Questions tagged [aliasing]

In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable (or aliases of one another) when sampled.

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12
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4answers
2k views

Is there such a thing as band-limited non-linear distortion?

So if you generate a square wave by just switching a signal between two values, at sample boundaries, it produces an infinite series of harmonics, which alias and produce tones below your fundamental, ...
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1answer
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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 ...
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322 views

Increasing Image Resolution

I know of some oscilloscopes (DSA8300) that repeatedly sample at a few hundred kS/s to reconstruct a few GHz signal. I was wondering if this could be extended to 2D signals (photographs). Can I take a ...
10
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1answer
687 views

What is the antialiasing algorithm employed by video cards?

What is the algorithm employed by video cards when one talks about for example 8xAA? I thought the algorithm was about looking at neighboring pixels for correction. ...
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3answers
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Alias frequency Formula

I'm taking a multimedia systems class in my MSc Computer Science, and I'm having some trouble understanding the formula for the alias frequency - this could stem from my misunderstanding of the alias ...
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What is the effect of aliasing on the magnitude of the autocorrelation?

I've a question about the effect of aliasing on the magnitude of autocorrelations. From a simulation in MATLAB, I don't see any effect of aliasing or any need to anti-alias filter when I take the ...
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3answers
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Demonstrating the effect of aliasing

How does the signal look when we don't use the Nyquist rate to remove aliasing from a signal during sampling? Let's suppose the signal is sinusoidal, with a frequency of 500 Hz and an amplitude ...
6
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2answers
848 views

Why is aliasing inherently non-linear?

E.g. when having 2 signals $x(t)$ and $y(t)$ and transforming each one with adding their spectra, the operation is linear, as the result would be the same as the transformation of $(x + y)(t)$. Even ...
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1answer
302 views

Can addition of two band limited signals create aliasing?

Can mixing/adding two band-limited signals create any frequencies above Nyquist?
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What's the impact of aliasing in the time domain?

I've been studying digital audio and come across something I can't understand. There appears to be something like a consensus (among those capable of understanding such things) that the impact of ...
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2answers
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Determine maximum frequency of input signal to make system LTI

This is question 4.26 from the third edition of Alan Oppenheim's textbook "Discrete-time Signal Processing". It is stated as follows: Firstly, the answer is the input signal should be bandlimited to ...
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2answers
728 views

Spectral images - How do they appear during ADC?

Why is it that spectral images appear? I know they appear for $F_s$, but why those replicas appear, I don't know. E.g., sampling $x(t) = sin(1000 * 2\pi t)$ with a sample frequency of 230 Hz. I know ...
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2answers
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Why is a square wave aliased?

I understand an ideal analog square wave contains sine-wave-components above Nyquist frequency (and towards infinity) and is thus subject to aliasing. So obviously we need an anti-aliasing-filter. Or ...
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1answer
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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 ...
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1answer
643 views

Illustration of time domain aliasing cancellation (TDAC, MDCT, lapped transform)

Can someone find or make a visual illustration of how time domain aliasing cancellation works in lapped transforms such as the MDCT? I think a graphic would make it a lot easier to understand, and I ...
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1answer
424 views

how to correct minBLEP sawtooth oscillator DC bias

In my implementation of minBLEP, overlapping minBLEPs result in a DC bias as the frequency increases. This is because each minBLEP correction (i.e. the minBLEP minus the aliased step) has a DC bias. ...
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2answers
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What is the proof of the equation for the aliased frequency fa = |R*n - fs|?

R (sampling rate) fs (signal being sampled fN (the Nyquist frequency) fa (aliased frequency) I was hoping someone could explain to me why this is true with a more mathematical proof. it seems ...
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1answer
84 views

Understading sampling theorem and aliasing

A real-valued analog signal with a flat spectrum between f = 0 Hz, and fmax is sampled at a sampling frequency fs = 24 kHz. The sampled signal is then processed with an ideal lowpass filter with ...
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3answers
179 views

Soft question: Why do we need to reconstruct a signal?

I am currently reviewing the Nyquist criterion which says I need to sample the signal at 2 times the maximum frequency in order to avoid aliasing in the reconstructed signal. My question is that in ...
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2answers
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Generating a sound where end frequency is 1/2 of the start frequency

I am very very new to signals, and I am trying to generate sounds based on some parameters from a database. So, the database stores the following: startFrequency endFrequency startAmplitude ...
2
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1answer
360 views

unexplainable aliases in spectrogram

My scope has 200 MHz bandwidth and can sample at 4GS/s I'm sampling a 138 MHz RF signal at 500 M Sa/s storing 14 M points. When I plot the spectogram in matlab I get the following spectrogram(Data,...
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2answers
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How to predict the spectrum of an aliased/folded signal without looking at the FFT?

This seems like a trivial concept but I just can't get the hold of it. I have a signal with two complex sinusoids having frequencies -25MHz and 17MHz. Currently it is being sampled at 64MHz so I get ...
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3answers
957 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, ...
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2answers
249 views

Alias-free digital nonlinear filter design

@Jazzmaniac has a good answer to the question of how to design an alias-free digital nonlinear time-invariant filter here: https://dsp.stackexchange.com/a/28787/18276 Basically, according to that ...
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1answer
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Avoid Aliasing when filming a moving car

I want to film a moving car with a rolling circumference: 190,5 cm and wheel circumference: 60 cm and 5 spokes. I record with 24Hz. What can be the maximal speed of the car to avoid aliasing?
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1answer
654 views

Why do Dyadic filterbanks downsample the high pass signal portions?

I'm currently programming a dyadic filter bank and have a question. I notice in all of the visual representations: (from here (Dyadic Analysis Filter Bank)), the high pass filtered output for each ...
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1answer
228 views

Why LFO should be alias-free?

I'm really tripping about this point! I try to reason with you and explain what's my trouble. Consider a scenario with Sample Rate 44100hz and a LFO square wave that will modulate some internal ...
2
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1answer
905 views

FFT and aliasing

I have got a txt file which contains voltage and time data. When I did fourier transform using fft function in matlab I got complex numbers. I understand that the real part is the magnitude and the ...
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1answer
309 views

Aliasing due to the Convolution of Gaussian Functions

For two discrete-time sequences $f[n]$ and $g[n]$, their linear convolution $(f*g)[n]$ is also given by $$f*g = \mathcal{F}^{-1}(\mathcal{F}(f) \cdot \mathcal{F}(g)),$$ where $\mathcal{F}$ and $\...
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1answer
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Sampling of band-limited white noise

The context is communication where we have a front-end that samples a signal and a noise (but here we focus only on the noise). My goal is to determine the noise power that I should use to simulate (...
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0answers
73 views

Apply timeshift to undersampled signal after bandpass after sampling?

I have sampled my continuous signal $x(t)$ at $x[n]=x(n/f_{\rm samp})$. The signal is undersampled, and I know I will not be able to recover my original signal anymore. After sampling a bandpass is ...
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3answers
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Visualizing aliasing in Matlab

I am trying to verify for myself that aliasing actually makes it impossible to distinguish the "real" and the aliased frequency. What I have done it set the sampling rate to 20 Hz and created two sine ...
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1answer
192 views

Understanding the mathematical proof for the alias frequencies in a sampled sine wave

I'm struggling to get my head round the mathematical proof for the alias frequencies in a sampled sine wave. I understand that sampling a sine wave of frequency $f_0$ every $t_s$ seconds gives you: $...
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2answers
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Does sampling in the frequency domain cause time-domain aliasing?

Let's say I have an impulse response $h[n]$. I analyze the power spectrum of that impulse response similar to fourier transformed $h[n]$ corresponding to roughly $H[f]$. Now I compare $H[f]$ with ...
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3answers
548 views

Applying Nyquist theorem to digital vs. analog audio quality

I had a recent conversation with a friend about analog vs. digital recordings. He shared his opinion that music recorded by analog techniques is of superior sound quality to music recorded by digital ...
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1answer
295 views

Downsampling impact on complex phase

For my application, I have to downsample a bandpass complex signal which spectrum is located on the second Nyquist zone. Knowing that this processing will cause a spectrum inversion, what would be ...
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4answers
5k views

Aliasing beating frequency

As I understand aliasing is because you sample a high frequency with a low sample rate, and the sample points don't have enough information about the high frequency component. Here is an example of ...
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2answers
3k views

frequency folding due to aliasing in FFT

I am having problem understanding why negative frequencies are showing up to the right of positive frequencies in discrete complex FFT. I am not sure how to apply aliasing concept to explain such ...
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2answers
282 views

Can aliasing happen in Laplacian Pyramids?

I want to use the Laplacian pyramid framework for representing images. The The Laplacian Pyramid as a Compact Image Code's language seems to provide freedom in the choice of blurring/interpolation ...
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1answer
691 views

why Sampling with higher than nyquist frequency is causing aliasing?

fm=20000 and I am sampling following signal at 65000 Hz: ...
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2answers
886 views

Circular convolution of length L of sequences of length greater than L

I'm trying to understand how may I obtain the circular convolution of length L when the sequences I'm trying to convolve are of length greater than L. For instance this Matlab code using sequences ...
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2answers
90 views

What does it mean that one frequency aliases another?

So I was watching a video on sampling and the professor asked which frequencies will alias to 0 and which to $B$ if we are sampling at frequency $f_{s1}$. What does mean that two frequencies alias ...
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1answer
73 views

How does the earphone cable in a mobile phone pick up the FM reception?

As I understand, the RF frequency-modulated signal centered in the range of $88-108\textrm{ MHz}$, is picked by the earphone antenna and is directly sampled by the sound card adc at $48\textrm{ kHz}$ ...
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2answers
146 views

Are time series data always contain noise?

I wonder if every time series data should contain noise or not. For example I am taking the price of a ticker, say Yahoo, every hour and noting the values. Does this data contains noise or not?
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1answer
464 views

How to ensure anti-aliasing in image resampling at lower resolution?

Suppose I have a 1D image with intensities $f_i$, $i=[0,n-1]$, where $n$ is the number of pixels. I want to downsample it to have $\frac{n}{2}$ pixels (let $n$ be an even number). For each new point I ...
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1answer
290 views

Which antialiasing filter before equispaced sampling

I am using MIT-BIH arrhythmical database where I have a digital signal of 1200 Hz so 1200 samples per second. This means that analog filters have already applied to remove the frequencies over Nyquist ...
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1answer
506 views

Using Spectrogram to check whether aliasing will occur

I was wondering if the spectrogram can be used as a MATLAB tool to check whether or not aliasing will occur when the signal is downsampled from let's say, 44.1kHz to 8kHz? And if yes, how do I go ...
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1answer
37 views

Calculate aliasing of $x_a(t) = \cos{(2\pi300t)} + \cos(2\pi600t)$ when sampled with $F_s = 1000$

I'm asked to sample the signal $$x_a(t) = \cos{(2\pi300t)} + \cos(2\pi600t)$$ with sampling frequency $F_s = 1000$ and plot the magnitude spectrum for the resulting sampled signal. My thinking is ...
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1answer
99 views

Designing an experiment to show blue noise not adding noise below nyquist and not aliasing above

I read this paper from 1983 "Spectral Consequences of Photoreceptor Sampling in the Rhesus Retina". This paper talks about blue noise sampling patterns in the retina and says that: The results (...
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1answer
50 views

Help with a product of a sum of infinite series [closed]

i would like some help with this: given: $$ h[n]=2^{-n} u[n] $$ calculate the series: $$ \sum_{r=- \infty }^ \infty h[n+4r] $$