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1500 questions
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2 answers

Pattern recognition for temporal data

I'm trying to detect and classify non-speech sounds. Currently, I'm using a series of moving overlapped power spectrums from training sounds as the features I am looking for. When I do analysis, I'm just computing the same amount of overlapped…
cufmo
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Does an asymmetric Bernoulli matrix satisfy the RIP?

Define an $n\times N$ sensing matrix $A$ by $A_{ij} = 0$ with probability $p$, and $A_{ij} = 1/\sqrt{n}$ with probability $1-p$. Does $A$ satisfy the restricted isometry property? For reference, the symmetric case is answered in the following…
olivia
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Sampling the Dirac function

I would like to ask a theoretical question concerning the Dirac function. The Fourier Transform of the Dirac function is the value 1 (DC) for every frequency. If we consider the Sampling Theorem, we have to find a maximum frequency in the signal $ \…
George Tseres
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How to make a signal conjugate symmetric?

Take the simple frequency-domain band-pass filtering operation below . . . NFFT = 128; x = randn(NFFT,1); H = zeros(NFFT,1); H(10:20) = 1; y = ifft(H.*fft(x), 'symmetric'); This gives a real output because I'm using the conjugate symmetric flag to…
learnvst
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How to find the color channel shifting of an RGB image that was supposed to be greyscale?

I am using a recording device that seems to shift the colors horizontally and I would like to find the amount of unshifting I have to do on 2/3 of the channels in order to obtain an image with minimal color distortion. You can see here that in the…
sorin
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Why do sinusoids have DFT magnitudes of N / 2 while we typically normalize by N?

I'm wondering why evaluating a sinusoid that matches one of the frequencies of the DFT basis functions has a magnitude of $N / 2$. Using this definition of the Discrete Fourier transform, it looks like this holds (I guess for non-zero $k$ and $N…
bluenote10
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Identify abrupt changes in an audio waveform

I have an audio file which was created by substituting multiple speech segments into an original utterance. I want to find the timestamps where such substitutions happened, so I guess that one thing I can do is looking for the abrupt changes in the…
Long
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Least Squares Solution Using the DFT vs Wiener-Hopf Equations

Least Squares channel estimation (or equalization) can be accomplished using the "Wiener Hopf Equations", or the Discrete Fourier Transform. Both appear to be least squares solutions. How do the two compare and contrast? Under what conditions would…
Dan Boschen
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Help in Retraining a HOG Descriptor using SVMLight

I am currently trying to count cars using OpenCV 2.4.4 with a HOG descriptor. Because a model for car detection is not available in OpenCV, I am creating my own model using SVMLight and a dataset of cars from the INRIA car dataset here (positive and…
bonchenko
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What is the frequency response of a filter with impulse response $h(t)=\frac{\cos(\omega_0t)}{\pi t}$?

Most of us have come across the impulse response of an ideal (continuous-time, unity gain) low pass filter with cut-off frequency $\omega_0$: $$h_{LP}(t)=\frac{\sin(\omega_0t)}{\pi t}\tag{1}$$ The question is what happens if we replace the sine in…
Matt L.
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When is it true that "the Fourier transform of the autocorrelation is the spectral density"?

I am currently struggling on the book "System Identification: Theory for the User" by Lenart Ljung (freely available here) about the definition of the spectrum (around page 33). As I have seen elsewhere, it is defined as the Fourier transform of a…
NokiYola
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How does a low-pass filter programmatically work?

I have been working on a simple low pass filter for < 100 Hz metering in my application. But so far, I am struggling with the theory behind it all. It's cool that I got it working, but I'd really enjoy it if I knew how/why it is working. I found the…
Niek van der Steen
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Surface detection

How would one segment large area's of gray (ranging from white to black) from an image ? (If you know this in opencv, you may answer by saying what you would do in opencv). For example given this picture: You see that this is a large area of gray…
Olivier_s_j
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Accurately measuring relative distance between a set of fiducials (Augmented reality application)

Let's say I have a set of 5 markers. I am trying to find the relative distances between each marker using an augmented reality framework such as ARToolkit. In my camera feed thee first 20 frames show me the first 2 markers only so I can work out the…
Jkh2
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Can humans hear Hilbert transform in audio?

I stumbled upon Hilbert transform when researching single sideband modulation. Apparently when the demodulator frequency is bit off by $\Delta f$, the signal after demodulation and low-pass filtering is not $m(t)$ where $m(t)$ is the original…
juhist
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