1. n. [Geophysics]

The exponential rate constant (τ) that determines how quickly the output amplitude of an electrical signal that is under automatic gain control (AGC) responds to a sudden increase or decrease in input signal amplitude. Mathematically,

A_f(t) = A_i(t) + ΔA_i (1 − e^−t/τ)

where A_f is the output signal amplitude, A_i is the input signal amplitude (A_i), ΔA_i is the change in input signal amplitude and t is time. When t equals τ, the function (1 − e^−t/τ) equals (1 − 1/e) equals 0.63. Therefore, the AGC time constant (τ) is the amount of time that elapses for the output signal of AGC to reflect 63% of the change in the input signal amplitude.

See related terms: automatic gain control,  AGC