Abstract: In an echo sounder including a cathode-ray tube display device for providing a visual display in the form of a succession of image frames each composed of a series of image lines extending in a first direction on the display and spaced apart in a second direction on the display, with each line being composed of a plurality of image points, and a memory having a number of addressable memory locations equal to at least the number of image points in each frame, with a representation of each image point being stored at a respective memory location, the memory is an addressable read-write random access memory, and the device includes circuitry connected for causing each memory location address to consist of a first part identifying the location of one image line in the second direction on the display and a second part identifying the location of one image point along any line in the first direction on the display, an address counter producing an output representing the address part corresponding to one direction,