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Define LTI system and impulse response

Short answer

LTI system obeys superposition and time invariance; output is convolution y(t)=x(t)*h(t). Impuse response h(t) is the output for a unit impulse δ(t) with zero initial conditions, fully characterizing the system; e.g., RC low-pass: h(t)=(1/RC)e^{-t/RC}u(t).

Define LTI system and impulse response.

LTI system: a system that is both linear (it obeys superposition: additivity and homogeneity) and time-invariant (a shift in the input produces the same shift in the output). Its output for any input can be found by convolution: y(t) = x(t) * h(t) = ∫ x(τ) h(t - τ) dτ. Examples: RC filters and ideal amplifiers.

Impulse response h(t): the output of a system when the input is a unit impulse δ(t) with zero initial conditions. It completely describes an LTI system: causality requires h(t) = 0 for t < 0, and stability requires ∫|h(t)| dt < ∞. For an RC low-pass filter, h(t) = (1/RC) e^(-t/RC) u(t).

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