A presentation board digitizer system for large boards preferably employs
at least three spaced-apart ultrasound receivers assemblies. A current
position of an ultrasound transmitter is assigned as a weighted centroid
of time-of-flight position measurements based on at least two pairs of
receiver assemblies. The weighting used varies as a function of the
position of the transmitter across the board. A preferred structure of an
ultrasound receiver assembly for use in the system employs a pair of
ultrasound receivers arranged side-by-side in a line perpendicular to the
surface of the presentation board. The receivers are connected so as to
generate a total output signal corresponding to the instantaneous sum of
the ultrasound signals received at each, such that the receiver assembly
is most sensitive to ultrasound signals incident from a plane adjacent to
the presentation board. Also described are a transmitter device for use
with a conventional pen in which the ultrasound transmitter is a
cylindrical element lying coaxial with the pen and adjacent to its tip,
and a jointed eraser structure.
