Abstract: A robust underwater communication system, for communication between an acoustic signal transmitter and a remotely positioned acoustic signal receiver, wherein transmitted data is carried by a plurality of symbols each having two components one of which comprises a distinctive bit code and the other of which appertains to the character of the symbol as a whole wherein the character of successive symbols is stepped through a predetermined continuously repeating sequence of distinctive steps each of which occurs once in the sequence, the signal receiver being operated synchronously with the signal transmitter and comprising a correlator arrangement responsive both to the bit code and to the character of received signals for effecting demodulation and having a plurality of outputs one for each symbol, so that as each symbol is detected a signal on the output to which it corresponds pre-dominates and amplitude detector means responsive to the outputs from the correlator arrangement for providing an output signal c

Abstract: A disk drive that protects stored data from stray magnetic fields is disclosed. Sensors for sensing stray magnetic fields are mounted to the base and/or cover of the drive and aligned with the trajectory of the movement of the read/write head. If the sensors, which have memory such as a toggle switch, are exposed to a high stray magnetic field, operation of the drive is suspended until a follow-up algorithm or protocol is implemented. The sensors can sense specific direction of stray fields. The sensor can change resistance if a high stray field is detected. The information in the sensors is collected and processed by the drive controller.