Raster display system

Apparatus for displaying data on a display in a raster format, such data representing echo returns obtained in a polar coordinate format comprising:

• (a) a scan converter means, fed by the echo returns in polar coordinates for converting the data in polar coordinates to data levels representative of the echo return levels in rectangular coordinates;
• (b) a bit image memory means for storing data levels fed thereto and for providing the data levels stored therein to the display when such stored data levels are read from the bit image memory means, the data levels stored therein representing levels of echo returns to be displayed in raster format;
• (c) a read-only memory means, addressed by the scan converter data levels and the bit image memory mean data levels read from the bit image memory means, for storing a plurality of display data levels at addressable locations of such read-only memory means, the display data level stored in the read-only memory means being greater than the bit image memory means data level addressing such read-only memory means when the scan converter data level addressing the read-only memory means is the same as the bit image memory means data level addressing the read-only memory means, data levels read from the read-only memory means being fed as data levels for storage in the bit image memory means;
• and
• (d) a pseudo-random pulse generator connected to provide an additional address to the read-only memory means.