Two types of echo exist, acoustic and electrical:

  • Acoustic when the received voice is caught by the microphone and sent back to the reciever
  • Electrical is when there is an inconsistancy in the telephone circuits, called impedence mismatch
Echo suppression - transmitting speak in the forward direction, whilst preventing it in the return direction, the return path is broken to prevent feedback.

Echo cancellation is much more sophisticated, it analyses the transmitted voice and subtracts it in the return path. Rather than stopping all return transmission, it removes specific parts.