Abstract: A time segment scrambling information encoding and decoding system wherein a multiplicity of scrambling algorithms are stored in a memory such as a ROM for use during transmission via a chosen medium and a multiplicity of correlated unscrambling algorithms are stored in memory for use during reception, the transmission equipment including apparatus which at different times substantially selects one of the scrambling algorithms to rely on in transmission until a different one of the scrambling algorithms is selected, the signals transmitted to receiving apparatus including finite-duration transmission of coordinating signal components by which the receiving and descrambling apparatus is caused to choose and rely on the coordinate algorithm there stored and to keep its restorative time-shifting of segments coordinated with the arrival of the time-shifted segments produced by the algorithm then being relied on at the transmitting point.

Abstract: A receiver of ultrasonic pulses having a known pulse width and at a known spacing includes a gate which is normally conditioned to accept the ultrasonic pulses. When the first pulse is received, however, the gate is opened thereafter only when subsequent pulses should be present and is closed when no pulses are expected. Such an arrangement prevents multipath signals being interpreted by the receiver as information.