WO1992019070A1

WO1992019070A1 - A switching apparatus for video signals

Info

Publication number: WO1992019070A1
Application number: PCT/AU1992/000173
Authority: WO
Inventors: Graham Peters; Joe Raine
Original assignee: Graham Peters
Priority date: 1991-04-19
Filing date: 1992-04-16
Publication date: 1992-10-29

Abstract

A system for switching a video display unit from a first source of video signals to a master source of video signals. The system comprising a plurality of switching apparatuses (40a) to (40f) and a master controller (41). Each switching apparatus comprises switching circuitry (14) to (19). Input ports of the switching apparatuses are connected with a student computer (5a) to (5f) and to one of the master controller outputs (c) to (h). The output of the switching apparatus is connected to a monitor (Aa) to (Af). The master controller has its inputs connected to a VDU(b) and master source of video signals (I). The master controller transmits the master source of video signals to each switching apparatus and has a control means for operating the switching apparatus to switch the output of each switching apparatus from the first source of video signals (5a) to (5f) to the master source of video signals transmitted from the master controller.

Description

A Switching Apparatus for Video Signals

The present invention relates to the computer industry and is particularly useful in computer based training applications. Typically if a new computer system is to be installed in a work environment, it is necessary to teach the persons who are to use the system all about its operating characteristics. This is usually done by way of special seminars outside of the work environment and in a location where the computer system is already set up so that computer terminal operators can have hands on experience of how to use each terminal as they are taught by an instructor.

Apart from the problem of having to move persons away from their normal working environment in order to avoid disruptions to the work environment, training methods which are used to teach computer terminal operators how to use a new system involved connecting together a number of computers terminals, with one of these terminals being the instructors terminal and the remainder being the students terminals. In such a situation, whenever the instructor wishes to show a student particular information on the instructors terminal, it is necessary for the students to move from their own terminals to the instructors terminal. In addition, if a student wishes to compare the information on their own terminal with the information on the instructors terminal, it is necessary for that person to continually move away from their terminal to the instructors terminal to make such a comparison.

The present invention is aimed at providing a switching apparatus which is aimed at alleviating problems associated with the above mentioned training procedures. The present invention provides a switching apparatus for switching video signals and may also include a master controller apparatus for controlling switching of the switching apparatus from a location remote from the switching apparatus. A system

SUBSTITUTE SHEET incorporating one or more switching apparatuses and one or more master controllers, also falls within the scope of the present invention.

According to a first aspect of the present invention there is provided a switching apparatus comprising first and second data inputs, a data output, a switching means interconnecting the first and second data input with the data output and a control input for receipt of a control signal which is arranged to operate the switching means, wherein in a first mode of operation, the switching means is arranged to connect the first data input with the output and in a second mode of operation the switching means is arranged to disconnect the first input from the output and connect the second input to the output.

Preferably, the control signal is provided by a switch of the switching apparatus. In an alternative embodiment^', the control signal is provided from a location remote from the switching apparatus. It is preferred that the switching apparatus comprise only two modes of operation, the first being when the first input is connected with the output and the second being when the second input is connected with the output. The switching apparatus may comprise a status indication means for indicating if the switching apparatus is in the first or second mode.

The indication means may comprise a visual display unit. Preferably, the switching means comprises at least a first and second relay, each with a plurality of contacts.

The data input and output may comprise red, green, blue, vertical synchronisation and horizontal synchronisation signals. _β

Preferably, the switching apparatus is arranged to be connected with a computer at its first input and with a monitor or VDU at its data output. The second data input is preferably arranged to be connected with an output of a second computer either directly or through a master controller having a control means for transmission of a control signal. The first and second relays may be driven by

Break-before-make relay drivers so that both relays cannot be energised at the same time.

It is preferred that the Break-before-make relay drivers each comprise a transistor having its input connected with a biasing means to an inverting buffer and its output (either the emitter or collector) coupled with its relay.

The output of each transistor is preferably connected to a status indication means such as an LED for status indication of the first or second mode.

Preferably, the relays are high frequency relays which may be low current relays whose contacts when de-energised are connected from their inputs to ground through resistors. The contacts, when energised, are preferably arranged to connect their input to the output.

The control input, preferably is connected with the Break-before-make relay drivers and the voltage at this input provides an indication of which one of the Break-before-make relay drivers is energised.

Preferably, the voltage at the control input is controlled by an external controller to energise or de-energise one of the Break-before-make relays.

A power supply may be provided in the switching apparatus or alternatively, the power supply may be provided by means of an external input.

Preferably, digital logic is provided to operate the Break-before-make relays.

Preferably, the digital logic comprises a bi-stable flip-flop which may be toggled by pressing a switch.

A status interface means is preferably provided between the digital logic and the control input. Preferably, the status interface comprises a buffer which is connected at its output to a voltage divider/filter whereby, the voltage at the junction of the divider provides an indication of the state of the switching apparatus (that is whether it is in the first or second mode) .

Preferably, the status interface includes a diode which enables a control signal to the control input to lower the voltage at the junction of the voltage divider and thus, reset the flip-flop of the digital logic to the second mode.

Preferably, the output of the flip-flop feeds two or more inverting buffers which control the Break-before-make relay drivers. Preferably, when the switching apparatus is in the first mode, this is designated "student" mode and when the switching apparatus is in the second mode, this is designated "instructor" mode.

According to another aspect of the present invention there is provided a master controller for a switching apparatus, the master controller comprising a video amplifier means for receiving a video signal and amplifying it for transmission to an input of the switching apparatus, and a control means which is arranged to be operable to operate the switching apparatus to disconnect any existing signals from its output and to switch the video signal received from the master controller to its output.

Preferably, the video signal comprises blue, red and green colour signals and vertical and horizontal synchronisation signals.

The master controller preferably comprises a switching apparatus, status detector for detecting a switching mode of the switching apparatus. Preferably, the master controller comprises an indicator means for indicating the mode of the switching apparatus detected by the status detector.

The control means preferably comprises a master switch which in use, is operable to transmit a control signal to the switching apparatus.

The status detector preferably comprises a comparator for comparing a detected voltage of the switching apparatus indicative of its switching mode with a reference, the comparator being arranged to produce a signal when the switching apparatus has changed from a first mode to a second mode.

The indication means, may be arranged to receive the signal from the comparator to provide a display of the mode of the switching apparatus.

The indication means preferably indicates if the switching apparatus is in its first or second mode.

The indication means, may comprise a pair of LED's.

The LED's may be connected across the emitter and collector of respective PNP and NPN transistors.

The master controller may comprise a signal loss detector for indicating when no video signal is received by the video amplifier means.

Preferably, the video amplifier means comprises a video duplicating amplifier for amplifying the colour signals and a horizontal and vertical synchronisation duplicating amplifier means for amplifying the horizontal and vertical synchronisation signals.

Preferably, the master controller comprises a power supply with an over current protection facility.

The control means may be arranged to be connected with the switching apparatus and the status detector may be arranged to detect the voltage on the line interconnecting the control means and switching apparatus.

Preferably, the master controller is arranged to control a plurality of switching apparatuses. The video amplifier means and the vertical and horizontal synchronisation amplifier means, are preferably arranged to provide a plurality of substantially identical colour signals and vertical and horizontal synchronisation signals respectively.

The control means may be arranged to produce a plurality of control signals.

The master controller may comprise a plurality of outputs each output having colour signals horizontal and vertical synchronisation signals and control signals for a separate switching apparatus.

Preferably, the control means can control each switching apparatus independently. The indication means preferably provides a display of the mode of each switching apparatus.

The power supply for each switching apparatus preferably is provided by the master controller.

According to another aspect of the present invention there is provided a system for switching a video display unit from a first source of video signals to a master source of video signals, the system comprising at least one switching apparatuses and a master controller, each switching apparatus being arranged to have its output connected to a VDU and its inputs connected with the master controller and its respective first source of video signals, the master controller being arranged to be connected with a master source of video signals, the master controller being arranged to transmit the master source video signals to each switching apparatus and further comprising a control means for operating the switching apparatuses to switch their output from their first source of video signals to the master source video signals transmitted from the master controller.

Preferably, the control means is arranged to operate each switching apparatus independently.

The master controller may be arranged to be connected in parallel with the master source of video signals and a master VDU to which the master source of video signals is arranged to be connected.

The master controller may comprise a distribution unit with inputs for connection with the master source of video signals and outputs each for connection to a separate switching apparatus, each output providing master source video signals and a control signal. Preferably, the master controller comprises a plurality of distribution units each connected in parallel with the master source of video signals.

Preferably, the master controller comprises a master distribution unit and a plurality of sub-distribution units, each sub-distribution unit being arranged to be connected to respective outputs of the master distribution unit and each sub-distribution unit being arranged to have its input connected to an output of a master distribution unit and its output connected to respective switching apparatuses. The input of the master distribution unit is arranged to be connected with the master source of video signals.

Preferably, the master distribution unit and each sub-distribution unit are substantially identical. The master distribution unit preferably has an indication means for indicating whether each sub-distribution unit is operating.

The master distribution unit preferably has an indication means for indicating whether switching apparatuses of each sub-distribution unit are in a first or second mode of operation.

The first and second mode of operation preferably indicates whether the switching apparatus has its output connected with the master source of video signals and is thus, in an instructor mode or alternatively, has its output connected to some other source of video signals and is thus, in a student mode.

The switching apparatus preferably comprises multiple outputs. Preferably, each of the outputs of the switching apparatus is connected to a different VDU, thus, providing a system whereby a number of VDUs are connected to a single switching apparatus. Preferably, operation of the switching apparatus switches all the VDUs of the switching apparatus to the master source of video signals.

Operation of the switching apparatus by the control means of the master controller, preferably switches the output of the switching apparatus from a first source of video signals to the master source of video signals provided from the master controller.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 shows a block diagram of a switching apparatus in accordance with the present invention;

Figure 2 shows a master controller for the switching apparatus shown in figure 1;

Figure 3 shows an electrical circuit diagram for the switching apparatus shown in figure 1;

Figure 4 shows the input connections to the master controller shown in figure 2; Figure 5 shows the status detectors and horizontal and vertical synchronisation duplicating amplifiers of the master controller shown in figure 2;

Figure 6 shows the status indication circuitry or indicator board of the master controller; Figure 7a shows the video duplicating amplifiers of the master controller;

Figure 7b shows the over current protection for a regulated power supply of the master controller;

Figure 8 shows the electrical circuitry for the video duplicating amplifiers;

Figure 9 shows the regulated power supply for the master controller;

Figure 10 shows a system according to one embodiment of the present invention; Figure 11 shows a system according to a second embodiment of the present invention; and

Figure 12 shows a system according to a third embodiment of the present invention. A description will first be provided of the switching apparatus shown in figures 1 and 3. As shown in figure 1, the switching apparatus has two input ports 11 and 12 and one output port 13. Input port 11 is for connection with a nine way cable coming from a master controller and inside the switching apparatus, the cabling associated with status and control is connected to the status interface unit 14 while the video signals; red, green, blue, horizontal synchronisation and vertical synchronisation are connected to one set of contacts of the high frequency relays indicated by item 15. The high frequency relays are controlled by Break-before-make relay drivers 16 which in turn are controlled by digital logic 17 connected directly with the status interface 14. The digital logic 17 is connected with a push button for providing local operation of the Break-before-make relay drivers 16. The Break-before-make relay drivers may also be connected with an indicator 19 to indicate the mode of operation of the switching apparatus. The digital logic 17 may also be provided with a power-on reset facility to ensure that the unit powers up with the high frequency relays connecting the video signals at input terminal 12 with output 13, that is, the switching apparatus is in mode 1. Figure 3 shows the actual electronic circuitry required to construct the switching apparatus shown in figure 1. For convenience, the circuit diagram in figure 3 has been divided up into various blocks which have been itemised to match the item numbers used in figure 1. The individual components shown in figure 3 and their connections are self explanatory and would be understandable to a person skilled in the art. A description is therefore, provided of the operation of the switching apparatus with reference to figures 1 and 3.

The switching apparatus in its preferred form is identified as a VGAS T3, high resolution video switching unit, which is to be used in conjunction with the master controller (which will be described in detail later) and which in its preferred form is a VGA6 video distribution system. The switching apparatus allows VGA signals or video signals to be alternately displayed on a single monitor A, as shown in figure 1. In a first mode of operation, the video signal from the students computers which is connected to input 12, is fed through the contacts of the high frequency relays 15 (in figure 3, relay 1) to the output 13 which is located on the right hand side of the box designated by item 15 in figure 3. In this mode, relay 1 is energised and the contacts of relay 1 are in the alternate position to that shown in figure 3. Energising of relay 1 is provided by a high output at pin 10 of ICl:E, this enables C4 to charge slowly towards +12 volts. When transistor Ql receives enough bias to conduct, it energies relay RLl and simultaneously pin 12 of IC1.F goes low discharging capacitor C5 rapidly and thus, de-energising relay RL2. This action causes relays RLl and RL2 to have a "Break-before-make" action so that both relays can never be energised together.

While transistor Ql is in conduction mode, LED2 is also in conduction mode and produces a green indication, thus, indicating that the monitor is connected to the students computer.

When it is desired to switch the switching apparatus from student mode to instructor mode, a master switch in the master controller is activated to force the voltage at the junction of resistors Rl, R2 and diode D2 to go low. This resets, the bi-stable flip-flop (ICl:a and IC1:B) forcing the output of IC1:E to go low and the output of IC1:F to go high. Transistor Ql thus, stops conducting and transistor Q2 begins conducting thus, resulting in relay RLl being de-energised and relay RL2 being energised. Because transistor Q2 is in conduction mode, current flows through LED1 and produces a red indication. At the same time, current ceases to flow through LED2 thus, the switching apparatus changes from "student" to "instructor" mode. Energising of relay RL2 connects the video signal from the master controller at input port 11 with the output port 13 as shown in figure 3. At the same time, relay RLl is de-energised and the contacts as shown are connected through 75 Ohm resistors to ground.

The bi-stable flip-flop (IC1:A and IC1:B) can also be toggled by pressing switch SW1 to thus, provide local control of the switching apparatus. ICl:d forces a high level via Dl onto pin 1 of IC1:A at turn-on to ensure that the unit powers up with "student" selected. ICl:c buffers and inverts the logic level at pin 2 of ICl:A and its output is connected to a voltage divider/filter (Rl, R2, Cl) . The voltage at the junction of R1/R2 is also an indication of the status of the switching apparatus (i.e. whether the switching apparatus is in instructor or student mode) and the voltage is fed via pin 9 ^'of PLG1 to the VGA6 (master controller) . D2 which is also connected to this junction, enables the master controller to force the voltage at this junction low, as previously mentioned.

A description will now be provided for the. master controller shown in figure 2 and as shown, the master controller which in its preferred form is referred to as a VGA6, is arranged to be connected in parallel with an instructor's computer I, which is connected to an instructor's monitor B. The master controller has connection ports so that the instructor's computer can be connected to port 20 and the instructor's monitor can be connected to port 21. Inside the master controller, ports 20 and 21 are connected to an input port 22 by means of wiring shown in figure 4. The different terminals of port 22 are arranged to carry different signals which can be split up into video colour signals red, green and blue, horizontal and vertical synchronisation signals and a master reset bus. As shown in figure 2, input port 22 is connected to video duplicating amplifiers 23, 24 and 25 for transmittal of the red, green and blue video signals respectively and other terminals of port 22 are connected to a horizontal synchronisation duplicating amplifier 26 and vertical synchronisation duplicating amplifier 27, these being respectively for receipt of the horizontal synchronisation signal and vertical synchronisation signal. The master reset bus is connected directly with the status detectors and display drivers 28. The status detectors and display drivers are connected to an indication means in the form of a front panel display 29 which also is provided with the master push button.

A regulated power supply 30 provides power in the form of -12 volts, +5 volts and +12 volts to the componentry in the master controller and also provides 12 volts through an over current protection unit 31 for transmission to each switching apparatus which is connected at the output of the master controller.

Operation of the master controller will now be described with reference to the circuit diagrams of the individual components as shown in figures 4, 5, 6, 7, 8 and 9. The function of the master controller can be divided into two main sections dealing with digital and analogue functions.

Referring to figure 5, the digital functions include synchronisation, i.e. synchronisation signals from the externally connected computer (the instructor's computer) are fed to the master controller through port

22 to amplifiers 26 and 27 via PLG1. The synchronisation signals are TTL-level signals and are fed to clamps formed from Rl, Dl and D2 (horizontal sync) and R2, D3 and D4 (vertical sync) to ensure that the inputs of IC1 and IC2 are not over stressed at any time (by ESD, etc) .

IC1 and IC2 are octal buffers configured to provide 6 outputs each of horizontal and vertical sync which are fed to the output connectors (SKT1 to 6, DB9 female) . A short or other fault on one output is unlikely to effect all the others due to the way in which the outputs have been matrixed as shown in figure 5. The outputs of IC1 and IC2 are fed to the output terminal 33 of the master controller for transmission to one or more switching apparatuses.

The master controller is also provided with a signal loss detector, a buffered output from pin 3 of IC2 is fed to a voltage doubler formed by C22, D9, D10, C23 and R9. This circuit only has an output when horizontal sync is present, that is when the instructors computer is connected and turned on. The DC voltage obtained is fed to comparator IC4:C which biases Ql on via R16. IC4.D is configured as a low frequency oscillator which is normally disabled by IC4:c in the absence of sync, IC4:d oscillates at about 1 hertz and causes Ql to turn on and off. Ql's collector supplies, +12 volts to the front panel board and thus, flashes the front panel LED's to indicate loss of the instructor's computer signal.

Status indication of the mode in which the switching apparatuses are in and control of the switching apparatuses is provided by status detectors and display drivers 28 as shown in detail in figure 5. Pin 6 of each output connector (SKT1-6) has a network, one of which will be described. C6 is a bypass capacitor to ensure that the line connected to pin 6 is an effective AC ground. R3 (1KW) is a current limiting resistor. D6 is connected to a bus which ultimately connects to the "master" reset push button which resets all the switch apparatuses to "instructor" mode. IC3:D is a comparator with a reference of 8.1 volts derived from R7 and R8. When the connected switching apparatus is in student mode, that is connected with a students computer, the status line will be at about +12 volts and so pin 14 of IC3:C will be high. If the student selects "instructor" mode, the status line will drop to about +6 volts and the output of IC3:D will change to a low state. In conjunction with the indicating circuitry on the front panel of the master controller, the status of each student may be monitored or forced to "instructor mode".

The analogue functions of the master controller will now be described.

The front panel of the master controller is provided with 6 identical circuits to indicate the status of each switching apparatus and a push button switch to pull the master bus low, thus resetting all the switching apparatuses to instructor mode. With reference to figure 6, one of the indicating circuits will now be described. The bases of Q7 (NPN) Ql (PNP) are commoned and connected via IDC1 to PLG2 on the Master Controller. When the output of IC3:D is high, R2 will bias Q7 to saturation, shorting out LED1, thus LED7 will be lit to indicate "student". When IC3:D's output goes low, Ql will saturate, LED7 will be short-circuited, and LED1 will light to indicate "instructor" mode. Rl limits the LED current to approximately 10mA.

Referring to figures 7(a) and (b) , three Video Duplicating Amplifier modules (VDA's) perform the task of amplifying the red, green and blue input signals to provide six identical outputs to feed the output connectors (SKT1-6) . They are fed from PLG1 via dc isolating capacitors C12, C13 and C14.

Pin 9 of each output connector (SKI-6) is fed with +12V to power the VGASWT3 boxes. PS1 is a self-resetting fuse to protect the VGA6 power supply in the event of a short-circuit. Cl - 5 (lOuf) bypass the pins 9 to ensure that they are effective ac grounds.

The input ports 20, 21 and 22 are located in a pcb which is merely a means of looping the input signals back out whilst picking off those signals for use by the VGA6. The single exception is pin 15 of both connectors, which carries the MASTER reset bus. This has been provided to allow any VGA6, in a system with several, to reset all VGASWT3's.

There are three VDA's in a VGA6 unit. One will be described with reference to figure 8. The input connector couples +12 volts, -12 volts, OV and the input video signal to the VDA. The video input signal is fed to a compensated adjustable voltage divider formed by Rl, Cl, VR1 and R2. Cl compensates for parasitic capacitive losses above about 20MHz. VR1 is a 20 turn linear element (bar-shaped) potentiometer which neatly avoids inductive effects and places the adjuster at the top of the pcb. Since the input impedance of this circuit is around 1K5 it is not advisable to parallel connect more than three units. This level of impedance strikes a compromise between parasitic losses and loading of the input signal. IC1 is a National Semiconductor LM6364. It has a Gain Bandwidth Product (GBW) of 175MHz, 300V / mS Slew Rate and minimum stable gain of 5. The feedback around IC1 y(R4,R3) yields this minimum gain and therefore the bandwidth of this circuit is 175/5=35MHz. C8 is a feedback phase correction capacitor. In a circuit such as this a gain of two is needed, so the input attenuator divides by approximately 2.5, but is adjustable to allow for cable losses, etc. R5 and R6 couple the output of IC1 to two unity-gain buffers IC2 and IC3. These LM6321 buffers have a bandwidth of 50MHz and can drive an impedance of 50Ohms. Each buffer has three 75 Ohm building-out resistors and these will feed a 75 Ohm load each, so the impedance seen by each buffer is 150/3=50 Ohms. The pcb is an important part of the design of these VDA modules. The top layer is a ground plane, which controls parasitic capacitances between tracks on the bottom layer and ensures a low impedance path for high frequency currents flowing to and from the OV (or ground) rail. The grounds for the outputs do not come from these modules, but directly from the power supply via similar OV planes on the mother board.

These modules have provision for cable equalisers which would attach to the rear of the board and place a complex impedance across R3 to compensate for high frequency losses in the cabling. This assumes that all cables in a system would be the same length.

No IC sockets have been used in the construction of these VDA's as the additional capacitance would have a disastrous effect upon their performance. Instead, to aid servicing, the complete module is able to be unplugged from the master controller section containing the video amplifiers and a replacement plugged in. this also assists in fault finding as the^' modules may be swapped freely. Note that this should never be done with power applied as the LM6321's are very sensitive to improper supply situations. Figures 10, 11 and 12 show a number of different systems embodying the master controller and switching apparatus as previously described. In figure 10, a single master controller 41 is utilised with its input terminals 41 a,b connected to a computer and VDU respectively. Six outputs c,d,e,f,g,h are connected to switching apparatuses 40 a,b,c,d,e,f respectively which are each connected to their own computer and VDU. In figure 11, to enable more students to be controlled by a teacher, two master controllers may be used. With the inputs of each master controller 42, 43 being connected in parallel with the teachers VDU and computer, each master controller has its output connected in the same manner as shown in figure 10.

In figure 12, an alternative embodiment is shown in which a single master controller acts as a master distribution controller 44, with its inputs connected to the teachers VDU and computer in the normal way but its output connected to the inputs of subsidiary master controllers 45 a to f. Each of the subsidiary distribution master controllers has their output connected to switching apparatuses in the way shown in figure 10.

It is important to realise that the cables used have a major effect upon the observed quality of VGA graphics and text. Multiple core data cable with an overall screen is unlikely to give a good result, whilst screened PTFE 75 Ohm co-axial ribbon will give excellent results. The difference in cost is about 100:1! High quality ribbon gives good pictures, and should be laid out flat (not coiled up) for best results. Interference from graphics cables may need to be shielded by metal ducting or flexible shielding if necessary. Shielded ribbon is more expensive, but should not need further shielding. The design of the VGA6's output connector configuration minimises interference pickup also.

The length of cables is determined not only by physical requirements, but also by the type of cable.

High loss cable, for example, cannot be employed over distances exceeding 10 to 12 Metres, but low loss, high quality cable will give good results over 30 Metres.

It is important that all the equipment in a system is plugged into the same power circuit.

Operation is simplicity. Turn on and use. If the LED's flash, the computer feeding the VGA6 is off or not connected properly. Press the master button to make all students watch the instructors screen. Observe the LED's to see if a student is watching the correct computer output.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A switching apparatus comprising first and second data inputs, a data output, a switching means interconnecting the first and second data input with the data output and a control input for receipt of a control signal which is arranged to operate the switching means, wherein in a first mode of operation, the switching means is arranged to connect the first data input with the output and in a second mode of operation the switching means is arranged to disconnect the first input from the output and connect the second input to the output.

2. A switching apparatus as claimed in claim 1 wherein, in the first mode of operation video signals are arranged to be transferred from the first data input to the data output and in the second mode of operation, video signals are arranged to be transferred from the second data input to the data output.

3. A switching apparatus as claimed in claim 1 or 2 comprising a status indication means for indicating if the switching apparatus is in the first or second mode.

4. A switching apparatus as claimed in any one of claims 1 to 3 comprising first and second relays each with a plurality of contacts, each relay being driven by a Break-before- make relay driver so that relays cannot be energised at the same time.

5. A switching apparatus as claimed in claim 4 wherein the Break-before- make relay drivers each comprise a transistor having its input connected with a biasing means to an inverting buffer and its output coupled with its relay.

6. A switching apparatus as claimed in claim 5 wherein the voltage at the control input is controlled by an external controller to energise or de-energise relays.

7. A switching apparatus as claimed in claim 6 comprising digital logic to operate the relays in response to a control signal at the control input.

8. A switching apparatus as claimed in claim 7 wherein, the digital logic comprises a bi-stable flip- flop which is arranged to be toggled by operation of a switch of the switching apparatus.

9. A switching apparatus as claimed in claim 8 having a status interface comprising a buffer connected between the digital logic and control input and at its output connected to a voltage divider, the voltage at the junction of the divider providing an indication as to the mode in which the switching apparatus is in.

10. A switching apparatus as claimed in claim 9 wherein, the status interface includes a diode which enables a control signal to the control input to lower the voltage at the junction of the voltage divider and thus, reset the flip-flop of the digital logic to the second mode.

11. A switching apparatus as claimed in claim 10 wherein, the output of the flip-flop feeds two or more inverting buffers which control the Break-before-make relay drivers.

12. A master controller for a switching apparatus, the master controller comprising a video amplifier means for receiving a video signal and amplifying it for transmission to an input of the switching apparatus, and a control means which is arranged to be operable to operate a switching apparatus to disconnect any existing signals from its output and to switch the video signal received from the master controller to its output.

13. A master controller as claimed in claim 12 having a first input for receiving the video signal, a first output for providing the video signal to a monitor and a second output for providing the amplified video signal for transmission to the input of the switching apparatus.

14. A master controller as claimed in claim 13 wherein, the video signal comprises blue, red and green colour signals and vertical and horizontal synchronisation signals.

15. A master controller as claimed in claim 14 comprising a switching apparatus status detector for detecting a switching mode of the switching apparatus.

16. A master controller as claimed in claim 15 comprising an indicator means for indicating the mode of the switching apparatus detected by the status detector.

17. A master controller as claimed in any one of claims 12 to 16 wherein the control means comprises a master switch which in use is operable to transmit a control signal to the switching apparatus.

18. A master controller as claimed in claim 15 or 16 wherein, the status detector comprises a comparator for comparing a detected voltage of the switching apparatus indicative of its switching mode with a reference, the comparator being arranged to produce a signal when the switching apparatus has changed from a first mode to a second mode.

19. A master controller as claimed in claim 16 wherein, the indication means is arranged to receive the signal from the comparator to provide a display of the mode of the switching apparatus.

20. A master controller as claimed in claim 19 wherein, the indication means comprises a pair of LED's connected with the emitter and collector of respective PNP and NPN transistors.

21. A master controller as claimed in claim 20 comprising a video duplicating amplifier for amplifying the colour signals and a horizontal and vertical synchronisation duplicating amplifier means for amplifying the horizontal and vertical synchronisation signals.

22. A master controller as claimed in claim 21 wherein the control means is arranged to be connected with the switching apparatus and the status detector is arranged to detect the voltage on the line interconnecting the control means and switching apparatus.

23. A master controller as claimed in any one of claims 12 to 22 which is arranged to be connected to a plurality of switching apparatuses to control operation of each of the switching apparatuses, the control means being arranged to produce a plurality of control signals.

24. A master controller according to claim 23 comprising a plurality of outputs each output having colour signals horizontal and vertical synchronisation signals and control signals for respective switching apparatuses.

25. A master controller as claimed in claim 24 wherein, the control means is arranged to control each switching apparatus with which the master controller is connected, independently.

26. A master controller as claimed in claim 25 wherein, the indication means provides a display of the mode of each switching apparatus to which the master controller is connected.

27. A master controller as claimed in claim 26 wherein a power supply for each switching apparatus is provided by the master controller.

28. A system for switching a video display unit from a first source of video signals to a master source of video signals, the system comprising at least one switching apparatus and a master controller, the or each switching apparatus being arranged to have its output connected to a VDU and its inputs connected with the master controller and its respective first source of video signals, the master controller being arranged to be connected with a master source of video signals, the master controller being arranged to transmit the master source video signals to the or each switching apparatus and further comprising a control means for operating the switching apparatus to switch the output of the or each switching apparatus from the first source of video signals to the master source video signals transmitted from the master controller.

29. A master controller as claimed in claim 28 comprising a plurality of switching apparatuses and wherein, the control means is arranged to operate each switching apparatus independently.

30. A master controller as claimed in claim 28 or 29 comprising a first input which is arranged to be connected with the master source of video signals and another input which is arranged to be connected with a master VDU and provide the video signals to the master VDU.

31. A master controller as claimed in claim 31 comprising a distribution unit with inputs for connection with the master source of video signals and outputs each for connection to a separate switching apparatus, each output providing master source video signals and a control signal.

32. A system as claimed in claim 31 wherein, the master controller comprises a plurality of distribution units each connected in parallel with the master source of video signals.

33. A system as claimed in claim 32 wherein, the master controller comprises a master distribution unit and a plurality of sub-distribution units each sub- distribution unit being arranged to be connected to respective outputs of the master distribution unit and each sub-distribution unit being arranged to have its input connected to an output of a master distribution unit and its output connected to respective switching apparatuses.

34. A system as claimed in claim 33 wherein, the master distribution unit has an indication means for indicating whether each sub-distribution unit is operating.

35. A system as claimed in claim 34 wherein, the master distribution unit has an indication means for indicating whether switching apparatuses of each sub- distribution unit are in a first or second mode of operation.

36. A system as claimed in claim 35 wherein, the first and second mode of operation indicates whether the switching apparatus has its output connected with the master source of video signals or has its output connected to some other source of video signals.

37. A system as claimed in claim 36 wherein, the switching apparatus comprises multiple outputs, each of the outputs being arranged to be connected to a different VDU.

38. The system as claimed in claim 37 wherein, operation of the switching apparatus switches all the VDU's with which it is connected to the master source of video signals.

39. A system as claimed in claim 38 wherein, operation of the switching apparatus by the control means of the master controller switches the output of the switching apparatus from a first source of video signals to the master source of video signals provided from the master controller.