Abstract:
A high-speed electrical data transmission system ( 10 ) includes a signal mixer ( 13 ) for receiving high-speed data from external network transceiver ( 12 ). The signal mixer ( 13 ) converts the data into an electrical current-fluctuating data signal. A signal processor ( 17 ) is located remotely of the signal mixer ( 13 ) and is connected electrically thereto by a simple/inexpensive cable ( 15 ) having only two operative conductors and receives the current-fluctuating data signal via the cable ( 15 ). The signal processor ( 17 ) converts the current-fluctuating data signal into a voltage-fluctuating data signal for distribution to a local area network.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a high-speed electrical data transmission system. The invention relates more particularly, although not exclusively, to a cost-effective system for electrically transmitting data in local area computer networks and other communications networks. 
     Modern methods of transmitting data signals by cable include fibre-optic communication, Low Voltage Differential Signalling (LVDS) and single ended signalling. 
     Fibre-optic systems are presently used to achieve high-speed data transmission over long distances. For example, high-speed broadband Internet and pay-TV networks and telephone networks have employed this technology. However, due to high costs of fibre-optic transceivers, the difficulty of splicing fibre-optic cables and the inability to convey electrical power in addition to data signals, fibre-optic networks have not extended the whole way to each end-connection point such as home or office modems, telephones or pay-TV receivers. Rather, the fibre-optic network extends to a fibre-optic transceiver “node” or “hub” employing optical-to-electrical (and vice versa) signal conversion and from which an electrical network extends to the various points of connection. For example, the signal from a fibre-optic transceiver having been converted into electrical voltage-fluctuation signals is connected by electrical cables to appliances such as home computers, telephones and pay-TV receivers in private premises and offices for example. For this reason, data signal from the fibre-optic transmission line to individual premises is by converting the optical signal into an electrical signal in a shared fibre-optic transceiver. The electrical signal is conveyed by LVDS transmission lines to electrical appliances such as personal computers (say in a local area network), telephones, cable televisions etc. 
     Although there have heretofore been disadvantages in the implementation of electrical cables from the transceiver to the various endpoints—particularly associated with cable length restrictions, there are advantages in adopting electrical cabling to transfer data and these include lower cable cost, ease of splicing electrical cables and the presence of existing in situ cable networks. Although LVDS transmission has replaced single-ended signalling transmission as it provides a higher data transfer rate and better resistance to electromagnetic interference, LVDS systems remain susceptible to electromagnetic interference and signal degradation over extended cable lengths. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages and/or more generally to provide an improved, cost-effective high-speed electrical data transmission system. 
     DISCLOSURE OF THE INVENTION 
     There is disclosed herein a high-speed electrical data transmission system, comprising:
         a first signal mixer for receiving high-speed data and comprising means for converting said data into an electrical current-fluctuating data signal; and   a first signal processor located remotely of the first signal mixer and connected electrically thereto for receiving the current-fluctuating data signal and comprising means for converting the current-fluctuating data signal into a voltage-fluctuating data signal.       

     Preferably, the system further a cable comprising only two operative conductors connected between the first signal mixer and the first signal processor for carrying said current-fluctuating data signal. 
     Preferably, the system further comprises:
         a transceiver located between the first signal mixer and an external network, the transceiver communicating a voltage-fluctuating data signal to the first signal mixer;   a second signal processor providing a voltage-fluctuating data signal to the transceiver.       

     Preferably, the voltage-fluctuating data signal provided by the first signal processor at said remote location is connected electrically to a network of one or more appliances, and wherein the system further comprises:
         a second signal mixer at said remote location providing a current-fluctuating data signal and being connected electrically with said second signal processor for conversion of the current-fluctuating data signal thereby into said voltage-fluctuating data signal presented to the transceiver.       

     Preferably, the transceiver is a fibre-optic transceiver and wherein the external network is a fibre-optic network. 
     The present invention can convey high bandwidth data at a rate comparable to that of fibre-optic systems, yet has the cost advantage and ease of splicing advantage of LVDS systems without susceptibility to signal degradation to which the latter is prone. Furthermore, the present system can carry electrical power in addition to data transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings, wherein: 
         FIG. 1  is a schematic block diagram depicting a high-speed electrical data transmission system communicating data between a fibre-optic cable and a local network; 
         FIG. 2  is a schematic circuit diagram of the one of the signal mixers and one of the signal processors connected to one another by a two-conductor transmission line cable; 
         FIG. 3A  is a graph showing a voltage signal waveform at the base of transistor Q 1  in the signal mixer of  FIG. 2 ; 
         FIG. 3B  is a graph showing the current signal waveform in the transmission line; and 
         FIG. 3C  is a graph showing the voltage output waveform of the amplifier U 1  of the signal processor shown in  FIG. 2 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The block diagram of  FIG. 1  depicts high-speed electrical data transmission system  10  communicating data between a fibre-optic public cable network  11  and a home or office local area network including for example a cable television receiver  19 , home automation appliances  20 , a telephone  21  and a personal computer  22 . Of course some of these features could be omitted and others added. In an office local area network, the various components might include a computer server, numerous workstations, photocopiers, fax machines and the like. 
     The fibre-optic cable  11  would typically extend under the street or footpath and service numerous offices and/or premises. At each residence or office building for example, there is provided a fibre-optic transceiver  12  which converts optical data signals into electrical data signals. 
     Connected electrically to the transceiver  12  is a signal mixer  13  and signal processor  14 . A two-conductor cable or “transmission line”  15  extends from the signal mixer  13  to a signal processor  17  inside the premises. The signal processor  17  would typically be housed in a plastics casing similar in style to an “external modem” or “broadband modem/hub”. To enable duplex data transmission, the fibre-optic transceiver is connected electrically with another signal processor  14  which is in turn connected by a two-conductor transmission line cable  16  to another signal mixer  18  alongside signal processors  17 . Signal processor  17  would typically be housed in the same “external modem” style housing and perhaps be integrated onto the same printed circuit board as signal processors  17 . Indeed, signal mixer  13  and signal process  14  might also be integrated onto the same printed circuit board. 
     The signal mixer  13  converts a voltage-fluctuating signal from the fibre-optic transceiver into a current-fluctuating signal for transmission along the transmission line  15 . The signal processor  17  detects current fluctuations in transmission line  15  and converts this back to voltage fluctuations. The voltage waveform produced by signal processor  17  matches the original voltage waveform communicated from the fibre-optic transceiver  12  to the signal mixer  13 . From signal processor  17 , the voltage fluctuation waveform is conveyed via the local network to the various components  19 - 22 . 
     Furthermore, and in order to facilitate duplex communication, the electrical appliances or components and  19 - 22  transmit voltage-fluctuation signals to the signal mixer  18  which functions the same way as signal mixer  13 —converting voltage fluctuations into current fluctuations for transmission along transmission line  16  to signal processor  14  for communication with the outside fibre-optic network via fibre-optic transceiver  12 . 
     As a further extension of the invention, the home computer  22  can be connected to another signal processor/signal mixer pair  23 ,  24  for communication via a pair of electrical transmission lines  25  and  26  to a distant remote location whereat a further signal mixer/signal processor pair  27 ,  28  is connected to another computer  29 . For example, appliances  19 - 22  might be located at the fifth floor of a high-rise office building, and the transmission lines  25  and  26  could extend to the twentieth floor of the same building whereat signal mixer  27 , signal processor  28  and computer  29  are located. 
       FIG. 2  is a detailed schematic wiring diagram of one of the signal mixers  13 , 18 , 27  and one of the signal processors  14 , 17 , 28 . The signal mixer receives DC power from an AC-to-DC power supply adapter or might alternatively receive DC supply voltage from an electrical appliance. The block identified as U 4  is a parallel-to-serial semiconductor IC used to convert parallel input signals into serial signals emitted through resistor R 6  to the base of transistor Q 1 . Transistor Q 1  converts the voltage signals into electrical current signals and sends these through the two-conductor transmission line  15 ,  16 ,  25 ,  26  as the case may be. 
     At the signal processor the current-fluctuation signal passes through resistor R 7  to ground and the amplifier U 1  receives the current fluctuations ahead of resistor R 7  via resistor R 4 . The amplifier U 1  converts the transmission line current-fluctuation signal into a voltage-fluctuation signal and passes this via resistor R 1  to the block identified as U 2  which comprises a serial-to-parallel semiconductor IC for reversion of serial signals to parallel signals. 
     As will be appreciated by reference to  FIGS. 3A ,  3 B and  3 C, the current signalling system of the present invention will produce a current signal waveform at the target end of the transmission line that is almost identical to the current waveform at the source. Line capacitance and inductance will affect a current signal waveform minimally and this compares favourably with degradation in the voltage waveform in LVDS systems resulting from line resistance and susceptibility to electromagnetic interference for example. An LVDS network having for example a 5V input at 100 mA and a line resistance of 10 ohms. The receiving end of the line will have a voltage amplitude of 4V representing a voltage distortion of 20%. With the present system of current signalling, a current of 100 mA at the sending end of the transmission line will present 100 mA (or a figure negligibly varied therefrom) at the target end of the transmission line—representing zero or negligible distortion. Furthermore, the present system has very high resistance to electromagnetic interference perceived by the transmission line itself. Indeed if a transmission line in an LVDS system is placed in close proximity to an AC motor supply line, the voltage signal waveform of the line suffers drastic distortion resulting from high back EMF from the motor winding. 
     Experimentation has shown that the transmission line of the present system suffers no detriment when placed in close proximity to a running AC motor. Furthermore, as the current signalling system of the present invention is less affected by line impedance, the signal waveform becomes less distorted compared with LVDS transmission. As a result, the choice of cable type for transmission line cabling adopted in the present system is left wide open. Even inexpensive Cat 1 cable (bell cable) can be used. For best performance however, the impedance of the cable should match or closely match the output impedance of the signal mixer and the input impedance of the signal processor. 
     It should be appreciated that modifications and alterations obvious to those skilled in the art are not to be considered as beyond the scope of the present invention. For example, the system is certainly not limited to connection to a fibre-optic transceiver. As coaxial cabling is also widely used to distribute signals over large distances.