• TECHNICAL FACILITIES FOR ENSURING SECURITY

    Automatic gain control in an underwater acoustic receiver

    Security & Future, Vol. 5 (2021), Issue 3, pg(s) 112-114

    The performance of two automatic gain control circuits is compared in an underwater acoustic receiver: a low-power hardware gain control and a software-controlled gain control that disables the gain change for a predetermined time interval after a valid signal level. Binary digits “0” and “1” are transmitted using bursts of the frequencies 33kHz and 43kHz, respectively, in two modes of operation. In the first mode, each digit is sent as one frequency burst followed by a pause. In the second mode, a frequency burst is transmitted at the beginning of a clock interval. Experiments are conducted in shallow waters. The results show that the hardware gain control circuit is suitable to equal time intervals between the transmitted bursts. The software gain control circuit allows arbitrary time intervals between the transmitted bursts. Increased power consumption of the device is detected as a disadvantage of the software gain control circuit.

  • TECHNICAL FACILITIES FOR ENSURING SECURITY

    Influence of seawater depth in hydroacoustic transmission of binary numbers

    Security & Future, Vol. 4 (2020), Issue 3, pg(s) 119-120

    The influence of seawater depth in hydroacoustic transmission of binary numbers is experimentally investigated. The binary digits are encoded as sequences of short and long rectangular pulses of a fixed duration. The experiments are carried out in coastal waters of Bulgarian South Black Sea shelf. The transmission of numbers is experimentally compared for three different water depths: 10m, 30m and 60m, at a transmitter-receiver distance of 1500m. Increasing the water depth improved the performance leading to a larger number of correct transmissions. Hydroacoustic transmission in very shallow water is more difficult due to strong interference from reflected signals. In 60m deep water the observed transfer of binary numbers is stable and correct. Certainty of correct transmission increases with increasing water depth in shallow seawater.

  • TECHNICAL FACILITIES FOR ENSURING SECURITY

    Influence of the clock of the encoding pulses in hydroacoustic transmission of binary numbers

    Security & Future, Vol. 4 (2020), Issue 2, pg(s) 78-79

    Hydroacoustic waves are used to experimentally transmit binary numbers underwater. The numbers are first encoded into pulse sequences, according to the encoding scheme: two consecutive short pulses for a digit zero and two consecutive long pulses for a digit one. A clock period determines the timing of the encoding pulses. The influence of the clock period duration on the number of correctly transmitted binary numbers is tested in 30m deep coastal seawater, at a 1500m distance. The performance is compared at three different periods of the clock: 10ms, 20ms and 40ms. Increasing the clock period of the encoding pulses leads to an increase in the number of correct transmissions. At the longest tested clock period of 40ms all the transmissions of binary numbers are successful. Certainty of correct transmission increases at the longer clock periods of the encoding pulses due to attenuated interference from reflected signals.