(a) Voltage or Electro-Magnetic Field Tablet and Pointer: This has a grid of wires, embedded in the tablet surface along with various voltages or magnetic fields consequent to various coordinates. Intermediate positions in a cell can be interpolated too.
(b) Acoustic or Sonic (Radio-Wave) Tablet and Pointer: The sound of a spark on the tip of the stylus is picked up via strip microphones beside two edges of the tablet. By the arrival time of the sound pulse in the microphones, the perpendicular distances of the stylus tip by the two axes are identified. The acoustic process suffers from its inherent noisiness and also its susceptibility to interference from the other noise.
A combination of time-delay and electric pulses detection via a sensor in the stylus that is termed as Electro-acoustic Table is also accessible.