Input Device for a Display System Using Phase-Encoded Signals

This invention relates to an input device for a display system such as a television receiver, a computer, a video game or the like.

Ser. No. 07/016805 concurrently filed herewith is a related application.

Display systems have become more complex in recent years. Television receivers, for example, have a greatly increased number of functions available to the user. These functions may include the routing and processing of stereo sound signals, the selection of various auxiliary video sources and the scheduling of future programs. Typically, in order to utilize these functions, a relatively large number of function selection switches are required. This makes the input device such as the hand-held remote control transmitter large, difficult and confusing to use and expensive to manufacture.

One way to minimize the number of switches on the remote control unit is to provide a display of a matrix of functions displayed on the screen of the display system in what is sometimes called a menu, and to provide input selection apparatus for selecting which ones of the displayed functions are to be exercised. Menu based systems for use in television receivers are disclosed in U.S. Pat. No. 4,641,205 (Beyers) issued 3 Feb. 1987 and U.S. Pat. No. 4,626,892 (Nortrup et al.) issued 2 Dec. 1986, both of which are assigned to the same assignee as the present application. The input device may be a so-called "mouse", a joy stick or a light pen, each of which provides X-axis and Y-axis position information to a display controller for determining the desired function. Typically, the user must use both hands or must have a flat surface on which to move the device. Most of these devices are wired to the controller and therefore do not lend themselves to remote control at distances usually associated with television viewing. It is desirable, however, that an input device make use of the human propensity to point at a desired item, thus providing ease of operation. Light pens and touchscreens provide such a pointing function but necessarily require the user to be within an arm's length of the display screen.

A wireless input device used for pointing to a particular item to be selected from a menu displayed on a display screen is disclosed in U.S. Pat. No. 4,565,999 (King et al.) issued on Jan. 21, 1986. This prior system determines the angular position of the remote transmitter unit by comparing the relative amplitudes of the "pointing" signals sequentially received by the display device.

In some applications, such as remotely controlling a television receiver, the user, and hence the remote control handunit, may be separated from the television receiver by as little as 3 feet and by as much as 30 feet (with a consequent 20 db signal amplitude variation). Thus, a system using amplitude modulated signals encounters difficulty in preserving the amplitude-proportionality of the samples, through the detection, preamplification, sampling and holding processes. Requiring the system to operate over the aforementioned distances, places severe linearity and stability constraints on the receiver circuitry. Any nonlinearity at the higher signal amplitudes (short distance) or instability at the lower signal amplitudes (long distance) will degrade the ability of the system to accurately convey user "pointing" information to its destination. Specifically, the AGC function of the signal amplifier circuits may be difficult to implement.

A remote control transmitter, when operated by a user, transmits pointing function information via a plurality of transducers mounted thereon for producing radiating energy beams at diverging angles from each other. Each of the transducers is driven by a unique waveform sequence with segments having particular phases of an energizing signal. The transmitted signals are combined at a receiver to produce a resultant signal with corresponding segments having phase angles representative of the vertical and horizontal angular positions of the remote control transmitter relative to the receiver.

It is herein recognized that if the pointing information is conveyed by exciting the transducers of the remote control handunit with phase-modulated signals, the amplitude-related problems mentioned above will be significantly reduced. As a result, the operation of the pointing function of the remote control handunit will be more reliable over the entire range of normal viewing distances.

FIG. 1 shows, in perspective diagram form, a display device and a remote transmitter according to an aspect of the present invention.

FIG. 2 shows a side view of a three-LED transducer and beam array suitable for use with the transmitter of FIG. 1 for Y-axis measurement.

FIG. 3 shows vector diagrams of signals, combined at a receiver, which were transmitted by the LED arrangement of FIG. 2.

FIGS. 4a and 4b show front and side views, respectively, of a typical LED array suitable for use on a remote transmitter unit.

FIGS. 5a and 5b illustrate two embodiments of a signal cycle sequence for use with the LED arrangement of FIG. 4.

FIG. 6 shows, in block diagram form, the circuitry of the remote control handunit of the invention.

FIG. 7 shows, in block diagram form, the infrared (IR) receiver circuitry of the invention.

FIGS. 8a and 8b illustrate the contents of the memory in the remote control handunit of FIG. 6.

FIG. 9 illustrates the contents of the memory used in the receiver circuitry of FIG. 7.

The display system of FIG. 1 includes a remote transmitter 101, subject to movement, which, when activated, transmits signals along divergent signal paths 102a, 102b, and 102c in a manner to be explained with respect to FIGS. 2, 3 and 4. Preferably, these signals are in the form of infrared (IR) radiation transmitted from respective light emitting diodes (LEDs). Transmitter 101 has a natural pointing axis which may be defined as the path along which it is expected by the user that signal will be emitted when the transmitter is activated. Nominally, the pointing axis is along the longitudinal axis of a rectangular enclosure encompassing the transmitter circuitry.

A sensor 103 for receiving signals from remote control transmitter handunit 101 is mounted adjacent to display screen 104 of display apparatus 105. Display screen 104 displays a menu of functions to be selected by a viewer. An index or cursor 106 is moved about the screen in response to pointing motions of the transmitter handunit, in a manner to be described below. When the cursor is detected proximate to the title of a function, a box 107 may be illuminated to highlight the function desired. A scroll bar 108 having a position indicator 109 may be illuminated on display screen 104 to assist the viewer in making adjustments of the selected function (i.e., volume, brightness, tint, color).

Briefly, angular movement of the transmitter with respect to the receiver will change the amplitude and phase of the resultant IR signal modulation components received from the transmitter LEDs in each of several cyclical time-multiplexed intervals. The changes in vector phase in X-axis and Y-axis measurement intervals will be reflected by the receiver circuitry in a change in the magnitudes of X-axis and Y-axis data, respectively. A microcomputer uses the X-axis and Y-axis data to position an illuminated aiming point such as 106 on video display screen 104 of FIG. 1. The illuminated aiming point provides convenient visual feedback for the user.

The term "phase modulation" as used herein means a fixed offset in electrical phase from a 0.degree. phase reference. In the present invention, in each measurement interval of the cyclical sequence, LEDs of the handunit having respective fixed physical angular offsets from the pointing axis of the remote control handunit are simultaneously energized with respective signals having corresponding fixed electrical phase shifts from a 0.degree. reference phase. In the present invention, the information to be conveyed (i.e. angular position of the remote control handunit relative to the receiver) is found in the phase angle of the resultant of the combined signals received from the energized LEDs. The information transmitted by a particular LED is representative of the relative angular position of that LED with respect to the pointing axis of the remote control handunit. The relative amplitudes of the IR signal components received by the sensor vary in response to transmitter handunit pointing angle. The sensor receives and combines the IR signal components to produce a signal exhibiting a resultant phase modulation, and generates an electrical signal having the same phase modulation as the resultant IR signal. Because the pointing angle information is represented by the phase angle of the electrical signal, the amplitude of the electrical signal does not need to be preserved and, therefore, may be amplified and limited. Thus, the problems affecting the prior art systems (i.e., preserving the relative amplitudes of each electrical or transmitted IR signal) are significantly reduced.

Referring to FIG. 2, infrared (IR) LEDs 202, 204 and 206 are mounted on a remote control transmitter handunit (not shown). Each of LEDs 202, 204 and 206 has an associated pointing axis 208, 210 and 212 respectively along which IR light will be transmitted with greatest intensity. Pointing axes 208, 210 and 212 correspond to pointing axes 102a, 102b and 102c of FIG. 1. The LEDs are physically disposed at divergent angles from the pointing axis of the handunit so as to enable a receiver to determine the angular position of the handunit and move the cursor accordingly. The IR light from LEDs 202, 204 and 206 is transmitted in radiation patterns 220, 222 and 224, respectively, with the radiant intensity decreasing monotonically with increasing radiating angle relative to the LED pointing axis. Upward-pointing LED 202 is excited by the application of an energizing signal which exhibits a +90.degree. phase shift from a reference-phase signal. Horizontally-pointing LED 204 is excited by the application of an energizing signal which exhibits a 0.degree. phase shift from a reference-phase signal. Downwardly pointing LED 206 is excited by the application of an energizing signal which exhibits a -90.degree. phase shift from a reference-phase signal. That is, oppositely oriented LEDs are energized with oppositely phase-shifted energizing signals. The above-mentioned energizing signals are applied, for example, during the Y-axis measurement interval of the cyclical sequence when a user presses a pointing function button on the handunit.

The three LED energizing signals (+90.degree., 0.degree., -90.degree.) are unidirectional squarewaves which cause the LEDs to radiate constant amplitude bursts of IR. The receiver sensor converts the received bursts of IR back to unidirectional squarewaves. As is well known, a squarewave may be considered to be a linear summation of DC, fundamental and harmonic terms. A bandpass filter, contained within the receiver sensor module, rejects the DC and harmonic terms, leaving only the three fundamental signals. These fundamental signals are sinusoids at the same modulation frequency having different phases and different amplitudes. The bandpass filtering should be done when the signals are still at a low level so that linearity is maintained. Since the three sinusoidal components are of the same frequency, they may be considered as vectors which sum to a single vector resultant as shown in FIGS. 3a, 3b and 3c. The resultant signal may be amplified and limited (i.e., clipped), because the information to be preserved is in the phase of the resultant signal and not in its amplitude. As will be explained below, this information is extracted by phase detector circuits which compare the phase of the resultant signal to a known reference phase.

FIGS. 3a-3c illustrate the IR modulation components of the signal received at the receiver, and is useful in understanding the operation of the system. Vector diagram 3a shows the relationship of the three signals at the receiver when the handunit is aimed directly at the receiver, that is, when the pointing axis of the handunit is parallel to the line of sight between the handunit and the receiver. Note that because the contribution received from upward pointing LED 202 (+90.degree. phase modulation represented vector 302) is equal and opposite to the contribution received from downward pointing LED 206 (-90.degree. phase modulation represented by vector 306), these two components cancel and the resultant vector 308 is equal to the horizontal component 304 contributed by horizontally-pointing LED 204 (0.degree. phase modulation). While vector 308 is shown below vector 304 for clarity, it should be understood that it actually lies along vector 304. Thus, a horizontally-pointed handunit may be determined by the receiver circuitry from a resultant vector which is in-phase with the reference phase.

Vector diagram 3b shows the relationship of the three signals at the receiver when the handunit is aimed upwardly with respect to the line of sight between the remote control transmitter handunit and the receiver. Because the handunit is pointing upwardly, the contribution of vector 322 from the upward-pointing LED 202 is less than in the previous example. Since the downward-pointing LED 206 however is now pointing more directly at the receiver, the contribution of vector 326 is greater than in the previous example. These amplitude changes are in accordance with radiation patterns 220, 222 and 224 described above. The contribution from the horizontally-pointing LED 204 as represented by vector 324 remains about the same as in the previous example. Since the vector 322 has a magnitude which is less than vector 326, the resultant vector 328 exhibits a negative excursion. Thus, an upwardly-pointing remote control transmitter handunit may be determined from a resultant signal exhibiting a negative phase angle and the amount of upward angular displacement of the remote control transmitter handunit from the horizontal position is represented by the amount of negative phase displacement from the reference phase.

Vector diagram 3c shows the relationship of the three signals at the receiver when the handunit is aimed downwardly with respect to the line of sight between the handunit and the receiver. Because the handunit is pointing downwardly, the contribution of vector 346 from the downward pointing LED 206 is less than in the first example. The upward-pointing LED 202 however is now pointing more directly at the receiver and the contribution of vector 242 is greater than in the first example. The contribution from horizontally-pointing LED 204 remains about the same as in the first example. Since the vector 346 has a magnitude which is less than vector 342, the resultant 348 exhibits a positive excursion. Thus, a downwardly-pointing remote control transmitter handunit may be determined from a resultant signal exhibiting a positive phase angle and the amount of downward angular displacement of the remote control transmitter handunit from the horizontal position is represented by the amount of positive phase displacement from the reference phase (0.degree.). X-axis measurement is accomplished in a manner substantially the same as described above except that the LEDs energized on the handunit are leftward pointing, center pointing and rightwardly pointing.

Two measurement intervals are included in a repetitive cycle during which the LEDs are individually driven at different phases of the carrier frequency. Each measurement interval allows the receiver circuitry to determine the pointing direction of the remote control transmitter handunit relative to the line of sight between the remote control transmitter handunit and the IR sensor at the receiver about one axis. One measurement interval is used for determining lateral (X-axis) pointing, and the second measurement interval is used for determining vertical (Y-axis) pointing.

FIGS. 4a and 4b show a front view 400 and a side view 420, respectively, of a typical embodiment of an LED array suitable for use on a remote control transmitter handunit in accordance with an aspect of the invention. Other LED configurations may also be used in the invention. However, when choosing one it is important that the pointing axes of the individual LEDs diverge from one another and from the pointing axis of the handunit, so that there is as little crosstalk between axes as possible. For good operation over a relatively long distance (30 feet) between the remote control transmitter handunit and the receiver, it is desirable that the amplitude of the resultant IR signal should be relatively uniform over the range of pointing angles of interest. A typical divergence angle between elements 401 and 405 is about 30 degrees. When the handunit is tilted downward more than 30.degree., for example, the strength of the IR signal received from a downwardly pointing LED is sharply reduced. Therefore, the range of pointing angles of interest of .+-.30.degree. corresponds to an LED divergence angle of 30.degree.. However, as the handunit of the present invention is rotated within the range of angles of interest, the amplitude of the resultant IR signal at the receiver sensor remains essentially constant because while the radiated IR signal received from one angularly disposed LED is decreasing, the radiated IR signal from the oppositely oriented LED is increasing. As noted above, the IR signal strength from the center pointing LED remains about the same within the range of angles of interest.

The four corner LEDs (401, 402, 403 and 404) are each aimed with equal X-axis and Y-axis offset angles from the handunit pointing axis 410. Each of the four side LEDs (405, 406, 407 and 408) has an X-axis or a Y-axis offset angle, respectively, but is centered about the other axis. FIGS. 5a and 5b illustrate that the IR transmission from the remote control handunit is partitioned into 4 principal intervals: synch and phase-reference, Y-axis measurement, "click" measurement, and X-axis measurement. Signals transmitted during the synch and phase-reference interval synchronize the receiver timing with that of the transmitter handunit, and establish a 0.degree. phase reference at the receiver to provide a reference from which to measure modulation phase angle displacement. In the Y-axis measurement interval, there occurs the simultaneous energization of horizontally pointing LEDs 405 and 406, upwardly pointing LEDs 401 and 402, and downwardly pointing LEDs 403 and 404 with their respective phase modulation signals. During the click interval, a signal representative of the state of a pushbutton switch is transmitted simultaneously by all of the LEDs using the same energizing signal. In the X-axis measurement interval, there occurs the simultaneous energization of center pointing LEDs 407 and 408, leftwardly pointing LEDs 401 and 403, and rightwardly pointing LEDs 402 and 404 with their respective phase modulated signals. During each of X and Y intervals, LEDs 401-408 are driven with signals having predetermined different phases. The phases of the LEDs for the X-axis and Y-axis measurement intervals for the embodiment shown in FIG. 5b described below is listed in Table 1.

The receiver circuitry, on detecting the presence of the modulated IR radiation, signals the microcomputer to cause the display apparatus to display a menu of functions. Various known menu display programs may be used for this purpose. By way of example, a menu display program similar to "Control Panel" of "Macintosh Finder" used with the Apple Macintosh computer may be used. When the X-axis and Y-axis measurement values have been passed from the receiver circuitry to the microcomputer, the microcomputer causes the display controller to display a cursor on the screen along with the menu. In this way, the user receives visual feedback which enables him to position the pointer until it is near the area of the screen in which the desired function is displayed. The pointer is positioned by moving the handunit as described above.

To cause the selection of the desired function, the display controller can be programmed to accept as a selection indication the loss of signal, due to release of the pointing functions button, after the cursor is within the display area for a predetermined time. Alternatively, the depression of a separate button or the further actuation of a two-stage pushbutton switch can be used to make the selection after the pointer is within the display area corresponding to the desired function. The latter avoids an unintentional selection due to a temporary loss of signal caused by, for example, the passage of an object between the receiver and transmitter.

The transmitter embodiment of FIG. 6 uses a two-stage switch 600, having switch contacts 600a and 600b, partial depression of which causes the illuminated point (such as cursor 106 of FIG. 1) to appear on the screen and full depression of which causes the actual selection of the desired function as described below. Partial depression of 2-stage pushbutton switch 600 closes switch contacts 600a applying supply voltage from battery 602 to all circuits in the remote control handunit. The application of power causes oscillator 604 to produce a 3.58 MHz signal which is applied to the input of divide-by-eight counter 606. Divide-by-eight counter 606 produces a 447 KHz signal which is applied to the input of 12-stage binary counter 608. The output lines of binary counter 608 provide most of the addressing to read-only memory (ROM) 610 with the remaining address line being connected to switch contact 600b of pushbutton switch 600. ROM 610 is arranged as 512 memory locations having 8 bits each. Each of the eight output bits is associated with a particular LED. As can be seen from FIG. 6, bit 0.sub.1 is connected to driver amplifier 621 which drives LED 631. Similarly bits 0.sub.2 -0.sub.8 are respectively connected to driver amplifiers 622-628 which drive LEDs 632-638, respectively.

The ROM address lines A.sub.0 -A.sub.8 are separated into three distinct groups. The first group comprises address lines A0 and A1 which are cycled through four states by counter 608 to cause the A.sub.0 and A.sub.1 outputs to produce a squarewave at a selected phase of the IR carrier frequency (56 KHz at the oscillator frequency shown). FIG. 8b shows that an individual LED may be energized with one of four possible phases of the 56 KHz IR carrier signal or may be turned off. For a particular LED to be energized with "-90.degree. phase", for example, the output bit associated with that LED must have a "low" level ("off") stored in those memory locations which are addressed when address lines A1 and A0 are energized with binary signals 00 and 11, and must have a "high" level ("on") stored in those memory locations which are addressed when address lines A1 and A0 are energized with 01 and 10 binary signals.

The second group of address lines comprises address lines A2-A7 which define 64 groups of memory locations with each group comprising 4 memory locations. Counter 608 repetitively cycles ROM 610 through these 64 groups. Because stages Q.sub.4 -Q.sub.6 of binary counter 608 are interposed between the stages which define carrier phase and those which define the 64 sequential groups, each group is cycled 8 successive times before the higher-order address (Q.sub.7 -Q.sub.12) advances ROM 610 to the next group. The 64 group sequence takes about 9.2 milliseconds (in the illustrated embodiment).

FIG. 8a lists the 7 sequence intervals defined by address lines A2-A7 and the groups within each sequence interval, and the output phases required to energize the individual LEDs for each group. The Xs in FIG. 8a indicate "don't care" states. That is, the output listed to the right will be produced for either value assumed by the address bit containing the X. The 7 sequence intervals are shown in FIG. 5b described below. Ignoring address bit 8 for the moment, it can thus be seen, for example, that the Sync and Carrier Phase-Reference interval is composed of three groups in which all of the LEDs will be driven with zero.degree. phase signals, followed by two groups of 180.degree. phase signals, and three more groups of zero.degree. phase signals. The Sync and Carrier Phase-Reference interval is followed by the Regular and Inverted Y-sense intervals, Regular and Inverted Click intervals, and Regular and Inverted X-sense intervals.

In the embodiment shown in FIG. 5b, a click interval is included in the cycle, during which the LEDs are driven in unison at one of two different phases of the carrier frequency. The choice of transmitted phase sends one bit of information to the receiver. The click transmission phase is based on the state of switch contact 600b of 2-stage pushbutton switch 600, and is used to convey the viewer's desire to select an item from a displayed menu on the display screen of the system.

The third group of address lines comprises address line A8 only. As can be seen from FIG. 6, address line A8 is coupled to switch contact 600b of 2-stage pushbutton switch 600 and is thus responsive to for both states of address line A8 are shown in FIG. 8a. The contents addressed by the closed condition of switch contacts 600b are shown below the main portion of the figure. When address line A8 is activated by user-actuated pushbutton switch 600, ROM 610 is addressed in its upper 256-address block (addresses 256-511) rather than its lower 256-address block (addresses 0-255).

Referring to FIGS. 5a and 5b, the system must provide means for synchronizing the timing of the receiving circuits at the receiver to the transmitted cycle sequence, and for establishing a reference phase of the carrier frequency. Two different systems for accomplishing these tasks will be described. Both systems require the receiver to have a phase-locked-loop (PLL) circuit which is designed to oscillate at the transmitted carrier frequency and to be locked in phase to the phase of the transmitted carrier.

In the system with the signal cycle sequence illustrated by FIG. 5a, the receiver has two states, designated by "acquire" and "locked". In the acquire state, the PLL circuit time-constant is made short enough so that the PLL phase will be slewed to match the signal phase within each interval of the cycle. The PLL error-signal then will be a pulse whenever there is a sudden phase-shift in the transmitted cycle. The sync/phase reference interval transmission 501 is a burst of 180 degree phase, with a duration different from that of any of the other intervals which comprise the transmitted cycle. The 180 degree phase of the sync/phase reference interval 501 is sufficiently different from the phase of the preceding and following intervals to assure a phase-shift (and PLL error pulse) at the beginning and end of that interval. The receive circuit resets a timer at each error pulse, and uses the timer to monitor the time-interval between successive error pulses. When the unique sync/phase reference duration is found, the timer may be assumed to have started at the beginning of the sync/phase reference interval, and the receiver may be put in the locked state. In the locked state the timer is no longer reset by error pulses, but is allowed to iteratively count out its full cycle, which matches the duration of the transmitted cycle. The PLL circuit time-constant is made long (for stability and noise-rejection), and the PLL error signal is gated on only during the sync/phase reference. The sync and phase reference interval 501 is followed by a Y-axis measurement interval 502, a click measurement interval 503, and an X-axis measurement interval 504.

In the alternative system with the signal cycle sequence illustrated by FIG. 5b, the synchronization and phase-locking function is as follows. The PLL circuit time-constant is made long enough for stability and noise-rejection. The transmitted cycle is made to have an average phase-angle of zero, as follows: Each measurement interval (X,Y, or "click") is divided into two equal halves. For purposes of explanation, the Y-axis measurement interval 512 will be described. This detailed description also applies to the X-axis 514 and click 513 intervals each having first and second halves 519a, 519b and 513a, 513b, respectively. During the first half, 512a, the LEDs are variously driven at +90.degree., 0.degree., and -90.degree. phases, as described above. During the second half, 512b, the LED drive phases are reversed, so that the LEDs driven at +90.degree. in the first half are driven at -90.degree. in the second half, and vice-versa. The LEDs driven at 0.degree. phase remain so through both halves of the measurement interval. The phase angle of the resultant signal at the receiver will then be of equal magnitude but of opposite signal in the two halves, and therefore will average to 0.degree. phase. Once the PLL circuit has had time to stabilize at the average (0.degree.=reference) phase, a burst of 180 deg. phase 511b, in the middle of sync and phase reference interval 511, may be detected and used to synchronize the receiver timing to that of the transmitter. If the sync burst interval 511b is kept relatively short it will not significantly pull the PLL circuit away from the reference phase. The contents of the ROMs shown in FIGS. 8a, 8b and 9 reflect the system illustrated by FIG. 5b.

The operation of the IR receiver circuitry of the invention will be explained with reference to FIG. 7. An IR wave from the remote control handunit is converted to an electrical signal by IR sensor and preamp unit 702, filtered to select the desired modulation frequency, and amplified. The output signal from IR sensor and preamp unit 702 is further amplified by amplifier 704 and applied to the input of amplitude detector 706 which produces a D.C. level representative of the amplitude of the received modulated IR signal. This D.C. level is compared in comparator 710 against a reference level, set by adjustment of a potentiometer 708. If the amplitude of the received IR is sufficient, comparator 710 generates an "IR-on" signal which is applied to interface circuit 770 for use in various functions such as enabling menu display by a microcomputer (not shown). The "IR-on" signal is also applied to one input of AND-gate 712 and to the trigger input of 500 ms one-shot 714. One-shot 714 generates an "acquire gate" signal which is coupled via OR-gate 716 to the other input of AND-gate 712. Since both inputs of AND-gate 712 are enabled, a "PLL and Sync Enable" signal is generated at its output, and applied to PLL phase-detector 730 and 9 microsecond one-shot 746.

The output of amplifier 704 is also coupled to limiter amplifier 718 where it is further amplified and limited to produce a squarewave signal having the same frequency and phase as the modulation of the signal sensed at IR sensor and preamp unit 702. This limited signal is fed to a vector phase detector 720, a PLL phase detector 730, and a sync phase detector 738.

The timing and control of most of the receiver circuit functions are provided by a phase-locked 3.58 MHz voltage-controlled crystal oscillator (VCXO) 736, the output of which is divided successively by a 6-stage binary counter 748 and a 9-stage binary counter 750. Nine particular outputs of these counters are used to address a 512 word by 8 output bits per word ROM integrated circuit 760 with contents as shown in FIG. 9. The least-significant address bits are provided by the 3 most-significant stages of 6-stage counter 748. These bits continuously cycle through 8 states at a rate matching the received IR carrier (about 56 KHz). The most-significant output bit of 6-stage counter 748, also designated "PLL zero-reference" is compared to the 56 KHz output of the limiter, by PLL phase-detector circuit 730. The resulting phase-error pulses are controlled by PLL gain potentiometer 732 and are low-pass filtered by low-pass filter 734, and used to control VCXO 736, thereby closing the loop. A PLL 90.degree. reference signal is also applied to PLL phase detector 730 to steer the phase detector during acquisition of lock.

The 6 most-significant ROM address bits are provided by the 6 most-significant stages of 9-stage counter 748. These bits cycle through a sequence (approximately 9.2 milliseconds in duration), corresponding to the transmitted sequence.

When the IR signal from the remote control handunit is first received, the leading-edge of the IR-on signal triggers one-shot 714 which outputs an "acquire gate" signal for a fixed interval (e.g., 500 milliseconds). This gate signal causes PLL phase-detector 730 to be enabled so that it pulls into the frequency and phase of the received 56 KHz, as rapidly as possible.

Once the PLL has been locked to the proper 56 KHz phase, sync phase-detector circuit 738 can use the PLL zero-reference to detect the short burst of 180 degree phase transmitted within the synch interval (see FIG. 8a). This detected sync pulse is low-pass filtered by LPF 740, compared to a theshold value by comparator 744. The result is used to trigger a 9 microsecond one-shot 746 which in turn causes 9-stage counter 750 to be reset, thereby causing it to be properly synchronized relative to the transmitted 9.2 millisecond cycle.

After the elapse of 500 milliseconds (enough time for proper carrier phase-lock and cycle-synchronization to be established), the acquire-gate signal is terminated. PLL phase detector 730 and reset one-shot 746 are then disabled except during the "sync gate", which is one of the ROM outputs (see FIG. 9). This change reduces the sync circuit's vulnerability to noise bursts, and improves the accuracy of the PLL circuit by allowing it to function only during the period when a known carrier phase is being transmitted by all the LEDs of the remote control handunit, in unison.

The other outputs of ROM 760 are used to measure the phase of the received carrier during the intervals in which the x-axis and y-axis differential phase transmissions occur (see FIG. 5b). The phase reference signal supplied to vector phase detector circuit 720 is a squarewave at +90.degree. phase during the "regular" X, Y, or click sense intervals (first half), and is at -90.degree. phase during the "inverted" intervals (second half). Vector phase detector circuit 720 is an exclusive-OR (XOR) gate, and its output is a double-frequency (112 KHz) waveform with a duty-cycle proportional to the phase difference between the received signal and the phase reference. This variable duty-cycle signal is ANDed in AND-gate 722 with a window waveform, and with a fixed high-frequency (19.6 MHz) signal from 19.6 MHz oscillator 724 to convert it to bursts of pulses, with the number of pulses per burst proportional to the detector output duty cycle. The output of AND-gate 722 is used to clock a 15-stage binary counter 728. Counter 728 is reset to zero by a pulse from ROM 760 prior to the start of each measurement interval (X, Y, or click). The total count accumulated during the measurement interval is proportional to the vector phase detector output, averaged over the whole interval. The purpose of the window waveform is to prevent excessive counts from accumulating in counter 728 by limiting the counting interval to that portion of the 112 KHz waveform in which the variation in count will take place. Also, the window waveform may be used to freeze the outputs of counter 728 at the end of the measurement interval.

The 12 most-significant bits of the binary count of counter 728 are applied to the inputs of two 12-bit latch circuits 762 and 764. Latch 762 is strobed by a pulse from ROM 760 after the X-measurement interval, and latch 764 is strobed similarly after the Y-measurement interval. After the click measurement interval, the count of counter 728 should be either very high or very low, according to the phases transmitted by the remote control handunit. The magnitude of the count of counter 728 is compared to a fixed threshold count by a magnitude comparator gate 766 to produce a binary 1 or 0 in response to the measurement of the data measurement interval. The binary result is then clocked into a flip-flop 768 by a pulse from ROM 760. Thus, flip-flop 768 contains a representation of the state of the "click" pushbutton on the remote control handunit. Finally, the outputs of latch 762, latch 764, and flip-flop 768, and the "IR-on" signal, are interfaced as data to the associated computer (not shown) by digital techniques known in the art. The microcomputer may be external to the display device and may be, for example, an IBM PCjr.. In such arrangement the X-axis and Y-axis signals may be connected to the joystick port X-axis and Y-axis inputs. A control program can then be written in IBM Basic which has statements which read the joystick port directly. Other statements can be used to move an illuminated point in response to the data read from the joystick port.

While the embodiments shown have included LEDs for IR transmission, it should be appreciated that any suitable medium of transmission may be employed, such as, ultrasonic, microwave or visible light.