there are many examples of preferred embodiments in this invention , which are addressed individually as the following : 5 . 1 embodiment 1 : a touch screen for multiple active touch - input devices using cdma ss signal structure the fundamental hardware function blocks of an active touch - input device are illustrated in fig8 . this embodiment generally comprises a method and apparatus for tracking one or more active touch - input devices on a touch sensitive surface , using the ss signal structure incorporating cdma ss codes . such touch - input devices may include a pen or stylus , knob , slider ( fader ), switch , joystick , mouse , or its equivalent . one example of an application for this embodiment is to provide a musician with a touch - screen - based mixer / editor platform . the apparatus of the invention includes at least one , and preferably a plurality of active touch - input devices 17 , as shown in fig7 . in terms of hardware implementation of the active touch - input devices , they all have generally the same electrical circuitry , with only a small modification according to their specific functionality . in fig8 there is a power supply unit 24 to provide power supply and power control to the device 27 . each touch - input device also includes a cdma code rom 25 and a waveform generator 28 , which periodically and / or continuously generate communication signals modulated by a unique cdma code of this device . these cdma signals are touch - excitation signals that are coupled into the resistive sensing layer 12 by a rx / tx unit 29 and an em transducer 30 . a control unit 31 collects the functional inputs of each device , such as on - off status of switches , etc ., and controls all the operational activities of this device . one example of implementing the waveform generator 28 in this invention in a cost - effective way , without using an expensive and power - consuming d / a circuit , is illustrated in fig9 . the concept of this implementation is first to generate the cdma signature in square wave then cut off the high order harmonics . as seen from the figure , the waveform generator 28 initiates its action by getting a clock signal from an external clock source 32 , then a column decoder 33 and a row decoder 25 a are used to address and fetch a 1 - bit data from the cdma code rom 25 . when a sequence of this data is clocked out , it forms a cdma code sequence 34 and is passed through a bandpass filter 35 to cut off the harmonics . the active touch - input device 17 may take any of several forms that have been disclosed in the prior art . as shown in fig1 a - 13 b , a knob controller 113 may include a base portion 114 , adhered to the sensing surface 84 , and upon which is mounted an upper portion 110 adapted for rotation about a common axis . the upper rotatable portion 110 also includes a device circuit 111 and an active touch stimulating tip 112 extending toward the screen layer 84 to emit excitation signals generated by circuit 111 , so that the screen assembly detects rotation of the upper portion by identifying the cdma code and tracking the movement of the tip 112 about the base 114 . with regard to fig1 a - 14 b , a fader controller ( slider ) 115 includes an elongated base portion 116 upon which a fader cap 117 is slidable to vary and select values in accordance with the position of the cap along the base . the upper slidable portion also includes a device circuit 119 and an active touch stimulating tip 118 extending toward the screen layer 84 , so that the screen assembly detects translation of the upper portion by identifying the cdma code generated by circuit 119 and tracking the movement of the tip 118 along the base 116 . with regard to fig1 a - 15 b , a joystick controller 121 includes a base portion 123 from which a control wand 120 extends upwardly . the wand 120 is coupled to the active touch stimulating tip 124 extending toward the screen layer 84 , so that the screen assembly detects translation of the wand 120 by identifying the cdma code and tracking the movement of the tip 124 on the screen . the tip movement corresponds to wand motion , whereby highly controllable input signals may be generated . the device 121 may operate in either joystick mode or mouse mode , as explained in the parent application . with regard to fig1 , a further embodiment of the active touch stimulating device is a pen or stylus 130 . the pen or stylus may include a signal generating circuit 132 and a tapered contact end supporting the active touch stimulating tip 131 extending toward the screen layer 84 to emit the excitation signal . this arrangement enables the screen assembly to detect and track the movement of the tip 131 . the stylus 130 may be wielded in the manner of a pen or pencil to mark or write on the sensing layer 84 . in addition to the active touch - input devices disclosed above , there are also embodiments to perform the system functions , which include the detection of the position of a touch - input device with respect to a sensing surface 13 , as shown in fig7 . in this embodiment , an em field 14 propagating through a conductive layer 12 is used as the physical carrier of the information embedded signals between the touch - input devices 17 and the touch - sensing surface 13 . this touch - sensing surface 13 comprises a conductive layer 12 that may be incorporated in a screen assembly , typically ( but not necessarily ) in combination with a flat panel display , monitor , or other graphic output device . the screen assembly may include a plurality of lamina , and the conductive layer 12 is typically protected by an outer layer of more durable material , such as glass or optical grade plastic material or a durable coating . alternatively , the conductive layer may be disposed at the back surface of the assembly . in the preferred embodiment the touch - sensing surface 13 is configured in a familiar rectangular format , though a wide variety of shapes are possible . at each vertex of the surface 13 there is disposed a transducer 15 to send and receive signals from the active touch - input devices 17 . the transducers are connected to a cdma rx / tx unit 19 , which does the typical signal transmission and reception jobs of a rx / tx chain in a communication channel , such as a / d , d / a , agc , power amplification , etc . after signals from touch - input devices are acquired , a cdma signal processing unit 20 is used to do the necessary signal processing procedures , such as synchronization , matched - filtering , rss estimation , etc . these results are then passed to a data processing unit 21 , to acquire data for the desired system operation , such as device identification , tracking and communication , etc . finally , this data are transferred to a master cpu controller 22 . this data may then be used in a computer system , or any electronic device that may employ a touch screen . it should be noted that in this embodiment two - way communication between the active touch - input devices and the touch - sensing surface is optional , while one - way communication from a device to the touch - sensing surface can perform the system functions well . this can greatly save the cost of system implementation . when only one - way communication is concerned , the rx / tx , unit 19 becomes a data acquisition circuit , an example of which is shown in fig1 . this implementation may include a signal source selector 40 to receive the signals from the transducers 15 and process them individually and selectively . the signal from each transducer is a mixture of cdma signatures from all the active devices operating on the screen . the signals are fed to a signal amplifier 41 and thence to an analog bandpass filter 42 to remove unnecessary noise . the filtered signal is fed to an a / d converter 43 to be converted into digital format and stored in data memory 44 , ready to be processed by the signal processing unit 20 . the operation flowchart of an active touch - input device is illustrated in fig1 . after power up , the device first checks for related input status . then , this status , together with other positioning information bits , are encoded into cdma signatures using the cdma code for this device . these signatures are then passed through a bandpass filter and then delivered to the touch - sensing surface 12 , completing the touch - excitation procedure . the operation flowchart of the touch - sensing architecture is illustrated in fig1 . after power up , the system looks for excitation signals from whatever active touch - input devices 17 are currently operating with the screen assembly 13 . these signals are processed to determine their respective cdma codes , and subsequently the position of each device 17 is determined . this data is transmitted from the cpu 22 to the computer system ( or equivalent ) that is associated with the touch screen assembly 13 . in this embodiment , a linear rrs model is used to calculate the position of each touch - input device . it is noted that the conductive layer 12 ( fig1 ) comprises an impedance that is distributed uniformly in the plane of the layer . the signal of each active touch stimulating device 17 is received by all of the transducers 15 , and the strength of each received signal is directly related to the distance from the active touch stimulating tip on the layer 12 to each of the transducers 15 . after matched - filtering the received signals with the cdma code from each touch - input device , the rss of that device can be determined . calculations may then be carried out to determine the active touch stimulating position relative to the transducers 15 , and thus to a x - y coordinate system . in this fashion a plurality of active touch stimulating devices may be tracked concurrently . to identify the presence of a touch - input device , if the rss of that device exceeds a preset threshold , it is recognized to be operating on the screen . thereafter , the determination of the touch stimulating device location is carried out based on rsss obtained from different transducers . as is known in the prior art , the rss is proportional to the distance from each active touch - input device to each transducer , and the position data for each active touch - input device may be calculated from ratios of the rss data from each transducer . the rss position locating arrangement of the invention is facilitated by the use of cdma signals regarding their orthogonality . as shown in fig1 a - 17 b , two examples of cdma codes of active touch stimulating device signals , cdma code a and cdma code b , are comprised of binary bits in series . these two codes occupy the same spectrum , which goes from f 1 - f 2 and is fairly flat across the entire bandwidth . generally speaking , the number of ones and zeros in a cdma code are approximately equal and evenly distributed in time so that the spectrum is generally flat . cdma methodology has been used for wireless communications by military organizations to encode communications information so that the carrier appears to be noise , and thus to be difficult to detect and intercept . cdma systems operate with high reliability in noisy environments , yet require relatively low power and have relatively high data rates . in this invention , cdma methodology is used in several unique ways : 1 ) the system may use only one - way communications from the active touch stimulating device ( s ) to the screen assembly , rather than two - way communications of prior systems ; 2 ) once the rss is obtained , it is used for both device detection and tracking . in typical cdma communications systems , rss is used only for detection . 3 ) this invention uses the sensing layer 12 as the signal propagation medium , rather than a broadcast em field used in wireless cdma communications . the sensing layer as a propagation medium enables the linear rss model for the positioning of the system . 4 ) the number of active touch stimulating devices may be one , or more than one , depending on the needs of the user . there are known in the prior art many methods for generating cdma coded transmissions . one technique involves using walsh functions to generate cdma codes according to the channel bandwidth of the system . another process is to generate a white noise signal , and then store it as a cdma code . any cdma code generating method known in the prior art may be used in this invention . with reference to fig8 the entire circuitry for an active touch stimulating device may be embodied in one custom asic 27 having approximately 2000 gates or less . that is , the power supply unit 24 , the clock logic 26 , the cdma code rom 25 , the waveform generator 28 , and the control unit 31 , and the rx / tx unit 29 may all be formed in one asic , thereby minimizing the device size and enabling a device of small dimensions . the use of a custom asic also makes the active touch stimulating devices more rugged by reducing component connections , and it minimizes power consumption . it should be noted that the screen assembly of this embodiment is also capable of being combined with components that register a finger touch . a finger touch typically acts to capacitively couple to the sensing layer the body attached to the touching finger . current is drawn from the sensing layer of the screen assembly , and current flow from each of the signal pickups may be determined and calculations made to locate the finger touch on the screen . another method for combining the finger touch with the cdma coded active touch stimulating arrangement is to assign the cdma to a bandwidth that does not interfere with the noise bandwidth coupled to the screen sensing layer by the finger touch . 5 . 2 embodiment 2 : an acoustic wave touch screen with multiple passive touch - input devices the acoustic wave touch detection system of this embodiment is illustrated in fig1 . this embodiment generally comprises a method and apparatus for tracking one or more passive touch - input devices on a display surface and / or touch sensitive surface , using a ss signal structure . such touch - input devices may include a pen or stylus , knob , slider ( fader ), switch , joystick , mouse , or the equivalent . typical applications of this embodiment include using it as a computer touch - screen , a digitizer , a writing pad , a mouse pad , a computer - game input apparatus such as joysticks , etc . the apparatus of the invention includes at least one , and preferably a plurality of passive touch - input devices 74 . one common feature of these passive touch - input devices is that they have one or more acoustic reflectors , and are devoid of any internal electronic components . these reflectors are used as the signal sources of each touch - input device . in the configuration , an acoustic wave field 70 propagating through air 71 is used as the physical carrier of the communication between the passive devices and the touch - sensing apparatus . in the system , an acoustic transmitter 79 and an rx / tx circuit 75 are used to generate the ss signal carrying acoustic wave field 70 , which is distributed across the surface of the display 72 . when a passive touch - input device 74 , such as a passive knob 86 ( fig2 a - 21 b ) with positioning reflector ( s ) 83 , is placed within this field , it will reflect the ss signals transmitted from the transmitter 79 . these reflected ss signals , termed echoes , are picked up by the acoustic receivers 73 placed on the surface of the display 72 ( this surface could be the surface of a touch screen ), and amplified and filtered by an acoustic rx / tx circuit 75 . these echoes are then processed by a signal processing unit 76 , using time - delay models , to track the position of the passive device . finally , the system operational results are processed by a data processing unit 77 and a mcu 78 is used to build up the data communication between the touch sensing system and the master pc 23 . it should be noted that in fig1 the number of the transmitter and receivers , as well as their placement geometry , are for illustration only . exact configuration may vary from one embodiment to another . for example , in one embodiment of a touch - screen for computers , the configuration could be a transmitter placed at the middle of one side and two receivers which are placed at the two top corners of the screen , respectively . in another embodiment of a square - shaped digitizer , the transducer configuration could be two transmitters at the middle of the right - hand and left - hand sides , and four receivers at the corners , respectively . the passive touch - input devices 74 may have different implementations according to their specific functional requirements . as shown in fig2 a - 20 c , a passive mouse 80 a may have 3 reflectors to perform its function . the positioning reflector 81 is fixed in the middle of its front to yield its position information , two other reflectors , the right button 82 and the left button 80 , are designed as moveable buttons to yield the clicking status . positioning resolution of these 3 reflectors may be designed to be different due to their different functionality . as shown in fig2 a , a regular pen or stylus 90 a may have only one positioning reflector 91 a to perform its function as a pen to write on the touch - sensing tablet . a mouse - pen 90 b ( fig2 b ) may have two more reflectors , right button 92 and left button 93 , to function as a pen as well as a mouse . referring to fig2 , a knob controller 86 may include a base portion 87 upon which an upper portion 85 that is mounted for rotation about a common axis . the upper rotatable portion also includes a positioning reflector 83 , so that the touch - sensing apparatus can detect the position and rotation of the upper portion by tracking this reflector . with regard to fig2 a - 23 b , a slider ( fader ) controller 96 a may have two portions in its structure . the base portion ( fader track ) 95 is adhered to the touch - sensing surface stage when the slider is in use and the sliding portion ( fader cap ) 96 can move along the fader track . on the fader cap , in order not to be covered by the operator &# 39 ; s finger , four positioning reflectors 94 are used to tell the relative position of the fader cap along the fader track . the system can automatically choose one of the reflectors 94 to do the positioning . with regard to fig2 , a joystick 99 a may also have two portions in its structure . the base portion 98 is fixed to a place near the touch - sensing tablet when the joystick is in use and the handle 99 can make 3 - d movements on this base . a positioning reflector 97 is used to sense the 3 - d movement of the handle and a fire - button reflector 96 is used to simulate the fire operation . it should be noted that , as air is used as the propagating medium , the acoustic wave is not confined to the surface of the touch - sensing tablet , and also not confined to a 2 - d plane . that is to say , with a proper transducer orientation arrangement , in this embodiment the system can perform 3 - d positioning of the passive touch - input devices . for example , suppose a touch screen system is designed to have a passive joystick 99 a as one of its touch - input devices . then the transducers could have their tx / rx beam patterns , the effective coverages of directional transducers , to cover the computer screen surface , as well as a 3 - d space within 1 - 2 meters from the screen . the joystick 99 a with passive reflectors 96 and 97 can be placed anywhere within the acoustic wave covered space to perform its functions . the touch system can perform 2 - d positioning for the touch - input devices on the screen surface , such as a pen 90 a and a knob 86 , and 3 - d positioning of the devices outside the screen surface , such as a joystick 99 a and a wireless mouse 80 a . in this embodiment , as seen from fig1 , the system operation is identical for each acoustic channel , which the reflectors of different passive touch - input devices may be assigned to . the ability to distinguish multiple touch input devices can be accomplished by many methods . one example is to use reflectors that have a unique reflectivity which responds to certain frequencies . for instance , one type of reflector may have very great reflectivity at one frequency but not at other frequencies and another reflector may have very great reflectivity at another frequency that is clearly distinguishable from the first reflector &# 39 ; s frequency and so on . another example is to use the 2 - d and / or 3 - d modeling ability of this system . this modeling capability enables the system to distinguish multiple devices that have been placed in different positions on the touch sensing area . multiple reflectors can be used on a single device where each reflector is distinguishable from the others . one method to do this is to use 3 - d modeling to distinguish multiple reflectors utilized on a single device , i . e ., a mouse or pen , where all of the reflectors on this device respond to the same frequency , but each individual reflector is distinguishable from each other by knowing the position of each reflector in 3 - d space . as shown in fig2 a - 20 c and fig2 b , the buttons 80 - 82 and 92 - 93 may comprise depressable surfaces , and their movement , when depressed , is detectable by the 3 - d modeling system and converted to button commands . arrangements in the prior art for forming depressable buttons are widely known and widely diverse . for the operation at each channel , the system first generates an acoustic wave field by periodically transmitting a ss code , such as a chirp , across the touch - sensing surface . echoes from the reflectors within this field are then collected by the receivers placed in a preset geometry . these recorded echoes are matched - filtered with the transmitted ss code to obtain the matched - filtered echoes , which are also called compressed echoes in that they are compressed in time . using the receiver geometry and the time instants of the peaks in the compressed echoes , the locations of the reflectors can then be identified . this positioning data is then passed through the master pc for further processing . time - delay positioning models based on acoustic wave technology have been widely used in both military and civilian applications for more than 100 years . the first patent regarding this technique can be traced to methods of detecting the range of underwater icebergs , after the tragedy of titanic shocked the world . since then sonar ( sound navigation and ranging ) and radar ( radio detection and ranging ) have been developed with a variety of applications , most of which are based on time - delay models . some times methods developed for either sonar or radar can be applied to each other without major modifications , after acquiring signals from their individual rx / tx front ends . in this embodiment , the following aspects are believed to be unique compared to current touch - screen technologies : 1 ) time - delay models , instead of rss models , are used . this yields higher resolution due to the relatively easy - to - achieve high resolution in time - delay estimation of the acoustic signal . 2 ) no sensing layer is required for approaches where air can be used as the propagating medium . 3 ) ss signal structures are used to eliminate multi - path , a severe problem in narrow band systems . 4 ) passive devices can be used , so the devices can be very low cost . 5 ) the touch receiving system can be low cost because no special sensing layer is required . 6 ) transducers can be placed anywhere near the display &# 39 ; s surface and / or a touch screen . as devices are placed on or near a display , calibration procedures can be used to locate and enable the operation of these transducers at any location . transducers are not confined to a preset place or geometry . 7 ) touch - input devices can be used to trigger touch - input events anywhere in a 3 - d space near the touch sensing - tablet . there is sufficient resolution in 3 - d spatial resolution to detect the actuation of control buttons ( mouse buttons , pen buttons , joystick buttons , etc .). such devices are not confined to a preset place or geometry . 8 ) more advanced and well - developed signal processing methods based on time - delay models can be utilized to increase the positioning accuracy ( touch resolution ) of any device . 9 ) using spread spectrum technology , ma can be realized for passive devices by assigning different frequency bands to different devices . this embodiment can be generally regarded as a simplified version of embodiment 2 , where the touch - input devices are limited to a single passive touch - input pen 90 c , as shown in fig2 c . in this embodiment , the touch - input pen 90 c is a passive pen which can be made by putting a small acoustic reflector 91 c at the lower part of a stylus . with the regular functionality disclosed in the embodiment 2 , this pen can function as an touch - input pen to perform the system functions such as touch - and - select , writing , etc . combined with the embodiment disclosed in companion patent application ser . no . ______ , filed ______ , the passive touch - input pen 90 c can be used to operate a graphic knob controller ( an emulated graphic knob on a display ). by placing the pen over the approximate center of a graphic image of a knob on a display and twisting the pen between one &# 39 ; s fingers , the graphic knob can be rotated and thereby be used as a physical controller . as disclosed in embodiment 2 of this invention , 3 - d position data of the reflector on the touch - input pen can be acquired by the touch system . this data can be then post - processed by the system of this invention to obtain further information such as rotary angle , moving speed , rotary axis , etc . this information is then used to operate the graphic knob - controller . in short , the passive touch - input pen 90 c in this embodiment can be used as a stylus , as well as a physical knob - controller , due to the 3 - d positioning capability of the touch - input system . the advantage of this embodiment , as compared to embodiment 2 , is its simplicity in hardware . as there is only one touch - input device , the system hardware configuration , such as rx / tx chain , mips requirement , could be very simple . a product example of this embodiment would be very low - cost touch - screens for computers , palm computers , high end cell phones , etc . this embodiment can be generally regarded as a simplified version of the embodiment of fig6 where the touch - input devices are limited to a single active touch - input pen 130 , and the excitation signals are not limited to cdma signal structures . in this embodiment , the touch - input pen 130 ( fig1 ) is an active pen which can emit an excitation signal with an ss signal structure . this excitation signal can be used to track the touch - position of the pen when the pen touches the touch - sensing tablet to introduce a touch event . in the instance where there is only one touch - input device allowed by the touch system , no ma method would be required . the excitation signal could take the form of one of any of the available ss signal structures , such as a chirp signal . this allows the pen to have maximum freedom in hardware configuration , while the advantage of using an ss signal structure remains unchanged . for example , to generate a ss signal in analog output form , in some cases a chirp signal could be much more easily generated than a cdma signal . fig2 is an example of the circuit used in the pen . in this figure , a comparator 101 is used to form a multivibrator 100 to generate a voltage increased by time . this voltage is then used to control the frequency of the vco ( voltage - controlled - oscillator ) 104 . the output of the vco is then an analog chirp signal , which is coupled by the em transmitter 102 ( a capacitor , for example ) onto the touch - sensing layer . since a chirp signal that is generated in this manners has less harmonic distortion than the cdma signals generated using the circuit 27 disclosed in fig8 a higher effective pg for the chirp can be expected . this embodiment can also be combined with the embodiment disclosed in the patent application ser . no . 09 / 139 , 078 , filed aug . 24 , 1998 , addressed above . the touch - input pen can be used to operate a graphic knob controller , in a way similar to the mouse disclosed in that patent application . however , here in this embodiment there are some improvements . with the more accurate 2 - d positioning of the pen tip using a ss signal structure ( and the handy drawing capability of using a pen ), this operation becomes much easier and more accurate . the foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description . it is not intended to be exhaustive or to limit the invention to the precise form disclosed , and many modifications and variations are possible in light of the above teaching without deviating from the spirit and the scope of the invention . the embodiments described is selected to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as suited to the particular purpose contemplated . it is intended that the scope of the invention be defined by the claims appended hereto .