AU640746B2 - Improved video display transfer - Google Patents

Description

WO 91/11887 PCT/AU91/00034 1 "IMPROVED VIDEO DISPLAY TRANSFER" THIS INVENTION relates to improvements in video display for a multicomputer network. In particular, the invention is directed to apparatus and method for transferring video signals between computers in a multi-computer network, with automatic level control.

BACKGROUND ART With the increasing demand and usage of computers, and their decreasing cost, multi-computer networks have become quite popular. Such networks typically comprise a number of microcomputers, such as personal computers which are connected in a network, for example by LAN (local area network) cablea. Each computer station typically comprises a microcomputer with associated keyboard, and a video display monitor.

Microcomputer networks are now becoming popular in classrooms for teaching purposes. A typical multi-computer teaching system comprises a number of student computer stations connected in a network which includes a teacher computer station. An interface circuit is provided between each computer station and the network bus, and a control unit is normally associated with the teacher computer station. The interface circuit may be housed on an adaptor card in each micro-computer. An example of a known multi-computer teaching system can be found in U.S. patent no. 4,759,717, In one known multi-computer teaching system, the video display monitor of each student computer station can display either an output from its own computer or an output WO 91/11887 PCT/AU91/00034 2 transmitted through the network from either the teacher computer station or another selected student computer station.

Each computer station is provided with an adaptor card containing a video logic circuit to enable video display signals to be transferred to and from that computer station.

The teacher can communicate with the whole class of students without moving away from the teacher's computer station. The teacher may transfer displays, information and data files, use the computer screen as a blackboard, correct the student's work, use a "mouse" as a pointer, and give different degrees of assistance to students with different abilities. The teacher can also obtain a full picture of the students' progress by scanning their displays. This can be done automatically, or by manual control so that the teacher may observe the individual students' displays for different periods. The multi-computer network therefore provides an efficient teaching tool.

However, a disadvantage of known multi-computer networks is that the brightness and quality of the monitor displays vary considerably, depending on the extent of the network, the number of computer stations connected to it, the length of LAN cables, the distaice between each receiving monitor and the computer station from which the video signals are being sent, and other factors. Such unwanted variation in display brightness and quality can have a disturbing effect on the student and detract significantly from the otherwise advantageous teaching qualities of the multi-computer system.

It is an object of the present invention to overcome WO 91/11887 PCT/AU91/00034 3 or at least substantially ameliorate the abovedescribed disadvantage by providing an apparatus and method for transferring video signals within a computer network such that the brightness and quality of the video display at the receiving monitors are maintained at a substantially constant level.

STATEMENT OF THE INVENTION In one broad form, the present invention provides a method of transferring video signals between computer stations in a multi-computer network, wherein said video signals include synchronising signals and colour signals, characterised in that a reference voltage is transmitted during at least some of the synchronising signals and said reference voltage is used by the receiving computer station(s) to control the amplification of the received colour signals so as to provide a substantially constant video display level at the monitor of the receiving computer station(s).

In the preferred embodiment, a reference voltage is derived at the transmitting station and transmitted during the horizontal synchronising pulses of the video display signals.

The reference voltage is sampled and held by each receiving station, and is used to provide a reference level for amplication of the received colour signals. (The reference voltage is removed before the received video display signal is sent to the monitor).

In another form, the present invention provides apparatus for transferring video signals between computer stations in a multi-computer network, wherein the video WO 91/1187 PC/AU9/00034 4 signals comprise synchronising signals and colour signals, said apparatus comprising means for detecting the occurrence of a synchronising signal; means for deriving and transmitting a reference voltage during the detected synchronising signal; means for receiving and retaining the transmitted reference voltage; and means for amplifying the received colour signals in accordance with the retained reference voltage in order to provide a substantially constant video display level.

Typically, the apparatus is embodied as a logic circuit on an adaptor card fitted to each computer to enable the computer to both transmit and receive a video display.

The means for retaining the reference voltage may suitably comprise a sample and hold circuit, but any other equivalent circuit will suffice.

The present invention overcomes the abovedescribed problem of known video display transfer systems since a reference voltage is transmitted with the displays, and this reference voltage is used to enable a constant display level to be achieved at the receiving station irrespective of cable and equipment loadings and other factors influencing the level of the received video signals.

In order that the invention may be more fully understood and put into practice, a preferred embodiment thereof will now be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1, 2 and 3 collectively form a schematic circuit diagram of the video signal transfer control apparatus WO 91/11887 PCT/AU91/00034 of an embodiment of the present invention; Fig. 4 illustrates the timing signals at the station receiving a transmitted video display; and Fig. 5 illustrates the timing signals at the station transmitting the video display.

DESCRIPTION OF PREFERRED EMBODIMENT The video display signals transmitted between computer stations in a network consist of analog colour video signals G, B) representing the red, green and blue primary colours together with horizontal (Hsync) and vertical (Vsync) synchronising signals. In general terms, the method of automatically maintaining constant brightness and quality of the video display according to this invention involves the transmission of a reference voltage to the receiving monitor during a synchronising signal. This reference voltage is retained by the receiving station and is used to provide the correct level of amplification to the received video colour signals. This is achieved by an improved video display transfer circuit, an embodiment of which is shown in Figs. 1 to 3.

The transfer circuit of Figs. 1 to 3 is typically provided on the adaptor card at each computer station which transmits or receives video signals. As illustrated in Fig.

1, each circuit comprises a multiplexer 1 which, in use, 25 receives a select signal ("sel from the control unit (not shown) to cause the multiplexer 1 to select either the Hsync signal transmitted by its own display adaptor card (H-VGA) or the Hsync signal transmitted through the network bus (H-Net).

WO 91/11887 PCT/AU91/00034 6 The waveform of the Hsync signal selected by the multiplexer 1 is shown as signal Sync A in the timing diagrams of Figs. 4 and The output signal sync A of the multiplexer 1 is fed to an exclusive-or gate 2 which provides an output pulse on each positive or negative level change in the sync A signal.

The waveform of the output signal of gate 2 (Exor B) is also shown in Figs. 4 and The Exor B signal pulses output from the exclusiveor gate circuit 2 are fed to a one-shot circuit 3 which acts as a masking circuit. The one-shot circuit 3 provides an output pulse on detecting a pulse from the exclusive-or gate 2, and the length of the output pulse from the one-shot circuit 3 is long enough to mask out the next received pulse from the exclusive-or circuit 2, thereby securing the front edge of the sync A pulse independent of polarity. The output of the one-shot circuit 3 is shown as the wave form "Sync Mask D" in figures 4 and The output signal of the one-shot circuit 3 is fed to a flip-flop 4 which acts as a sample control circuit. The flip-flop 4 provides an outpuc timing pulse C which is fed to the multiplexer circuit 1 and is used to form a Sample signal provided by the multiplexer 1 to a sample and hold circuit 11 via a level translator 10 (Fig. The Sample signal is used to indicate to the sample and hold circuit 11 when the signal from the network is to be sampled, namely, during the synchronising pulse. The wave form of the output signal from the flip-flop 4 is shown as "Sample control FF C" in Fig. 4.

WO 91/11887 PCT/AU91/00034 7 The output of the one-shot circuit 3 is also fed to a reference one-shot circuit 17 which outputs a reference signal pulse "Tx Ref E" whose waveform is shown in Figures 4 and 5. The Tx Ref signal has a pulse length of preferably about 20% more than sync pulse sync A so as to ensure that the reference signal is transmitted for sufficient time to be sampled by all receiving circuits.

The output of the one-shot circuit 3 is also fed to AND gate 5 the output of which controls switches for the analog video signals R, G, B (as described later) to prevent the reception of the video colour signals during the transmission of a reference voltage as well as preventing the transmission of the reference voltage from one station to its own monitor. The AND gate also receives "select own display" or "select network display" signals from the control unit.

During transfer of video display signals, the blue, red and green colour signals are fed from the VGA controller of the transmitting computer station to the network. A switch 6 disconnects the transfer circuit from the blue colour signal B from the VGA controller card of the micro-computer during the Hsync signal, thereby allowing a reference voltage to be sent on the Blue line to the LAN network. That is, the reference voltage is not affected by the blue signal.

The reference voltage is derived from a voltage reference circuit 7 which is switched into the network by a transistor switch 7A controlled by the output signal Tx Ref E of one-shot circuit 17. Thus the reference voltage is transmitted on the blue line during the duration of the pulses WO 91/11887 PCT/AU91/00034

B

on the Tx Ref E waveform.

The reference voltage signal is passed through video buffer circuit 8A and a transmit switch 8B. The transmit switch 8B is controlled by a transmit display signal from the control unit to connect only the transmitting computer station to the network, i.e. the transmit display signal disconnects transmitting portions of all other computer stations from the network so that only one station may transmit at a time.

An analog switch 9 controlled by the "own direct" output signal from AND gate circuit 5 also ensures that the transmitted reference voltage is not fed to the monitor of the transmitting computer station while the reference signal is transmitted to the network during the sync pulse.

When a computer station is receiving a video display from the network bus, the "sample" signal from the multiplexer 1 indicates when the received signal is to be sampled and held so as to capture the reference voltage being transmitted during the sync pulse of the received video signal. As described above, the sample signal from multiplexer 1 is fed to a level translator circuit 10, the output of which is connected to sample and hold circuit 11. During this sample pulse F, as shown in Fig. 4, the sample and hold circuit 11 will sample the received reference voltage by charging a capacitor through FET switches throughout the sample signal, and hold the charged voltage at the negative going edge of the sample signal.

An amplifier 12 is used to amplify the voltage level held by the sample and hold circuit 11, and the amplifier WO 91/11887 PCT/AU91/00034 9 output is then buffered to provide a reference level to the individual video amplifiers for the R, G, and B colour signals. Preferably, the amplifier 12 has gain adjustment and reference adjustment to allow the proper voltage levels for the video amplifiers of the R, G and B colour signals to be pre-set.

The colour video signal received from the network on the blue line (after the sync pulse) is then amplified by amplifier 13. The previously sampled and buffered reference voltage is now used to control a gain control FET switch which acts as a voltage controlled resistance and varies the amplification provided to the blue colour video signal by the amplifier 13 so as to produce a substantially constant level signal.

The blue colour signal output from amplifier 13 is then transmitted to the monitor of the computer station via video buffer Thus, irrespective of the level of the received blue colour signal, the output of the amplifier 13 will be of substantially consistent level as determined by the reference voltage transmitted during the sync pulse. The variable degree of amplification thereby compensates for variations to the video signal, for example, as a result of the network cable loading.

A network analog switch 14, controlled by the "net display" signal from the AND gate circuit 5, is interposed between amplifier 13 and the monitor to prevent the reference voltage transmitted during the sync signals from being WO 91/11887 PCT/AU91/00034 transmitted to the monitor.

The video loic circuits 16 for the red and green colour signals are shown in Fig. 3. These logic circuits are similar to the logic circuit for the blue colour signal, except that there is no need to include circuitry for transmitting, or receiving and capturing, the reference voltage as the output of the gain amplifier 12 in the blue signal logic circuit is used to control the individual colour signal amplifiers in the red and green signal circuits as well.

In use, when video display signals ar transmitted from a particular computer station, a reference voltage is transmitted by that computer station during the synchronising pulses of the video signal. The receiving computer stations use that reference voltage to maintain a substantially constant output display level, irrespective of factors such as cable and equipment loading on the transmitted video signals.

As a result, the displays at all of the computer stations are of zubjtantially constant level and quality.

The foregoing describes only one embodiment of the invention, and modifications which are obvious to those skilled in the art may be made thereto without departing from the scope of the invention as defined in the following claims.

For example, the reference voltage may be transmitted during the vertical synchronising signals rather than the horizontal synchronising signals. Furthermore, the reference voltage may be transmitted on the red or green colour signals rather than the blue colour signal.

WO 91/11887 PCT/AU91/00034 11 The invention may also be used for monochrome video displays, and the term "colour" as used in this specification is to be interpreted accordingly when used in reference to monochrome video signals.

Claims (11)

1. A method of transferring video signals between computer stations in a multi-computer network, wherein the video signals include at least one synchronising signal and at least one colour signal, characterised in that a reference voltage is transmitted during at least part of a synchronising signal and the reference voltage is used by the receiving computer station(s) to control the amplification of the received colour signal(s) so as to provide a substantially constant video display level at the monitor of the receiving computer station(s).

2. A method as claimed in claim 1, wherein the reference voltage ia transmitted during either the horizontal or vertical synchronising pulses for a particular colour signal.

3. A method as claimed in claim 1, wherein the timing of the transmission of the reference voltage is controlled by a controller station of the network.

4. Apparatus for transferring vidso signals between computer stations in a multicomputer network, wherein the video signals comprise at least one synchro*.ising signal and at least one colour signal, said apparatus comprising meaus for transmitting from a computer station, a reference voltage during at least part of a synchronising signal, and means at each receiving computer station for receiving the reference voltage and using same to control the amplification of the received colour signal(s) ao as to provide a substantially constant video display level at the monitor of the receiving 'VO 91/11887 PCT/AU91/O0034 13 computer station(s). Apparatus as claimed in claim 4, wherein the transmitting means includes means for detecting the occurrence of one of either a horizontal or vertical synchronising signal, and means for deriving and transmitting a reference vol'age during the dttected synchronising signal.

6. Apparatus as claimed in claim 4, further comprising means for inhibiting the transmission of video signals to the network from other computer stations while a particular computer station is transmitting video signals to the network.

7. Apparatus as claimed in claim 4, further comprising means for inhibiting the transmission of the reference voltage from a computer station to its own display monitor.

8. Apparatus as claimed in claim 4, wherein the receiving means comprises means for retaining the received reference voltage, and means for amplifying received colour signals in accordance with the retained reference voltage.

9. Apparatus as claimed in claim 8, wherein the retaining means comprises a sample and hold circuit. Apparatus as claimed in claim 8, wherein the amplifying means cc-prises an amplifier, a voltage control resistance connected to the amplifier for varying the gain thereof, the voltage control resistance being connected to the received reference voltage.

11. Apparatus as clzimed in claim 4, wherein the apparatus is mounted on an adaptor card fitted to each computer station.

12. Apparatus as claimed in claim 4, further comprising WO 91/11887 PCT/AU91/00034 14 a control unit for controlling the timing of the transmitting and receiving means, the control unit being connected to one of the computer stations.

13. A multicomputer network comprising the apparatus of claim 4.