A color filter substrate for a liquid crystal display includes a transparent substrate, and color filters formed on the substrate each with a groove. These residual distributed long period frequency components in the output of a swept DDS pseudo random noise generator are suppressed by making the sweep itself irregular. Optionally, a grass collection system may be mounted on the mower, with much of the system being located laterally rearward of the rear drive wheels so that the center of gravity of the mower is shifted rearwardly. If the object’s vertical height is less than the user defined clearance height, an alarm is sounded.