PURPOSE:To perform signal conversion so that the surface of the sea shows a straight line even if a depth is changed, by using the value of a writing address corresponding to the depth shown by a depth signal as the starting value of a reading address, changing the address in the reverse direction from that at the time of writing, and performing reading. CONSTITUTION:Key impulses are projected toward the water surface from a transducer 12 in the water. The device 12 transmits the received wave signal indicating a reflected wave. The received wave signal is digitized in an A/D converter 13 and a received-wave digital signal 131 is outputted. A pair of line memories 14a and 14b alternately perform the writing and reading of the signals 131 every time the key impulse is transmitted. A depth operating part 17 computes the value of a writing address 151 corresponding to the depth shown by a depth signal 112. A writing counter 15 sends the address 151 to the line memories, where the memories 14a and 14b undergo writing. A reading counter 16 perform counting reversely than the counter 15 does, with the output value of the operating part 17 as a count starting value. A reading address 161 is sent to the line memories, where the memories 14a and 14b undergo reading.
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