What is the main characteristic of a linear phase filter?

A) Its output is not proportional to the input.

B) It introduces constant delay across all frequencies.

C) It only allows certain frequencies to pass.

D) It cannot be implemented digitally.

Answer: B) It introduces constant delay across all frequencies.

Explanation: A linear phase filter maintains a constant delay for all frequency components, which is important for preserving the waveform shape of signals during processing.