GB2250162A

GB2250162A - Automatic line equaliser

Info

Publication number: GB2250162A
Application number: GB9117133A
Authority: GB
Inventors: Owen Thomas Drumm
Original assignee: OSABA Ltd
Current assignee: OSABA Ltd
Priority date: 1990-09-24
Filing date: 1991-08-08
Publication date: 1992-05-27
Anticipated expiration: 2011-08-08
Also published as: GB2250162B; GB9117133D0; IE68688B1

Abstract

Apparatus and a method for optimising transmission of a signal between a source and a remote receiving point e.g. a video monitor connected by a transmission line. The apparatus may include an encoder which monitors an input signal, generates a test signal and adds it to the input signal, and a decoder which may include a synchronisation pulse detector 19, a test signal generator 20, a subtractor 21 and analyser 23 components. The components of the decoder in combination detect any change in the quality of the transmitted signal and make suitable adjustments to restore the transmitted signal to the quality of the input signal. <IMAGE>

Description

AUTOMATIC LINE EQUALISER This invention relates to apparatus and a method for optimising transmission of a signal between a source and a remote receiving point connected by a transmission line.

In a transmission line, such as a cable, canying a signal, variations in the line will cause "distortion". These variations can result from manufacturing flaws, subsequent damage or environmental factors such as electrical fields. When a signal travelling along a cable encounters a variation, some of energy from the signal is "reflected" back along the cable towards the source.

Another source of distortion can be the actual device being driven which may also cause some of the signal to be reflected back along the cable.

In cases where the source, the device, for example a monitor, and the transmission cable have been deliberately and properly set up, the device itself will not cause distortion. In such a case, the device is said to be "matched" to the transmission line. The device which receives the signal at the end of the transmission line must appear to the signal to be a continuation of the cable to avoid reflection at this point. However, in certain situations the device may be driven by a number of sources at different times, and in such circumstances it can never be matched with all of them.

Front Office Systems for the financial industry are used for example in dealing rooms in banks where up-to-the-minute information on currency values around the world is required. Such systems include computer based systems which provide access to external information via a PC and associated monitor at a dealer's desk or via a monitor connected by a cable link to an equipment room at a point distal from the dealer's work station. One such system is a completely softwarebased system marketed by the Applicants under the Trade Mark InVision. The latter system resides on a Local Area Network and as such can handle digital information only.

There is a need for Front Office Systems which can process nondigital information. Many of the financial information services such as Reuters, Telerate and Topic provide their information in a non-digital format. Thus Front Office Systems for the dissemination of information to a number of dealers in a financial institution desirably are capable of handling analogue services as well as digital services.

One way in which information is received from the Reuters network is by way of a telephone line, typically by way of a stream of tones through a modem, and thence to a Reuters box' located in a backroom or equipment room of a bank or broking house. The Reuters' 'box' generates a video signal based on the information received.

Information from other similar sources is received in essentially the same way. A video switch or key press operation may be used by a dealer to select which source is to be displayed on his video monitor. It will be appreciated that even in a Local Area Network system the hardware at individual work stations may differ perceptibly which will affect the interaction of such hardware with external information sources.

Reuters also provide a digital service and dealers will commonly wish to switch between the two different Reuters services and the other services mentioned above To disseminate received information within financial institutions through a system such as that marketed under the Trade Mark InVision, the information is transmitted through co-axial cable which connects the various components of the system.

Normally, a high quality co-axial cable, of uniform construction along its length, is used and there is little reflected energy. However, even if high quality cable is used, a mis-matched device at the end of the cable will cause reflections and distortion of the signal. In a system which is intended to deliver high quality signals over a transmission line, matching is critical and the termination devices (i.e., matching networks) should ideally be individually adjusted to match the cables and sources to which they are connected. This is not usually practical and a small amount of distortion will be found even in very expensive installations.

The Applicants have found that in transmitting a video signal there is a serious degradation in the quality of the image with progressive length of standard specification co-axial cable. Such degradation of the video signal manifests itself as ghosting, shadowing and poor definition of characters, as the case may be. To overcome this problem one can use fully shielded co-axial cables such as camera cable.

However, the cost of, and difficulty of, handling shielded cable is not a practical and realistic solution to the problem.

Hence there is a need for a system that would allow one to send a signal over a long length of standard specification cable, while maintaining the quality of the signal.

To overcome the requirement for tedious manual matching of transmission lines, the termination device on a monitor for example could be constructed to automatically and continually optimise the match to its cable. In its simplest form, this can be achieved by maximising received signal strength, by adjusting the internal matching network of the termination device: since reflections consist of energy taken from the original signal, maximum signal strength means minimum reflected energy and so, optimum matching. Such a simple automatic matching system may suffice for some applications, but is not adequate for transmission lines which carry complex signals or in networks which contain junctions and taps onto the cable. In such networks, a reflection may itself be partially reflected at a cable junction and thus arrive at the device slightly late.This delayed signal may contribute to the main signal being received and so it will be seen by the simple automatic matching system as being stronger than it actually is.

Accordingly, the invention provides apparatus for optimising transmission of a signal between a source and a remote receiving point connected by a transmission line which comprises: means for selecting a component of a received signal to serve as a test signal or superimposing a test signal on a component of said received signal; means for optionally generating a further and identical test signal at said remote receiving point; a matching network integral with said remote receiving point and having one or more variable component(s); means for observing any difference between the respective test signals; and means for adjusting the or each variable component to compensate for any observed difference.

The invention also provides a method for optimising transmission of a signal between a source and a remote receiving point connected by a transmission line, which method comprises: a) selecting a component of a received signal to serve as a test signal or superimposing a test signal on a component of said received signal; b) optionally generating a further and identical test signal at said remote receiving point; c) observing any difference between the respective test signals; and d) adjusting one or more variable component(s) in a matching network integral with said remote receiving point to compensate for any observed difference.

The apparatus and method in accordance with the invention overcome the problems of reflections in a transmission line hereinbefore mentioned by providing for automatic determination and correction of any mis-match.

The introduction of a test signal on the signal to be transmitted to the remote receiving point is optional. Thus if the normal signal contains a suitable equivalent, such as when the signal is in the form of a digital data stream, the shape of the received data bits can be examined for determining any degradation of the transmitted signal, assuming that the shape of the transmitted bits is consistent and known.

In video transmission cables, the pulses used for synchronisation (synchronisation pulses) could serve as the test signal. There is a specification which defines what a synchronisation pulse should look like within accepted tolerances. However, a synchronisation pulse is normally a low frequency signal, whereas a video signal will exhibit large changes of voltage. Accordingly, it is preferable to superimpose a test signal on the synchronisation pulse of said video signal. If choosing a test signal to superimpose on a synchronisation pulse one should ensure that the test signal contains a broad range of frequency signals from high to low, especially high frequency signals. Thus the test signal should basically simulate the video signal by having the same frequency content as said video signal.If the apparatus is adjusted to perform well at the high and low ends of the frequency range, one can generally interpolate the intervening frequencies. An identical test signal may be generated at the remote receiving end or point for comparison with that sent over the line. By observing the difference between the two test signals it is possible to determine which variable component(s) of the matching network need(s) adjustment. This adjustment can be made automatically and the results verified if necessary by looking for the expected improvement in the next burst of test signal.

Preferably, the adjustment is achieved by means of a closed loop feedback system.

However, various types of adjustment systems may be used which constantly and continuously seek to minimise distortions and optimise the signal quality received. Such systems can include electronic circuitry specific to a given application, whereby an adjustment is made at any given time and the adjustment verified. Verification involves the system 'looking' for an improvement or deterioration. If an improvement is observed the system will continue to adjust in that direction by adjusting the appropriate parameters of the matching network. As an example, if, in a start-up situation, the impedance of a standard cable (75 ohms) has been reduced to 70 ohms by variation and the internal matching network of the receiving point is at 80 ohms, the adjustment system can be programmed to adjust the resistance of the matching network in stepwise fashion until it finds an improvement.Thus if a first adjustment involves changing the resistance of the matching network to 79 ohms and the system 'finds' an improvement, it will continue to decrease the resistance as long as it 'sees' an improvement. As the adjustment passes 69 ohms a marginal disimprovement is detected which will result in an adjustment back to 70 ohms until no further change is observed. In fact, the system will be programmed to make continuous fractional changes. The system will continuously hunt for a local minimum value and, thereby, ensure that distortion effects are minimised at any given time.

The systems used for adjusting the matching network will include both adjustment and verification functions. However, the systems will be basically of two types: i) an adjustment system which operates by making incremental adjustments on a hit or miss basis followed by verification and ii) an adjustment system which makes calculated adjustments followed by verification.

Thus the apparatus in accordance with the invention by continuous observation and adjustment can compensate for variations in a transmission line caused by factors such as changes in temperature which vary the dimensions of said transmission line, which would otherwise require manual re-matching. This is of particular benefit in switched systems where a single device such as a video monitor is switched between several sources. The inevitable differences between the cables from each of the sources means that there is no one fixed setting of the matching network integral with the monitor which will match all of the cables.

The automatically adjustable matching network of the apparatus according to the invention is instantly adjustable on being switched from one transmission line to another. The end result is uncompromised performance regardless of which source is selected. The automatically adjustable matching network of the apparatus according to the invention is located upstream of the standard fixed matching network located in the connector to the video monitor or other device which displays the adjusted signal. Such a fixed matching network, which is a standard component of a video monitor and other units connected to a transmission line, includes circuitry which simulates in the connector the environment of the transmission line and prevents reflection of the signal back down the line as it arrives at the termination device.

The apparatus according to the invention will minimise signal distortion due to any junction(s) and/or tap(s) in the transmission line and will thus optimise received signal quality.

The adjustable matching network of the apparatus according to the invention may have a single variable component, more particularly a variable resistor, the capacitor and inductor being fixed. However, for optimum performance all three components should be variable.

The cable used as the transmission line in accordance with the invention may be a standard co-axial cable with a characteristic impedance of 75 ohms. By using the apparatus and method in accordance with the invention it is possible to assume the acceptability of such a cable without the need for testing the cable to verify its impedance properties. Thus there is a relaxation on the precision requirements for cable used in accordance with the invention.

The means for superimposing a test signal on a component of the received signal preferably forms part of an encoder unit. The remaining elements of the apparatus according to the invention preferably form a part of a decoder and adjustment unit.

Thus advantages of the apparatus and method according to the invention include better quality images, cost savings in terms of both installation and cable costs and saving in floor duct space.

The invention will be further illustrated with reference to the following description of a preferred embodiment thereof with reference to the accompanying Drawings in which: Fig. 1 is a schematic representation of apparatus according to the invention; Fig. 2 is a schematic representation of an encoder unit forming part of the apparatus of Fig. 1; and Fig. 3 is a schematic representation of a decoder unit forming part of the apparatus of Fig. 1.

Referring to Fig. 1 of the Drawings, there is indicated generally at 10, apparatus in accordance with the invention for optimising transmission of a video signal, indicated by the sequence of arrows, between a source and a termination device 11 which is a video monitor on a dealer's desk in a bank or broking house. The apparatus 10 comprises an encoder unit 12 and a decoder and adjustment unit 13 connected by a transmission line 14 of standard co-axial cable with a characteristic impedance of 75 ohms.

Encoder unit 12 (see Fig. 2) monitors the video signal received from an external source to detect the synchronisation pulse by means of a synchronisation pulse detector 15. The output from the synchronisation pulse detector 15 is a trigger pulse to start a test signal generator 16. A test signal is generated by the test signal generator 16 and the test signal is superimposed on the synchronisation pulse by means of an adder 17. The resultant modified signal is transmitted down the transmission line 14 to the decoder and adjustment unit 13.

The decoder unit 13 (see Fig. 3), which includes a matching network 18, scans the signal for the synchronisation pulse by means of a synchronisation pulse detector 19. A test signal identical to the test signal generated by the encoder unit 12, is generated locally by means of a test signal generator 20 and subtracted from the received modified signal by means of a subtractor 21. The output from the synchronisation pulse detector 19 is a trigger pulse to start the test signal generator 20 and the output of the latter generator 20 is a test signal identical to that superimposed on the incoming signal in the encoder unit 12. Feedback path 22 is a difference signal received by an analyser 23. The output of the analyser 23 is the corrective adjustment to the matching network 18. Continuous and constant adjustment of the matching network 18 is achieved by means of the closed loop feedback system, resulting in minimal distortion and optimum signal quality on the video monitor 11.

Claims

CLAIMS:

1. Apparatus for optimising transmission of a signal between a source and a remote receiving point connected by a transmission line which comprises: means for selecting a component of a received signal to serve as a test signal or superimposing a test signal on a component of said received signal; means for optionally generating a further and identical test signal at said remote receiving point; a matching network integral with said remote receiving point and having one or more variable component(s); means for observing any difference between the respective test signals; and means for adjusting the or each variable component to compensate for any observed difference.

2. Apparatus according to Claim 1, wherein the signal to be transmitted includes a video signal and a test signal is superimposed on the synchronisation pulse of the received signal.

3. Apparatus according to Claim 1 or 2, wherein the transmission line includes one or more junction(s) and/or tap(s).

4. Apparatus according to any preceding claim, wherein the transmission line consists of standard co-axial cable with a characteristic impedance of 75 ohms.

5. Apparatus according to any preceding claim, wherein the matching network has a single variable component.

6. Apparatus according to Claim 5, wherein the single variable component is a resistor.

7. Apparatus according to any one of Claims 1-4, wherein all of the components of the matching network includes a plurality of variable components.

8. Apparatus according to any preceding claim, wherein the adjustment means is a closed loop feedback system.

9. A method for optimising transmission of a signal between a source and a remote receiving point connected by a transmission line, which method comprises: a) selecting a component of a received signal to serve as a test signal or superimposing a test signal on a component of said received signal; b) optionally generating a further and identical test signal at said remote receiving point; c) observing any difference between the respective test signals; and d) adjusting one or more variable component(s) in a matching network integral with said remote receiving point to compensate for any observed difference.

10. Apparatus according to Claim 1 for optimising transmission of a signal between a source and a remote receiving point connected by a transmission line, substantially as hereinbefore described with particular reference to the accompanying Drawings.

11. A method according to Claim 9 for optimising transmission of a signal between a source and a remote receiving point connected by a transmission line, substantially as hereinbefore described.