Undersampled signals alias because high-frequency components mirror below the Nyquist frequency

Evidence

When converting between analogue and digital signals, the sampling theorem (or the Nyquist theorem) requires that the sampling rate must be at least twice as high as the highest signal component to be converted. Otherwise aliasing will occur, whereby signal components of frequency higher than the Nyquist frequency (half of the sampling rate) will be mirrored to below the Nyquist frequency, thus distorting the signal. To avoid aliasing, an A/D-converter normally includes a low-pass filter that yields enough attenuation above the Nyquist frequency. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 404

Categorization

• DEFINE: Defines the Nyquist/sampling theorem and aliasing, including the standard low-pass anti-aliasing filter remedy.
• corroborated: 2 independent passages support this claim — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 404
• XREF: connects to: Adjacent-band aliasing cancels by opposite phase when bands are unmodified; Aliasing creates signal components that never existed in the original; Perfect pattern control across the audio range is physically infeasible due to aliasing cost (+1 more)

Broader Connections

• Aliasing creates signal components that never existed in the original #corroborates (0.95)
• Array spacing beyond half a wavelength causes spatial aliasing with multiple lobes #exemplifies (0.85) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2890
• Adjacent-band aliasing cancels by opposite phase when bands are unmodified #corroborates (0.80)
• Perfect pattern control across the audio range is physically infeasible due to aliasing cost #qualifies (0.70)