FREE SPACE COMMUNICATIONS SYSTEM EMPLOYING LINE OF SIGHT RADIATION

Abstract: A signal transmission system comprises a transmitter arranged to transmit a coded signal as a series of pulses of line of sight radiation having a predetermined minimum interval therebetween, a receiver responsive to such radiated pulses received at a corresponding minimum interval, and a plurality of repeater devices. Each repeater device comprises a radiation receiving element, a radiation transmitting element, and means responsive to receipt of a pulse of transmitted energy by said receiving element for causing said transmitting element to emit a corresponding pulse of transmitted energy and for causing said receiving element to be disabled for a predetermined period of time corresponding to at least a multiple of the duration of the pulse transmitted by said transmitting element. A plurality of the repeater devices can thus be arrayed between said transmitter and receiver to provide a plurality of line of sight radiation propagation paths with a plurality of repeaters each in line of sight of the transmitter and/or other repeaters and with the receiver in line of sight of a plurality of said repeaters so that random interruption of the signal propagation paths does not interrupt the transmitted signal. Each repeater may comprise its own transmitter and keypad to provide an audience polling system.

Description: - 2 - keypad responds to a different address and as the controlling computer polls the keypad addresses they each return their delegate' s response. Such a system uses serial bi-directional communications over cables. Most conferences are held in venues which do not provide group response systems so that such a system would be hired in for the duration and installed on a temporary basis. The installation of such a system is labour intensive in view of the requirement for the provision of cabling, and thus a system that avoided the use of cables would provide a significant advance on the state of the art. The present invention provides a signal repeater device comprising a radiation receiving element, a radiation transmitting element, and means responsive to receipt of a pulse of transmitted energy by said receiving element for causing said transmitting element to emit a corresponding pulse of transmitted energy and for causing said receiving element to be disabled for a predetermined period of time corresponding to at least a multiple of the duration of the pulse transmitted by said transmitting element. The invention further provides a signal transmission system comprising a transmitter arranged to transmit a coded signal as a series of pulses of line of sight radiation having a predetermined minimum interval therebetween, a receiver responsive to such radiated pulses received at a corresponding minimum interval, and a plurality of repeater devices as referred to above, the arrangement being such that said plurality of repeater devices can be arrayed between said transmitter and receiver to provide a plurality of line of sight radiation propagation paths with a plurality of repeaters each in line of sight of the transmitter and/or other repeaters and with the receiver in line of sight of a plurality of said repeaters, the longest propagation path and FREE SPACE COMMUNICATIONS SYSTEM EMPLOYING LINE OF SIGHT RADIATION This invention concerns signalling devices employing line of sight radiation, and more especially, but not exclusively, to infra red transmitters and receivers of the kind that are designed to communicate using coded signals. Such devices have particular application in circumstances where relatively short range line of sight communication is appropriate, and avoid disadvantages inherent in, for example, radio communication or the use of cables . They do have the disadvantage, however, that signal communication is interrupted when there is no direct line of sight between transmitter and receiver. Where relatively short range communication is involved, it has been proposed to overcome the latter disadvantage by providing infra red reflectors or repeaters to enable transmission via an indirect path. This presupposes however that the path or paths of transmission can be planned in advance and does not allow for possible random interruption of signal paths. It is accordingly an object of the present invention to provide an improved means of line of sight signal transmission that is less susceptible to failure as a result of random blockage of signal paths and that is therefore usable in systems of a kind that has hitherto required the use of radio or direct cabling. One known communication system, for example, makes use of a communication link for polling a number of hand held keypad devices used in gathering opinions from delegates at meetings . At such a meeting, each delegate is equipped with a hand held keypad. The presenter displays a question on a video screen and the delegates enter their response on their keypad. Each the response time of each repeater being such that all repeaters in said longest path will have responded to a given pulse from said transmitter within the said predetermined period of time, and the said predetermined period of time being less than, or at most equal to the said minimum interval. Advantageously such a signal transmission system may be embodied in a polling system as described above, in which case each hand held keypad may comprise a repeater as defined above in addition to a said transmitter and a said receiver and may incorporate a microprocessor to determine whether the keypad device is to operate as a repeater, a receiver or a transmitter. The invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is a plan view illustrating one arrangement of a signal transmission system according to the invention. Figure 2 is a timing diagram illustrating the operation of a system as shown in Fig. 1, Figure 3 is a circuit diagram of a repeater of the system of Fig. 1, and Figure 4 is a plan view in diagrammatic form of a hand held keypad/repeater unit. Figure 1 shows a typical seating plan of a room containing a polling system of the kind described above and embodying a signal transmission system according to the invention utilising infra red links to eliminate the cabling of the known system. Keypads are shown in four regions labelled B, C, D, and E. A transceiver A for polling the keypads is shown in the top right hand corner of the room and the regions indicate the general progression of the communications using the proposed repeater method. The polling command and keypad address first emanate from A and act directly with region B. Region B repeats the signals to region C and so on to region D and then region E. The keypad that is being polled, lets say in region E then radiates its signal to region D which repeats it to region C and so on back to the transceiver A. The key factors to be considered, to make this method successful, are feedback loops and propagation delays. Figure 2 shows a timing chart with the transceiver signal A being successively delayed through each region Bl in region B, Cl in region C, Dl in region D, El in region E at times tl, t2, t3, t4 and t5. To prevent feedback loops, each repeater has a lockout period B2, C2, D2, E2 which is initiated by a received pulse. Looking at figure 2 keypads in region B could be retriggered by keypads in region D. To prevent this the receiving circuit in B is disabled for the lockout period B2. This strategy works provided that there are not so many propagation stages that keypads could be producing pulses after the lockout period as shown by XI in figure 2. However as such a key pad would be out of range of keypads that have ended their lockout period this would not be a problem. The lockout period may, for example, be only 4 to 10 times the propagation delay. Figure 3 shows how the repeater is implemented using standard electronics techniques. The received signal at the diode detector A is amplified and differentiated B. The signal is passed through a comparator C with its threshold Cl set above noise levels. The output pulse is then retimed by monostable D to produce a signal for repetition. The lockout monostable E is also triggered by monostable D. Monostable E disables monostable D during the lockout period. The lockout period is less than or at most equal to the interval between consecutive pulses of a coded signal. As shown in Fig. 2 the lockout period B2 ends at time t6 before the next pulse of transceiver signal A at time t7. Each keypad thus recognises the individual pulses originating from the transceiver whilst ignoring repetitions of the same pulse. Whilst repeating the transmitted pulses each keypad also decodes the signal via the microprocessor shown in Fig. 3 to determine whether its own address has been transmitted and it should transmit a response to the poll. Such a response includes data input manually via the keypad and stored by the microprocessor as is conventional in the known systems referred to above. When a keypad is transmitting its delegate's response it changes from repeater mode to originator mode using the mode control line and mode switch F. After it has completed its transmission the keypad returns to repeater mode. In the illustrated embodiment the normal non-return-to-zero (NRZ) serial data format of the microprocessor is converted to return-to-zero (RZ) serial data format for transmission. This improves the integrity of the signal once it is converted into pulses. The RZ signal is likewise converted to NRZ at the input to the microprocessor. Advantageously each keypad has IR detectors and emitters on its sides as well as the front edge. Such an arrangement is shown in Fig. 4, wherein IR detectors are illustrated at 10 and emitters at 11. This allows the signal to pass across a row of delegates sitting next to each other. In addition to keys 14 for manual input, the keypad may also have a microphone 12 which transmits using infra red and preferably also the repeater method. In the latter case the mode switch F will be set to originator mode upon use of the microphone, for example, in response to actuation of a transmission button 13 associated with the microphone. The keypad is held up to the speaker's mouth when it is being used, hence the importance of sidelooking transceivers which retain their general sideways view when the keypad is held like this. It will be appreciated that this novel arrangement of detectors, receivers and microphone may be utilised independently of the repeater transmission system without departing from the scope of any invention comprehended herein as alternative means may be provided for the reception of microphone signals, such as dedicated IR receivers at the ends of each row of seating.

Claims: CLAIMS 1. A signal repeater device comprising a radiation receiving element, a radiation transmitting element, and means responsive to receipt of a pulse of transmitted energy by said receiving element for causing said transmitting element to emit a corresponding pulse of transmitted energy and for causing said receiving element to be disabled for a predetermined period of time corresponding to at least a multiple of the duration of the pulse transmitted by said transmitting element. 2. A device according to Claim 1, further including means for generating an original signal for transmission from the device and mode control means for selecting one of two modes of operation of the device in a first of which the device responds as a repeater to said pulses received by said receiving element and in a second of which the device operates as a transmitter of said original signal and any said received pulses are ignored. 3. A device according to Claim 1, including a keypad enabling the manual input of data, the said means for generating said original signal being responsive to said keypad to derive therefrom data to be incorporated in said original signal. 4. A device according to Claim 3, wherein said means for generating said original signal is arranged to store data input via said keypad and to transmit said original signal including said data in response to a prompting signal received via said radiation receiving element. 5. A device according to Claim 4 wherein said means for generating said original signal and said mode control means comprises a microprocessor. 6. A device according to Claim 5, wherein said microprocessor stores an identity address identifying the said signal, repeater device, and said microprocessor is programmed to respond to said identity address when included in a signal repeated by the device in order to select said second mode of operation and to transmit said original signal. 7. A device according to Claim 2, wherein said means for generating said original signal comprises a microphone, and said mode control means is arranged to set said device to said second mode of operation when said microphone is in use. 8. A device according to Claim 7, comprising a plurality of said radiation receiving and transmitting elements at least some of which are located on one or more side edges of the device so that radiation can be transmitted sideways along a row of such devices when held in an upright position for use as a microphone. 9. A device according to Claim 8 further comprising said transmitting and/or receiving elements along a front edge of the device, when laid at rest. 10. A signal transmission system comprising a transmitter arranged to transmit a coded signal as a series of pulses of line of sight radiation having a predetermined minimum interval therebetween, a receiver responsive to such radiated pulses received at a corresponding minimum interval, and a plurality of repeater devices as claimed in Claim 1, the arrangement being such that said plurality of repeater devices can be arrayed between said transmitter and receiver to provide a plurality of line of sight radiation propagation paths with a plurality of repeaters each in line of sight of the transmitter and/or other repeaters and with the receiver in line of sight of a plurality of said repeaters, the longest propagation path and the response time of each repeater being such that all repeaters in said longest path will have responded to a given pulse from said transmitter within the said predetermined period of time, and the said predetermined period of time being less than, or at most equal to the said minimum interval.

Inventors: KNOWLES MICHAEL, KNOWLES, MICHAEL
Applicants: IML LTD, KNOWLES MICHAEL, IML LTD., KNOWLES, MICHAEL
Application year: 1995
Main classification: H04B10
IPC codes: H04B7, H04B10, H04Q7