Abstract:
A receiving unit for receiving an acoustic signal from a digital pen, comprises: at least two ultrasound receivers, for receiving ultrasound signals from the digital pen; an electric circuit connected to said ultrasound receivers, and configured to extract ultrasound signals received by said ultrasound receivers, to enable tracking a position of said pen, and a biased clip for holding paper in fixed relationship to said ultrasound receivers. The clip may be metal, and alignment and location features may be provided for fitting sheets of paper for writing.

Description:
RELATED APPLICATION/S 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/884,894 filed on Aug. 22, 2007, which is a National Phase of PCT Patent Application No. PCT/IL2006/000373 filed on Mar. 23, 2006, which claims priority of U.S. Provisional Patent Application No. 60/664,202 filed on Mar. 23, 2005. The contents of the above applications are incorporated herein by reference. 
     
    
     FIELD AND BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to acoustic positioning methods, and more particularly, but not exclusively to a method and an apparatus for data entry using an acoustic signal transmitting pen input device. 
         [0003]    Digital writing instruments, interchangeably referred to herein as Digital Pens, regardless of whether they actually write on paper, can be used to capture pen strokes on paper and digitize them. 
         [0004]    For example, with a digital pen, pen strokes may be converted by handwriting recognition software to a digitally stored record of the writing. In this way, a laborious, tedious step in modern written communication, namely, the manual transcribing of handwriting into a computerized word processor, is eliminated, greatly increasing productivity. 
         [0005]    Sensing a time-dependent position of the pen and converting the positions to pen strokes may be used for input of digital representations of the pen strokes to a handwriting recognition device. 
         [0006]    As known in the art, ultrasonic systems can be used in which a special pen generates or alters an ultrasonic signal, as the pen is moved across a piece of paper. 
         [0007]    The ultrasonic signal is sensed by receivers and correlated to a position vis-à-vis each receiver, as the outputs of the receivers is triangulated and correlated to absolute pen positions. A sequence of pen positions can then be digitized for input into handwriting recognition engines. 
         [0008]    An advantage with ultrasonic systems is that the user of the ultrasonic signal emitting device can use the device to write on an ordinary piece of paper that is placed on or nearby a base station, which receives the ultrasonic signals and converts the signals to alpha-numeric characters. 
         [0009]    There are many currently known in the art methods for data entry using an acoustic impulse transmitting pen input device. 
         [0010]    U.S. Pat. No. 4,814,552, to Stefik, filed on Dec. 2, 1987, entitled “Ultrasonic position input device”, describes an input device, or stylus, for entering hand drawn forms into a computer comprising a writing instrument, a pressure switch for determining whether the instrument is in contact with the writing surface, an acoustic transmitter for triangulating the position of the stylus on the surface, and a wireless transmitter for transmitting data and timing information to the computer. 
         [0011]    In operation, the stylus described by Stefik transmits an infrared signal which the system receives immediately, and an ultrasound pulse which two microphones receive after a delay which is a function of the speed of sound and the distance of the stylus from each microphone. 
         [0012]    U.S. Pat. No. 6,654,008, to Ikeda, filed on Nov. 27, 2001, entitled “Electronic whiteboard and penholder used for the same”, describes an electronic whiteboard capable of being drawn, using marker pens of several colors, and one penholder for use in such an electronic whiteboard. 
         [0013]    In Ikeda&#39;s patent, an infrared light emitting unit emits infrared light containing color information of the marker pen, an ultrasonic wave emitting unit emits the ultrasonic wave, and color information changeover means changes over color information depending on the color of marker pen. The electronic whiteboard main body receives the infrared light and ultrasonic wave emitted from the penholder, and issues information about a position of the penholder depending on the reception timing of the infrared light and ultrasonic wave. 
         [0014]    U.S. Pat. No. 6,876,356 to Zloter, filed on Mar. 18, 2002, entitled “Digitizer pen”, describes a digitizer pen system including a pen having a means protruding from the pen&#39;s writing tip, for preventing fingers blocking communication with a base unit. 
         [0015]    U.S. Pat. No. 6,184,873 to Ward, filed on Jan. 20, 1998, entitled “Pen positioning system”, describes a pen positioning system including a pen. The pen has multiple output elements and is adapted to accurately determine the location of the pointing tip of the pen, in relation to an electronic tablet. 
         [0016]    The output elements, preferably ultrasonic transmitters having distinct frequencies, are located a fixed distance from each other, and are also related in space to the pointing tip of the pen. 
         [0017]    A detection system is used to receive the output signals from the output elements, isolate the output signals from each other, and process them independently, to determine the location of the output elements and of the pointing tip of the pen. 
         [0018]    U.S. Pat. No. 6,703,570 to Russel, filed on May 10, 2000, entitled “Digital pen using ultrasonic tracking”, describes a digital pen system. Russel&#39;s system includes an elongated pen defining a writing tip, and an ultrasonic transducer oriented on the pen to direct frames of ultrasonic energy outwardly from the pen, with each frame including plural receive pulses. 
         [0019]    The digital pen system in Russel&#39;s patent further includes two or more detectors positioned on a base, such as a laptop computer, for receiving the pulses, with each pulse being associated with at least one pulse time of arrival (TOA) relative to at least one detector. Russel&#39;s system further includes a processor positioned on the base, receiving signals from the detectors, and outputting position signals representative of positions of the pen, based on the received signals. 
         [0020]    However, there are inherent problems in current acoustical technology and in the implementation of the current acoustical technology in digital pens, such as the digital pens described in the patents cited hereinabove. 
         [0021]    Among the disadvantages of current acoustic technology are: lack of accuracy, lack of multi-devices support, high power consumption, etc. The problems have implications on the mechanical design of existing data entry using an acoustic impulse transmitting devices. 
         [0022]    Apart from that, there are manufacturing problems related to the assembly of the acoustic transmitter and its incorporation in a digital pen or the like. For instance, such problems may arise in connecting an acoustic transmitter to a flexible printed circuit board (PCB). There are also marketing issues, such as differentiation between products by changing their appearance, while keeping the functional parts the same. 
         [0023]    There is thus a widely recognized need for, and it would be highly advantageous to have, an apparatus or a method devoid of the above limitations. 
       SUMMARY OF THE INVENTION 
       [0024]    According to one aspect of the present invention there is provided a receiving unit for receiving an acoustic signal from a digital pen, comprising: 
         [0025]    at least two ultrasound receivers, for receiving ultrasound signals from the digital pen; and 
         [0026]    an electric circuit connected to the ultrasound receivers, and configured to extract ultrasound signals received by the ultrasound receivers, to enable tracking a position of the pen, and 
         [0027]    a biased clip for holding paper in fixed relationship to the ultrasound receivers. 
         [0028]    The clip may be metal. 
         [0029]    An embodiment may comprise a depression for accurate location of the paper. 
         [0030]    In an embodiment, the depression is for accurate location of a corner of the paper. 
         [0031]    In an embodiment, the clip comprises a release-extension for opposing the bias to release the clip. 
         [0032]    In an embodiment, the extraction comprises referencing a reference model comprising data pertaining to expected reference signals. 
         [0033]    In an embodiment, friction pads may be located in association with the clip to improve grip of the paper. 
         [0034]    In an embodiment, a surface of the receiving unit opposite the clip is shaped to cause deformation of the paper held by the clip to improve grip of the paper. 
         [0035]    An embodiment may comprise elongated grips located to hold the paper at an oblique angle. 
         [0036]    In an embodiment, ultrasound receivers may be electret microphones or MEMS microphones. 
         [0037]    An embodiment may comprise an attachment device, for removably attaching the receiving unit to another item. 
         [0038]    In an embodiment, the at least two ultrasound receivers are positioned less than 65 mm apart from each other. 
         [0039]    In an embodiment, the receiving unit further comprises a housing, configured to house the ultrasound receiver and having a changeable receiving unit cover element. 
         [0040]    An embodiment may be used with a digital pen comprising a skeleton and a removable skin: 
         [0041]    the skeleton holding together in fixed relationship:
       an electric circuit;   an acoustic transmitter, detached from the electric circuit, and configured to transmit acoustic signals; and       
 
         [0044]    the removable skin comprising a pen housing fitting over the skeleton. 
         [0045]    The pen housing and the receiving unit cover element may correspond. 
         [0046]    According to a second aspect of the present invention there is provided a digital pen system, comprising:
       a digital pen having an elongated body terminating in a writing tip, a writing element protruding from the writing tip, and an acoustic signal transmitter deployed adjacent to the writing tip and configured to transmit an acoustic signal;   at least one receiving unit as discussed above; and   a processor, associated with the at least one receiving unit, configured to process the received acoustic signal, for determining presence of the digital pen in a predefined area, and to trigger a predefined functionality upon the determining presence; and   a map, configured to graphically map the predefined area, so as to assist a user in positioning the digital pen in the predefined area.       
 
         [0051]    According to a third aspect of the present invention there is provided a digital pen system comprising:
       a digital pen having an electric circuit, an acoustic transmitter configured to transmit acoustic signals, detached from the electric circuit, and a resilient holder, configured to press the electric circuit into contact with the transmitter upon applying a mechanical pressure to the resilient holder, so as to electrically connect the electric circuit and the transmitter;   at least one receiving unit as discussed above; and   a processor, associated with the at least one receiving unit, configured to process the received acoustic signals for determining location of the digital pen.       
 
         [0055]    According to a fourth aspect of the present invention there is provided a digital pen system comprising:
       a digital pen having an acoustic transmitter, configured to transmit acoustic signals and a switch assembly having two switching points for pressing the assembly to achieve first and second switching modes respectively, the assembly further having a third mode selectable upon the two switching points being pressed substantially simultaneously   at least one receiving unit as discussed above; and   a processor, associated with the at least one receiving unit, configured to process the received acoustic signals for determining location of the digital pen.       
 
         [0059]    In an embodiment, the processor is further configured to detect a mode of the digital pen. 
         [0060]    According to a sixth aspect of the present invention there is provided a digital pen system comprising:
       a digital pen having an acoustic transmitter, configured to transmit acoustic signals, and a smooth contact switch configured to actuate the digital pen upon applying a pressure on the smooth contact switch;   at least one of the receiving unit of claim  1 ; and   a processor, associated with the at least one receiving unit, configured to process the received acoustic signals, for determining location of the digital pen, the smooth contact switch comprises a resilient element, mounted on a first side of an open electric circuit and disconnected from a second side of the electric circuit, the resilient element being compressible into a position where the resilient element contacts the second side of the electric circuit, thereby closing the electric circuit, upon applying a writing pressure compressing the resilient element into the position.       
 
         [0064]    In an embodiment, the processor is further configured to detect the actuation of the digital pen. 
         [0065]    According to a seventh aspect of the present invention there is provided a digital pen system comprising:
       a digital pen having an acoustic transmitter, configured to transmit acoustic signals;   at least one receiving unit according to claim  1 ; and   a processor, associated with the at least one receiving unit, configured to process the ultrasound signals, for extracting location of the digital pen, the extraction comprises referencing a reference model comprising data pertaining to expected reference signals.       
 
         [0069]    An embodiment may comprise at least one attachment device, for removably attaching the receiving unit to another item. 
         [0070]    According to an eighth aspect of the present invention there is provided a digital pen system comprising:
       a digital pen having an acoustic transmitter, configured to transmit acoustic signals;   at least one receiving unit as discussed above, the receiving unit further comprising a housing, and wherein the two acoustic signal receivers are positioned inside the housing, less than 60 mm apart from each other, and configured to receive an acoustic signal from the digital pen; and   a processor, associated with the at least one receiving unit, configured to process the acoustic signal, for determining location of the digital pen.       
 
         [0074]    In an embodiment, the housing comprises a changeable cover element. 
         [0075]    According to a ninth aspect there is provided a receiving unit for receiving an acoustic signal from a digital pen, comprising: 
         [0076]    at least two ultrasound receivers, for receiving ultrasound signals from the digital pen; 
         [0077]    an electric circuit connected to said ultrasound receivers, and configured to extract ultrasound signals received by said ultrasound receivers, to enable tracking a position of said pen, 
         [0078]    a biased clip for holding paper in fixed relationship to said ultrasound receivers, the biased clip comprising a metal clip, a depression for accurate location of said paper and wherein said depression is for accurate location of a corner of said paper. 
         [0079]    According to a tenth aspect there is provided a receiving unit for receiving an acoustic signal from a digital pen, comprising: 
         [0080]    at least two ultrasound receivers, for receiving ultrasound signals from the digital pen; 
         [0081]    an electric circuit connected to said ultrasound receivers, and configured to extract ultrasound signals received by said ultrasound receivers, to enable tracking a position of said pen, 
         [0082]    a biased clip for holding paper in fixed relationship to said ultrasound receivers, the biased clip comprising a metal clip, a depression for accurate location of said paper and wherein said depression is for accurate location of a middle of a side of said paper. 
         [0083]    Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The materials, methods, and examples provided herein are illustrative only and not intended to be limiting. 
         [0084]    Implementation of the method and system of the present invention involves performing or completing certain selected tasks or steps manually, automatically, or a combination thereof. Moreover, according to actual instrumentation and equipment of preferred embodiments of the method and system of the present invention, several selected steps could be implemented by hardware or by software on any operating system of any firmware or a combination thereof. For example, as hardware, selected steps of the invention could be implemented as a chip or a circuit. As software, selected steps of the invention could be implemented as a plurality of software instructions being executed by a computer using any suitable operating system. In any case, selected steps of the method and system of the invention could be described as being performed by a data processor, such as a computing platform for executing a plurality of instructions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0085]    The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
           [0086]    The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in order to provide what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
           [0087]    In the drawings: 
           [0088]      FIG. 1  a simplified block diagram illustrating a digital pen, according to a preferred embodiment of the present invention. 
           [0089]      FIGS. 2   a  and  2   b  are exemplary depictions of a resilient holder, deployed inside a digital pen, according to a preferred embodiment of the present invention. 
           [0090]      FIG. 3  is an exemplary depiction of a digital pen having a switch assembly comprising two switches, according to a preferred embodiment of the present invention. 
           [0091]      FIG. 4   a  is a simplified block diagram schematically illustrating an exemplary switch assembly mechanical design, according to a preferred embodiment of the present invention. 
           [0092]      FIG. 4   b  is a diagram showing a first exemplary cover element for a switch assembly, according to a preferred embodiment of the present invention. 
           [0093]      FIG. 4   c  is a diagram showing a second exemplary cover element for a switch assembly, according to a preferred embodiment of the present invention. 
           [0094]      FIGS. 5   a - 1  and  5   a - 2  are simplified block diagrams depicting a touch switch, according to a preferred embodiment of the present invention. 
           [0095]      FIG. 5   b  is a simplified diagram, illustrating an adhesive having a vent, according to a preferred embodiment of the present invention. 
           [0096]      FIG. 6   a  is a simplified diagram illustrating a first digital pen having a changeable cover element, according to a preferred embodiment of the present invention. 
           [0097]      FIG. 6   b  is a simplified diagram illustrating a second digital pen having a changeable cover element, according to a preferred embodiment of the present invention. 
           [0098]      FIG. 6   c  is a simplified diagram illustrating a third digital pen having a changeable cover element, according to a preferred embodiment of the present invention. 
           [0099]      FIG. 6   d  is a simplified diagram illustrating a fourth digital pen having a changeable cover element, according to a preferred embodiment of the present invention. 
           [0100]      FIG. 7   a  is a simplified block diagram illustrating a first retractable digital pen according to a preferred embodiment of the present invention. 
           [0101]      FIG. 7   b  is a simplified diagram illustrating a second retractable digital pen according to a preferred embodiment of the present invention. 
           [0102]      FIG. 8   a  is a simplified block diagram illustrating a second retractable digital pen, according to a preferred embodiment of the present invention. 
           [0103]      FIG. 8   b  is a simplified diagram illustrating a second retractable digital pen, according to a preferred embodiment of the present invention. 
           [0104]      FIG. 9  is a simplified block diagram schematically illustrating a digital pen having two acoustic transmitters according to a preferred embodiment of the present invention. 
           [0105]      FIG. 10  is a diagram schematically illustrating a digital sleeve for a writing instrument, according to a preferred embodiment of the present invention. 
           [0106]      FIG. 11   a - 11   e  are schematic depictions of a digital pen&#39;s grating for a writing instrument, according to a preferred embodiment of the present invention. 
           [0107]      FIG. 12  is a schematic depiction of a first receiving unit for receiving an acoustic signal from a digital pen, according to a preferred embodiment of the present invention. 
           [0108]      FIG. 13  is a schematic depiction of a second receiving unit for receiving an acoustic signal from a digital pen, according to a preferred embodiment of the present invention. 
           [0109]      FIG. 14  is a simplified block diagram illustrating a digital pen system, according to a preferred embodiment of the present invention. 
           [0110]      FIG. 15  is a simplified block diagram illustrating a decoding unit, according to a preferred embodiment of the present invention. 
           [0111]      FIG. 16  is a simplified block diagram illustrating exemplary components of a mathematical model for incorporating into a maximum likelihood detector, according to a preferred embodiment of the present invention. 
           [0112]      FIG. 17  is a two-part graph showing an exemplary correlation function, according to a preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0113]    The present embodiments comprise a digital pen, a digital sleeve, a receiving unit, and a digital pen system. 
         [0114]    The principles and operation of a digital pen, a digital sleeve, a receiving unit, and a digital pen system according to the present invention may be better understood with reference to the drawings and accompanying description. 
         [0115]    The present invention attempts to overcome drawbacks of traditional technologies, some of which are described hereinabove in the background and field of invention section. The present invention attempts to improve current technologies by introducing and implementing new ideas into the design of a viable product, be it a digital pen, a digital sleeve, a receiver for acoustic signals transmitted from a digital pen, or a digital pen system. 
         [0116]    Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0117]    Reference is now made to  FIG. 1 , which is a simplified block diagram illustrating a digital pen, according to a preferred embodiment of the present invention. 
         [0118]    A digital pen  1000 , according to a preferred embodiment of the present invention includes at least one acoustic transmitter  100 , preferably deployed adjacent to the pen&#39;s tip, and an electric circuit  110  such as a flexible printed electric circuit board (PCB)  120  which includes a connection to an electric power source, such as a miniature battery. 
         [0119]    Preferably, the acoustic transmitter  100  is an ultrasound transducer. 
         [0120]    Optionally, the ultrasound transducer is a piezoelectric transducer which converts electrical energy into ultrasound signals. Piezoelectric crystals have the property of changing size when an electric voltage is applied to them. By applying an alternating electric voltage (AC) on a piezoelectric crystal, the crystal is caused to oscillate at very high frequencies producing ultrasound signals comprised of very high frequency sound waves. 
         [0121]    Preferably, the ultrasound transducer is made of Polyvinylidene Fluoride (PVDF), which is flexible plastic polymer, bearing piezoelectric properties. 
         [0122]    The acoustic transmitter  100  is electrically connected to the circuit  110 , which may be detached from the transmitter  110 , say to allow assembling an ink refill inside in the pen. 
         [0123]    However, the acoustic transmitter  100  is too sensitive to allow heating for soldering, for electrically connecting the transmitter  100  to the electric circuit  110 , or even the attachment of plastic to the transmitter  100 . Though screwing the transmitter is optional, it is not suitable for fast high volume production. 
         [0124]    A preferred embodiment of the present invention attempts to overcome the difficulty in electrically connecting the transmitter  100  and the electric circuit  110 , using a resilient holder  120 . 
         [0125]    The resilient holder  120  presses the electric circuit  110  into contact with the acoustic transmitter  100 , upon applying a mechanical pressure on the resilient holder  120 , so as to electrically connect the electric circuit  110  and the acoustic transmitter  100 . 
         [0126]    Optionally, the resilient holder  120  is further configured to push the flexible PCB into a position, in order to allow the placement of components such as IR transmitters in certain positions. 
         [0127]    Preferably, the resilient holder  120  facilitates the digital pen&#39;s  1000  being smaller than known in the art digital pens. 
         [0128]    Reference is now made to  FIG. 2   a  which is an exemplary depiction of a resilient holder deployed inside a digital pen, according to a preferred embodiment of the present invention. 
         [0129]    A digital pen  2000  according to a preferred embodiment of the present invention has an elongated body terminating in a writing tip, a writing element  220  protruding from the writing tip, an acoustic transmitter  210  deployed adjacent to the writing tip, and an electric circuit  240  such as a flexible PCB  240  (flexible printed circuit board) or conductors. 
         [0130]    The digital pen&#39;s writing element may be, but is not limited to an ink refill, a pencil tip, a marker, etc. The digital pen may also include an eraser. The digital pen may also allow a user to change the color of writing, say using the switch assembly, described in detail herein below. Optionally, the writing element is rather a sharpened tip which does not physically write. 
         [0131]    Preferably, the digital pen  2000  further includes a resilient holder  250  pressing the flex PCB  240  (or the conductors) into contact with the acoustic transmitter  210 , for electrically connecting the flex PCB  240  and the acoustic transmitter  210  (and to the pen body). 
         [0132]    Preferably, the resilient holder  250  may be made of conductive material, in order to increase electrical conductivity between the Flex PCB  240  and the acoustic transmitter  210 . 
         [0133]    Optionally, the electric conductivity between the Flex PCB  240  and the acoustic transmitter  210  may be increased by deploying gold contacts on the resilient holder  250 . 
         [0134]    The resilient holder  250  secures electrical contact between the Flex PCB  240  and the acoustic transmitter  210 . Optionally, as a result of mechanical pressure applied on the resilient holder  250 , say from the pen&#39;s housing. 
         [0135]    In a preferred embodiment, the resilient holder  250  is in the shape of a “U”: solid above and open below, such that the pen body is kept tight by the bottom part of the resilient holder  250 . The “U” shaped resilient holder  250  comprises a base and extensions arising perpendicularly from the base, and is configured for location of the flex PCB  240  or any other first electric circuit, and a second electric circuit, within the confines of a housing. The resilient holder  250  brings about an electric contact between the two electric circuit, due the confinement within the housing, thereby connecting the two circuits. By connecting the two circuits, the resilient holder  250  electrically connects the flexible PCB  240  and the acoustic transmitter  210 . 
         [0136]    Preferably, the resilient holder  250  has some elasticity so as to enable easy assembly, by putting all parts and sliding the resilient holder  250  to position. The mechanical force is kept by elastic lugs on the upper part consisting of the base hereinabove. The elastic lugs push the holder up (while the bottom part is secured to the pen&#39;s body as explained hereinabove), as explained hereinabove. 
         [0137]    Reference is now made to  FIG. 2   b  which shows an exemplary a digital pen having a resilient holder, according to a preferred embodiment of the present invention. 
         [0138]    A digital pen  2000  has a resilient holder  290 , as described hereinabove. The resilient holder  290  further has leaf springs  292 . The leaf springs  292  are configured to apply pressure on an acoustic transmitter&#39;s ribbon, thereby connecting the acoustic transmitter to the flexible PCB  295 , as described hereinabove. 
         [0139]    Reference is now made to  FIG. 3  which an exemplary depiction of a digital pen having a switch assembly comprising two switches, according to a preferred embodiment of the present invention. 
         [0140]    A digital pen  3000 , according to a preferred embodiment of the present invention includes at least one acoustic transmitter  310 , preferably an ultrasound transducer. 
         [0141]    The digital pen  3000  further comprises a switch assembly  320  having at least two switches. The digital pen  3000  has a certain mode which a user may select by pressing at least two of the switches substantially simultaneously. 
         [0142]    Optionally, the switch assembly  320  is mechanically designed, according to known in the art techniques, with a position associated with the certain mode of the pen. Preferably, the position is accessible only when the user presses the two switches of the switch assembly  320  simultaneously, or almost simultaneously. 
         [0143]    Reference is now made to  FIG. 4   a , which is a block diagram schematically illustrating an exemplary switch assembly mechanical design, according to a preferred embodiment of the present invention. 
         [0144]    A switch assembly, according to a preferred embodiment of the present invention, has two switches  410 , 420 , mounted on a switching rod  450 . The switching rod is balanced about a fulcrum  470 . Preferably, the fulcrum  470  is urged up by a spring. 
         [0145]    A user may push the first switch  410 , thus putting the assembly in a first position (a), or push the second switch  420 , thus putting the assembly in a second position (b), or toggle between the two positions (a,b). 
         [0146]    When a user pushes the two switches  410 , 420 , at the same time, or almost at the same time, the pressure applied on the two switches  410 , 420  simultaneously pushes the fulcrum  470  against the spring, and puts the assembly in a third position (c) where both switches are pressed. The third position may be associated with a certain mode, as described hereinabove. 
         [0147]    Optionally, the switch assembly  320  is electrically designed, according to known in the art techniques, with a position associated with the certain mode of the pen. The position is accessible only when the user presses the two switches of the switch assembly  320  substantially simultaneously. 
         [0148]    Reference is now made to  FIG. 4   b , which a block diagram showing a first exemplary cover element for a switch assembly, according to a preferred embodiment of the present invention. 
         [0149]    Switch assembly  320 , may further have cover element  4000 . The cover element  4000  has right and left protrusions  4100  (or regressions), guiding a user press one of the two switches  410 , 420 , as explained in greater detail hereinabove. The cover element  4000  further has a central protrusion  4200  (or regression), guiding the user, to apply a pressure substantially simultaneously on the two switches  410 , 420 , thus bringing the switch assembly into the third position, described hereinabove. 
         [0150]    Reference is now made to  FIG. 4   c , which is a block diagram showing a second exemplary cover element for a switch assembly, according to a preferred embodiment of the present invention. 
         [0151]    Cover element  4500  is mount on the switch assembly, such that the protrusions (regressions) are positioned above their two switches  4520 , described hereinabove. 
         [0152]    Reference is now made to  FIG. 5   a , which is a simplified block diagram depicting a touch switch, according to a preferred embodiment of the present invention. 
         [0153]    A digital pen according to a referred embodiment of the present invention includes a smooth contact switch, configured to actuate the digital pen upon applying a mechanical pressure on the smooth contact switch. 
         [0154]    The smooth touch switch is assembled inside the digital pen, such that the mechanical pressure is applied on the smooth contact switch when the pen touches a surface, such as a sheet of paper, say when a user writes using the digital pen. 
         [0155]    Preferably, the applied pressure may be very small, preferably less than twenty five grams. More preferably, the switch activation travel distance is very small (say, less than 0.1 mm), and not sensed by the user who uses the pen. 
         [0156]    The smooth touch switch may be mounted on an open electric circuit, such as a flexible printed circuit board (PCB), a regular circuit, or on two detached circles of conductive material, etc As a result, no wires or ribbons are needed to connect the switch to the electrical circuit. 
         [0157]    In a preferred embodiment, a concentric adhesive  510  with electrical conduction properties is applied on a resilient element  530 , electrically connected to one side of an open electric circuit. 
         [0158]    The upper part of the smooth touch switch is the flat and compressible resilient element  530  having conducting properties, mounted on the adhesive  510 , as shown in a cross sectional view ( 5   a - 2 ) along the AA line of the bottom view ( 5   a - 1 ). Optionally, the resilient element is made of a conductive material or the conductive properties are given to the resilient element  530 , by a adding an additive, such as a conductive ink or glue to the resilient element  530 . 
         [0159]    A pressure in the center of the upper part of the resilient element  530  of the smooth touch switch compresses the resilient element  530  into a position forming an electric path from a second side of the open electric circuit, through the resilient element  530 , through the concentric adhesive  510 , and to the first side of the electric circuit, thus closing the electric circuit, thereby actuating the digital pen. 
         [0160]    The actuating pressure is controlled by the thickness of the concentric adhesive  510 , inner diameter of the concentric adhesive  510  and thickness of the resilient element  530 . 
         [0161]    Optionally, the resilient element is made of Polyethylene Terepthalate—(PET) material covered with conductive ink, and the concentric adhesive is a very thin layer, of no more then 0.1 mm, and made of 3M™ Z-Axis, or similar products. Optionally the resilient element  530  is made of conductive metal. 
         [0162]    Reference is now made to  FIG. 5   b  which is simplified block diagram, illustrating an adhesive having a vent, according to a preferred embodiment of the present invention. 
         [0163]    Preferably, the adhesive  5100  used in the smooth touch switch, described hereinabove, includes vent holes  5150 , for relieving air pressure trapped inside the cavity formed by the resilient element  530 , the adhesive, and the electric circuit closed by the smooth touch switch upon compressing the resilient element  530 . 
         [0164]    Reference is now made to  FIG. 6   a , which is a simplified diagram illustrating a first digital pen having a changeable cover element, according to a preferred embodiment of the present invention. 
         [0165]    Preferably, a digital pen according a preferred embodiment of the present invention, has an inner structure which holds the functional parts together and a housing having a changeable cover element (skin). 
         [0166]    The inner part may hold an acoustical transducer, IR emitters, an electric circuit such as a flexible PCB, switches, etc. The housing covers the inner part and has some mechanical interfaces which allow its connection to the inner part. The housing may have additional functional properties, such as a battery holder. 
         [0167]    In a preferred embodiment, there is introduced a variety of colorful and fashioned changeable cover elements, thus providing a range of covers (skins) for the digital pen. 
         [0168]    Optionally, a manufacturer of the pen assembles the pen with one cover element of a variety of cover elements and the end user does not change the cover element. Preferably, an end-user is allowed to change the cover element of the housing, thus giving the digital pen different appearance and different feel or texture 
         [0169]    For example, a digital pen  6100  has a housing which includes a central changeable cover element (skin)  610 , connected to battery support chassis  611  on one side, and to a pen tip  612 , on the other side. Optionally, the central changeable cover element (skin)  610  is connected to the battery support chassis  611  and the pen tip  612 , utilizing snap locks  615 , visible or hidden, as known in the art. 
         [0170]    Reference is now made to  FIG. 6   b , which is a simplified diagram illustrating a second digital pen having a changeable cover element, according to a preferred embodiment of the present invention. 
         [0171]    a digital pen  6200  has a housing which includes a changeable cover element (skin)  6210 , connected to a pen tip  6220 . 
         [0172]    Reference is now made to  FIG. 6   c , which is a simplified diagram illustrating a third digital pen having a changeable cover element, according to a preferred embodiment of the present invention. 
         [0173]    a digital pen  6300  has a housing which includes a changeable cover element (skin)  6310 , connected to a battery cover  6320 . 
         [0174]    Reference is now made to  FIG. 6   d , which is a simplified diagram illustrating a fourth digital pen having a changeable cover element, according to a preferred embodiment of the present invention. 
         [0175]    a digital pen  6400  has a housing which includes an upper changeable cover element (skin)  6410 , connected to a lower cover  6420 . 
         [0176]    Preferably, the digital pen is a retractable digital pen allowing covering the writing element at the tip of the pen, say at the tip of an ink cartridge deployed inside the pen, (or stylus, or pencil). 
         [0177]    Reference is now made to  FIG. 7   a  which a simplified block diagram illustrating a first retractable digital pen according to a preferred embodiment of the present invention. 
         [0178]    According to a preferred embodiment, the digital pen  700  has a rotating part  710  which moves forward or backwards when a user rotates the part  710 . 
         [0179]    The rotating part  710  moves forward and covers a writing element  720 , protruding from the tip  715  of the digital pen  700 , when the user rotates the part  710  in one direction. The rotating part  710  moves backwards, and exposes the writing element  720 , as the user rotates the part  710  in an opposite direction. 
         [0180]    Optionally, the rotational movement of the rotating part  710  is transformed into a linear movement where the rotating part  710  moves forward, for covering the writing element  720 , or backwards, for exposing the writing element  720 . The transformation may be facilitated by a helical track, guiding the rotating part  710 , as known in the art. 
         [0181]    Reference is now made to  FIG. 7   b  which a simplified diagram illustrating a second retractable digital pen according to a preferred embodiment of the present invention. 
         [0182]    A digital pen&#39;s housing includes a skin  7120  and a retractable tip  7110 . The retractable tip  7110  is connected by a spiral mechanism  7100  to the skin  7120 . The spiral mechanism  7100  causes a linear movement of the tip in and out. The rotation motion is applied by the user between the tip  7110  and the skin  7120 . 
         [0183]    Reference is now made to  FIG. 8   a  which a simplified block diagram illustrating a third retractable digital pen, according to a preferred embodiment of the present invention. 
         [0184]    A retractable digital pen  800  comprises an elongated housing  805 , covering an elongated body  820  terminating in a writing tip, wherefrom a writing element  810 , such as a tip of an ink refill protrudes. 
         [0185]    The elongated body  820  may be moved forward, to expose the writing element  810 , and backwards to cover the writing element inside the housing  805  of the digital pen  800 . 
         [0186]    Optionally, the elongated body  820  is urged backwards by a spring  830 , thus pushing the elongated body  820  into a position where the writing element  810  is covered by the housing. 
         [0187]    Preferably, the elongated body  820  is securable into a position where the writing element  810  is exposed, by a securing means  850 . Optionally, there may be used a snap, a lock, etc, for locking the elongated body on the edge of the housing  805 . 
         [0188]    Optionally, a digital pen, according to a preferred embodiment may have moving parts, such a refill, a skeleton, a tip, a battery house, any other part, or a combination thereof. The movement between the moving parts may be facilitated utilizing designs similar to the designed described above, using  FIG. 7-8 . 
         [0189]    According to a preferred embodiment of the present invention, there are put several infrared (IR) emitters, on several points of the digital pen for more robustness. 
         [0190]    As a result, if one of the IR emitters is covered, say by the hand of a user while holding the digital pen, the other parts maintain the link with a receiver. 
         [0191]    Examples of the possible points on the digital pen where the IR emitters may be deployed include, but are not limited to: the bottom part of the digital pen, the upper part, on the top of the pen, on a flexible PCB installed in the digital pen (as described hereinabove), etc. 
         [0192]    Preferably, the housing of the digital pen includes a soft material such as rubber, so as to provide better convenience for a user holding the digital pen. 
         [0193]    Reference is now made to  FIG. 8   b  which a simplified block diagram illustrating a fourth retractable digital pen, according to a preferred embodiment of the present invention 
         [0194]    A digital pen may have a retractable skeleton  7250 , pushed by a button  7270  mounted on top of the digital pen, utilizing and a locking mechanism  7200 . 
         [0195]    Reference is now made to  FIG. 9  which is a block diagram schematically illustrating a digital pen having two acoustic transmitters according to a preferred embodiment of the present invention. 
         [0196]    A digital pen  900  may have two acoustic transmitters  930 . Installing two acoustic transmitters in a digital pen may have several advantages, which may include, but are not limited to the following:
   1) Allowing the receiver to estimate the five dimensional (5D) location of the pen which includes the three dimensional location, and leaning angles of the digital pen, or a six dimensional (6D) location of the pen, which includes the five directional (5D) location as well data relating to rotation of the digital pen.   2) Estimating more accurately the writing element&#39;s position and compensating for the distance difference between the transducer and position of the writing element.   3) Allows gaming functions, using the digital pen as a joystick.   
 
         [0200]    Reference is now made to  FIG. 10  which is a diagram schematically illustrating a digital sleeve for a writing instrument, according to a preferred embodiment of the present invention. 
         [0201]    The digital sleeve  1000  comprises an acoustic signal transmitter, for transmitting an acoustic signal. The digital sleeve  1000  may also comprise an electric circuit, a power source, or other elements, as described for a digital pen hereinabove. 
         [0202]    A digital sleeve  1000 , according to a preferred embodiment, may be mounted on a regular writing instrument  1100 , such as a pen, a pencil, a marker, etc. 
         [0203]    According to a preferred embodiment of the present invention, the digital sleeve  1000  may be worn on a finger. For example, Epos Technologies™ provides a stylus-at-your-fingertip product. 
         [0204]    Preferably, the digital sleeve  10000  further includes a writing sensor  10200 . The writing sensor  10200  is configured for detecting a movement (or a friction) of the writing device  10100 , relative to the digital sleeve  10000  mounted thereon. 
         [0205]    That is to say, as a user, holding a pen mounted with the sleeve  1000 , starts writing with the pen, a relative movement (or friction) occurs between the pen touching a paper and the sleeve  1000 . The relative movement (or friction) is sensed by the writing sensor  10200 . The writing sensor  10200  in turn, actuates the acoustic transmitter, through electric circuitry. Then, the acoustic transmitter transmits the acoustic signals, say to a receiving unit, as described in greater detail for a digital pen system herein below. 
         [0206]    Reference is now made to  FIG. 11   a - e,  which are schematic depictions of a digital pen&#39;s grating for a writing instrument, according to a preferred embodiment of the present invention. 
         [0207]    Typically, an acoustic transmitter, specifically—an ultra sound transducer has some irregularities. The irregularities make the transducer not entirely omni directional. 
         [0208]    The irregularities result from a part of the transducer having an inherent defect, because the transducer is made from a rectangular foil laminated to form a cylinder. The lamination forms a passive part which does not radiate acoustic energy. The inherent defect causes the signal in front of the defect to be much weaker than in front of other parts of the ultrasound transducer. 
         [0209]    Typically, the position of the digital pen is determined utilizing an algorithm, based on a measurement of TOA (time of arrival) of the acoustic signals from the acoustic transmitter. Usually the algorithm compares the TOA of the signals with IR signals transmitted from the digital pen. 
         [0210]    As a result of the inherent defect, the sum of the signals received at a given point in the space surrounding the acoustic transmitter has a phase shift, in comparison to other points at a similar distance away from the acoustic transmitter. 
         [0211]    A digital pen, according to a preferred embodiment, includes an acoustic wave guide, positioned adjacent to an acoustic transmitter of the digital pen. 
         [0212]    Preferably, the acoustic wave guide comprises a plurality of fins  1110  radiating outwardly in a direction away from the acoustic signal transmitter. 
         [0213]    More preferably, the fins  1110  are positioned so as to spatially divide the space surrounding the acoustic transmitter into directional sectors. 
         [0214]    The fins  1110  substantially isolate acoustic signals transmitted by the acoustic transmitter through one of the sectors from acoustic signals transmitted from the acoustic transmitter through the other sectors. 
         [0215]    That is to say, to eliminate the shift in location, the fins  1110  are positioned so to as to divide the space around the acoustic transmitter into sectors, such that each sector is decoupled or isolated from the other sectors. 
         [0216]    As a result of the division of space around the acoustic transmitter into significantly isolated sectors, the phase shift is significantly eliminated. The elimination of phase shift may improve the results of acoustic signal correlation based position decoding techniques. However, the amplitude of the sum of signals transmitted through each point in one of the sectors around the acoustic transmitter is reduced, as signals from the other sectors are significantly eliminated from the sector. 
         [0217]    Optionally, the grating around the acoustic transmitter may be designed differently than the above described fin design. 
         [0218]    For example, the grating may comprise a spiral opening keeping a single opening, a grating coming upwards combined with a grating coming downwards (keeping an opening to free air in between), etc. 
         [0219]    According to a preferred embodiment of the present invention, there is provided a receiver configured to receive acoustic signals transmitted from a digital pen, to be used for determining location of the digital pen, say for automatically digitizing hand writing carried out using the digital pen. 
         [0220]    Reference is now made to  FIG. 12  which is a schematic depiction of a first receiving unit for receiving an acoustic signal from a digital pen, according to a preferred embodiment of the present invention. 
         [0221]    A receiving unit  1200 , configured to receive acoustic signals from a digital pen may have a metal plate  1210  mounted on the body  1220  of the receiving unit, for securing the receiving unit  1200  to a sheet of paper. 
         [0222]    Pressing the metal plate one end  1210 - a  makes the other end  1210 - b  open a gap between the other end  1210 - b  and the body  1220  of the receiving unit  1200 . Through the opened gap, a sheet of paper may be inserted between the plate&#39;s end  1210 - b  and the body  1220  of the receiving unit  1200 . 
         [0223]    Releasing the metal plate pressed end  1210 - a  makes the other end  1210 - b  get back to its natural position and embed a force on the paper sheet which is pressed between the plate&#39;s end  1210 - b  and the body  1220  of the receiving unit  1210 . 
         [0224]    The metal plate  1210  and the body  1220  of the receiving unit  1200  may have additional non-flat surface properties (such as rubber pads) which allow more friction between the paper and the receiving unit&#39;s body  1220 . 
         [0225]    Preferably, the metal plate  1210  may be shaped, so as to cause a slight deformation of the paper, in order to have a better grip of the paper sheet. 
         [0226]    One or more receiving unit(s)  1200  may be fit on the paper sheet&#39;s center, or on the sheet&#39;s edges. 
         [0227]    Preferably, the receiving unit body  1200  and plate have stoppers  1212  that fit the 90 degrees of a paper sheet&#39;s corner (and hold the receiving unit at 45 degrees). 
         [0228]    The placement of the receiving unit  1200  on the corner of the paper sheet instead on the sheet&#39;s middle has several benefits, such as: Repeatability, Accuracy—a receiving unit placed on the corner has a better perspective, improving its accuracy, Less dead zones—as the operating angle of a receiving unit placed at the corner of the paper sheet is much smaller than when a receiving unit is place in the middle of the paper. 
         [0229]    Reference is now made to  FIG. 13  which is a schematic depiction of a second receiving unit for receiving an acoustic signal from a digital pen, according to a preferred embodiment of the present invention. 
         [0230]    A receiving unit  1300 , according to a preferred embodiment of the present invention includes two microphones  1330 . 
         [0231]    Optionally, the two microphones are ultrasound receivers, as known in the art. 
         [0232]    Preferably, the two microphones are electret microphones or alternatively MEMS microphones. Electret microphones are miniature microphones that work on condenser microphone principles, as known in the art, but have permanently charged polymer diaphragms. Electret microphones have miniature preamplifiers built in, and require low voltage direct current (DC) power (typically from a 1.5 to 18 volts battery). 
         [0233]    Electret microphones are widely used in hand held devices—such as mobile computer games, mobile phones, etc. 
         [0234]    The receiving unit  1300  further includes an electric circuit. 
         [0235]    The electric circuit is configured to extract ultrasound signal, received by the microphones  1330 , say by implementing frequency down conversion, signal filtration, signal amplification techniques, or other methods. 
         [0236]    Some of the methods used by the electric circuit are described in greater detail in the applicant&#39;s International Application No. PCT/IL03/00309, entitled “Method and system for obtaining positional data”, filed on Apr. 14, 2003. 
         [0237]    According to a preferred embodiment of the present invention, the two microphones  1330  are positioned in a distance of less then 65 mm from each other. 
         [0238]    The signals received form the two microphones  1330  positioned less than 65 mm away from one another, may be processed for generating positional data relating to the digital pen. 
         [0239]    The processing may be carried out using decoding methods, say utilizing models of the transmitted and received signals as described in grater detail herein below. 
         [0240]    According to a preferred embodiment of the present invention, a processor, connected with one or more receiving unit(s), is configured to process acoustic signal, received at the receiving unit(s), for determining presence of the digital pen in a predefined area. 
         [0241]    Preferably, the processor may be configured to trigger a predefined functionality when a user places the digital pen in a predefined area. 
         [0242]    Optionally, the user may be provided a printed map or menus, and position the receiving unit(s) on the map or menus. When the user positions the digital pen on an icon, representing the predefined area, printed on the paper, the digital pen is present in the predefined area. Consequently, the predefined functionality is triggered by the processor. 
         [0243]    For example, the user may be provided a printed menu having drawn icons such as an eraser, a marker, etc. The user may deploy the receiving unit(s) on the printed menus. If the user places the digital pen on the eraser icon, the processor switches into an erasing mode and the digital pen functions as an eraser. If the user places the digital pen on the marker icon, the processor switches into a marker mode and the digital pen functions as a marker. 
         [0244]    Preferably, the housing of  1320  of the receiving unit  1300  is used as an assembly jig. A worker assembling the receiving unit  1300  may insert the microphones  1330  into their position inside the body  1320 , and solder a printed electric circuit board (PCB) into a position inside the body  1320 . The worker may then connect the PCB to the microphones  1330 . 
         [0245]    Optionally, the receiving unit  1300  may be removable attached to another item, such as a paper clipboard used by a student, etc. 
         [0246]    Preferably, the housing  1320  of the receiving unit  1300  includes a changeable cover element. 
         [0247]    The changeable element may provide a user of the receiving unit  1300 , a manufacturer of the receiving unit  1300 , or both, with the option to change the color and appearance of the receiving unit  1300 . 
         [0248]    Optionally, the housing  1320  of the receiving unit  1300  may also house a serial interface cable, rolled in and out from the housing. Preferably, a connector at the end of the interface cable may be clipped to the housing  1320 . The housed interface cable helps to keep the receiving compact. 
         [0249]    Reference is now made to  FIG. 14  which is a simplified block diagram illustrating a digital pen system, according to a preferred embodiment of the present invention. 
         [0250]    A digital pen system  1400  includes a digital pen  1410 , and one or more digital pen receiver(s)  1420 , as described in greater detail hereinabove. 
         [0251]    The system  1400  further includes a processor  1450 , communicating with the receiving units(s)  1420 . 
         [0252]    The processor  1450  is configured to process acoustic signals, transmitted from the digital pen  1410  and received by the receiving unit(s)  1420 . 
         [0253]    Through the processing of the received acoustic signals, the processor  1420  determines the location of the digital pen  1410 . 
         [0254]    Optionally, the processing further includes determining the presence of the digital pen  1410  in a predefined area, and triggering a predefined functionality upon the determined presence in the predefined area, as described hereinabove. 
         [0255]    According to a preferred embodiment of the present invention, the location of the digital pen according to the acoustic signals transmitted from the digital pen is carried out utilizing a decoding algorithm. The decoding algorithm may be implemented in a decoding unit  1470 . The decoding unit  1470  may be implemented as a part of the processor  1450 , as a part of a device communicating with to the processor  1450 , as a part of the receiving unit(s)  1420 , etc. 
         [0256]    Reference is now made to  FIG. 15  which is a simplified block diagram illustrating a decoding unit, according to a preferred embodiment of the present invention. 
         [0257]    A decoding unit  70  includes a maximum likelihood detector  72 , which uses a channel mathematical signal model  77 , a correlator  71 , a maximum likelihood detector, a path estimator and transmitter timing estimator. 
         [0258]    The maximum likelihood detector  72  generates most likely distance data, relating to the distance of the digital pen from a receiving unit, based on the acoustic signals received from the digital pen, and feeds the path estimator  73  with the most likely distance data. 
         [0259]    The maximum likelihood detector  72  estimates the transmitter position and feeds the path estimator  73  with several options for location of the transmitter, each option having a probability associated therewith. The path estimator  73  further uses previously calculated possible positions from a sampling bank  75  (and their probabilities), provided by a transmitter timing estimator  76 , in order to choose the right estimated coordinates  74  of the position of the transmitter 
         [0260]    The decoding algorithm is used to convert digitized versions of the digital pen&#39;s acoustic signals into position coordinates for passing to a local computer operating system, a computer application, or the like. 
         [0261]    The decoding algorithm preferably takes into account the relatively low sampling frequency capabilities likely to be available, by carrying out frequency down conversion. Preferably, the path estimator  73  uses known in the art methods of interpolation, for compensating for the relatively low sampling rate. 
         [0262]    In addition, the algorithm preferably includes an ability to handle noise. 
         [0263]    The algorithm is preferably adapted for other specific issues, involved in the handling of the acoustic signals transmitted from the digital pen. 
         [0264]    Traditional position location methods concentrate on the use of very short and energetic acoustic signals, as the location signal. In order to achieve good resolution, the traditional methods dictate high sampling frequencies, typically higher than 400 KHz, in order to be able to find such short location signals and not miss them entirely. 
         [0265]    By contrast, the present embodiments preferably do not use sampling rates higher than 44.1 KHz, since such frequencies are incompatible with the installed base of sound processing equipment, such as the electret microphones. 
         [0266]    Furthermore, it is recommended to keep the beacon signal sound frequency higher than 20 KHz, that is within the ultrasonic range, so that users do not hear it. 
         [0267]    In another preferred embodiment of the invention, the sampling rate may be higher than the 44.1 KHz, say 100 KHz. This is possible by a receiving unit which is configured for a high sampling rate. The higher sampling rate enables better noise rejection of the audio band and higher bandwidth of the transmitted signal. 
         [0268]    A preferred embodiment of the present invention uses a solution in which data is modulated over an ultrasonic carrier signal or waveform. The data can be frequency modulated (FM), or phase modulated (PM), onto the carrier comprising the ultrasonic signal. Optionally, other known method may be used. 
         [0269]    The decoding algorithm preferably decodes the modulated signal and reconstructs the original position-information bearing signal from the results of sampling thereof. In the present embodiment, it is preferred to use band-limited signals in order to achieve a desired resolution level. 
         [0270]    Preferably, continuous wave (CW) modulations such as spread spectrum and frequency hopping are used in acoustic position finding, to overcome reverberation and multi-path effects. 
         [0271]    A preferred embodiment of the present invention uses the maximum likelihood detector  72 , for decoding the signals received from the receiving units, to determine the distances of the digital pen from the individual receiving unit(s). 
         [0272]    At the maximum likelihood detector  72 , the acoustic signals received from the receiving units are compared to reference signals in a look-up table (LUT)  68 . 
         [0273]    The comparison indicates a most likely signal, and from the most likely signal, a distance is determined as the distance from which the signal was most likely transmitted. 
         [0274]    The maximum likelihood detector  72  preferably uses a full mathematical signal model  77  of the channel, against which to compare received signals, so that a best match distance can be found. 
         [0275]    As an alternative, the expected waveform can be sampled at the Nyquist rate, and any timing mismatch between the sampling points can be overcome by extrapolation functions, to reveal the distance. 
         [0276]    Reference is now made to  FIG. 16 , which is a simplified block diagram illustrating exemplary components of a mathematical model for incorporating into a maximum likelihood detector, according to a preferred embodiment of the present invention. 
         [0277]    The model  20  comprises an initial signal sequence S(t), generated in the signal generator, which is fed into the transfer function of the acoustic transmitter  26  with its filter  25 . The digital pen  14  is followed by the channel  27 . The result is then fed to the reception path in the receiver which includes transfer function  29  for the ultrasound receiver, and filtering  30 . 
         [0278]    The full modeling of the channel is useful in the design of the maximum likelihood detector  72 , in that it allows accurate expected signals to be constructed against which the received acoustic signals, ideally, differ only in phase. 
         [0279]    The detector (estimator)  70  is then relatively easily able to distinguish the most likely signal, which in turn corresponds to the most likely distance of the digital pen from the receiving unit. 
         [0280]    Preferably, the infrared (IR) signal transmitted from the IR transmitters, spread on the face of the digital pen, are used to set the start of the delay, and also to synchronize clocks between the digital pen and the receivers. 
         [0281]    In  FIG. 15 , synchronization path  76  is also indicated on the model. 
         [0282]    A skilled person will appreciate that acoustic signals have differing angular transfer functions. An equalizer may be used in order to compensate for this fact. 
         [0283]    The skilled person will appreciate that, instead of a model, a look-up table may be used. Furthermore, other detectors may be used, and there are several known decoders of FM signals, such as PLL (An electronic circuit that consists of a phase detector, low pass filter and voltage-controlled oscillator), I/Q demodulation, phase multiplication etc. 
         [0284]    Reference is briefly made to  FIG. 17 , which is a two-part graph showing an exemplary correlation function, according to a preferred embodiment of the present invention. 
         [0285]    The top part  1710  of the graph shows the function, and the lower part  1720  of the graph is an enlarged or zoomed view of the upper central part of the graph. 
         [0286]    It is expected that during the life of this patent many relevant devices and systems will be developed and the scope of the terms herein, particularly of the terms “Digital”, “Pen”, “Acoustic transmitter”, “Ultrasound transducer”, “Microphone”, and “Processor” is intended to include all such new technologies a priori. 
         [0287]    Additional objects, advantages, and novel features of the present invention will become apparent to one ordinarily skilled in the art upon examination of the following examples, which are not intended to be limiting. Additionally, each of the various embodiments and aspects of the present invention as delineated hereinabove and as claimed in the claims section below finds experimental support in the following examples. 
         [0288]    It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. 
         [0289]    Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. All publications, patents and patent applications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual publication, patent or patent application was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art to the present invention.