Browse Prior Art Database

Pulse-Coded Modulated Pen Digitizer Signal

IP.com Disclosure Number: IPCOM000115574D
Original Publication Date: 1995-May-01
Included in the Prior Art Database: 2005-Mar-30
Document File: 4 page(s) / 109K

Publishing Venue

IBM

Related People

Beers, GE: AUTHOR [+3]

Abstract

Disclosed is a method of adding function to the signal of a cordless pen of an electrostatic digitizer while improving the signal in the presence of noise.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Pulse-Coded Modulated Pen Digitizer Signal

      Disclosed is a method of adding function to the signal of a
cordless pen of an electrostatic digitizer while improving the signal
in the presence of noise.

      Tablet computers with digitizing screens using pen radiated
input are susceptible to noise coupling from the LCD driving signals,
power supply switching signals, backlight driver, and other system
digital noise sources.  The noise coupling mechanism is mostly
electrostatic and because of the large digitizing surface area much
noise is picked up and transferred to the low level input circuits of
the sensor amplifiers.  The pen signal being electrostaticly coupled
into the digitizer tablet also has to overcome the large noise signal
levels.  It is starting out at a disadvantage being such a small
level signal as compared to the noise signal.

      Additional information added to the pen signal is encoded with
coded pulses which have a higher noise immunity than other types of
modulation such as AM.  Described is a method to overcome the noise
problems with a very poor signal to noise ratio (S/N).  The method
proposed uses a Pulse Code Modulated (PCM) signal from the pen to
encode a signal on top of a carrier signal Fc which is typically in
the frequency range of 50KHz to 200KHz.  To encode, the carrier Fo is
switched on and off (modulated) by a digital signal that represents
information.  The frequency of modulation should be selected to be
spaced in frequency so that it is between all of the major system
noise source frequencies.  In addition to the pen position, alternate
functions can be encoded by this method.  One of the useful functions
would be a mouse button emulation, such as single click or double
click button press.  Also, mouse buttons one, two, and three are
convenient functions to emulate.

      The details of the circuit are described next.  The PCM data is
applied to the pen carrier frequency Fc in a serial format which is
comprised of a quiet zone or non-information zone and a coded data
information zone.  Refer to Figure for additional details.  Tframe is
composed of several information subdivisions which are described
here:
  (Tquiet)  -  N (16 for example) Tcode periods of no signal data for
                synchronization.
  (Tcode)   -  4 bit periods (Tbit) forming a 16 level code.
  (Tbit)    -  A bit period made up of 2 halves; a signal and a non
                signal section.
  (Thbit)   -  The signal containing half of the bit time which
                contains a pulse width (Tpw) of a certain number of
                carrier cycles.
  (Tpw)     -  The pulse width which contains a whole number of
             ...