Aliasing occurs when the sampling frequency is not greater than twice the frequency of the input signal.

• A. True
• B. False
• C. Only in color images
• D. Only with low-resolution sensors

Answer: (A)

Aliasing happens when you don’t sample fast enough to capture the highest frequency present in the signal. The practical rule is the Nyquist criterion: to reconstruct a signal without distortion, the sampling rate must be more than twice the maximum frequency component. If the sampling rate is not greater than twice that frequency—that is, it’s 2f_input or less—those higher-frequency components can’t be distinguished from lower frequencies and end up “folding” into the spectrum, creating misleading or distorted results.

So the statement is true. If you sample at or below twice the input frequency, aliasing can occur. For example, a 60 Hz tone sampled at 100 Hz falls below twice its frequency (the Nyquist rate would be 120 Hz for 60 Hz), so the high-frequency content can alias into the observed signal. Even at the exact 2x boundary, real-world non-idealities mean aliasing is still a risk, so designers typically aim for a sampling rate well above twice the highest frequency.