FIELD OF THE INVENTION

The invention relates to a location and communication device and in particular to an ultrasonic locator operable and effective in high noise environments.

BACKGROUND OF THE INVENTION

Recreational SCUBA diving experience has identified that maintaining dive buddy contact in accordance with recognised recreational diving practise can be difficult in low visibility conditions. The same is also true in other applications such as fire fighting where smoke, breathing apparatus and background noise can compromise sight and hearing.

Underwater incidents in recreational SCUBA diving, including low on air, equipment failure, entanglement or entrapment that might readily be resolved by the presence of a dive buddy can escalate to accidents or fatalities when dive buddies become separated. Recommended dive practise requires aborting a dive when buddy separation occurs reducing the recreational utility of the sport.

When people operating in teams for safety become separated any aid that assists in early location can be expected to have significant benefits.

Ultrasonic locating devices and navigational aids are known to be used in different applications and environments for the purposes of locating buddies and/or other team mates. US 6272073, for example, discloses a device that uses ultrasonic signals to indicate the direction and distance between divers. The device incorporates an array of ultrasonic transmitters and multiplexed adaptive band-pass filtered receivers. The device is intended to be worn on a belt and provides directional information based on maximum signal strength from an array of receivers and range information based on transmission time between devices. The device incorporates frequency channel selection and device specific coding, a variable maximum range alarm, and an LCD display with audio and vibration alarms.

Ultrasound is used by mammals including bats and dolphins as a means of object location, ranging and communication. In the marine environment certain crustaceans and echinoderms also emit high intensity ultrasound, as do boat-mounted ultrasonic sonar and fish-finders. In man's built environment electrical equipment such as electronic fluorescent lamp ballasts can also generate unintended ultrasonic signals and electromagnetic interference (background noise). Field signal measurements in both air and water have identified that background noise can be a significant impediment to the reception of desired ultrasonic signals. While the above described device, and others known in the art, may have desirable capabilities, they cannot operate appropriately in high background noise environments in water and air. Furthermore, while background noise may be overcome by relatively high-powered transmission signals this requires physically large transducers and power sources which preclude incorporation in a hand-held device, places restrictions on battery life, and/ or present significant noise pollution in the marine environment.

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.

It is an object of the present invention to provide an improved ultrasonic locator for use in high noise environments or to at least provide the public with a useful choice.

SUMMARY OF THE INVENTION

In a first aspect the invention may broadly be said to consist of a method for identifying a target signal having a predetermined pulse width and a predetermined interval of transmission between pulses in the presence of background noise, the method comprising the steps of:

receiving a signal,

periodically comparing the signal to an average noise value of the environment to determine an occurrence of a pulse within the signal,

determining the presence of the target signal by correlating the presence of the pulse with the presence of at least one previous pulse determined within the signal and occurring substantially one or more predetermined time intervals prior to the current pulse.

Preferably the step of periodically comparing the signal to the average noise value to determine the occurrence of a pulse comprises:

a) determining a long-term average noise value from a plurality of past values of the received signal, b) comparing a current value of the signal to the long-term average noise value, c) periodically repeating steps a) and b) at a first frequency to identify the occurrence of a cycle from a predefined number of consecutive values higher than the average noise, and

d) periodically repeating step c) at a second frequency to determine the presence or absence of the pulse from the presence or absence of a predefined number of consecutive cycles defining the pulse.

Preferably the method further comprises after identifying the occurrence of the pulse, associating a value indicative of the presence or a value indicative of the absence of the pulse against the last of said consecutive cycles.

Preferably the step of determining the presence or absence of the target signal comprises correlating the value associated with the last of the consecutive cycles with the value associated with one or more previous cycles separated by increasing multiples of the predetermined interval of transmission between pulses.

Preferably the method further comprises before determining the presence or absence of a target signal, determining an indication of the strength of a possible target signal by determining the median of the strengths of the consecutive cycles defining the pulse associated with the target signal, wherein the strength of each of the consecutive cycles defining the pulse is determined from a maximum value of the consecutive values identifying the cycle.

Preferably the step of receiving the signal comprises passing the output of a transducer through a passive diode bridge transmit /receive switch, followed by a Low Frequency (LF) band-pass response amplifier for extracting a band pass filtered transmitted signal, followed by a down- converting mixer and Local Oscillator (LO) for converting the band-pass signal to a base-band filtered signal, followed by a base-band Ultra Low Frequency (ULF) band-pass amplifier comprising a buffer, an active band-pass filter, and DC level shifter for extracting the base-band signal, followed by an analogue to digital converter (ADC) for digitizing the base-band signal to extract the received signal.

Preferably the output of the transducer is a signal with a frequency in the range of 20 kHz to 200 kHz and the LO frequency is in the range of 18 kHz to 200 kHz. More preferably the output of the transducer is a 40 kHz signal and the LO frequency is 38.4615 kHz for converting a 40kHz LF band-pass response signal to a 1.5385 kHz nominal base-band signal.

Preferably the current value of the received signal is determined from the output of the ADC.

Preferably the first frequency for repeating steps a) and b) above is determined by the resonant frequency of the transducer and the speed of the ADC.

Preferably the second frequency for repeating step c) is 1.5385 kHz. Alternatively frequencies in the ULF band (300 Hz to 3 kHz) may also be used.

Preferably the target signal has a predetermined pulse width of 4.55 ms and a predetermined channel dependent interval of transmission between pulses of 113.75 ms or 143.65 ms. Preferably the output of the transducer is a signal having 40 kHz cycles in a pulse of 4.55 ms width and repeated every 118.3 ms or 148.2 ms (channel dependant). Preferably step c) above comprises periodically repeating steps a) and b) at the first frequency to identify the occurrence of a 1.5385 kHz cycle from at least 6 consecutive values higher than the average noise. Preferably step d) above comprises periodically repeating step c) at the second frequency to determine the presence or absence of the pulse from the presence or absence of at least 6 consecutive 1.5385 kHz cycles respectively.

Preferably the long-term average noise is determined from the average of 2 ¹⁶ previous digital values of the received signal. In a second aspect the invention may broadly be said to consist of a location device comprising: a case,

a display for providing an indication of range and/or distance between corresponding devices,

a single ultrasonic transducer for providing reception of signals, and

a processor configured to detect a target signal transmitted from another device in the presence of background noise by:

periodically comparing a received signal to an average noise value of the environment to determine an occurrence of a transmitted pulse within the received signal, and determining the presence of the target signal transmitted from another device by correlating the presence of the pulse with the presence of at least one previous pulses determined within the received signal and occurring substantially one or more predetermined time intervals prior to the current pulse.

Preferably the case is a resonant cavity case.

Preferably the ultrasonic transducer is capable of transmitting and receiving signals and the other device is a similar location device.

Preferably the processor is configured to correlate the presence of the pulse with the presence of at least two previous pulses determined from the received signal.

Preferably the step of periodically comparing the signal to the average noise value to determine the occurrence of a pulse comprises:

a) determining a long-term average noise value from a plurality of past measurements of the received signal,

b) comparing the current value of the signal to the average long-term noise,

c) periodically repeating steps a) and b) at a channel dependent first frequency to identify the occurrence of a cycle from a predefined number of consecutive values higher than the long-term average noise, and

d) periodically repeating step c) at a second frequency to determine the presence or absence of a transmitted pulse from the presence or absence of a predefined number of consecutive cycles defining the transmitted pulse.

Preferably the device further comprises a memory component associated with the processor and the processor is further configured to, after identifying the occurrence of the pulse, associate a value indicative of the presence or a value indicative of the absence of the pulse against the last of said consecutive cycles and storing said value in memory against the last of the consecutive cycles.

Preferably the step of determining the presence or absence of the target signal comprises retrieving from memory the one or more values associated with one or more previous cycles separated by increasing multiples of the predetermined interval of transmission between pulses and correlating the value associated with the last of the consecutive cycles with the retrieved values associated with one or more previous cycles.

Preferably the memory component is a cyclic buffer. Preferably the processor is further configured to, having determined the presence or absence of a target signal, assigning an indication of the strength of the target signal from the median of the strengths of the consecutive cycles defining the pulse associated with the target signal, wherein the strength of each of the consecutive 40 kHz cycles defining the pulse is determined from a maximum value of the consecutive values identifying the cycle.

Preferably the processor is configured to send a signal to the display indicative of the strength of the target signal and the relative distance of the device from which the target signal was sent. Preferably the display comprises a plurality of light emitting diodes (LEDs) and the relative distance is indicated by the number of activated LEDs in the display.

Preferably the device comprises electronic circuitry having a receiver stage, the receiver stage comprising: a passive diode bridge transmit/ receive switch connected to the transducer output, a low noise LF band-pass amplifier connected to the diode bridge switch, a down-converting mixer and LO connected to the output of the LF amplifier, a base-band ULF band-pass amplifier comprising a buffer, active band-pass filter and DC level converter connected to the output of the down converting mixer, and a signal processor connected to the ULF amplifier output by an ADC.

Preferably the electronic circuitry further comprises a transmitter stage having a two phase 40 kHz processor controlled synthesized sinusoidal oscillator across the primaries of a tuned pulse transformer. Preferably the processor is further configured to operate the device in one of a plurality of modes, comprising:

a transmitting mode wherein the processor generates one or more signals for transmitter stage to transmit a target signal, and

a receiving mode wherein the processor is configured to detect a target signal sent from another device in the presence of background noise.

Preferably the device further comprises an ambient-pressure c scrirninating mode switch for providing an interface for a user to control operation of the device and switch between modes of operation. Preferably the mode switch is a piezoelectric momentary normally-open extended- pulse push button connected to an RC filter that provides immunity from impulse transients and ultrasonic transmission signals. Preferably the processor is further configured to de-bounce the mode switch.

Preferably the electronic circuitry further comprises a bridge circuit for enabling the device to interface with an external computer, for examining usage parameters, and for reprogramming.

In a third aspect the invention may broadly be said to consist of a method for identifying a pulse modulated signal having a predetermined interval of transmission between pulses in the presence of background noise, the method comprising the steps of:

frequency shifting the pulse modulated signal to a base-band frequency,

detecting whether or not a target signal might be present by comparing a number of baseband samples with the long-term average noise and storing in a buffer a value indicative of the presence of a signal, and

utilising the predetermined interval of transmission and the transmitted signal's pulse modulation characteristics to perform logic comparisons on two or more values in the storage buffer separated by one or more of the predetermined interval of transmission.

In a fourth aspect the invention may broadly be said to consist of a location device comprising: a resonant cavity case,

a mode switch coupled to the case for operational control of the device,

a display for providing range and/ or distance estimates between corresponding devices, a single piezoelectric signal transducer hermetically sealed to the case for providing transmission and reception of ultrasonic signals, and

a power supply and other electronic circuitry housed within the case for controlling the operation of the device including a processor arranged to discriminate a target signal from a received signal in the presence of background noise, the processor configured to:

periodically compare a received signal to a long-term average noise value of the environment to determine an occurrence of a pulse within the signal, and

determine the presence of a target signal sent from another communication device by correlating the presence of the pulse with the presence of at least one previous pulse determined within the received signal, the current and previous pulse being separated from one another by substantially one or more of the predetermined time intervals.

The term "comprising" as used in this specification means "consisting at least in part of. When interpreting each statement in this specification that includes the term "comprising", features other than that or those prefaced by the term may also be present. Related terms such as "comprise" and "comprises" are to be interpreted in the same manner.

The invention consists in the foregoing and also envisages constructions of which the following gives examples only.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will be described by way of example only and with reference to the drawings, in which:

Figure 1 is a perspective external view of a preferred form communication device of the invention,

Figures 2a and 2b are flow diagrams showing an overview of the noise suppression method of the invention and in particular the processes for cycle detection and pulse detection respectively,

Figure 3 is a schematic circuit diagram of the Diode Switch, Low Frequency Amplifier, Mixer/Down Convertor with Local Oscillator, Analogue PSU Regulator and Voltage Doubler of the preferred embodiment of Figure 1,

Figure 4 is a schematic circuit diagram of the Ultra Low Frequency Band Pass Amplifier of the preferred embodiment of Figure 1,

Figure 5 is a schematic diagram of the Microcontroller, Mode Switch, Battery Management Circuitry, Transmitter Drive, LED Display, and ICCP/In-Water/l-Wire Interface of preferred embodiment of Figure 1,

Figure 6a is a graphical representation of a 20 uV, 40 kHz, pulse-modulated transmission signal at the receive transducer with a pulse duration and period of 4.55 ms and 118.3 ms respectively,

Figure 6b is a graphical representation of the signal of figure 6a converted to a 0.2V, 1.5385 kHz, baseband signal at the ADC input after the device,

Figure 6c is a close-up view of the base-band signal showing a 4.55 ms pulse comprising seven 1.5385 kHz cycles and showing the long term average noise, Figure 6d is a close-up view of the base-band signal showing an ADC cycle sampling window of 25 samples per 1.5385 kHz cycle, Figure 6e shows a 90 degree phase-shifted ADC sampling window at 25 samples per 1.5385 kHz cycle,

Figure 7 is a graphical representation of cyclic buffer implementation of the two period 118.3 ms autocorrelation software of the preferred embodiment,

Figure 8a is a graph showing an ADC sampled 250 mV base-band signal with 1.6 V impulse marine noise,

Figure 8b is a graph of a Digital Signal Processing (DSP) recovered 250 mV signal from Figure 8a,

Figure 9a is a graph showing an ADC sampled 250 mV base-band signal with 2.5 V impulse marine noise,

Figure 9b is a graph of a DSP recovered 250 mV signal from Figure 9a,

Figure 10a is a graph showing an ADC sampled 100 mV base-band signal with 50 mV electromagnetic interference (EMI) in air, and

Figure 10b is a graph of a DSP recovered 100 mV signal from Figure 10a.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

1. Overview

The invention relates to location and communication devices and in particular to a device for estimating range and direction between devices in low-visibility high background noise environments by means of ultrasonic signals. The preferred embodiment is an ultrasonic handheld location device that is operable in high background noise environments for ranging and direction to one or more other similar hand-held devices. The method used to suppress the background noise and determine the presence of a valid target signal received from another device will be described with reference to the preferred embodiment, however it will be appreciated that other communication devices operating in high noise environments may also incorporate the noise suppression method of the invention and such communication devices are not intended to be excluded from the scope of the invention.

Figure 1 shows a low power electronic hand-held device 100 for use in salt or fresh water or air that estimates range and direction between two or more devices or units 100 in the presence of high levels of background noise. The device 100 has a streamlined robust hermetically-sealed resonant cavity case 140 rated for use under water to depths of at least 50 m, a single hermetically sealed ambient-pressure discriminating mode switch 120 providing full operational control, a contrast enhanced LED bar graph display user interface 130, a single piezoelectric signal transducer (hereinafter also referred to as a transceiver 110) hermetically sealed to the case, a tuned pulse transformer providing narrow band transmission and reception of ultrasonic signals, a fastening anchor point 150 to provide secure attachment of the device 100 to the user, and electronic circuitry and software contained within the case 140 for signal conditioning and processing, power supply management, and operational control of the device 100.

The device 100 is intended for application in any low visibility environment (air or water) in the presence of high levels of background noise where users desire to maintain contact, locate a remote unit 100, or for man-down search. The default active mode of the device 100 is as a low power pulse position modulated transmitter. When switched to receive mode the device 100 provides direction and range information to any on-channel regional transmitters (i.e. other devices 100 that are transmitting on the same channel) in the presence of high levels of background noise. The direction is realised upon a user determining the direction of the strongest received signal (or in the alternative, the opposite direction to the weakest received signal) and the range is estimated based on the processed received signal amplitude displayed on the LED display 130. The receive function is facilitated through body-blocking and a high gain band-pass filtered direct conversion receiver circuit combined with real time noise compensation and DSP implemented in software as will be described in detail further below.

Overview of Noise Suppression

The device 100 is arranged to send a unique pulse of ultrasonic signals at predetermined time intervals constituting a transmission signal with period (x). The transmission signal consists of a continuous train of pulses having a predetermined unique duration or width (i.e. predetermined unique signal duty cycle) with an interval of transmission, x, between the start of successive pulses. The frequency of the transmission signal is preferably higher than the pulse width such that each pulse constitutes/contains a predetermined number of frequency cycles. The transmitted signal is attenuated as it propagates through the communication channel, and mixed with delayed alternative path transmission signals and background noise when it received at other devices 100. Referring to Figures 2a and 2b, when a device 100 is in receive mode 200, it will digitise 210 and process any received signal 205 (normally with any combination of filtering, amplification and frequency shifting beforehand) to detect the unique pulse embedded within background noise. Depending on the transducer resonant frequency, the speed of the ADC, the width of a transmission pulse and the extent of any frequency shifting, each cycle within a pulse will have a number of digital readings associated with it. A received cycle is recognised 230 when a certain consecutive number (n) of those readings are higher than the long-term average noise of the environment (215 and 225). The average noise is generally computed 215 dynamically over a period that is comparatively longer than the length of the transmission signal period (x) cycle. The number of consecutive digital readings (n) required for establishing the presence of a cycle 230 is dependent on the cycle width, the ADC capability (speed and precision) of the processor used, and the desired accuracy of the system. If a valid cycle is recognised by the device 100, the device 100 looks at the immediate history of received cycles to determine whether the unique pulse width that constitutes a transmitted signal has been received 235 (n cycles in a row in this case but any other pattern can be used to define a transmitted pulse). If so, the device stores a logical 1 (or a measure of the amplitude of the cycle or pulse) against the current cycle to indicate the occurrence of a pulse 250, otherwise a logical zero is stored against the cycle 240. A circular buffer is updated with the pulse occurrence value stored against the current cycle. This buffer is then auto-correlated 255 with itself over the transmission period (x) at least once but preferably over several transmission periods. In other words, the occurrence of a pulse is correlated with the occurrence of previous pulses received at one or more transmission periods prior to the current cycle to identify reception of a valid transmitted signal.

The process of autocorrelation provides a measure of the similarity between a given signal and a delayed replica of that signal. In the preferred embodiment the sampled base-band history from two complete transmission periods is stored in a circular memory buffer. As each successive cycle is added to the buffer it is correlated with two ideal periods of the transmitted signal. By reducing the ideal pulse train to binary and making use of the known period of the transmitted signal, the correlation simplifies to three simple logic comparisons that can be completed in real time with modest computational resources. The number of correlations required to identify a valid signal is a design choice with increased correlation providing additional signal integrity at the expense of increasing delay in the time to valid signal detection. If the output of the autocorrelation is a logical 1 260 then there is a high probability that a valid transmitted signal has been detected 265.

When correlation is achieved the signal amplitude information is recovered from the median of the base-band cycle samples. The strength of the signal is determined to indicate the relative distance of the transmitter from the device 100 which is then indicated on the LED display 130. 2. Preferred Embodiment

A preferred embodiment of a device 100 employing the above method of noise suppression will be fully described below. It will be appreciated that other devices that communicate via predetermined signals (e.g. ones having predetermined pulse patterns and intervals of transmission) can make use of the above method to identify valid communication signals in the presence of high background noise. The invention is therefore not intended to be limited to the working embodiment described below and much of the components described are preferable but not essential for defining the scope of the invention.

2.1 Preferred Mechanical Properties

The device 100 is housed in a rugged, hermetically sealed resonant case 140 which is preferably plastic and rated for use at absolute pressures in excess of 6 bar. The case 140 is unobtrusive, streamlined ergonomic and, when fully assembled, approximately neutrally buoyant in sea water. The case 140 incorporates a contrast enhancing display window 143 adjacent the LED display 130 to enhance LED visibility in bright light. The rear of the case provides a secure anchor attachment point 150. The signal transducer 110 is mounted on the front of the case and the mode switch 120 is mounted on either the left or right hand side (shown on right side in preferred embodiment but this is subject to the user handedness requirements).

Referring to Figure 1, the case 140 comprises of a main body 141 and a cover 142 of injection moulded ABS plastic (or similar material). The piezoelectric elements 110 and 120, display window 143 and cover 142 are adhered to the case body with a suitable adhesive and/or welded to provide a hermetically sealed enclosure. Preferably the enclosure is rated for submersion to depths of up to 50 m in seawater. The case 140 forms a resonant cavity for transmission signals, which enhances omni-directional signal transmission beyond the inherent directional characteristics of the transducer 110. The external dimensions of the case 140 in the preferred embodiment are 95 mm long by 36 mm wide by 22 mm high but in alternative embodiments the case 140 may have different dimensions, preferably suitable for use as a handheld device, but potentially configured with other equipment. The weight and volume of the device 100 provide approximately neutral buoyancy.

Figure 1 shows a right-handed variant of the single piezoelectric mode switch 120; however a left-handed variant is also envisaged. The mode switch 120 is mounted for ergonomic thumb or forefinger control on the side of the device 100. In the preferred embodiment, the mode switch facilitates all operating modes including power on, cycling between transmit and receive modes, changing channel, power off, and access to other modes including search and rescue, maintenance testing and data recall. The mode switch also provides the anode connection for the associated battery charger. In alternative embodiments however any number of control switches may be incorporated in the device.

In the preferred embodiment, the device uses impressed current cathodic protection (ICCP) to reduce potential corrosion of the aluminium piezoelectric element housings 110/120 when immersed in salt water. In the preferred embodiment a single nickel plated brass anode is provided external to the case 140 to impart an impressed current of approximately 0.01 A/m ² of cathodic area. The impressed current is derived from the device's 100 DC battery power supply under processor control. The magnitude of the current is determined by resistor R27. Alternative embodiments may use more than one anode comprising other materials and plating, or passivated piezoelectric element housings 110/120 (including but not limited to anodising, nickel or gold plating).

2.2 Preferred Electronic Properties

The internal electronic circuitry shown in the schematic circuit diagram of Figures 3 to 5 contributes to the unique capabilities of the present invention. The circuit comprises six functional stages being a microcontroller (also referred to as a processor), an LED display, a mode switch, a receiver, a transmitter, and a power supply. Each of these stages is described in detail as follows.

The preferred embodiment of the invention is for a locator device 100 operating in the ultrasonic frequency range. As such, the piezoelectric transducer element 110 preferably outputs/receives signals with a frequency above 20 kHz and more preferably in the range of 20 kHz to 200 kHz. Similarly the local oscillator (LO) of the receiver stage described in more detail below preferably operates in the frequency range of 18 kHz to 200 kHz accordingly. In the preferred embodiment described below the transducer's 110 operating frequency is 40 kHz and the LO frequency is 38.4615 kHz for converting a 40 kHz band-pass signal to a 1.5385 kHz base-band signal. The invention is not intended to be limited to a specific frequency or frequency range of operation; however, the electronic circuitry described below has been optimised for a device 100 operating to send/receive 40 kHz ultrasonic signals. It will be appreciated that the stages of the electronic circuitry are only preferred and that other components known in the art of communication can be employed in place of or in addition to the componentry described for the preferred embodiment if desired without departing from the scope of the invention. 2.21 Microcontt oiler U29

Referring to Figure 5, the microcontroller stage is central to the effective operation of the device 100. It provides precision timing functions, full mode control, LO, ADC and the real-time DSP that permits signal recovery in the presence of high levels of background noise. In the preferred embodiment, the microcontroller U29 is an Atmel ATMega8 series integrated circuit with an external crystal controlled 8 MHz oscillator Q3, power supply decoupling, power on reset circuit, and printed circuit board serial peripheral interface (SPI) programmability. The software program may be hardware locked to prevent attempts at reverse engineering. The microprocessor U29 utilises sleep modes to minimise power consumption in both active modes and in the off state. External interrupts are used to interface to the mode switch and detect connection of an external battery charger. The ADC reference is externally filtered by capacitor C23, and a power supply filter comprising inductor U22 and capacitor C36. Automatic Gain Control (AGC) is implemented by software selection of the ADC reference voltage.

In the preferred embodiment, the microcontroller U29 incorporates a high speed ADC with sample and hold, AGC, LO generation, dynamic noise adjustment, real time DSP for target signal identification and noise suppression, signal transmission time-base and phasing, mode and timing control, integrated power management, LED display control, non- volatile electronic data recording and recall of operating parameters and device-specific assignments, maintenance and testing functionality. The device 100 may comprise the necessary hardware and software components for re- programming the microcontroller U29. In the preferred embodiment, the device 100 comprises both an on-board SPI interface and a proprietary implementation of the Dallas 1 Wire Bus system for interfacing with the microcontroller U29 for re-programming purposes. Software for error detection, such as cyclic redundancy check (CRC) code and embedded copyright detection is also employed in the preferred embodiment for checking the integrity and detecting accidental or deliberate changes to internal program and data associated with the microcontroller U29. The device 100 may alternatively utilise other hardware and software components known in the art for interfacing with and reprogramming the microcontroller U29.

2.22 LED Display 130

The eight segment LED display 130 provides the user with visual cues to enhance mode switch operation, information about display functionality, battery status, serial number, operating mode, selected channel, and shut-down. When in receive mode the LED display 130 provides information about the direction and range of regional on-channel transmitters. During battery charging the LED display 130 indicates charge status and charge completion. In maintenance mode the LED display 130 provides test information and recalls usage data from non-volatile EEPROM. The device 100 may also be configured to allow querying of the serial number associated with the device 100. In this case the LED display 130 is configured to provide an indication of the serial number, such as the lowest binary digits of the serial number stored in memory, depending on the number of LEDs available on the display 130. In alternative embodiments any number of LEDs, an LCD or other forms of display can be used instead, or in addition, to provide information to the user about the operating condition of the device. In the preferred embodiment, the LED display 130 comprises four red U19, U21, U25 and U28 and four green U16, U30, U31 and U32 LEDs configured on the printed circuit board as a bar graph. Each LED is individually addressable via the microcontroller Input/ Output (IO) ports. LED current is established by series resistors R18, 40, 41, 42, and 47 to 50 and is typically 3.5 mA in the preferred embodiment.

2.23 Mode Control Switch 120

The mode control switch 120 is implemented as an ambient-pressure discriminating piezoelectric momentary normally-open extended-pulse push button. Circuitry comprises a passive low pass RC filter (R23 and C9), discharge resistor R22 and MOSFET amplifier U20 interfacing to a drain load RC filter comprising a programmable resistor internal to the microcontroller U29 external interrupt INTO IO Port and capacitor (Cll). The resistive voltage divider established by R23 and R22 provides over-voltage protection for the MOSFET U20 gate. The time constant of the RC filter provides immunity from impulse transients and ultrasonic transmission signals, and extended pulse operation. Further switch de-bouncing is implemented in software. The case of transducer 120 is a loose ground connection via resistor R16 to permit its use as the battery charger anode connection.

2.24 Receiver Stage 160

Referring now to Figures 3 and 4, the receiver stage 160 comprises the following sub-stages: a passive diode bridge transmit/receive switch 161, a low noise LF band-pass amplifier 162, a down-converting mixer and LO 163, and a base-band ULF band-pass amplifier 164 comprising a buffer, active band-pass filter and DC level converter. Each sub-stage is described as follows.

Ultrasonic pressure waves at the resonant frequency of the piezoelectric signal transducer 110 and noise impulses result in voltage signals between the output pins of the transducer 110. The transducer's case is connected to signal ground. The ground-referenced signal voltage on the active pin of the transducer is capacitively coupled (CI 2) to a passive diode bridge switch 161 (Ull to 14 and R5 and 32). The switch 161 isolates high voltage transmission signals from the input of the first integrated circuit operational amplifier 162 while providing minimal transmission signal loading through the series resistance of R5 and R32. The diode bridge 161 also provides mid-rail DC bias for operational amplifier Ul (1.65 V in the preferred embodiment). A protection diode U18 is incorporated in the bridge design 161 as the analogue electronics power supply voltage is switched off in transmit mode. The front end low noise LF band-pass response amplifier 162 is implemented using two ultra low noise LMP 7732 integrated circuit operational amplifiers (both labelled Ul) and associated analogue components. In the preferred embodiment, the amplifier provides a 40 kHz band-pass response with a gain of 40 dB and a - 3 dB bandwidth of 2.8 kHz. The mixer 163 functions as a single ended direct-conversion receiver (frequency down converter) with a double balanced output having a gain of approximately 16 dB using an NE612 mixer integrated circuit (U2) with a 38.4615 kHz crystal controlled LO implemented in the processor resulting in a 1.5385 kHz base-band signal from a 40 kHz transmission signal. Nyquest's Sampling Theorem requires a sampling rate of 80 kHz to establish the presence of band-width limited 40 kHz signals. While this is achievable with state-of-the-art electronics, high frequency real-time sampling and processing is relatively expensive in terms of commercial cost, power consumption, memory and computational burden. Hence the implementation of a direct conversion receiver to permit signal processing at a more modest base-band frequency of 1.5385 kHz is desirable and employed in the preferred embodiment of the invention. The LO signal is generated by the processor providing a square wave output of 0.3 mV across 1.5 K Ohms (-13 dBm) in accordance with mixer manufacturer's literature. The third (and higher) harmonic products from the square wave LO are effectively attenuated by the ULF sub-stage.

The base-band ULF band-pass amplifier 164 comprises a quad TLV 2774 operational amplifier (all labelled U3) configured to provide an differential impedance buffer, active low-pass filter, active band-pass filter, and DC level converter with a 1.5385 kHz modified band-pass response having an overall gain of up to 26 dB and with a -3 dB bandwidth of approximately 680 Hz. Each stage has been optimised for transient and frequency response, component-count and tolerancing. The DC level converter uses the input diodes of the associated integrated circuit operational amplifier to limit negative transitions on the non-inverting input.

2.25 Transmitter Stage 170

Referring to Figure 5, the transmitter stage 170 comprises a two phase 40 kHz microcontroller oscillator switching IFR7807 MOSFETs (U26 and 27) across the push-pull five turn bifillar wound primaries of a tuned toroidal pulse transformer. The transformer is wound on an N87 ferrite core. The 84 turn transformer secondary provides 140 V ppk across the Audiowell Type TR40-16B ultrasonic transducer. The MOSFET gates are connected to ground via resistors R43 and 44 to prevent inadvertent switching during processor transients. The transmitted signal in the preferred embodiment comprises 4.55 ms of 40 kHz synthesised sine wave pulses with a repetition period (pulse transmission interval) of 113.75 ms or 143.65 ms subject to channel selection. In alternative embodiments any other suitable pulse modulation technique (i.e. any other frequency and pulse periods) may be employed to transmit the communication information between devices 100 and the scope of the invention is not intended to be limited to this particular modulation scheme.

The current embodiment incorporates a two phase 125 ns pulse- width modulation (PWM) scheme for synthesised sinusoidal transducer drive implemented through processor control. The experimentally optimised PWM compensates for changes in the electromechanical properties of the transducers in water and in air (in conjunction with the in-water sensor described below) to ensure a sinusoidal transducer excitation voltage with minimal transient overshoot. The purpose of this design feature is to minimise the risk of premature transducer failure through transient shock and associated over-voltage. 2.26 Power Supply

In the preferred embodiment the power supply is contained within the case 140 with associated circuitry providing full charge and discharge management, short circuit and over voltage protection, over temperature shutdown, battery condition display and associated software and hardware to provide extended battery life. Referring to Figures 3 to 5, the power supply stage comprises the following sub-stages: a battery, a charge management controller, a MOSFET power switch, a voltage regulator, and a charge pump voltage doubler. Each sub-stage is described as follows.

The battery comprises a single nominal 3.7 Volt Lithium Polymer cell with a rated capacity (C) of 900 mAH in the preferred embodiment. The battery contains integral short circuit and over- discharge protection. Any other suitable battery may be used by in alternative embodiments however.

The charge management controller 171 comprises an MCP73832 4.2 V integrated circuit (U15) with input and output filter capacitors C24 and C27 and programming resistor R15 selected for C/2 constant current charging. Inherent characteristics of the integrated circuit are conditioning charge, constant current charge, constant voltage charge, minimum charge current shutdown and over-temperature limits. In the preferred embodiment, the integrated circuit layout is optimised for heat dissipation through the printed circuit board. Resistor R16 provides a novel loose ground connection for the mode switch case which acts as the charging anode connection. The resistor prevents charging of the input filter capacitor C27 through reverse leakage currents. The charge management controller is integrated with microcontroller U29 via charger connection sensing and current limiting resistor R25, an open drain full charge status logic connection internal to the charge controller integrated circuit, and a power supply voltage divider comprising R17 and R21. However other battery charging configurations, including dedicated external contacts, are envisaged.

The microcontroller U29 provides charge status through the LED display 130 and power shutdown on charge completion. Battery charge termination occurs when the constant voltage charge current reduces below 0.1 C (typical). In operation the device 100 will automatically turn off after ninety minutes of inactivity if it is out of the water to preserve battery life. Voltage regulator TS9001 (U10) shown in Figure 3 provides a low- voltage-drop 3.3 V analogue supply rail switchable under processor control to permit optimised power consumption during transmit and off modes.

The charge pump voltage doubler 172 shown in Figure 3 comprises an ADM660 integrated circuit (U6) with input and output filter capacitors C31 and 32 and charge pump capacitor C21 driven from the analogue 3.3 V power supply regulator. Schottky diode Dl prevents latch-up of the integrated circuit during turn on and is appropriately rated for the charging current of the output capacitor C32 during turn-on transients to avoid instability. The voltage doubler provides a stable 6.6 V supply for the mixer integrated circuit U2 and power supply decoupling from the LF 162 and ULF 164 gain stages.

The LF gain stage 162, and the ULF gain stage 164 are powered by voltage regulator TS9001 (U10). The regulator and individual integrated circuit power supply connections are decoupled to reduce inter-stage coupling through the power supply and provides stable and predictable gain of the analogue stages over the life of the batteries. The regulator is decoupled by 0.1 uF ceramic capacitor CI 8 at the input and 22 uF Tantalum capacitor C6 at the output. A decoupled mid-rail voltage reference is established by resistive voltage divider R24 and 31 and capacitor C2 from the regulated supply rail for the ULF stages. A further mid-rail voltage reference is established by the diode bridge transmit/receive switch and the RC filter comprising R19 and C36 for the LF band pass response gain stages.

The microcontroller U29 and the transmitter toroidal transformer primary windings are connected direcdy to the battery in parallel with a low Equivalent Series Resistance (ESR) 22 uF Tantalum capacitor C24 to reduce voltage transients during transmit pulses and prevent inductive instabilities with the charge management integrated circuit U15.

2.27 Other features

Multi-channel operation In the preferred embodiment, the device 100 comprises two or more transmit and receive channels for enabling communication between multiple different groups of divers or to ensure a pair or group of divers are communicating with one another and not a different group of divers. Multiple channels are implemented through pulse position modulation (PPM) implemented in software. In the preferred embodiment pulse separation is 118.3 ms and 148.2 ms for each of two implemented channels respectively. Alternative and/or additional PPM schemes are envisaged.

Other software and/or hardware implemented methods for achieving multi-channel operation may also be employed.

In water detection

In the preferred embodiment, the device 100 further comprises circuitry for detecting whether the device is submerged in water or not. In particular, the device 100 comprises resistor R27, two configurable processor IO ports, and an electrode external to the case. Resistor R27 forms a simple voltage divider with the resistivity of the immersion medium to the grounded piezoelectric transducer cases 110/120. The presence or absence of water is sensed directiy by the processor under software control through the voltage presented at IO Port PD7 which provides a ADC capability.

Device to computer interface

In the preferred embodiment, the device 100 further comprises a separate Bridge circuit providing an interface between the device 100 and an external computer. The Bridge connects to the device 100 through an external electrode (also serving to provide immersion sensing and ICCP) via a proprietary implementation of the Dallas 1 Wire Bus protocol. The interface between the Bridge and an external computer may be via any standard communication means such as a serial port, as used in the preferred embodiment, or a Universal Serial Bus (USB) or wireless (using Wi-Fi or Bluetooth standards for example). The Bridge interface provides the capability of downloading data stored in device 100 memory such as serial number, software and hardware version, transmit and receive mode cycles and times, and charge cycles and time. This information is intended to aid in establishing warranty claims and assist in the provision of servicing. The Bridge interface also enables uploading of software revisions to device 100 to enhance and improve device 100 functionality and capability over time. Maintenance Mode The preferred embodiment of the device 100 further comprises a maintenance mode, not normally available to the end user, that permits direct reading of non-volatile EEPROM within device 100, and completes a sequence of hardware functionality tests to aid in operational testing and quality assurance during manufacture. Maintenance mode is activated through the use of the mode switch 120, requiring a precise sequence of presses at specific times during the standard device 100 power-up sequence. Once in maintenance mode device 100 will revert to standard operation after 5 minutes of inactivity or at any time on three consecutive presses of the mode switch 120 at approximately one second intervals. 2.3 Digital Signal Processing for Noise Suppression 2.31 Normal Transmit/Receive Operating Modes

The receiver stage 160 outputs base-band signal to the ADC input of the microcontroller U29 which digitizes the signal for further processing by the microcontroller U29 and in particular for identifying valid low level transmission signals of interest in the presence of background noise.

In the preferred embodiment, a 40 kHz transmission frequency is used by device 100 on the basis of understood noise sources in marine environments to minimise intermittent, local and prevailing noise from seismic activity, precipitation, shipping, sea ice, surface agitation and other sonar devices. However marine biological noise remains prevalent across the LF band.

Equipment was constructed to measure background noise in order to better understand the noise problem. Figure 9a shows typical high levels of marine background noise measured in an area of known high noise. The noise signal can be considered to comprise random impulses with intensity inversely proportional to occurrence, as might reasonably be expected from a large number of random pulse emitters dispersed over the sea- floor and throughout the regional body of water.

Figure 10a shows high levels of noise from two electronically ballasted florescent lamps in close proximity to the receiver electronic circuitry. The noise is primarily EMI and is continuous with a base-band frequency of between 1.7 and 2.2 kHz and cyclic peaks at 50 Hz.

Range, direction and the time of signal transmission are unknown at the receiver so the valid signal detection algorithm contained within microprocessor U29 is restricted to processing pulse width and duly cycle, both of which are known a priori. The DSP as applied may be generally described as digital filtering, autocorrelation, and linearisation.

The operation of the noise suppression functionality of the DSP will be described with reference to the preferred form of transmission signal on one channel. It will be appreciated that the same conceptual method of target signal discrimination may be employed on systems or devices using alternative transmission signals and channels, and the scope is not intended to exclude such other systems and devices. The operation of the device is described below with reference to the first channel of the preferred embodiment only. The processor of the device 100 is also configured to transmit and process received signals using a second channel (as described in the section 2.27) where the predetermined interval/period of transmission between pulses is 148.2 ms (instead of 113.75 ms). As such, it will be apparent to a person skilled in the art that necessary alterations in timing from what is described below is required to transmit and process received signals through the second channel.

The pulse modulated 40 kHz transmitted signal shown in Figure 6a has a pulse width of 4.55 ms and a period of 113.75 ms. At the receiver stage 160, the received signal is amplified, filtered, shifted in frequency, and subject to further amplification, filtering and DC level-shifting by the analogue electronics to produce a 1.5385 kHz base-band signal as shown in Figure 6b which is applied to the input of the microcontroller's ADC.

A received base-band transmission pulse at the input to the ADC comprises 7 cycles (4.55 ms) of 1.5385 kHz signal as shown in Figure 6c.

The base-band frequency of 1.5385 kHz was selected for the current embodiment after consideration of the characteristics of selected microcontroller including its capability to provide a free-running crystal controlled LO, its computational processing ability, and the need to discriminate between impulse noise and transmission signals of interest.

In the preferred embodiment 2 ¹⁶ consecutive ADC values are summed to establish a computationally efficient measurement of the long-term average noise which is dynamically updated every 1.7 s. It will be appreciated that any number of consecutive ADC values may be summed to establish the long-term average noise provided the number of samples is relatively long in comparison with the transmission signal period. In the current embodiment the microcontroller's ADC completes a free-running conversion every 26 us providing 25 samples per cycle of the 1.5385 kHz base-band signal as shown in Figure 6d. The peak amplitude of each base-band cycle is determined from the maximum of 6 or more consecutive ADC readings each required to be greater than the measured long-term average noise. Otherwise a cycle is considered to have zero amplitude. As shown in Figure 6e, 6 consecutive samples greater than the long-term average noise accommodates for potential phase shift between the transmitted signal and the ADC 25 reading sample window. This initial signal processing effectively constitutes a Finite Impulse Response (FIR) 6.4 kHz low-pass filter with infinite attenuation in the stop-band.

In the preferred embodiment valid transmission pulse detection is realised on the occurrence of 6 or more consecutive non-zero 1.5385 kHz cycles, being slighdy less than the full (7 cycle) transmitted pulse width to accommodate frequency variations between devices and relative (Doppler effect) movement between transceivers. The amplitude value assigned to a valid pulse is the median of the most recent 5 base-band cycles (the calculation of a median of 5 values providing computational expedience). The use of the median as opposed to the arithmetic mean or peak amplitude reduces the influence of high amplitude one or two cycle impulses - an observed characteristic of marine impulse noise. In alternative embodiments however, other known methods of calculating the average of the amplitude can be used. This DSP strategy has been found to provide excellent noise discrimination in practise in both water and air (refer to Figures 8, 9 and 10). However signal detection can be further improved under specified noise conditions by reducing the cycle count required to detect valid signal. Tuning of this aspect of the preferred embodiment can assist in recovery of valid signals buried deep within noise.

The previous 236.6 ms of base-band cycle history is stored in a circular buffer implemented in 364 contiguous Random Access Memory (RAM) locations as shown in Figure 7. When a valid pulse is detected a logical 1 (or an indication of the amplitude of the pulse) is stored in the current RAM location of the cyclic buffer, otherwise the RAM location is cleared to logical zero.

The occurrence of a valid pulse is auto-correlated with the cycles that occurred 118.3 ms and 236.6 ms previously by simple logical comparisons between the contents of appropriately spaced cyclic buffer locations. Binary autocorrelation indicates either a high probability that a valid signal of interest has been received at the current instant in time, or alternatively a high probability that no valid signal has been received. The preferred embodiment incorporates autocorrelation over two 118.3 ms signal periods, with an initial capture delay of 236.6 ms while the cyclic buffer is initially filled. This has been found to be effective in practical application. Where additional signal integrity is desired additional correlation cycles could be incorporated with minimal software overhead subject to available microcontroller RAM and a proportionate increase in initial signal capture time. Calculation of the autocorrelation function in real time, even at relatively low frequencies, can be computationally expensive in terms of processing power and storage requirements. However the device 00 effectively completes autocorrelation in the real time with low processing and storage requirements. This is facilitated by frequency shifting the pulse modulated 40 kHz signal to a base-band frequency of 1.5385 kHz, detecting whether or not a signal might be present by comparing a number of base-band samples with the long-term average noise, and making use of a priori knowledge of the transmitted signal's pulse modulation characteristics to reduce calculations to a limited number of logic comparisons.

The LED display 130 is updated every 118.3 ms, or 148.2 ms subject to channel selection, with the highest correlated median pulse amplitude received in this time interval or alternatively cleared to zero. In the preferred embodiment the lowest LED is also polled for 0.1 s in every 2 s to indicate that a unit is in receive mode in the absence of a valid received signal.

Theoretically the received signal strength from a remote transmitter in open space (that is in the absence of ultrasonic channelling, reflection and refraction) varies in proportion to the reciprocal of the square of the range. The received signal display is linearised by means of a lookup-table based on field measurements such that each additional LED lit in the display 130 after the first corresponds with a reduction in the relative distance between the transmitter and receiver by approximately l/8 ^th . The resulting display provides an indication of range relative to the receiver/ transmitter orientation and actual range.

The use of a priori knowledge of the transmit signal pulse width and duty cycle combined with binary reduction for DSP greatly simplifies the correlation process from computationally expensive multi-byte multiplication across the full length of the circular buffer to a few discrete logic tests, enabling effective real-time DSP with modest processing capabilities.

Inherent in the current embodiment is detection of the valid signal with the highest amplitude in the presence of multiple transmitters and noise. However the use of alternative pulse modulation schemes also permits the identification of specific transmitters, enabling the channel separation between several transceiver pairs operating in the same region at the same frequency in the presence of background noise.

2.32 Search and rescue mode

In the preferred embodiment, the device 100 comprises a search and rescue operating mode which does not utilise noise suppression, ACG or signal linearisation as described above. In this mode, the device 100 is configured to continuously display the maximum received signal at the highest gain at the fastest possible ADC sampling rate. Preferably device 100's operation in this mode is channel independent so that all channels are open/operating to receive, detect and display ultrasonic signals (including background noise) within the environment. Field trials have demonstrated that, with appropriate operator training, this mode allows directional detection of a transmitting device independent of transmission channel.

3. Experimental Work

Application of the DSP algorithm in the presence of high levels of noise was simulated using Microsoft Excel using base-band data obtained from field measurements.

Figure 8a shows 1 s of ADC-sampled valid base-band signal with a 250 mV peak amplitude in the presence of moderate repetition rate 1.60 V impulse amplitude marine noise. Figure 8b shows the result of application of the described DSP. The signal information is entirely recovered, less the first two pulses required for initial autocorrelation.

Figure 9a shows 1 s of ADC-sampled valid base-band signal with a peak amplitude of 250 mV embedded in high intensity 2.5 V impulse amplitude marine noise. Figure 9b shows the result of application of the described DSP. The signal information is entirely recovered from the noise, less the first two pulses required for initial autocorrelation.

Figure 10a shows 1 s of ADC-sampled valid base-band signal from a non-aligned transmitter at a range of 20 m with a reflected and aperture-limited signal path. The signal has a peak amplitude of 100 mV in the presence of 50 mV of EMI from two fluorescent lamps located within 0.25 m of the receiving device. Figure 10b shows the result of application of the described DSP. The signal information is entirely recovered from the noise, less the first two pulses required for initial autocorrelation.

Example Mode of Operation of the Device Briefly depressing the mode switch 120 once turns the device 100 on. All eight LEDs 130 will light for a one second lamp test followed by a battery condition display and a channel number display (see Table 1 below).

[] Red LED On [] Green LED On | LED Off

Table 1: Power on, Lamp Test, Battery Condition and Channel Display The bottom red and top green LEDs indicate the channel. If the battery is depleted (after about 50 hours of use) the device 100 will indicate a depleted battery condition by displaying two red LEDs and turn off after thirty seconds. This feature has been incorporated to ensure that the device 100 will have sufficient power to operate reliably during use and to avoid deep-discharge damage to the battery. A depleted battery can be avoided by recharging at the earliest opportunity when a marginal battery condition is indicated.

The channel can be changed by depressing the mode switch 120 five times at approximately one second intervals during the battery condition display. This long key sequence has been incorporated to reduce the possibility of inadvertently changing channels.

Briefly depressing the mode switch 120 once more from the battery display activates the transmit mode of the device 100. If transmit mode is not activated within 30 seconds the device 100 will automatically shut down. In transmit mode the upper green LED will flash approximately every second as shown in Table 2. A unit will stay in transmit mode until receive mode is activated, it is turned off, or it automatically turns off after 90 minutes and is out of water. Transmit mode is the default mode for diving applications. LED Display Condition

■mm Buddy Locator is in transmit mode.

ft Flashing Green LED | LED Off

Table 2: Transmit Mode Display

Briefly depressing the mode switch 120 once again activates the receive mode. The LED display 130 will indicate received signal strength as shown in Table 3 (ranges stated for aligned units in water). The signal strength is indicative of both direction and range subject to the orientation of the transmitter /receiver pair and the environment.

Flashing Red LED [] Red LED On [] Green LED On | LED Off Table 3: Receive Mode Signal Strength Display More LEDs lit equates to increasing received signal strength as identified by the above described DSP unit. Each LED in the bar-graph display 130 corresponds to approximately 1/8* of the distance to a transmitter. Each unit 100 will remain in receive mode for 5 minutes before it automatically switches back to transmit mode. This safety feature ensures that each unit 100 will change to transmit mode if it has been inadvertently placed in receive or should a user be unable to operate it. Briefly depressing the mode switch 120 returns the device 100 to receive mode. Briefly depressing the mode switch 120 once swaps between transmit and receive modes at any time.

The device 100 can be turned off by depressing the mode switch 120 three times at approximately 1 second intervals. The LEDs 130 will go through a Cylon display sequence indicating that the device 100 is shutting down.

Each unit will automatically turn off after 90 minutes of inactivity if it is out of the water. This design feature is intended to conserve battery life if a unit is inadvertently left on after a dive while ensuring that a unit will remain operating until the batteries are exhausted when submerged to aid with lost diver search.

Pressing the mode switch 120 once during the Cylon turn-off display will display an indication of the device's serial number. The LEDs will indicate the lowest binary digits of the device's serial number. The device 100 will automatically turn off after 30 seconds from displaying the serial number digits.

The device 100 is also configured to provide a search and rescue mode of operation. Depressing the mode switch 120 three times at approximately one second intervals during the Lamp Test display (shown in Table 1) will initiate search and rescue mode. In this mode there is no marine noise suppression, AGC or signal linearisation. In search and rescue mode the device 100 will not time out and stay in this mode until the batteries are depleted. Pressing the mode switch three time at approximately one second intervals in this mode will turn the device 100 off.

The device 100 is preferably designed to identify certain hardware and software fault conditions and display a particular LED combination indicative of the fault such as shown in Table 4 below. Memory Corrupted and Program Mode Initiated

The foregoing description of the invention includes preferred forms thereof. Modifications be made thereto without departing from the scope of the invention as defined by the accompanying claims.