Signal processingLesson 3 of 4 in the course Sampling

Aliasing: Sampling Ambiguity, Nyquist Limits, and Anti-Aliasing

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

Aliasing occurs when discrete samples make a signal appear to contain a frequency or pattern that was not actually present. This lesson introduces the basic idea and shows how aliasing appears in audio, video, and images.

A continuous signal can vary smoothly over time or space. Sampling records only selected points—for example, audio samples taken at regular time intervals or pixels arranged on a camera sensor. A reconstruction system uses those samples to estimate the original signal.

All flashcards

What is aliasing?
Aliasing is ambiguity or distortion in which sampled or reconstructed data appears to represent a different signal or pattern from the original.
What is an alias?
An alias is a misleading frequency or signal interpretation that is consistent with the samples, even though it may not be the original component.
How do temporal and spatial aliasing differ?
Temporal aliasing results from sampling over time, as in audio or video. Spatial aliasing results from sampling across space, as in a digital image.
What is the wagon-wheel effect?
It is a temporal aliasing artifact in which a rotating wheel appears to move more slowly than it does or to rotate backward in filmed footage.
What is a moiré pattern?
A moiré pattern is an unintended visual pattern, often bands or ripples, that can appear when fine repeated details are inadequately represented by image pixels.

Quiz questions

  1. For a sampling frequency of 48,00048{,}000 samples per second, what is the Nyquist frequency?Answer: 24,00024{,}000 Hz
  2. A real sinusoid at 17,00017{,}000 Hz is sampled at 32,00032{,}000 samples per second. What is its lower-frequency alias?Answer: 15,00015{,}000 Hz
  3. What is the purpose of a low-pass anti-aliasing filter placed before a sampler?Answer: To attenuate frequencies above the intended Nyquist limit before sampling.
  4. Why can reconstruction filtering not restore information lost through aliasing?Answer: Different original signals can produce identical samples, leaving the original components indistinguishable.
  5. Why might a system deliberately undersample a suitably chosen bandpass signal?Answer: Its high-frequency content can appear in a lower alias band for more efficient processing.

Based on the Wikipedia article Aliasing, licensed CC BY-SA 4.0. Adapted into a summary, flashcards and quizzes.

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