Optical fibre communication link.

0001The present invention relates to an analogue optical interface, more particularly for use as part of a communications link between a computer system and a peripheral device such as a data display device.

0002Prior art communication links between computer systems and peripheral devices typically involve the transmission of a video signal in the form of a time-variant current passing through electrically conductive copper wires.

0003Within the art, a video signal consists of a continuous flow of analogue information which is sequenced in a manner compatible with a display device and to this end a periodic "blanking", interval can be included in such a sequence for control purposes.

0004By the nature of display apparatus, the frequency of a video signal is typically several tens of Megahertz. The signal carrying wires are therefore conventionally clad in a conductive screen connected to earth, such that radio frequency emissions are minimised. This is necessary to prevent undesirable radio frequency interference (RFI) which can clearly exist between a host computer system and peripheral apparatus disturbing the normal operation of the host computer system or any other sensitive electrical equipment in the vicinity of the communications link.

0005With the trend towards increased image definition, requiring faster video signals, the need has arisen to examine new techniques for connecting the display device to a display adapter. Conventional screening methods, an example of which is described above, have bandwidth limitations such that RFI from electrical cabling becomes significant when such video rates are used. Furthermore a signal attenuation factor can be associated with these conditions.

0006EP-A-0,174,099 describes a fibre optic terminal interface which overcomes the RFI problems mentioned above by multiplexing monochrome digital video signal with Manchester-coded ancillary information, such as keyboard control data and sound synthesis data, onto a single optical fibre link between a host data processor and a user display terminal. The Manchester-encoded data has a clock rate which is tied to a video dot clock rate and is multiplexed onto the optical link during a horizontal scan retrace (blanking) period.

0007A feature of low cost opto-electronic devices is poor electron to photon conversion efficiency which can vary significantly between nominally identical devices. A typical optical link, composed of a light emitting diode (LED) connected to PIN diode via an optical fibre, may provide only 20uA output current for 100mA input. It follows therefore that a high gain, low noise amplifier would be required in the receiver. Although the application of an optical link to the transmission of an analogue video signal is suggested in EP-A-0,174,099, there is no facility for regulating the transfer function of such a link under the conditions discussed, and therefore a communication scheme as specified cannot be extended to the matching of a plurality of nominally identical optical communication channels (for example, the red, green, and blue video channels of a colour display system).

0008The aim of the present invention is to prevent a communications link from emitting objectionable levels of RFI by replacing electrical wires with optical fibres which are compatible with analogue video communication; the video signals being represented by time variant luminance signals.

0009In accordance with the present invention, there is now provided, an optical fibre communications link, for connecting a peripheral device to a computer system is presented, which includes means for generating a video signal, means for transmitting such a signal along an optical fibre between the computer system and the peripheral device, characterised in that the transmitted video signal includes a reference pulse of predetermined amplitude, compensation means for correcting changes in the characteristics of a received video signal which result from said optical communication link,and means for using said reference pulse in a received video signal in order to control said compensation means.

0010A dynamic range, conventionally assigned to an optical communication link, defines maximum and minimum signal amplitudes allowed therein.

0011By way of a second aspect of the invention, the aforementioned dynamic range is further partitioned into a plurality of subsections such that said reference pulse occupies a first range of amplitude, said video signal occupies a second range of amplitude, with the provision for video synchronisation pulses to occupy a third range of amplitude.

0012In one preferred arrangement the reference pulse is included in the video signal during periodic blanking intervals.

0013This construction achieves the advantage that a communicated signal is both transmitted and received most conveniently by electrical means. It follows that where the compensation means consists of a plurality of electrical circuits, a partition of the dynamic range corresponds to a threshold voltage determined by the compensation means.

0014Furthermore, in the event of the operation of a peripheral device and the dynamic range of the optical communication link permitting, the aforementioned reference pulse may take the form of a video synchronisation pulse.

0015A specific embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrams in which: <ul id="ul0001" list-style="none"><li>Figure 1. illustrates a partitioning of the dynamic range of the communications link.</li><li>Figure 2. is a functional block diagram of a system for transmitting information along the communication link.</li><li>Figure 3. is a functional block diagram of a system for receiving information from the communication link.</li></ul>

0016Figure 1. shows a sequence of data suitable for communication via an optical link, consisting of: <ul id="ul0002" list-style="none"><li>a) An amplitude reference pulse 1 of amplitude V4-V2 occuring during a blanking period 2.</li><li>b) Option for control signals 3 of amplitude V2-V1 such as synchronisation pulses.</li><li>c) An analogue information signal 4 such as a video signal, occuring for an active video period 5, with an amplitude bounded by extremes V2 and V3. where V1,V2,V3,V4 are electrical voltages and the range between V1 and V4 is the electrical signal space available to the optoelectronic interface circuitry of both the transmitter and the receiver.</li></ul>

0017Figure 2 shows one system structure which can be used to generate such a signal as that shown in Figure 1. Block 6 is a summing amplifier. Block 7 is a difference amplifier. Block 8 is reference pulse generator. Block 9 is a pulse delay circuit. An amplitude reference pulse 10 is generated by block 8 whenever a delayed synchronisation pulse 11 triggers a circuit such as a monostable multivibrator, for example. The delayed synchronisation pulse 11 is generated from a synchronisation pulse 12 in order to prevent the reference pulse from adding objectionable detail to a displayed image. The amplitude reference pulse is added to input video signal 13 by summing amplifier 6. The synchronisation pulse 12 can be included in the output video signal 14 via difference amplifier 7.

0018In an alternative to the example shown in Figure 2, a digital to analogue convertor in a display adapter system can also generate a signal such as that shown in Figure 1.

0019Figure 3 shows one receiver structure for such a signal as that shown in figure 1. Block 15 is a variable gain amplifier (VGA). Block 16 is a unity gain DC level shifting circuit. Block 17 is a positive peak voltage detector. Block 18 is a negative peak voltage detector. Blocks 19 and 20 are both difference amplifiers.

0020The most positive level of the receiver output signal 21 is maintained at V4 in a feedback process whereby the most positive peak of the receiver output signal is extracted by block 17 and compared with a fixed V4 reference level at the input to block 19. The voltage output from block 19, which indicates an error in the receiver output amplitude, is used in block 16 to adjust the DC level of the receiver output signal.

0021The most negative level of the receiver output signal is maintained at V1 by a feedback process whereby the most negative peak of the receiver output signal is extracted by block 18 and compared with a fixed V1 reference level at the input to block 20. The voltage output from block 20, which indicates an error in the receiver output amplitude, is used in block 15 to adjust the gain of a variable gain amplifier acting on the input 22 to the receiver.

0022Recovery of any information synchronisation 23 signals which may be present can be simply effected by comparing 24 the receiver output signal to a threshold voltage equal to: 0.5(V1+V2)

0023In summary, a communication link between a display device and a computer system has been described by way of example of the present invention. It will be appreciated however that the present invention could be used to connect a computer system to other peripheral devices.

0024Furthermore, for the purpose of the present invention, a video signal can be any continuous flow of analogue data which is sequenced in a manner compatible with a Particular receiving. Accordingly, it will be appreciated that the receivng device need not be a display device.