WO1995022812A1

WO1995022812A1 - Improved switching apparatus for video signals

Info

Publication number: WO1995022812A1
Application number: PCT/AU1995/000051
Authority: WO
Inventors: Graham Peters; Steve Thomson; David Shenker
Original assignee: Graham Peters; Steve Thomson; David Shenker
Priority date: 1994-02-17
Filing date: 1995-02-06
Publication date: 1995-08-24
Also published as: AUPM397094A0

Abstract

The present invention relates to an apparatus for enabling switching a video display data between a plurality of video display units. In particular, it relates to an apparatus which is useful in computer base training to enable video signals generated by a number of computers to be switched between respective video display units associated with the computers. The system comprises a student unit associated with each student computer apparatus (1) and a master unit associated with the teachers computer apparatus (10). The student unit and master unit are connected by a single video transmission cable (17). The student can selectively view his own display or a display generated by a video signal being received on the transmission cable (17), which may be a video signal from the teacher computer (10) or another student computer (1) also connected to the master unit. The teacher can override student control and can force a student to view data intertransmitted on the video transmission cable (17) or can view the students display on his own screen via the video transmission cable (17).

Description

IMPROVED SWITCHING APPARATUS FOR VIDEO SIGNALS

The present invention relates to an apparatus for enabling switching of video display data between a plurality of video display units, and, particularly, but not exclusively, relates to an apparatus which is useful in computer based training and general education applications.

Typically, if a new computer system is to be installed in a work environment, it is necessary to teach prospective users of the system how to operate the system. This is usually done by organising seminars outside the work environment in a location where the particular system is already installed, so that the users can be given hands-on experience on how to operate the system.

Training methods generally involve prospective users sitting at respective computer terminals (a computer terminal comprising a computer apparatus incorporating a processing unit [computer] , a keyboard and an associated visual display unit arranged to produce a display in accordance with video display data generated by the computer) and operating their terminal in accordance with instructions from an instructor

(teacher) , who will be seated at his own computer terminal.

At. times, it may be desirable for the instructor to demonstrate a particular operation with reference to a display appearing on his own screen. In order for the students to view the display, it would previously have been necessary for them to gather around the instructor's computer terminal. Similarly if a student wishes to compare information on his own screen with information on the instructors screen.

Further, an instructor may at times wish to view a particular students display to determine whether he is carrying out a particular operation correctly. To do this, the instructor must physically move to the students terminal. Further, where the instructor may wish to demonstrate a display appearing on a particular student ' s terminal, the other students must gather around the particular students terminal .

The applicants own earlier international patent application, WO92/19070, the disclosure of which is incorporated herein by reference, discloses an apparatus which enables a display appearing on the instructor's screen to be viewed on the student ' s screens. The instructor's terminal is associated with an instructor's master control unit by which the instructor may ensure that what is appearing on his display screen appears on all the student's display screens. Further, the students each have their own selection switch to enable them to view the display on the instructors screen at any time. This prior art apparatus does not overcome the problem which arises when the instructor wishes to view a display on one particular student's screen, or wishes to show the other students what is on a particular student ' s screen. In the earlier invention, the students may only view the instructors screen display or their own display.

German patent DE 3438907 discloses a video switching apparatus which purports to enable switching of video signals between a plurality of student computers, a master computer and their respective visual display units (VDU's) so that the teacher may view the displays generated by the respective students computers on his VDU, the teacher may cause a display generated by his computer to appear on the respective students VDU's and the teacher may cause a selected students computer video signal to generate displays on other students VDU's. The switching apparatus utilises a central switch box for switching the video signals from the respective computers. The video transmission cable (VTC) from each computer in the system is connected directly to the central switch box. Further VTC's then lead from the central switch box to each VDU in the system. There is no direct connection between the computer and VDU of each respective computer apparatus. Operation of the switch box routes signals as desired from computers to VDU's.

This is a primitive system, merely comprising a multi-way switching apparatus for switching between a plurality of video cable connections. It has a number of disadvantages, among which are the following:

1. If the central switch box is not in operation, it is not possible for the connected computer apparatus to be operated. Power to the central switch box is required before each computer is connected to its respective VDU via the switch box. A fault in the power supply for the switch box or a fault in the switch box itself can therefore prevent normal operation of associated computer apparatus.

2. In a classroom situation, where a plurality of students computers are connected to the central switch box controlled by a teacher, the students have no control whatsoever over the display appearing on their respective VDU.

3. Each computer apparatus connected to the switch box requires a pair of relatively long VTC's, one each for the computer/switch box connection and switch box/VDU connection. Two cables are therefore required for each computer/switch box connection.

United Kingdom patent application number GB 2149617A also discloses a video switching system for computer apparatus which requires a central switch box and dual cable connections between the central switch box and the respective computer apparatus. This has similar disadvantages to those discussed above, and the further disadvantage that it is a digital system and therefore not suited to present requirements for video resolution.

From a first aspect, the present invention provides a system for enabling switching of video display data between a plurality of video display units, the system comprising: a plurality of student units, each respective student unit being arranged to be connected to a respective student source of video display data, to a respective student video display unit and to one end of at least one respective video transmission cable which is arranged to be connected at its other end to a master unit, each student unit including student switch means for connecting the student video display unit to receive video display data from the respective student source or to receive further video display data from the video transmission cable, and for placing student video display data from the student source onto the video transmission cable, and student control means for controlling the student switch means; a master unit arranged to be connected to the student units via the other ends of the respective video transmission cables, and also connected to a master video display unit, the master unit including master switch means and master control means for connecting the master video display unit to receive video display data from a selected one of the plurality of at least one video transmission cables, and for placing further video display data onto the video transmission cables.

Preferably, the master unit is arranged to be connected to a master source of video display data, the master switch and control means being arranged to connect the master video display unit to receive video display data from the master source.

Preferably, the master control and switch means are arranged to place video display data from the master source onto the video transmission cables as the further video display data. Preferably, the master control and switch means are operable to place video display data received from a selected one of the video transmission cables onto the other connected video transmission cables as the further video display data. Preferably, the student control means is arranged to operate the master control means to cause the further video display data to be placed on the respective students video transmission cable, for providing a source of video display signals for the respective students video display unit.

Preferably, the master control means is operable to force the student unit control and switch means to cause the student video display unit to receive further video display data from the respective video transmission cable.

Preferably, the master control means is operable to disable the student control means and cause the student switch means to connect the student video display unit to receive video display data only from the respective student source.

Preferably, the master control unit further comprises a display means including indicator means for each student video display unit, the indicator means providing an indication of whether each video display unit is receiving video display data from its associated student source or is receiving further video display data from the transmission cable, and, where further video display data is received, the indicator means provides a directional indication of the source of origin of the further video display data.

Preferably, the indicator means is in the form of a dynamic flow diagram. In a preferred embodiment, each student unit is connected to the master unit by a single video transmission cable which, under control of the respective student and master control means, is arranged to transmit video display data in both directions along the cable, i.e., from the student source to the master control unit or from the master control unit (further video display data) to the student unit.

The master and student control means are preferably connected to a control line which preferably extends along the video transmission cable. Preferably, a logic high or low on this control line is used to provide a control signal from the student unit to the master control unit . In one logic state the student VDU receives signals from the associated source of video signals and places the video display data from the student source onto the transmission line, so that it may be selectively accessed by the master unit. In the alternative logic state, the student control unit causes the master control unit to place video display data from a further source onto the transmission cable, to drive the student VDU. Actuation means, preferably in the form of a toggle button, are provided to enable the student to control the student control means, and thus the student in normal operation can determine which video display data drives his VDU.

Preferably, the master control unit, using the same control line, is able to override student control and control switching of the student unit, and the video display data transmitted on the VTC, from the master control uni .

Preferably, if the system (master unit and student units) is not turned on or there is a power failure to the system, the respective switch means switches to a "fail-safe" position, where each respective VDU is connected to its own respective source of video signals.

The video transmission cable utilised in the preferred embodiment is a nine way cable, including three coaxial, incorporating their respective central conductors and shields, and three signal conductors. Analog Red (R) , Green (G) , Blue (B) analog signals are transmitted on the coaxial conductors. Horizontal (H) and Vertical (V) synchronisation signals are transmitted on two of the wire conductors and the third conductor is used for control, as discussed above. It will be appreciated that the present invention may be adapted to use other types of video transmission cables . In the preferred embodiment, therefore, the system may be used with a plurality of student computer apparatus and a master computer apparatus, the master may view video display data generated by any of the respective students computers, each student may view a video display generated by his own computer or select video display data appearing on the connected video transmission cable. This further video display data may be determined by the teacher, but where the teacher is not operating the master display unit it will normally be video display data generated by his own computer, so that each student may selectively view the teachers video display. Where the teacher is viewing a selected student, however, the system is preferably arranged so that operation of the student control unit enables the student to view what the master is viewing, i.e., the selected other student. The master may also "force" each student to view what he is viewing, i.e., either his own display or that of a selected student, and may also "lock out" (bar) the student units so that each student can only view the display generated by his own respective computer. It will be appreciated that the system may be configured to operate in other modes, but these are the preferred modes, in particular for the classroom situation.

The master unit preferably comprises a number of "plug-in modules" associated with the students. In a preferred embodiment, each student plug-in module is connected via a pair of VTC's to two student units. Each module includes switch means and control means for controlling connections to the pair of student units. A teacher unit is also incorporated, which controls connections to the teacher computer apparatus. The teacher unit and plug-in units are connected by a master unit system BUS.

Each student unit and the master control unit include amplification means connected to at least the respective R, G, B conductors on the VTC's to overcome power loss with shared loads and to additionally buffer connections. Characteristic impedances are also preferably maintained throughout the system by placement of characteristic impedances "Z" at required points in the system to match the characteristic impedance of the VTC's and therefore to prevent signal reflection problems.

The present invention has, in at least a preferred embodiment, the advantage that video display data can be transmitted from a student computer apparatus to a master computer apparatus or from a master computer apparatus to a student computer apparatus along the same video transmission cable. In a preferred embodiment, the system also enables the student to exercise control over his own video display unit via the provision of a student control means associated with his computer apparatus.

Further, if the system is not operational, each connected computer apparatus will operate as normal .

It will be appreciated that the terms "student" and "master/teacher" are non-limiting. The present system is particularly suited for application in classroom situations, but could also be adapted to other applications requiring switching of video display data between VDU' s.

The present invention further provides a student unit for use with a system for enabling switching of video display data between a plurality of video display units, the student unit being arranged to be connected to a respective student source of video display data, to a respective student video display unit and to one end of at least one respective video transmission cable, which is arranged to be connected at its other end to a master unit, the student unit including student switch means for connecting the student video display unit to receive video display data from the student source, or to receive further video display data from the video transmission cable, and for placing video display data from the student source onto the video transmission cable, and student control means for controlling the student switch means.

The student unit may include any or all of the preferred features of the student unit discussed above in relation to the first aspect of this invention.

The present invention further provides a master unit for use with a system for enabling switching of video display data between a plurality of video display units, the master unit being arranged to be connected to a plurality of video transmission cables, which are also arranged to be connected to a respective plurality of student units, the master unit also being arranged to be connected to a master video display unit, the master unit including master switch means and master control means for connecting the master video display unit to receive video display data from a selected one of the video transmission cables and for placing further video display data onto the transmission cables.

The master unit may include any or all of the preferred features of the master unit discussed above in relation to the first aspect of this invention.

The present invention yet further provides a display means for a system for enabling switching of video display data between a plurality of video display units and associated sources of video display data, the display means including indicator means for each video display unit, the indicator means providing an indication of whether each video display unit is receiving video display data from its associated video source or is receiving video display data from another video source, and where video display data is received from another source, the indicator means is arranged to provide a directional indication of the source of origin of the display data.

Preferably, the indicator means is in the form of a dynamic flow diagram.

By "dynamic flow diagram" (both in relation to this aspect of the invention and the first aspect of the invention discussed above) is meant a diagram which indicates the direction of video signals from/to each particular VDU in the system and changes display to indicate changes in signal flow from/to VDU's.

In a preferred embodiment, the display means of this aspect of the present invention is used with the system of the first aspect of the invention discussed above. The display means is associated with the master unit. The teacher is able to view the display and immediately determine the status of each of the student

VDU's, whether the student VDU is viewing the signal from its own respective source of video signals, or whether it is viewing video display data transmitted to it on the video transmission cable. Where it is viewing video transmission data from the video transmission cable, the display gives an indication of where this particular video display data originates. The display advantageously facilitates operation of the system by the teacher.

Features and advantages of the present invention will become apparent from the following description of an embodiment thereof, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram showing a system in accordance with the embodiment of the present invention connected to a plurality of computer apparatus;

Figure 2 is a schematic block diagram for illustrating operation of the system of figure 1;

Figures 3 to 5 are connection diagrams showing various connection modes of the system as illustrated in figure 2;

Figure 6 is a schematic diagram illustrating a control connection between a teacher unit and a student unit of the system of figure 1;

Figure 7 is a front view of a display panel of the master unit of the system of figure 1;

Figure 7A is a circuit diagram of a time delay circuit for push button operation of the master unit;

Figure 8 is a top view of a student control module associated with each student unit of the embodiment of figure 1; Figure 9A is a circuit diagram for a student unit of the system of figure 1;

Figure 9B is a circuit diagram for the student control module of figure 8; Figure 10A is a circuit diagram for part of a student plug-in card incorporated within the master unit of figure 1;

Figure 10B is a continuation of the circuit diagram of figure 10A showing more of the student plug-in card;

Figure 11 is a circuit diagram of a teacher unit incorporated in the master unit.

Figure 12A and 12B are circuit diagrams of strobe generators used to generate control signals; and Figure 13 is a circuit diagram of an amplifier utilised in the system.

With reference to figure 1, an apparatus for enabling switching of video display data between a plurality of video display units (VDU's) is illustrated connected to a plurality of computer apparatus, being student computer apparatus 1 and master computer apparatus 10. Note that the master 10 and student 1 computer apparatus may be exactly the same type of computer apparatus, e.g., conventional PC's. Separate reference numerals are used for convenience because the master computer 10 is connected to a different part of the system of this embodiment of the present invention than the student computer apparatus 1.

The system comprises a master unit 11 and a plurality of student units 2, a student unit 2 being associated with each student computer apparatus 1. Each computer apparatus 1 comprises a student source of video signals (i.e., student computer) 3 and an associated student VDU . A keyboard 5 is also provided for student operation of the computer apparatus.

Similarly, the master computer apparatus includes a master source of video signals 12, a master VDU 13 and a master keyboard 14. Each student unit 2 is connected to the associated student computer 3 and VDU 4, as shown by connecting line 6 and 7. Similarly, the master unit 11 is connected to the master computer 12 and master VDU 13 by lines 15 and 16, respectively. The master unit 11 is also connected to each respective student unit 2 by a single 9-way video transmission cable (VTC) 17, including three coaxial cables for the transmission of R, G, B video signals and three wire conductors, two for the H and V sync signals and one for control.

Each student unit is also connected to a student control module 8A to enable the student to control the unit 2.

The student unit includes control means and switch means for connecting the student VDU 4 to receive video display data from the student source 3 or to receive video display data from VTC 17. Further, the student unit 2 also operates to place video display data from the student source 3 onto VTC 17 for access by master unit 11.

The master unit 11 includes master switch means and master control means for connecting the master VDU 13 to receive video display data from any selected one' s of the plurality of VTC's 17. Further, the master switching and control means is operable to connect the master source 12 to provide video display data to the master VDU 13, to place on the VTC 17 further video display data, which may comprise video display data produced by the master source 12 or a selected one of the student sources 1.

The system therefore allows the VDU's 4, 13 to be driven by video display data from any of the computers 3, 12. In particular:

A. The teacher may view on his VDU 13 the display being generated by the computer 3 of any student he selects;

B. A student may operate the student control unit 2 to enable his VDU 4 to generate a display in accordance with video display data from his own computer 3 or in accordance with video display data on VTC 17. In this embodiment, if the student selects to view video display data on VTC 17, then this data will correspond to video display data being viewed on the master VDU 13 , whether being video display data from a selected student computer 3 or from the master computer 12.

C. The instructor may "force" the students to view a display generated by video display data on line 17, i.e., he may force them to view his own display or the display of a selected student. In the force mode, student control is disabled and the student has no control over his display.

D. The master unit 11 is operable to "lock-out" (bar view) the student units 2, so that the students cannot view any other display but that generated by their own computer 3.

Figure 2 shows a schematic block diagram of the system to illustrate in more detail operation thereof. The same reference numerals are used in figure 2 to designate the same components as in figure 1. Only two computer apparatus 1 are shown in the figure. It will be appreciated that connections are provided for the others.

To assist clarity, only a single coaxial connection RED is shown for each VTC 17. The RED coaxe is for transmission of the R video signal, and switching and control in the system will be described in relation to switching only of the R signal. The B, G, H sync and V sync signals are switched in a similar manner, although the H sync and V sync are switched utilising digital switches in some parts of the system, as will become clear from the following description. R, G and B signals are switched at all times using relays, as these are analog signals. Nevertheless, the switching paths are the same for R, G, B, H sync and V sync for all switching modes, as all these signals are required to drive a VDU. The following description of the switching path for the R signal can therefore be considered to apply similarly to all other signals making up the video signal .

The master unit 11 is organised in a modular form. It comprises a teacher unit 20 which incorporates teacher control 21 and switch means RL1, RL3, RL6 and RL9 (further switch means [not shown] being provided for the G, B, H sync and V sync) for switching an R signal between an out BUS, and in BUS, the teacher VDU 13 and teacher computer 12. The teacher control 21 is connected to control the switches, as indicated by the arrow head lines in figure 2 and is also connected by a further line to a control BUS in the master unit 11.

Also connected to the BUSES are a plurality of plug-in units 22 for connection to the respective student units 2 via VTCs 17. In the preferred embodiment, each plug-in unit

22 comprises a plug-in circuit board and is arranged to be connected to two student units. This is illustrated by the plug-in unit 22 in the bottom half of the figure, which shows connections to two VTCs 17. For connection co two student units. Each student plug-in unit 22 includes a student plug-in control 23 (only shown in the top plug-in unit in figure 2. The bottom plug-in unit will also have a student plug-in unit control, not shown) . The student plug-in unit control controls switch means in the plug-in unit 22 RL1, RL3 (for switching RED signal only, G, B, H sync and V sync switches are not shown but are also controlled by the control unit 23) . A control line from control unit 23 is also connected to a teacher/student control line which runs in the cable 17 (only shown in the top half of the figure) .

It will be appreciated that the lower student switch means RL7, RL5 of the bottom half of the figure will have its own associated unit control 23.

Each student unit 2 comprises student switch means RL3, RL1 (as with the other units, other switch means are provided for G, B, H sync and V sync signals but these switch means are not shown) . A student unit control 24 is operable to control the switches RL1, RL3 (and the other switches) and is also connected to the other end of the teacher/student control line in the cable 17. Again, control is not shown on the lower student unit 2 in figure 2. Although similar reference numerals are used for the switches in the different units 2, 22 and 20, -these are in fact different switches.

The switches in figure 2 are shown in no particular switching mode. Video amplifiers V are provided in the system to overcome power loss in the system with shared loads. The video amplifier V is necessary only for the R, G, B signals and not necessary for the H sync and V sync signals. The video amplifiers for G and B are no shown in figure 2. Characteristic impedances are maintained throughout the system by placement of characteristic impedances Z at the correct points to match the characteristic impedance of the video cables preventing signal reflection problems. H sync and V sync do not require an impedance of value Z (see later) .

In operation, the switches are controlled by the respective control units, 2, 23, 21 and ultimately by the student or teacher by the student control module 8 and teacher display and push button console 25 to provide the switching modes discussed above in relation to figure 1.

Figures 3 through 5 are switching diagrams illustrating various switching modes for the R signal. For reasons of clarity, those sections of the circuit not connected in a particular mode are not shown. Figure 3 shows the "observe selected student state". The observe student (top most) views his own screen and his signal is broadcast on the system BUS via his student plug-in unit. The other students (bottom) may observe the selected students screen (on operation of the student control module 8 to view what is incoming on respective VTC 17) and the teacher also observes the selected student screen.

Figure 4 shows the "teacher broadcast mode" . The teachers signal is broadcast on the system BUS (the teacher observes his own screen) . The top most student is seen to be observing the teachers screen whilst the bottom student elects to watch his own (both under student control) . Each student uses his respective student control module 8 co select which screen to watch so that his switching structure will alternate between the top and bottom structures. The teacher may also override so that all students are forced to the top switching structure (must view state) or the bottom switching structure (bar view, or "lock-out") .

Figure 5 shows "power down mode" . It is essential in this mode that each student and the teacher at least has unconditional access to his/her own screen. The switches and control means are arranged to provide this switching mode when there is no power.

It will be appreciated that the control units in the system may be configured to provide other modes of operation. In this preferred embodiment, however, the only modes of operation provided are those discussed above in relation to figure 1.

Control is implemented by logic. A person skilled in the art will be able to devise appropriate logic to provide the switching modes discussed above. The following points are worth noting, however.

Each VTC 17 includes a single conductor for transmitting control signals between the master unit 11 and the student unit 2, the TEACHER/STUDENT control line. Two logic states are implemented on this one wire. Logic high means that the student views the signal from his own computer 3 and also places the video signal from his computer 3 onto the line 17, for access by the teacher, if desired. Logic low means that the student observes a teacher signal or, more specifically, observes the signal which is present on the out BUS which, as will be realised from the above description, may be the teacher signal or may be a signal from a student being viewed by the teacher. Normally the state of this signal is dictated by the student via the student unit 2 and student control module 8. The student may "toggle" between his own video signal and the video signal on the out BUS of the master unit 11. The teacher, however, has the capability to override the student control for the "must view" and "bar view" modes. Because there is only a single teacher/student control line, this override capability must be done via that single line. This is achieved by the circuit schematically illustrated in figure 6. The teacher signal is connectable to the student/teacher control line via a low impedance Z₀ and via a tri-state logic gate 30. On the other hand, the student unit 2 is connected by a high/low logic arrangement 31 and a relatively high impedance Zl to the student/teacher line. In normal operation, the teacher signal is tri-stated so that the student's control signal dictates the state of the signal on the student/teacher control line. When the teacher signal is tri-state disabled, however, its signal, being seen by the student/teacher line through a lower impedance, dominates and thus establishes the new signal state.

One further control signal is transmitted from the master unit 11 down the teacher/student line. This signal is only transmitted when the particular student signal is being broadcast to all the other students in the must view mode. Because only a single student/teacher control wire is available, this signal is transmitted using a high frequency narrow pulse. A low pass filter and pulse discriminator at the student end receive the pulse before it affects any control logic in the student control unit. The pulse is used to drive a display on the student control module 8 to indicate to the student that his signal is being broadcast .

Figure 7 is a front view of a display and push button control 25 of the master unit 11. The display is in the form of dynamic flow diagram. Three LED's A, B and C are associated with each student unit connected in the system. The console 25 has room for displaying and operating 20 student units i.e., up to 10 plug-in cards (one for every two students) can be plugged into this particular master unit 11. Underneath each set of three LED's A, B, C is a push button, 1 through 20. "Bar view", "must view" and "view self" LED indicators D,E,F and associated push buttons 21, 22, 23 are positioned on the right hand side of the panel 25. A further directional indicator G provides further information to the instructor on which signals are being viewed. Finally, the console is also provided with a power button 24, for powering up the system.

The display panel 25 is specially designed to facilitate operation of the system in accordance with this embodiment of the invention. Each of the student control buttons 1 through 20 is connected to the respective plug-in units 22. Operation of any one of these buttons enables the teacher to view the display being generated by that particular students computer 3.

In normal operation, when the teacher is viewing his own display and all the students are viewing their own displays, all the LED's B are lit and the view self LED F is also lit. If the teacher wishes to view a selected student, say student 6, he actuates the appropriate button. LED C of student 6 illuminates, in addition to LED B LED C is in the form of a pointer, pointing downwards, along line 26 to "viewing" indicator G. This indicator also illuminates. The view self indicator F is extinguished. It is therefore immediately clear from the display that the teacher is viewing the display of student 6.

If another student, say student 3, wishes to view what is on the master unit out BUS (i.e., the teacher signal or, if the teacher is viewing another students signal, that selected students signal) , then the student operates his unit and the top LED A student 3 illuminates. LED B of student 3 is extinguished. Again, LED A is a downward pointing indicator connected to another line 27 and indicating that student 3 is viewing whatever is coming from line 27. Where the teacher is still viewing student 6, it will be clear from LED C of student 6, line 26, the viewing LED G, line 27 and LED A of student 3, that student 3 is now viewing the display of student 6.

If the teacher wishes to allow the students the option of viewing his own display, he merely actuates button 23, which extinguishes the LED C of student 6 and LED G. It is now clear that student 3 is viewing the teachers display, while the teacher is viewing his own (view self) .

If a teacher wishes all other students to view a display he is viewing, he hits the must view button 22. All the indicators A then light up indicating those students are viewing what the teacher is viewing. Further, to indicate that this is a must view state, and not a state which has been selected by the students, the LED's A are arranged to flash. If the teacher has selected a particular student screen for broadcasting, that will be indicated by the particular students LED C.

LED E (Must View) is a bi-colour LED, in the preferred embodiment. If the teacher is viewing his own screen, LED F will be alight and if he has also selected the Must View button 22, so that all students are viewing the instructors screen, LED E will be illuminated in a first colour. If, on the other hand, the teacher is viewing a selected students screen and selects Must View 22, LED E will be illuminated in a second colour.

Finally, if the teacher wishes to prevent the students from viewing anything but their own signals, he hits the bar view button 21 and all the LED's B will then be lit. Note that in this mode he may still look at the student signals.

In the preferred embodiment LED A is amber, LED B is red and LED C is green. LED F is red and LED E is either red or green, being red if the students are viewing the teachers screen on Must View and green if the students are viewing a selected students screen on Must View . LED D is red .

In the preferred embodiments, the push buttons are connected to the student plug-in unit control 23 for each student. This is indicated by line 50 in figure 2. To prevent two signals appearing on the out BUS/in BUS at the same time, each connection to the student plug-in unit control 23 from the display and push button console 25 is made via a time delay circuit, so that when a push button is actuated any previous signal on the out BUS/in BUS is switched off before the selected signal is placed on the BUS. Appropriate time delay circuitry may easily be devised by a person skilled in the art. An example circuit is shown in figure 7A, however. Two of the push buttons on the console 2, and 3 are shown. All buttons are connected to a common RESET line via diodes 60. A time delay circuit 61 and flip flop 62 are connected between the push button 2, 3 and respective student plug-in control units 23. Time delay circuits 61, 62 are connected to the set input of the flip flop 63 and the reset line is connected directly to the reset inputs R. On operation of push button 2, for example, all flip flops connected to the reset line will automatically be reset and will therefore provide an appropriate control input to student plug-in control 23 for each student (note that the teacher control 21 is also connected to RESET) so that any signal already on the BUS is removed from the BUS by the switch means. After a time governed by ΔT 62, the set input of the flip flop 63 is actuated to alter the output Q to the plug-in control 23 for push button 2, to put the respective students video signal onto the master unit BUS. Because of the diodes 60 on the set lines, none of the other set inputs of the other flip flop 63 are affected.

Appropriate circuitry for implementing operation of the LED's on the console 25 as discussed above may be implemented by a person skilled in the art. In the present embodiment, the LEDs are driven by logic.

Figure 8 is a top plan view of a student module for controlling the student unit 2. It includes a toggle switch 70 and three LED's 71, 72 and 73. In operation, this is attached to the student unit 2 by cable (see figure 1) . Module 8 can therefore be kept by the students computer, in a similar fashion to a "mouse" . The relatively bulky student unit 2 may be mounted out of the way, underneath the students desk, for example.

The toggle switch 70 is operable to toggle the student VDU 4 between the video signal produced by the students computer 3 and the video signal on VTC 17. In the preferred embodiment, LED 73 is red and flashes when the student is being broadcast to the other students in must view mode. LED 72 is green and is lit when the student is viewing himself. LED 71 is amber and is lit when the student is viewing what is on the master unit BUS (i.e., whatever the teacher is viewing, either the teachers own signal or a selected students signal) . The LEDs are controlled by appropriate logic.

Figures 9B through 13 are circuit diagrams showing the various components of the system of the preferred embodiment . Detailed description is not given of these circuits, as it is considered that the features will be apparent to a skilled engineer from the circuit diagrams and the following general description. Conventional terminology is used to designate various components, e.g., "C" stands for a capacitor. All the circuit component values designated are by way of example only and should not be considered limiting.

In each circuit diagram, much of the labelling is self explanatory. The following may, however, assist in interpretation of the diagrams.

RED on a line indicates that this line carries a R video signal. Similarly, GREEN, BLUE, HS and VS on respective lines indicate that these lines carry G, B, H sync and V sync signals respectively.

Labels associated with the R, G, B, HS and VS signal lines indicate the transmission direction of the signal. For example, in figure 9A, in the top left hand corner, "SOUT" indicates that the respective signals are going out on the jack plug J3 to the student VDU 3.

Terms VRIN, VGIN, VBIN, VROUT, VGOUT and VBOUT on a line generally indicate respective R, G, B inputs to video amplifiers and the respective outputs.

The terms TOUT and TIN are used for teacher signals. Other labels will be explained in the following general description, or will be apparent to a skilled person. Figure 9A is a circuit diagram of the student unit 2. Connector J3 connects to the student VDU 4, outputting the signals (R,G,B, HS and VS) indicated on the connection lines. J2 connects from the student computer 3, inputting the line signals (R,G,B,HS and VS) as labelled. The relays are labelled similarly to the switches in figure 2. The circuit diagram also shows the relays (RL2, RL4) for GREEN and BLUE, but it is considered that no further explanation is required, as the switching path is the same as for the RL3, RL1 relays described with reference to figure 2, for switching the R signal.

One point worth noting is that the H sync and V sync signals from the student computer 3 are switched to the student output at J3 via relay switches RL4 and RL5. In the majority of the rest of the system, H sync and V sync are switched via logic circuitry (see inputs to IC1 from the SINHS and SINVS lines respectively) . Relay switching of the sync signals is required for the power down mode, however, to ensure that the student can operate his own computer without power to the system.

R15/C19 act to save power to the relays. At switch on the relays see 12V as an "impulse". Subsequently, the voltage drops due to the R15/C19 combination, to reduce the current required for holding the relay.

Note that all power to system units is provided by a power module in the master unit 11. Power regulators are provided for providing the regulated power sources of each respective unit. The regulators are not shown. They are conventional and will be easily realisable by a skilled person.

VDD is the power source for the video amplifiers V (one amplifier for each of R,G and B) . When the video amplifiers V are not required, i.e. when the student signal is not being put on the VCT 17 and the relay RL5 is in the alternate position to that shown in the diagram, the power supply to VDD is disconnected. This is another power saving feature.

C15, C16 and C17 operate to ac couple the R, G and B signals to the respective video amplifiers V. Video amplifiers V are not shown. An amplifier circuit will be described later with reference to figure 13. R5, R6 and R7 provide the characteristic impedance Z referred to above. IC1 is a programmable logic device (PLD) and IC2 is a relay driver. It will be appreciated by the skilled person that the logic device and relay driver are configured to provide the switching modes and control signals (on the STUDENT/TEACHER line) as discussed above.

R4 and RIO are provided to prevent the H sync and V sync signals from loading down the PLD. C6/C8 and Rll provide a low pass filter for allowing only low frequency signals to input 3 of the PLD. Any high frequency pulse on the STUDENT/TEACHER control line, therefore, will not affect this input. As discussed above such a pulse is produced when the particular student is being broadcast to the other students in the must view mode. High frequency volts affects control input for of IC1. R12 and C18, in conjunction with IC1, provide the discriminator circuit referred to earlier. R2, R3, C7 and R9 are connected to the toggle switch SI for student control on the student "mouse" (figure 9B) . R8 provides Zl of figure 6, discussed above.

Jl is the connection to the VTC 17. It provides connections for R, G and B signals, H-sync, V- sync, STUDENT/TEACHER control, +15, -15. A student control "mouse" is shown in figure 9B. It is connected by connector J5 to corresponding connector J4, connected to IC1 and IC2. LI, L2, L3 and associated resistors R14, R13 and Rl comprise the LEDs 73, 72 and 71 of figure 8, respectively, and associated operating circuitry. Toggle switch SI corresponds to switch 70 discussed in relation to figure 8. Operation will be apparent to the skilled person from the above description. Figures 10A and 10B are circuit diagrams of a student plug-in unit 22. As discussed above, each plug-in unit 22 provides for two student units. J2 and J3 are the respective connections to the respective VTCs 17. A single video amplifier V can be used for each R, G and B signal from the system bus (ie. for receiving the teacher's video signal or a selected student's video signal from the bus) , for transmission to the pair of student units connected to the card.

Again, switch operations for each of the signals are the same as that described in relation to figure 2 and no further detailed description is required here. RL1, RL2, RL5 and RL6 provide lines for receiving the R, G, B video signals from respective student lines, VTCs 17. RL3, RL4, RL7 and RL8 provide the switches for switching the video signal to the VTC 17 from the master BUS or from the VTC line 17 to the master BUS.

IC3 and IC4 are programmable logic devices and IC5 is a relay/light emitting diode driver. Together these ICs constitute control means for the plug-in unit 22 and also operate to drive the LEDs on the teacher console 25. SI and S2 are inputs from the respective student's push buttons on the console 25, shown with time delay circuitry R7, R9, Dl, D2, IC1.

Logic devices and relay/LED drivers are configured in accordance with the switching modes required, as discussed above.

Rl, R2, R3, R4, R5, R6. R64, R65, R66, R61, R62, R63 all correspond to matching impedance Z. Rll, RIO, R68 and R67 provide some impedance matching for the H- sync and V-sync signals to prevent reflection, but not large enough to load the logic circuitry which drives the H-sync and V-sync signals. Similar power saving circuits are provided for the relays and video amplifiers as were provided in the circuit of Figure 9A. These are provided by R59 and C25, R13 and C3, R12 and R60.

Figure 10B shows the connection of the plug-in unit 22 to the system bus. The connections are self- explanatory. The H stroke and L stroke signals will be referred to later. FORCE and BAR relate to the MUST VIEW and BAR VIEW functions. A RESET line is also provided.

Appropriate power regulators are provided to regulate the various required power supplies . These are not shown.

Figure 11 is a circuit diagram of the teaching unit 20. Jl is the connection to the system BUS. Signals on the respective lines are self-explanatory. IC7 and IC8 are PLDs and IC9 is a relay driver. IC7, 8 and 9 are configured to provide the switching modes discussed above and no further explanation will be given. Z-matching impedance is provided by the 75 ohm resistors R1-R6, Rll, R12, R13 and the relays are provided with appropriate power saving circuits, as previously described in relation to the other units in the system R7, R8 provide some impedance matching for H sync and V sync lines. RL6 and RL7 provide switching connections from the master unit BUS to the teacher VDU (on lines TROUT, TGOUT and TBOUT) . Power saving is provided by R15 and C14. RL1, RL2, RL3 provide further switch to the teacher VDU. RL4 and RL5 provide switches from the teacher computer (TRIN, TGIN, TBIN) . RL8 and RL9 provide input switches to the distant BUS for video signals R, G, B. Power saving is provided by R16 and C15. Incidently, power saving on relays RL1 through RL5 is provided by R14 and C13. RL2, RL3 and RL5 also operate to switch H sync and V sync for the teachers signal. The switching arrangement is the same as described in relation to figure 2, but reproduced for the G, B, H-sync and V-sync signals. U4, A B and C in the bottom left hand corner of the circuit and the connected components R17-19 and C7, C16, C7 provide switch bounce protection for the VIEW SELF, BAR VIEW and MUST VIEW (force) switches. A3 connects to the teacher computer and J2 to the teacher VDU.

Figure 12A and Figure 12B shows circuits for providing a low frequency (RIO, C19, U4D) and high frequency strobe signal, respectively. High frequency strobe signal provides a narrow pulse for the discriminator circuit

A low frequency strobe is operable to flash LEDs on the panel 25 where a force (MUST VIEW) condition is in operation.

There is also a Power On reset line (R21, C20, U4:F) which automatically operates to reset all connections so that every VDU is getting signals from its own computer apparatus.

Finally, figure 13 shows the amplifier V. The amplifier comprises discrete components, including transistors Q19, Q20, Q23, Q22, Q21, Q24, resistors R41, R51, R45, R44, R50, R49, R43, R48, R47, R77, R42, R52 and R46, and capacitors C23, C21 and C22. It is believed that operation of this amplifier is clear from the circuit diagram.

It will be appreciated that the master unit need not necessarily be associated with a master computer. A master could easily be operated solely to switch signals between student computers, and to enable the teacher to monitor what the students are doing on his own VDU.

The source of video signals need not, in fact, be a computer. It could be any source of video signals.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A system for enabling switching of video display data between a plurality of video display units, the system comprising: a plurality of student units, each respective student unit being arranged to be connected to a respective student source of video display data, to a respective student video display unit and to one end of at least one respective video transmission cable which is arranged to be connected at its other end to a master unit, each student unit including student switch means for connecting the student video display unit to receive video display data from the respective student source or to receive further video display data from the video transmission cable and for placing student video display data from the student source onto the video transmission cable, and student control means for controlling the student switch means; a master unit arranged to be connected to the student units via the other ends of the respective video transmission cables, and also connected to a master video display unit, the master unit including master switch means and master control means for connecting the master video display unit to receive video display data from a selected one of the plurality of at least one video transmission cables, and for placing further video display data onto the video transmission cables.

2. A system in accordance with claim 1, the master unit further being arranged to be connected to a master source of video display data, the master switch and control means being arranged to connect the master video display unit to receive video display data from the master source.

3. A system in accordance with claim 2, the master control and switch means being arranged to place video display data from the master source onto the video transmission cables as the further video display data.

4. A system in accordance with claim 1, 2 or 3, the master control and switch means being operable to place video display data received from a selected one of the video transmission cables onto the other connected video transmission cables as the further video display data.

5. A system in accordance with any preceding claim, the student control means being arranged to operate the master control means to cause the further video display data to be placed on the respective students video transmission cable, for providing a source of video display signals for the respective students video display unit.

6. A system in accordance with any preceding claim, wherein the master control means is operable to force the student unit control and switch means to cause the student video display unit to receive further video display data from the respective video transmission cable.

7. A system in accordance with any preceding claim, the master control means being operable to disable the student control means and cause the student switch means to connect the student video display unit to receive video display data only from the respective student source.

8. A system in accordance with any preceding claim, the master control unit further comprising a display means including indicator means for each student video display unit, the indicator means providing an indication of whether each video display unit is receiving video display data from its associated student source or is receiving further video display data from the transmission cable, and, where further video display data is received, the indicator means provides a directional indication of the source of origin of the further video display data.

9. A system in accordance with claim 8, wherein the indicator means is in the form of a dynamic flow diagram.

10. A student unit for use with a system for enabling switching of video display data between a plurality of video display units, the student unit being arranged to be connected to a respective student source of video display data, to a respective student video display unit and to one end of at least one respective video transmission cable, which is arranged to be connected at its other end to a master unit, the student unit including student switch means for connecting the student video display unit to receive video display data from the student source, or to receive further video display data from the video transmission cable, and for placing video display data from the student source onto the video transmission cable, and student control means for controlling the student switch means.

11. A master unit for use with a system for enabling switching of video display data between a plurality of video display units, the master unit being arranged to be connected to ends of a plurality of video transmission cables, the other ends of which are arranged to be connected to a respective plurality of student units, the master unit also being arranged to be connected to a master video display unit, the master unit including master switch means and master control means for connecting the master video display unit to receive video display data from a selected one of the video transmission cables and for placing further video display data onto the transmission cables.

12. A display means for a system for enabling switching of video display data between a plurality of video display units and associated sources of video display data, the display means including indicator means for each video display unit, the indicator means providing an indication of whether each video display unit is receiving video display data from its associated video source or is receiving video display data from another video source, and where video display data is received from another source, the indicator means is arraned to provide a directional indication of the source of origin of the display data.

13. A system in accordance with claim 12, wherein the indicator means is in the form of a dynamic flow diagram.