U.S. patent application number 13/171601 was filed with the patent office on 2011-10-20 for noise reduction in digitizer system.
This patent application is currently assigned to N-trig Ltd.. Invention is credited to Haim PERSKI, Ori Rimon.
Application Number | 20110254807 13/171601 |
Document ID | / |
Family ID | 34910939 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110254807 |
Kind Code |
A1 |
PERSKI; Haim ; et
al. |
October 20, 2011 |
NOISE REDUCTION IN DIGITIZER SYSTEM
Abstract
A digitizer that includes plurality of detecting elements for
detecting an electromagnetic signal. A method for noise reduction
in the digitizer includes determining a location of a stylus with a
known frequency of emission over a first sampling period, sampling
output from the plurality of detecting elements substantially
simultaneously over a second sampling period, the second sampling
period subsequent to the first sampling period, identifying
detecting elements with output above a defined threshold in a
selected frequency that is close to but different than the known
frequency, wherein the detecting elements identified are identified
based on the output sampled over the second sampling period,
selecting from the detecting elements identified in the second
sampling period, a detecting element that is spaced away from the
location of the stylus, wherein the detecting element selected is
selected as a candidate carrier of mere noise, and reducing output
sampled in the second sampling period of at least one other
detecting element in accordance with output of the detecting
element selected as a candidate carrier of mere noise.
Inventors: |
PERSKI; Haim; (Hod-HaSharon,
IL) ; Rimon; Ori; (Tel-Aviv, IL) |
Assignee: |
N-trig Ltd.
Kfar-Saba
IL
|
Family ID: |
34910939 |
Appl. No.: |
13/171601 |
Filed: |
June 29, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11063535 |
Feb 24, 2005 |
7995036 |
|
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13171601 |
|
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60547772 |
Feb 27, 2004 |
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Current U.S.
Class: |
345/174 |
Current CPC
Class: |
G06F 3/046 20130101;
G06F 3/0412 20130101; G06F 2203/04108 20130101; G06F 3/04162
20190501; G06F 3/0441 20190501; G06F 3/0442 20190501; G06F 3/04182
20190501; G06F 3/03545 20130101; G06F 2203/04106 20130101 |
Class at
Publication: |
345/174 |
International
Class: |
G06F 3/044 20060101
G06F003/044 |
Claims
1. A method for noise reduction in a digitizer, the digitizer
comprising a plurality of detecting elements for detecting an
electromagnetic signal, comprising: determining a location of a
stylus with a known frequency of emission over a first sampling
period; sampling output from the plurality of detecting elements
substantially simultaneously over a second sampling period, the
second sampling period subsequent to the first sampling period;
identifying detecting elements with output above a defined
threshold in a selected frequency that is close to but different
than the known frequency, wherein the detecting elements identified
are identified based on the output sampled over the second sampling
period; selecting from the detecting elements identified in the
second sampling period, a detecting element that is spaced away
from the location of the stylus, wherein the detecting element
selected is selected as a candidate carrier of mere noise; and
reducing values of output sampled in the second sampling period of
at least one other detecting element, in accordance with output of
the detecting element selected as a candidate carrier of mere
noise.
2. The method of claim 1, comprising locating the stylus in a
second sampling period based on a signal level of the reduced value
of output of the at least one other detecting element.
3. The method of claim 1, comprising detecting from each of the
plurality of detecting element, output in the selected frequency
and output in the known frequency.
4. The method of claim 1, comprising utilizing signal level of
output in the selected frequency to estimate a noise level in the
known frequency of the stylus.
5. The method of claim 1, wherein output of candidate carrier of
mere noise is at least in part a consequence of presence of any
member of a group of: a finger, hand or palm.
6. The method of claim 1, wherein said digitizer is adapted for
finger touch detection and wherein the selected frequency is a
frequency used for said finger touch detection.
7. The method of claim 1, wherein the detecting element that is
spaced away from the location of the stylus is spaced away from the
location of the stylus by a distance that is above a predefined
distance.
8. The method of claim 1, wherein the detecting element selected
from the detecting elements identified is a detecting element that
is furthest from the location of the stylus.
9. The method of claim 1, wherein the selected frequency is a
frequency within a defined range of frequencies around the known
frequency of the stylus.
10. The method of claim 9, wherein the selected frequency is a
frequency having a highest level output in the defined range.
11. The method of claim 1 further comprising: detecting output in
each of a plurality of frequencies within a defined range of
frequencies around the known frequency of the stylus; and defining
the selected frequency as a frequency with a highest output from
the output in each of the plurality of frequencies.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 11/063,535 filed on Feb. 24, 2005, which
claims the benefit of priority of U.S. Provisional Patent
Application No. 60/547,772 filed on Feb. 27, 2004. The contents of
the above applications are all incorporated herein by
reference.
FIELD AND BACKGROUND OF THE INVENTION
[0002] The present invention relates to noise reduction in a system
comprising a digitizer and, more particularly, but not exclusively
to noise reduction in a system comprising a digitizer associated
with a display screen.
[0003] U.S. Pat. No. 6,690,156 "Physical Object Location Apparatus
and Method and a Platform using the same" assigned to N-trig Ltd,
and U.S. patent application Ser. No. 10/649,708 "Transparent
Digitizer" also assigned to N-trig Ltd, describe a positioning
device capable of detecting multiple physical objects, preferably
styluses, located on a flat screen display. One of the preferred
embodiments in both patents describes a system built of transparent
foils containing a matrix of vertical and horizontal conductors. In
one embodiment the stylus includes a passive resonance circuit,
which is triggered by an excitation coil that surrounds the foils.
The stylus is excited at a predetermined range of frequencies
depending on the capacitance and inductance of the resonant
circuit. Other embodiments may include a different kind of EM
stylus. The exact position of the stylus is determined by
processing the signals that are sensed by the matrix of horizontal
and vertical conductors.
[0004] Existing digitizer systems use several noise removal methods
to improve the detection precision. For example the received signal
is processed through a band pass filter leaving a window of
frequencies including the stylus frequency. The filtered signal may
then be passed through a Fourier transform selecting the single
frequency of the stylus.
[0005] Elements that induce an equal amount of noise on each
conductive line regardless of the line location may then be
eliminated through the use of differential amplifiers. For example,
objects that are far enough from the sensor will have the same
effect on all the sensor lines.
[0006] There are other examples of noise reduction methods that do
not eliminate noise at the stylus frequency.
[0007] Using the various prior art systems, much of the noise is
removed, but one element of noise necessarily remains because it
cannot be identified and filtered out and that is noise that is at
the same frequency as the stylus.
[0008] Preferred Application
[0009] The preferred application to which the embodiments to be
described hereinbelow are applicable is a transparent digitizer for
a mobile computing device that uses a flat panel display (FPD)
screen. The digitizer detects the position of one stylus at a very
high resolution and update rate. The stylus is used for pointing,
painting, writing (hand write recognition) and any other activity
that is typical for a stylus. The digitizer supports full mouse
emulation. As long as the stylus hovers above the FPD, a mouse
cursor follows the stylus position. Touching the screen stands for
left click and a special switch located on the stylus emulates
right click operation.
[0010] The application may utilize a passive EM stylus. External
excitation coils that surround the sensor are utilized to energize
the stylus. However, other versions may include an active stylus,
battery operated or wire connected, which does not require external
excitation circuitry.
[0011] In one application the electromagnetic object responding to
the excitation is a stylus. However, other embodiments may include
other physical objects comprising a resonant circuit or active
oscillators, such as gaming pieces. Applications describing gaming
tokens comprising resonant circuits are described in U.S. Pat. No.
6,690,156 ("physical object location apparatus and method and a
platform using the same").
[0012] In the preferred application, the digitizer can detect
simultaneous and separate inputs from an electromagnetic stylus and
a user finger. Hence, it is capable of functioning as a touch
detector as well as detecting a stylus. However, other embodiments
may include a digitizer capable of detecting only an
electromagnetic stylus.
[0013] In a preferred application, the stylus supports full mouse
emulation. However, in different applications the stylus could
support additional functionality such as an Eraser, change of
color, etc. In other embodiments the stylus could be pressure
sensitive and changes its frequency or changes other signal
characteristics in response to user pressure.
[0014] In a preferred application, the mobile device is an
independent computer system having its own CPU. In different
embodiments the mobile device might only be a part of system such
as a wireless mobile screen for a Personal Computer.
[0015] In a preferred application, the digitizer is integrated into
the host device on top of the FPD screen. In additional application
the transparent digitizer can be provided as an accessory that
could be placed on top of a screen. Such a configuration can be
very useful for laptop computers, which are already in the market
in very large numbers. Such systems can turn a laptop into a
powerful device that supports hand writing, painting or any other
operation enabled by the transparent digitizer.
[0016] In a preferred application, the digitizer supports one
stylus. However, in different applications more than one stylus may
operate simultaneously on the same screen. Such a configuration is
very useful for entertainment application where multiple users can
paint or write to the same paper-like screen.
[0017] In one application, the digitizer is implemented on a set of
transparent foils. Alternatively such a digitizer may be
implemented using either a transparent or a non-transparent sensor.
One example is a Write Pad device, which is a thin digitizer that
is placed below normal paper. In this example, the stylus combines
real ink with electro magnetic functionality. The user writes on
the normal paper and the input is simultaneously transferred to a
host computer to store or analyze the data.
[0018] An additional example of a non-transparent sensor is an
electronic entertainment board. The digitizer, in this example, is
mounted below the graphic image of the board, and detects the
position and identity of gaming figures that are placed on top the
board. The graphic image in this case is static, but it could be
manually replaced from time to time (such as when switching to a
different game).
[0019] In some applications a non-transparent sensor could be
integrated in the back of a FPD. One example for such an embodiment
is an electronic entertainment device with a FPD display. The
device could be used for gaming, in which the digitizer detects the
position and identity of gaming figures. It could also be used for
painting and/or writing in which the digitizer detects one or more
styluses. In most cases, a configuration of non-transparent sensor
with a FPD will be used when high performance is not critical for
the application.
TECHNICAL DESCRIPTION
[0020] Transparent Digitizer
[0021] A preferred digitizer allows for the location and
identification of physical objects, such as styluses and user's
fingers. Identifying the location of the physical objects is sensed
by an electro magnetic transparent digitizer that is mounted on top
of a display. The transparent digitizer is described in U.S. Pat.
No. 6,690,156 and detailed in U.S. patent application Ser. No.
10/649,708.
[0022] The various components and functionality manner of the
transparent digitizer are as follows.
[0023] Sensor
[0024] In the preferred digitizer, the sensor is a grid of
conductive lines made of conductive materials, such as ITO or
conductive polymers, patterned on a transparent foil or substrate.
For further information please refer to U.S. patent application
Ser. No. 10/649,708, sub-heading: "Sensor", the contents of which
are hereby incorporated herein by reference.
[0025] Front End
[0026] In the preferred digitizer the Front end is the first stage
where sensor signals are processed. Differential amplifiers amplify
the signals and forward them to a switch, which selects the inputs
to be further processed. The selected signal is amplified and
filtered by a filter & amplifier prior to sampling. The signal
is then sampled by an A2D and sent to a digital unit via a serial
buffer. For further information please refer to U.S. patent
application Ser. No. 10/649,708, under the heading "Front end", the
contents of which are hereby incorporated by reference herein.
[0027] Digital Unit
[0028] In the preferred digitizer the digital unit functions as
follows: The front-end interface receives serial inputs of sampled
signals from the various front-ends and packs them into parallel
representation. A processing unit, such as a DSP core or a
processor, which performs the digital unit processing, reads the
sampled data, processes it and determines the position of the
physical objects, such as stylus or finger. The Digital unit can be
embedded in an ASIC component. The calculated position coordinates
are sent to the host computer via link. For further information
please refer to subheading: "Digital unit" in U.S. patent
application Ser. No. 10/649,708, the contents of which are hereby
incorporated by reference.
[0029] Detector
[0030] The detector consists of the digital unit and the Front
end.
[0031] Detection of Stylus
[0032] The preferred digitizer utilizes a passive electromagnetic
(EM) stylus. The stylus comprises two main sections; the first
section is an energy pick-up circuit and the second section is an
active oscillator which is coupled to the stylus tip. An external
excitation coil that surrounds the sensor supply energy to the
energy pick up circuit. The received energy is transferred to the
active oscillator through a rectifying component such as a diode
bridge. The exact position of the stylus is determined by the
detector, which processes the signals sensed by the sensor. In the
preferred embodiment only the electric wave of the electromagnetic
signal generated by the stylus, is utilized; However, other
embodiments may utilize the magnetic portion in addition or instead
of the electric wave. For further information please refer to U.S.
patent application Ser. No. 10/649,708 assigned to N-trig, and US
provisional patent application "Electromagnetic Stylus for a
Digitizer System" filed December 2004, also assigned to N-trig, the
contents of both applications are hereby incorporated by
reference.
[0033] In the preferred digitizer, the basic operation cycle
consists of windowing, FFT/DFT, peak detection, interpolation,
filtering and smoothing. For further information please refer to
U.S. patent application Ser. No. 10/649,708, sub-title:
"Algorithms".
[0034] Noise Sources
[0035] There may be a variety of noise sources in the stylus
frequency range. The most common signals interfering with the
stylus signals are signals that originate from conductive objects,
such as a user finger, touching the screen. FIG. 1 is an electrical
equivalent of a user finger touching one of the digitizer's
antennas. When the user touches an antenna 11 a capacitance 12 is
formed between the finger and the sensor conductors.
[0036] The noise situation is best explained with respect to finger
induced noise signals.
[0037] There are two main scenarios that cause finger induced
signals--
[0038] 1. When the system is not connected to the common ground,
electrical network vibrations lead to system oscillations 10 in
reference to the ground. Since the user's body is not oscillating,
the capacitance 12 between the user's finger and the system induces
leakage current 13 through the user's finger to the ground.
[0039] 2. When the user's body is subjected to electromagnetic
interferences from the environment, it, and any associated finger,
oscillates in reference to the system; as a result a leakage
current is induced from the user's finger to the conductive
antennas.
[0040] In both cases, the digitizer senses a leakage current
originating from the user touching the sensor. When the leakage
current induces a signal that is at the same frequency of the
stylus, the leakage current can be mistaken for a stylus
signal.
[0041] A second possible source of noise is the electronic
components within the system, which radiate at many frequencies.
These components may induce noise signals at the stylus frequency;
thus interfering with stylus detection. Electronic devices placed
in proximity to the system, such as cellular phones, may also
radiate in frequencies that interfere with the stylus
detection.
[0042] FIG. 2 is an example of a noise and stylus affecting the
sensor at the same time. In this case the noise source is the
user's finger touching the screen. The sensor 20 comprises a matrix
of conductive lines. When stylus tip 21 is present at the surface
of the sensor it affects the antennas in its proximity. One or more
antennas in proximity to the stylus may suffer noise signals
induced by a finger 22 touching the sensor. For example, antenna 23
exhibits signals induced by both stylus 21 and finger 22.
[0043] Erroneous Stylus Detection
[0044] In a digitizer of this kind, the stylus detection comprises
two detection steps. The first step is to find the antenna
exhibiting the maximum stylus signal. The second step is
calculating the stylus position by interpolating the signals on the
maximum signal antenna and its surrounding antennas.
[0045] A digitizer system designed to detect an electromagnetic
stylus may suffer from two kinds of problems. The first kind is
when the unwanted signals are stronger then the stylus signals,
thus interfering with the first detection step. In this case the
digitizer system should sample and employ the noise removal
algorithm on all the antennas in order to reveal the antenna
exhibiting the maximum stylus signal.
[0046] Reference is now made to FIG. 3, which describes a case when
a user's finger 22 touching the screen induces a stronger signal 33
than the stylus 21, causing the digitizer to mistake the finger for
the stylus. As a result the digitizer chooses the wrong antennas
for interpolation.
[0047] The second kind of problem is when the stylus signal is
stronger than the finger-induced signal. However, an error in the
stylus detection may still occur during the interpolation step of
the detection. FIG. 4, to which reference is now made, describes a
case when a user's finger 22 touching the screen induces a signal
that causes the digitizer to miscalculate the stylus position 34.
The user finger 22 induces a signal on one of the X axis antennas
31 while the stylus 21 is located closer to a different X axis
antenna 32. The signal received on the stylus antenna 32 is weaker
than the signal 33 received on the finger antenna 31. Hence, the
digitizer will miscalculate the stylus position 34.
[0048] The object of the present invention is to solve both cases
and eliminate noise above and below the level of the stylus
signal.
[0049] There is thus a widely recognized need for, and it would be
highly advantageous to have, a noise reduction system devoid of the
above limitations.
SUMMARY OF THE INVENTION
[0050] According to one aspect of the present invention there is
provided a method for noise reduction in a digitizer, the digitizer
comprising a plurality of detecting elements for detecting an
electromagnetic signal at one of a number of predetermined
frequencies: the method comprising:
[0051] sampling at least two of the detecting elements
substantially simultaneously to obtain outputs therefrom, and
[0052] reducing the output on one of the two elements in accordance
with the output on the other of the elements.
[0053] Preferably, one of the at least two elements is selected as
a candidate carrier of a stylus signal and the other of the at
least two elements is selected as a candidate carrier of mere
noise.
[0054] The method preferably comprises detecting at each of the
elements the one predetermined frequency and another arbitrary
frequency.
[0055] The method preferably comprises utilizing the arbitrary
frequency to estimate the amount of noise at the predetermined
frequency.
[0056] The method preferably further comprises:
[0057] determining a ratio between signals of the one predetermined
frequency and the arbitrary frequency at the candidate carrier of
mere noise,
[0058] from the ratio and the arbitrary signal at the candidate
carrier of the stylus signal determining an amount of noise at the
one predetermined frequency, and
[0059] reducing a signal at the one predetermined frequency at the
candidate carrier of a stylus signal by the determined amount of
noise.
[0060] Preferably, the arbitrary frequency is selected as a
frequency within a preset detection range having a relatively high
noise.
[0061] Preferably, the digitizer is also for touch detection and
the arbitrary frequency is selected as a frequency already used for
the touch detection.
[0062] The method preferably further comprises deliberately
generating noise at the arbitrary frequency.
[0063] Preferably, the predetermined frequency is changed to a new
frequency during use, the method comprising changing the arbitrary
frequency from a frequency relatively close to the predetermined
frequency to a second frequency relatively close to the new
frequency.
[0064] Preferably, the candidate carrier of mere noise is selected
from a group of elements exhibiting more than a threshold amount of
noise, as the element in the group which is furthest away from a
stylus previously known location.
[0065] Alternatively, the candidate carrier of mere noise may be
selected as an element exhibiting a strongest noise signal as long
as it is beyond a determined distance from a stylus previously
known location.
[0066] Alternatively, the candidate carrier of mere noise is
arbitrarily selected,
[0067] noise subtraction is carried out over a group of elements on
the basis of the selection being correct,
[0068] a resulting signal pattern over the group of elements is
analyzed, and
[0069] the arbitrary selection is allowed if the resulting signal
pattern is indicative of a correct selection, otherwise a new
arbitrary selection is made.
[0070] The method may comprise using as patterns indicative of a
correct selection a first pattern indicative of a stylus at one of
non-selected elements, and a second pattern indicative of no stylus
being present.
[0071] The method may comprise verifying the presence of a stylus
before carrying out the reduction.
[0072] The method may comprise verifying the presence of a stylus,
before carrying out the reduction, by comparing magnitudes at the
arbitrary frequency and magnitudes at the predetermined
frequency.
[0073] In one preferred embodiment:
[0074] the candidate carrier of mere noise is arbitrarily
selected,
[0075] noise subtraction is carried out over a group of antennas on
the basis of the selection being correct,
[0076] a resulting signal pattern over the group of antennas is
analyzed, and
[0077] the arbitrary selection is rejected if the resulting signal
pattern is indicative of an incorrect selection, and a new
arbitrary selection is made.
[0078] The method may comprise using as a pattern indicative of an
incorrect selection a pattern indicative of a stylus at or near the
selected candidate.
[0079] The method may comprise using a complex proportion to
compensate for at least one of phase and magnitude differences
between respective antennas during the compensating.
[0080] The method may comprise sampling a group of antennas of an
array, selecting at least one antenna which is least affected by
the stylus and reducing respective outputs of at least some
remaining antennas in accordance with the output of the selected
antenna.
[0081] The method may comprise using a plurality of arbitrarily
selected frequencies to calculate the reduction.
[0082] The method may comprise using an average output of the
plurality of arbitrarily selected frequencies to calculate the
reduction.
[0083] Preferably, the detecting elements are conductive detectors
of a flat array of the digitizer for digitizing signals of a
movable object to indicate location of the object.
[0084] Preferably, the noise is at least in part a consequence of
the presence of a finger.
[0085] Preferably, the digitizer comprises a detection surface for
carrying the detecting elements, the detecting surface further
being touch sensitive.
[0086] According to a second aspect of the present invention there
is provided apparatus for noise reduction in a digitizer, the
digitizer comprising a plurality of detecting elements for
detecting an electromagnetic signal at one predetermined frequency
of a plurality of predetermined frequencies: the apparatus
comprising:
[0087] a sampler for sampling at least two of the detecting
elements substantially simultaneously to obtain outputs therefrom,
and
[0088] a noise reduction unit for reducing the output on one of the
two detecting elements in accordance with the output on the other
of the detecting elements.
[0089] Preferably, one of the detecting elements is selected as a
candidate carrier of a stylus signal and the other of the detecting
elements is selected as a candidate carrier of mere noise.
[0090] The apparatus may comprise a frequency detector for
detecting outputs at each of the detecting elements at the
predetermined frequency and another arbitrary frequency.
[0091] The apparatus may comprise
[0092] a ratio finder for determining a ratio between outputs at
the predetermined frequency and the arbitrary frequency at the
candidate carrier of mere noise,
[0093] and wherein the noise reduction unit is operable with the
ratio finder to: [0094] determine from the ratio and the arbitrary
signal at the candidate carrier of the stylus signal determining an
amount of noise at the predetermined frequency, and [0095] reduce
the output at the predetermined frequency at the candidate carrier
of a stylus signal by the determined amount of noise.
[0096] Preferably, the detecting elements are transparent
conductors.
[0097] Preferably, the arbitrary frequency is selected as a
frequency within a preset detection range having a relatively high
noise.
[0098] Preferably, the arbitrary frequency is selected as a
frequency already used for finger detection.
[0099] The apparatus may comprise a noise generator for
deliberately generating noise at the arbitrary frequency
[0100] Preferably, the predetermined frequencies are liable to
change during use, the apparatus accordingly being configured to
change the arbitrary frequency from a frequency relatively close to
a first predetermined frequency to a second frequency relatively
close to a second predetermined frequency.
[0101] Preferably, the candidate carrier of mere noise is selected
from a group of detection elements exhibiting more than a threshold
amount of noise, as the element in the group which is furthest away
from a stylus previously known location.
[0102] Preferably, the candidate carrier of mere noise is selected
as an element exhibiting a strongest noise signal as long as it is
beyond a determined distance from a stylus previously known
location.
[0103] In an embodiment, the candidate carrier of mere noise is
arbitrarily selected or otherwise chosen, say using a selection
algorithm,
[0104] noise subtraction is carried out over a group of elements on
the basis of the selection being correct,
[0105] a resulting signal pattern over the group of elements is
analyzed, and
[0106] the arbitrary selection is allowed if the resulting signal
pattern is indicative of a correct selection, otherwise a new
selection is made.
[0107] The apparatus may comprise using as patterns indicative of a
correct selection a first pattern indicative of a stylus at one of
non-selected elements, and a second pattern indicative of no stylus
being present.
[0108] The analysis may comprise determining a number of elements
wherein an output exceeds a predetermined threshold.
[0109] In an alternative embodiment, the candidate carrier of mere
noise is arbitrarily selected,
[0110] noise subtraction is carried out over a group of elements on
the basis of the selection being correct,
[0111] a resulting signal pattern over the group of elements is
analyzed, and
[0112] the arbitrary selection is rejected if the resulting signal
pattern is indicative of an incorrect selection, and a new
arbitrary selection is made.
[0113] The apparatus may comprise at least one of a phase
compensator and a magnitude compensator using a complex proportion
to compensate for at least one of phase and magnitude differences
between respective elements during the compensating.
[0114] Preferably, the elements comprise an array, the apparatus
configured to sample at least some elements of the array, the noise
reduction unit being configured to choose at least one element and
reduce respective outputs of at least some remaining elements in
accordance with the output of another of the elements.
[0115] Preferably, the noise reduction unit is configured to use a
plurality of arbitrarily selected frequencies to calculate the
reduction.
[0116] Preferably, the noise reduction unit is configured to use an
average noise level of the plurality of arbitrarily selected
frequencies to calculate the reduction.
[0117] Preferably, the elements are conductive detectors of a flat
array of the digitizer for digitizing signals of a movable object
to indicate location of the object.
[0118] Preferably, the movable object is a stylus.
[0119] Preferably, the stylus is an electromagnetic stylus.
[0120] Preferably, the stylus comprises an active oscillator and an
energy pick up circuit.
[0121] Preferably, the stylus comprises a resonance circuit.
[0122] Preferably, the detecting elements are arranged in a grid
array.
[0123] Preferably, the detecting elements are loop elements.
[0124] Preferably, the noise is at least in part a consequence of
the presence of a finger.
[0125] 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.
[0126] 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
[0127] 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.
[0128] In the drawings:
[0129] FIG. 1 is a simplified diagram showing a theoretical
equivalent circuit of a finger on a digitizer surface;
[0130] FIG. 2 is a simplified diagram showing a sensor array of a
digitizer circuit with a finger at one location and a stylus at
another location;
[0131] FIG. 3 is a simplified diagram showing the sensor array of
FIG. 2 in which an erroneous detection is made;
[0132] FIG. 4 is a simplified diagram illustrating the sensor array
of FIG. 2 in an alternative scenario in which an erroneous
detection is made;
[0133] FIG. 5 is a simplified diagram illustrating a digitizer
suitable for use with a preferred embodiment of the present
invention;
[0134] FIG. 6 is a simplified flow diagram illustrating a procedure
in accordance with a first preferred embodiment of the present
invention;
[0135] FIG. 7 is a simplified flow diagram illustrating in greater
detail the procedure shown in FIG. 6, according to a further
preferred embodiment of the present invention;
[0136] FIG. 8 is a simplified diagram illustrating a method of
selecting an arbitrary frequency according to one preferred
embodiment of the present invention;
[0137] FIG. 9 is a simplified diagram illustrating a method of
selecting a candidate carrier of pure noise according to a
preferred embodiment of the present invention;
[0138] FIG. 10 is a simplified flow diagram illustrating a second
method of selecting a candidate carrier of pure noise according to
another preferred embodiment of the present invention;
[0139] FIG. 11 is a simplified diagram illustrating operation of a
digitizer according to an idealized embodiment of the present
invention;
[0140] FIG. 12 is a simplified diagram illustrating operation of
the digitizer of FIG. 11 in a less idealized case;
[0141] FIG. 13 is a simplified diagram illustrating operation of a
digitizer according to a preferred embodiment of the present
invention in which the method of FIG. 10 is used to identify a
candidate carrier of pure noise;
[0142] FIG. 14 is a simplified diagram illustrating the operation
of FIG. 10 in an alternative outcome;
[0143] FIG. 15 is a simplified diagram showing the equivalent
components that contribute to noise detected by the digitizer;
[0144] FIG. 16 is a simplified diagram showing how the situation
shown in FIG. 15 varies for two noise sources; and
[0145] FIGS. 17 and 18 are two graphs illustrating signals sampled
at antennas according to preferred embodiments of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0146] The present embodiments comprise a noise reduction system
for stylus detection in a digitizer. More particularly, the present
embodiments comprise a system for noise identification and
reduction at the signal frequency being used for detection of the
stylus. It is noted that if the digitizer is combined with a touch
detector, then the finger involved in touch detection is likely to
be a source of noise specifically at the stylus detection
frequency.
[0147] Such noise reduction specifically at the detection frequency
may thus improve the detection of an electromagnetic stylus in a
digitizer system. The digitizer is a computer associated input
device capable of tracking user interactions via the stylus or
other locatable objects. In general the digitizer is associated
with a display screen, on which the results of stylus detection may
be displayed. The digitizer may further enable touch detection.
[0148] The present invention does not require that the digitizer be
placed directly on the display screen. Rather, it is applicable
both to transparent digitizers where the stylus is moved over the
display screen and to other types of stylus, which are moved over
tablets or paper or whiteboards.
[0149] The present invention further applies to any kind of stylus
or other pointer device which has a detection frequency and not
merely to a passive electromagnetic stylus. The noise reduction
algorithm described herein can be implemented on any digitizer
system capable of tracking one or more electromagnetic styluses.
The present invention is furthermore applicable in systems designed
to detect both stylus and touch interactions, as will be explained
hereinbelow.
[0150] The preferred embodiments are able to identify noise at the
stylus frequency, as will be explained below, and subtract the
identified noise from the stylus signals, to leave the stylus
signals as the only source of output at the stylus frequency.
[0151] It is noted that whilst the digitizer is designed to detect
an electromagnetic stylus, other conductive objects touching the
screen may induce noise that can interfere with the stylus signal.
Conventional noise removal methods, such as band pass filters and
Fourier transform are often used to filter unwanted signals from
the stylus signal. However, these methods can not remove unwanted
signals at the same frequency as the stylus, and, as mentioned
above, a high percentage of the output at the desired frequency can
be due to such induced noise.
[0152] The principles and operation of a noise reduction system
according to the present invention may be better understood with
reference to the drawings and accompanying description.
[0153] 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.
[0154] Reference is now made to FIG. 5, which illustrates a simple
digitizer. As mentioned, a digitizer is a device which detects the
movement of an object such as a stylus and converts it into a
digital signal which can be made available to software
applications. A stylus or other pointing object 110 is moved over a
surface 112. Detectors 114 within the surface 112 detect the
current position of the pointer and send an output signal
indicating that location to operating system 116 of a computer. The
operating system ensures that the location information is made
available to currently active application 118, so that the
application knows where the pointer is and can incorporate the
position into its current operation.
[0155] Reference is now made to FIG. 6, which is a simplified
diagram illustrating the basic principle behind noise reduction
according to the present embodiments. The digitizer as shown in
FIG. 5 has several detecting elements which between them detect
signals from the stylus. In a world without noise the detectors
having the strongest signal would be taken as the location of the
stylus. If two detectors give strong signals then the location of
the stylus may be taken as being somewhere between them.
[0156] The real world has noise. Much of the noise is at
frequencies other than the detection frequency being used by the
stylus and can therefore be removed simply by filtering or like
techniques referred to in the background. However filtering cannot
be used for the frequency being used by the stylus. In the process
shown in FIG. 6, two detecting elements are read at the same time
at the frequency used by the stylus. Then the output obtained at
one of the elements is subtracted from the output at the other
element.
[0157] It will be appreciated that if one of the elements is an
element currently detecting a combination of stylus and noise
output, and the other element is an element currently detecting
only noise, then by obtaining the difference between the two
signals one obtains a pure stylus detection signal. The method
therefore preferably includes initial stage 120 of selecting an
element as a candidate carrier of pure noise and a stage 122 of
selecting another element as a candidate carrier of the stylus
signal. Several methods of selecting these candidates will be
discussed hereinbelow. The outputs are measured at the two elements
in stage 124. As will be explained below, more than two candidates
may be used in practice. Finally in stage 126, the output at the
candidate carrier of the stylus signal is reduced in accordance
with the noise detection at the other element. It is pointed out
that the above applies to an "ideal" digitizer, where all the
antennas and amplifiers are exactly the same. In such a case one
can take the noise output from one antenna and subtract it from a
second antenna. Such a simplified solution can be implemented in
cases such as that of FIG. 11 which will be described below.
[0158] Reference is now made to FIG. 7, which is a simplified flow
chart illustrating preferred outputs that can be used at the two
carriers to obtain the necessary reduction. In stage 130, an
arbitrary frequency is selected which is close to the frequency
being used by the stylus so that noise levels at the two
frequencies may be expected to be related. In stage 132 the outputs
of both elements are measured at the two frequencies. In stage 134
a ratio is determined between the outputs at said two frequencies
at the noise carrying element. In stage 136 that ratio is used to
reduce the signal at the stylus frequency at the stylus carrying
element.
[0159] In one preferred embodiment the arbitrary frequency is
chosen using the procedure shown in the flow chart of FIG. 8. A
range of frequencies around the stylus frequency is monitored in
stage 138. Then, in stage 140, the arbitrary frequency is chosen as
the frequency having the highest output level.
[0160] In an alternative embodiment, the digitizer is additionally
used for touch detection and a given frequency is used for touch
detection. In such an embodiment a frequency that can usefully be
selected as the arbitrary frequency is the frequency already being
used for touch detection.
[0161] In one embodiment, it is possible to use the detection
elements or other parts within the digitizer system to deliberately
generate noise at the arbitrary frequency.
[0162] It is noted that in the case of a single stylus using a
single frequency at all times, the arbitrary frequency may be
fixed. But in many embodiments the stylus may change its frequency
during use. For example the frequency may change in order to
indicate particular states of the stylus. In such a case the
arbitrary frequency may also be changed.
[0163] Reference is now made to FIG. 9, which is a simplified
diagram showing how a candidate carrier of mere noise may be
selected. In FIG. 9 the stylus's most recent position is known, so
it is assumed that the current position is relatively close to that
most recent position. In stage 142, measurements are taken to
determine all of the detecting elements that are currently emitting
an output at the arbitrary frequency which exceeds a threshold
signal level. There is thus established a group of elements
emitting above the threshold. In stage 144 the one element in the
group that is furthest away from the stylus's most recently known
position is selected as the candidate carrier of mere noise.
[0164] In a variation of stage 144, the candidate carrier of mere
noise is selected as that element exhibiting a strongest noise
signal, as long as it is beyond a determined distance from the
previously known stylus location.
[0165] Reference is now made to FIG. 10, which illustrates a more
general method of finding a candidate carrier of mere noise which
does not rely on having a recent stylus location. The method of
FIG. 10 may be used at all times or may be substituted for the
method of FIG. 9 when a recent stylus location is not available. In
FIG. 10 a candidate carrier of mere noise is arbitrarily selected
in stage 146. Then detection and noise subtraction are carried out
over all elements of the digitizer on the basis of the selection
being correct, in stage 148. In stage 150 the resulting detection
pattern is analyzed. In stage 152, a decision is made as to whether
the detection pattern in fact indicates a correct selection or not.
If a correct selection is indicated then the results are accepted
in stage 154, otherwise the results are rejected and a new
arbitrary selection is made. In alternative or complimentary
embodiments, either unlikely patterns are rejected, or likely
patterns are positively selected.
[0166] As will be explained in greater detail below, the different
elements are at different spatial locations and may have different
characteristics, as a result of which the same signals may be at
different phases and magnitudes. A preferred embodiment thus uses a
complex proportion to compensate for either or both of phase and
magnitude differences.
[0167] In one variation the digitizer is a grid array of detection
elements. One of the elements is selected as the candidate carrier
of mere noise using any of the methods described above or other
methods that will occur to the skilled person, and then the outputs
of either all or part of the other elements in the grid are reduced
in accordance with the detected noise in the candidate carrier of
mere noise.
[0168] In another preferred embodiment, the method involves using,
not just one, but a group of arbitrarily selected frequencies to
calculate the reduction. The reduction calculation may thus be
based on an average output within the group.
[0169] As will be explained in greater detail below, the detecting
elements may be conductive detectors of a flat array. The detected
object may be a stylus but may alternatively be any moving object
emitting location signals at a given frequency.
[0170] The noise is often at least in part a consequence of the
presence of a finger.
[0171] As explained above, noise reduction is carried out by
identifying noise levels at the stylus frequency and subtracting
them from the signals received on the sensing elements or antennas
sensing the stylus. To do so, the digitizer must first find an
antenna exhibiting pure noise (i.e. an antenna that is least
effected by the stylus), and then use the output at that antenna to
calculate the noise components on the stylus-detecting
antennas.
[0172] FIG. 11 is an illustration of the solution described above
in an ideal digitizer system where the amplifiers are identical and
have infinite input resistances. In particular the user induced
signals do not suffer magnitude or phase shifts due to resistance
of sensor conductors, differences in parasitic input capacitances
and the like. This scenario corresponds to the simplified flow
chart described in FIG. 6.
[0173] FIG. 11 shows a sensor 220 comprising a grid of detecting
elements, including elements 252 and 253. A finger is at location
222 and a stylus at location 221. A mixed output 251 is obtained
from detecting element 252 due to the finger and the stylus.
However element 253 is not affected by the finger.
[0174] In accordance with the above assumptions, a detected noise
signal induced by a user finger 222 touching two different antennas
252 and 253 is similar. The stylus antenna 252 receives signals 251
from both finger location 222 and stylus location 221. As long as
both antennas 252 and 253 are sampled simultaneously the signal 254
detected on the finger antenna 253 can be subtracted from the mixed
signal 251 received on the stylus antenna 252. As a result the pure
stylus signal is obtained at the stylus antenna 252.
[0175] A Solution for Non-Ideal Digitizers
[0176] Detecting Noise Signals
[0177] As explained above, a preferred implementation of the
present invention utilizes noise signals at an arbitrary frequency,
different from the stylus frequency, in order to reduce noise
signals in the stylus frequency. For purposes of the present
disclosure the frequency used for noise signals that are not at the
stylus frequency will be referred to as f.sub.arb. As described
above, the digitizer preferably finds an antenna exhibiting pure
noise signals in order to eliminate the noise signal from the
stylus antenna. One way to find a pure noise antenna lies in the
ability to detect noise signals in f.sub.arb.
[0178] The noise spectrum is usually wider then the stylus
frequency range; thus sampling a range of frequencies around the
stylus frequency is most likely to reveal noise signals at other
frequencies as well. In this case each time the digitizer samples
the antennas it scans a range of frequencies around the stylus
frequency. Signals received in frequencies other than the stylus
frequency are evidently noise. In one embodiment, the digitizer
selects a frequency showing the strongest noise level, as long as
it is above a certain threshold, to be the frequency f.sub.arb. In
other embodiments, the noise frequency can be selected according to
various system considerations. For example, some embodiments may
implement touch detection at one frequency (f.sub.1) and stylus
detection at a second frequency (f.sub.2). Since the system is
already configured to examine signals at f.sub.1, it may be
preferable to utilize f.sub.1 as the noise frequency
(f.sub.arb=f.sub.1).
[0179] In some cases it is possible to deliberately generate a
noise signal at a predetermined frequency (f.sub.arb) that is
induced by the same source of unwanted noise. An example for such a
case is a conductive object, such as a user's finger, touching the
screen and inducing noise signals. Thus, oscillating the
digitizer's antennas intentionally will create finger-induced
signals at the frequency of the oscillating antennas. The antenna
oscillations may be at any frequency (f.sub.arb) other than the
stylus frequency. However, the best results are achieved when
f.sub.arb is close to the stylus frequency. In this case f.sub.arb
is determined according to the frequency utilized to generate the
noise outputs as opposed to the first scenario where it is
determined on the fly. Note that in most applications the stylus
may oscillate at more than one frequency at different times. For
example, when the stylus is hovering above the sensor it may be set
to oscillate at one frequency (f.sub.1) and when it is in contact
with the sensor (`tip-down`) it may be set to oscillate at a second
frequency (f.sub.2). The digitizer preferably recognizes the status
of the stylus (i.e. hovering or `tip-down`) according to the
frequency of the stylus signal. In this case, the selected noise
frequency (f.sub.arb) corresponds to the relevant stylus frequency.
When the stylus is in the hovering state, f.sub.arb is close to
f.sub.1. When the stylus is in contact with the sensor a second
noise frequency is utilized, closer to f.sub.2.
[0180] Finding the Pure Noise Antenna
[0181] It is reasonable to assume that in any case when noise
sources are present, the system can identify at least one antenna
that exhibits pure noise signals. This assumption is based on the
fact that the stylus is affecting a relatively small area in the
proximity of its tip and the fact that physically the stylus and
noise source can not be located at the exact same place.
[0182] The preferred embodiments use two different methods in order
to find a pure noise antenna. The first method, described above
with respect to FIG. 9, is applicable only when the previous
location of the stylus is known. FIG. 12 is a simplified diagram
illustrating a digitizer on which is carried out the method of FIG.
9, namely of finding the pure noise antenna using the stylus'
previous location. FIG. 12 shows the same grid arrangement as in
FIG. 11, with a finger at one location and the stylus at another
location. The method of FIG. 12 relies on the fact that the stylus
movements across the sensor are continuous. In addition, the
antennas sampling rate is such that the previous location of the
stylus is a good indication of its current whereabouts. The
following steps are used to identify the pure noise antenna:
[0183] 1. Detecting antennas exhibiting noise signals in
f.sub.arb--This stage can be preformed in any of the ways described
hereinabove or any other methods that may occur to the skilled
person.
[0184] 2. Ignore any antennas that might be used for stylus
detection, such as antenna 261. The algorithm chooses the antenna
farthest away from the stylus' previous location, but still
exhibiting a noise signal exceeding a certain threshold. In this
case antenna 262 is most likely to be chosen as it has a noise
source located thereon, namely a finger.
[0185] Alternatively, the algorithm may choose the antenna
exhibiting the strongest noise signal as long as it is sufficiently
distanced from the stylus previous location, again as described
above.
[0186] The second method, as illustrated above in FIG. 10, does not
rely on the previous location of the stylus, thus it can be
implemented even when the stylus is not present at the sensor
surface or when a previous location is not known. The procedure of
finding the noise antenna, using the second method is as
follows--
[0187] 1. Detecting several antennas exhibiting a strong enough
noise signal in f.sub.arb--as described hereinabove.
[0188] 2. Choose one of the antennas to calculate the noise
component in the stylus frequency on all the other antennas, as
described elsewhere herein. The choice may be made arbitrarily or
using any suitable algorithm.
[0189] 3. If there is no stylus in the region of the sensor then
the selected antenna must exhibit a pure noise signal and after
subtraction all other signals will be very low. On the other hand,
if there is a stylus detected by the sensor, there are two options:
[0190] The selection was correct and the chosen antenna exhibit
pure noise signals. In this case the noise is subtracted from all
other antennas and the stylus is detected correctly. [0191] The
selection was wrong and the chosen antenna exhibits a mixed output
of noise and of stylus signal. In such a case, after subtraction of
the incorrect pure noise elements from all the other antennas, the
pattern of signals exhibited on the entire sensor will not match
the kind of pattern induced by a real stylus. For example, a
pattern may be considered invalid if the number of antennas
exhibiting stylus signals is above a certain threshold. An invalid
pattern can also be identified by the distance between the stylus
antennas, according to the spatial EM field emitted from the stylus
tip.
[0192] The system thus identifies an invalid pattern and goes on to
select a further candidate for being a pure noise antenna,
preferably the antenna exhibiting the largest noise signal
(f.sub.arb) on the other axis. Noise is subtracted from all other
antennas and the system looks for a valid stylus pattern. This
process of antenna selection and noise subtraction is repeated
until choosing a real pure noise antenna thus detecting a valid
stylus pattern after noise subtraction.
[0193] Reference is now made to FIG. 13 which illustrates the grid
of FIG. 11 on which the method of FIG. 10 is being carried out. In
FIG. 13 the digitizer identifies antenna 268 which is the antenna
exhibiting a large noise signal on the X axis. The digitizer
further identifies antenna 263 exhibiting a large noise signal on
the Y axis. FIG. 13 clearly shows that only antenna 263 is
exhibiting pure noise, output 264. Antenna 268 by contrast exhibits
a mixed signal 267 including elements from both noise source 265
and a stylus 266.
[0194] The algorithm chooses one of the noise antennas as a pure
noise antenna and uses it to calculate the noise component in the
stylus frequency on all the other antennas, as described
hereinbelow. As will be explained below, the antennas affected by
these calculations are those exhibiting noise signals in f.sub.arb.
For example, the stylus antenna 261 is unaffected by the noise
source, therefore its signals are not altered.
[0195] Now we review the case when the algorithm chooses the
antenna 268 exhibiting mixed signals 267 instead of the antenna 263
exhibiting pure noise 264. As a result of choosing the wrong
antenna the digitizer will not be able to determine the stylus
position. Referring now to FIG. 14, we describe the signals
received at antenna 263 and antenna 268 after subtracting the mixed
signals 267 on the chosen noise antenna (68). Antenna 268 does not
exhibit any signals in the stylus frequency while antenna 263
exhibits a signal 269 at the stylus frequency. These indications
imply that the stylus affects only one axis, and thus its location
cannot be determined.
[0196] Since the stylus cannot in reality only affect a single axis
an erroneous selection of the noise antenna is a clear conclusion.
The algorithm thus proceeds to choose the noise antenna on the
other axis as the pure noise antenna.
[0197] For purposes of the present disclosure the antenna
exhibiting pure noise signals is referred to hereinbelow as the
`Noise Antenna`.
[0198] When the previous location of the stylus is unknown, the
noise removal algorithm may consist of several iterations. In this
case it would be preferable to avoid unnecessary processing, and
issue the noise removal algorithm only when there is high certainty
that a stylus is indeed present. Once an antenna exhibits a strong
enough signal in the stylus frequency, the digitizer compares the
signals at the stylus frequency (f.sub.s) to signals received at
the noise frequency (f.sub.arb). The algorithm checks the following
ratio--
Mag ( f arb , X ) Mag ( f arb , Y ) Mag ( f s , X ) Mag ( f s , Y )
> ? Threshold ##EQU00001##
[0199] Where, Mag(f.sub.arb,X) is the magnitude of the signal in
the noise frequency (f.sub.arb), received on the antenna exhibiting
the strongest signal on the X axis.
[0200] Mag(f.sub.arb,Y) is the magnitude of the signal in the noise
frequency (f.sub.arb), received on the antenna exhibiting the
strongest signal on the Y axis.
[0201] Mag(f.sub.s,X)--The magnitude of the signal in the stylus
frequency (f.sub.s), received on the antenna exhibiting the
strongest signal on the X axis.
[0202] Mag(f.sub.s,Y)--The magnitude of the signal in the stylus
frequency (f.sub.s), received on the antenna exhibiting the
strongest signal on the Y axis.
[0203] As stated above, the ratio is calculated using the antennas
exhibiting the highest signals on each axis. When the ratio exceeds
a certain predetermined threshold it means that the signals are
most likely originating from noise in the stylus frequency rather
than a stylus. In this case the signals are preferably
discarded.
[0204] Proportion Coefficient Between a Pair of Antennas
[0205] As explained above, when a signal is induced by a single
source, the digitizer may sense that signal in a different
magnitude or phase on different antennas. Some of the reasons for
the differences in phase and magnitude are: [0206] The distance of
the signal source form the different antennas [0207] The fact that
the amplifiers at the end of the antennas are not necessarily
identical, they may for example have different input resistances.
[0208] The different locations of the signal source in respect to
the inputs to the amplifiers.
[0209] Since the preferred embodiments utilize the signal received
on one antenna to calculate the signal received on a second
antenna, it is important to compensate for the variations in phase
and magnitude. To do so a complex proportion coefficient is
preferably used to correlate between signals received on the
different antennas.
[0210] Reference is now made to FIG. 15 which is a simplified
circuit equivalent showing two antennas, 273 and 277, being
affected by a single oscillating source 270 which represents a user
finger. Each antenna 273, 277, is connected to the oscillating
source 270 through a different set of resistors and capacitors.
Capacitor C1 271 represents the capacitance between the user's
finger 270, and antenna 273. Resistor R1 272, represents the
internal resistance of antenna 273. Capacitor C2 275 represents the
capacitance between the user's finger 270 and antenna 277. Resistor
R2 276 represent the internal resistance of the second antenna 277.
Capacitors C3 278 and C4 274 represent the parasitic capacitance
between the antennas and the surrounding components. It will be
apparent that since the antennas have different characteristics and
locations they are affected differently by the oscillating
source.
[0211] The signal at first antenna 273 may be expressed
as--S.sub.1=Z1F.
[0212] where F is the signal induced by the finger and Z1 is a
complex number representing the phase shift and magnitude change
due to capacitors C1 C4 and resistor R1. Z1 also incorporates the
phase shift and magnitude change due to the distance of the finger
from the first antenna 273.
[0213] The signal on the second antenna 277 may be expressed
as--S.sub.2=Z2F
[0214] where F is the signal induced by the finger and Z2 is a
complex number representing the phase shift and magnitude change
due to capacitors C2 C3 and resistor R2. Z2 also incorporates the
phase shift and magnitude change due to the distance of the finger
from the second antenna 277.
[0215] Dividing the above equations gives a proportion coefficient
(C=Z1/Z2) that can be used to reduce the signal on one antenna
(S.sub.1) based on the signal received on a second antenna
(S.sub.2) as follows:
S 1 S 2 = Z 1 Z 2 S 1 = Z 1 Z 2 S 2 S 1 = C S 2 Equation 1
##EQU00002##
[0216] Since Z1 and Z2 are complex numbers, their ratio is also a
complex number representing the phase shift and magnitude
difference between the signals received in the first antenna 273
and the second antenna 277.
[0217] The phase shift is calculated by subtracting the phase part
of Z2 from the phase part of Z1--The magnitude difference is
calculated by the ratio between the magnitude parts of Z1 and
Z2--
MAG { C } = MAG { Z 1 } MAG ( Z 2 ) . ##EQU00003##
[0218] The proportion coefficient (C) is a function of many
parameters such as the distance between the antennas and the
signal's source, the resistances of the antennas, environmental
conditions, different parasitic capacitances on each antenna etc.
However, for a small enough frequency range the proportion
coefficient is unchanged. Thus, the same proportion coefficient can
be used when sampling signals of close frequencies on the same pair
of antennas, at the same time. It is also possible to establish one
proportion coefficient (C.sub.1), corresponding to a first range of
frequencies around f.sub.1, from a second proportion coefficient
(C.sub.2), corresponding to a second range of frequencies around
f.sub.2.
[0219] Referring now to FIG. 16, which is a simplified diagram
showing two sources 280 and 281 oscillating at close but different
frequencies, f.sub.1 and f.sub.2, affecting a pair of antennas 273
and. 277. Parts that are the same as in previous figures are given
the same reference numerals and are not referred to again except as
necessary for understanding the present embodiment.
[0220] The oscillating signals 280 and 281 oscillate at two
different yet relatively close frequencies--f.sub.1 and f.sub.2.
The oscillating energy is transmitted to the antennas through the
equivalent of a set of resistors and capacitors as previously
described in FIG. 16. Signals induced by first oscillator 280 are
received on first antenna 273 as signal S.sub.1(f.sub.1) 282 and as
signal S.sub.2(f.sub.1) 283 on second antenna 277. Signals induced
by the second oscillator 281 are received on first antenna 273 as
signal S.sub.1(f.sub.2) 284 and as signal S.sub.2(f.sub.2) 285 on
second antenna 277. The oscillating frequencies are such that the
same proportion coefficient can be used for signals at both
frequencies.
[0221] As long as the above signals are sampled at the same time, a
proportion coefficient can be used for calculating signals received
on the first antenna by sampling the signals received on the second
antenna:
S.sub.1(f.sub.1)=CS.sub.2(f.sub.1) Equation 2
S.sub.1(f.sub.2)=CS.sub.2(f.sub.2) Equation 3
[0222] Since the proportion coefficient is approximately the same
for both of the above equations--
S 1 ( f 1 ) S 1 ( f 2 ) = S 2 ( f 1 ) S 2 ( f 2 ) Equation 4
##EQU00004##
[0223] The present invention uses Equation 4 in order to calculate
the noise component on the stylus frequency as will be elaborated
hereinbelow.
[0224] Subtracting the Noise Component from the Stylus Signal
[0225] The present embodiments may be implemented on any antenna,
whether or not it exhibits a stylus signal. It is possible to
calculate the pure noise signal on each and every antenna and
subtract it from the overall signal received on said antenna. When
a stylus signal is indeed present, the result will be a pure stylus
signal. When the antenna is unaffected by the stylus the
subtraction of the pure noise signal will indicate that no signals
are present on the antenna. In both cases precise detection of the
stylus is achieved.
[0226] The above principle is now explained using one antenna
exhibiting mixed stylus and noise signals and another antenna
exhibiting pure noise signals. Reference is now made to FIG. 17,
which is a graph showing intensity vs. frequency of the signals
sampled on one of the antennas sensing mixed stylus and noise
signals. This antenna is now referred to as the stylus antenna.
[0227] The pure noise signal in f.sub.arb received on the stylus
antenna is marked N'(f.sub.arb) 300. The pure noise component
induced by the noise source in the stylus frequency is N'(f.sub.s)
301. The pure stylus signal is marked S(f.sub.s) 302. Noise removal
according to the present embodiments comprises distinguishing
between N'(f.sub.s) and S(f.sub.s). Note that the noise reduction
algorithm can be applied even if S(f.sub.s)=0.
[0228] Reference is now made to FIG. 18 which describes the signals
sampled on an antenna that is unaffected by the stylus. This
antenna is referred to as the pure noise antenna, since it exhibits
noise signals alone. The signal received on the pure noise antenna
in f.sub.arb is marked N(f.sub.arb) 305. The pure noise signal
received on the noise antenna in the stylus frequency is marked
N(f.sub.s) 304.
[0229] The intensity of the signals is arbitrary, and used for
illustrating the kinds of signals received on each antenna. The
present embodiments identify N'(f.sub.s) 301 in order to subtract
it from the signals received on the antenna sensing the stylus
[0230] FIGS. 17 and 18 describe stylus induced signals on one
specific antenna. However, the method can be implemented on any
antenna or group of antennas.
[0231] Signals N(f.sub.arb), N'(f.sub.arb) and N(f.sub.s) are
sampled simultaneously, hence (based on equation 4)--
N ( f arb ) N ' ( f arb ) = N ( f s ) N ' ( f s ) Equation 5
##EQU00005##
[0232] The frequency chosen for detecting the noise signals is
preferably different from the frequency currently used by the
stylus, yet close enough for equation 4 to remain valid.
[0233] Notice that the only unknown parameter in equation 5 is
N'(f.sub.s), therefore--
N ' ( f s ) = N ' ( f arb ) N ( f arb ) N ( f s ) Equation 6
##EQU00006##
[0234] Once N'(f.sub.s) is calculated it is subtracted from the
signal received on the stylus antenna revealing the signal induced
by the stylus alone S(f.sub.s).
[0235] Improving the Proportion Coefficient Calculations
[0236] The preferred embodiments use the ratio between detections
at the pure noise antenna and detections received on other antennas
at different frequencies in order to calculate the noise component
on the stylus frequency as explained.
[0237] Averaging Over Several Arbitrary Frequencies
[0238] The embodiments of the present invention described above use
one arbitrary frequency in order to calculate the above ratio.
However, in an alternative embodiment several frequencies are used,
and an average is taken of the detections induced on the respective
antennas for the different frequencies. The use of more then one
arbitrary frequency reduces the proportion coefficient dependence
on frequency.
[0239] The digitizer according to this embodiment thus uses
different arbitrary frequencies (f.sup.1.sub.arb, f.sup.2.sub.arb,
. . . , f.sup.n.sub.arb) in order to detect noise signals in
several frequencies. The noise signals themselves are detected as
described hereinabove.
[0240] Sampling the conductive lines provides detections at
frequencies--f.sup.1.sub.arb, f.sup.2.sub.arb, . . . ,
f.sup.n.sub.arb.
[0241] The average output received on a stylus antenna due to
finger noise alone is--
N _ ' ( f arb ) = i = 1 n N ' ( f arb i ) n . ##EQU00007##
[0242] The average output received on the noise antenna due to
finger noise is--
N _ ( f arb ) = i = 1 n N ( f arb i ) n . ##EQU00008##
[0243] Since all detections are sampled simultaneously, the ratio
between the averaged signals and the signals received at the stylus
frequency is--
N _ ( f arb ) N _ ' ( f arb ) = N ( f s ) N ' ( f s ) Equation 7
##EQU00009##
[0244] Once again, the only unknown parameter is N'(f.sub.s),
hence--
N ' ( f s ) = N _ ' ( f arb ) N _ ( f arb ) N ( f s ) Equation 8
##EQU00010##
[0245] The stylus signal, S(f.sub.s), is then calculated by
subtracting N'(f.sub.s) from the signal received on the stylus
antenna.
[0246] It is expected that during the life of this patent many
relevant stylus devices and digitizer systems will be developed and
the scope of the corresponding terms herein, is intended to include
all such new technologies a priori.
[0247] 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.
[0248] 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.
* * * * *