A linear phase channel with phase delay τp and group delay

1. τ_p = τ_g = constant
2. τ_p ∝ f and τ_g ∝ f
3. τ_p constant and τ_g ∝ f
4. τ_p ∝ f and τ_g = constant

For a linear phase channel both phase delay τ_p and group delay τ_g should be constant.