Abstract: To detect echoes on a transmit path of signals on a receive path, an echo detector stores averages of the signals on the receive path each for a period equal to or just greater than the maximum end delay for received signals to reach the transmit path, and determines the greatest magnitude of the stored signals. This greatest magnitude, reduced by the worst case echo return loss, is compared with the current signal average on the transmit path, and an echo decision is produced in dependence upon this comparison and a detection of speed signals on each of the transmit and receive paths. The echo detector is particularly useful in a digital speech interpolation transmission system.