Producing amplitude values for controlling pixel illumination on a sonar display

A method and apparatus for producing an amplitude value for use in controlling illumination of a pixel on a display, in response to a sampled sonar signal produced by an element of a sonar array. A pixel time associated with the pixel is found, the pixel time representing a time required for sound to travel a distance represented by the pixel, according to a range of distance to be viewed on the display. A sample point of the sampled sonar signal is then found, the sample point having a sample time nearest the pixel time. A delay value associated with the pixel is then found, the delay value including an array element delay value. A representation of a synthesized waveform is then produced, in response to the sonar signal, the representation comprising a plurality of sample points coinciding in time with the sample points of the sonar signal. An amplitude value representing an amplitude of the synthesized waveform at a time corresponding to the pixel time is then produced by finding an amplitude value of the waveform at a time value corresponding to the sum of the sample time and the delay value. The amplitude value is then made available for use in illuminating the pixel.