Practice question · Multiple choice
A digital sampling system reproduces a 20 kHz tone accurately and turns a 30 kHz tone into a spurious low-frequency one. What has happened?
Hints
- Nyquist: you need more than two samples per cycle. Ask what happens with fewer.
- Think of a wagon wheel filmed at 24 fps appearing to turn backwards.
Show the answer
D. Aliasing, the sampling rate is too low to represent 30 kHz
Why
Below two samples per cycle, a high frequency produces the same sample values as a low one and is reconstructed as the impostor. It is the wagon-wheel effect in audio, and it is why every ADC has a low-pass filter in front of it, you cannot remove an alias after sampling.
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