryhoyt
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