GB2429868A

GB2429868A - Video display with automatic colour component amplitude adjustment

Info

Publication number: GB2429868A
Application number: GB0616762A
Authority: GB
Inventors: Alex Kidger
Original assignee: Amulet Electronics Ltd
Current assignee: Amulet Electronics Ltd
Priority date: 2005-09-03
Filing date: 2006-08-24
Publication date: 2007-03-07
Anticipated expiration: 2026-08-24
Also published as: GB0616762D0; GB2429868B

Abstract

A video source 1 at a transmitting end is connected to a remote display 5 at a receiving end via a cable system 3 carrying the separate RGB components of a colour video signal. The video signal path incorporates equalisation amplifiers 10 for adjusting the relative amplitude and d.c. offset of the three components. At the receiving end, receive controller 8 signals the transmitting end, via an additional communication channel, to generate a rectangular test signal which is multiplexed with all three components of the video signal. The receive controller is arranged to monitor the amplitudes and black levels of the RGB components during transmission of the test signal and generates signals to adjust the equalisation amplifiers. This ensures compensation is applied to the individual colour components to achieve amplitude equalisation.

Description

Amulet Electronics Limited

VIDEO DISPLAY SYSTEM

TECHNICAL FIELD OF THE INVENTION

This invention relates to video display systems in which the RGB components of a colour video signal are sent along a cable to a remote display.

BACKGROUND

In computer systems, colour video signals can be sent over long cables which carry the RGB (red, green and blue) components separately using three separate channels. A common cabling medium is so-called "twisted pair' cable such as CAT5 or CAT6 (Category 5 or Category 6) as used in Ethernet networks. Such cable conveniently contains four pairs of wires, so that three of the pairs can be used for red, green and blue components. Whilst this medium is relatively cheap and widespread, it imposes two significant problems, namely attenuation and delay.

Attenuation reduces the amplitude of the signal. Higher frequencies generally suffer greater attenuation so that the signal does not merely get smaller but is also distorted. In a black-and-white image for example, a sudden change from black to white or vice versa is a high-frequency edge. Cable attenuation softens these edges so that the received image looks smeared.

Delay in the cable depends on the cable construction. CAT5 and CAT6 Ethernet cable, for example, give different delays on different pairs, so that red, green and blue components arrive at the far end at different times and are skewed relative to each other. In a black-and-white image, a black-to-white edge may be displayed as black-green-yellow-white, for

example.

In addition, the imperfect nature of the electronic circuits used at each end of the cable means that a third factor may need to be taken into account.

DC offsets in circuits and components used in the system can result in the zero-signal level being above or below the true circuit ground or zero-volt potential. This can result in errors in the signal levels and a poor quality video display.

To combat these problems, manufacturers have used equalisation amplifiers and delay lines to correct the shape and alignment of the video signal components. This has been done using discrete components, and although integrated chipsets are now available for the purpose it is still necessary to determine the best settings for these elements in a particular installation. The usual method of doing this is to observe the resulting video display and manually adjust the settings to achieve a displayed image which looks acceptable. However, this is time consuming and subjective, requiring the application of skill and judgement in assessing which of the three RGB channels to adjust, and whether to change the amplitude or delay, or both. Furthermore, significant alterations to the signal path may require the setting up procedure to be repeated.

The present invention seeks to provide a new and inventive form of remote video display system which eliminates subjective adjustments in the setting up process.

SUMMARY OF THE INVENTION

The present invention proposes a video display system in which a transmitting end is connected to a remote display at a receiving end via a cable medium carrying separate RGB components of a colour video signal, and the video signal path incorporates compensation means for adjusting the amplitude of the three components, in which the transmitting end is arranged to send a video test signal along the cable medium, and the receiving end includes a receive controller which is arranged to monitor the amplitudes of the RGB components during transmission of the test signal and control the compensation means to equalise the amplitudes of the three components.

An additional communication channel may be used to carry control signals between the receive end and the transmitting end.

The video test signal preferably comprises three substantially identical waveforms which are sent along the respective RGB channels.

Preferably, the video test signal comprises an edge which is transmitted simultaneously in all three RGB channels, and the receiving end includes means for detecting the said edges in each of the three received channels. The receive controller preferably measures the signal levels in all three of the RGB channels at corresponding positions in the received video test signal and adjusts the compensation means to vary the frequency dependent and frequency independent gain in the three RGB channels.

The signal levels may also be measured in a black portion of the video test signal and said measured signal level used to adjust the compensation means to set the d.c. offset level of the three RGB channels.

The three channels may be adjusted together or independently of each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Ftgure I is a schematic diagram of a video display system which incorporates signal compensation in accordance with the invention; Figure 2 is a schematic diagram showing one implementation of a test pattern generator which may be included in the system; Fiqure 3 is a waveform diagram showing three possible signal equalisation settings which may be obtained using a typical rectangular test signal; FiQure 4 is another waveform diagram showing typical measurement points used in the signal equalisation process; Figure 5 is a block diagram showing one implementation of a signal level measurement stage used in the video compensation system; Figure 6 is a block diagram showing one implementation of a d.c. offset compensation stage which is included in the video compensation system; Figure 7 is a further waveform diagram showing two possible skew adjustment settings which may be obtained using a typical rectangular test signal; and Figure 8 is a block diagram showing one implementation of a skew measurement stage used in the video compensation system.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring firstly to Fig. 1, a video source 1 such as a computer is connected via a transmitter 2 to a long distance transmission medium such as CAT5 or CAT 6 Ethernet cable 3. The far end of the cable is connected to a receiver 4, which is in turn connected to a video display unit 5 such as a computer monitor.

It should be noted that the red, green and blue (RGB) components of the video travel separately along mutually discrete pathways, but for convenience of illustration only a single video pathway is generally shown in the drawings. Thus, the three RGB components travel along separate communication channels 6, which may comprise separate twisted pairs, within the cable 3. The transmitter and receiver, 2 and 4, contain respective controllers 7 and 8 which are arranged to communicate with each other over a fourth channel 9 within the cable 3. Each of the RGB channels 6 includes a variable gain amplifier 10 to compensate for cable attenuation and d.c. offset, and a delay line 11 to compensate for skew introduced by the cable 3. In the drawing the amplifiers 10 and delay lines 11 are included in the receiver, but either or both of these elements could be provided in the transmitter, or indeed they could each be partly in the transmitter and partly in the receiver, as circumstances require.

Upon arriving at the transmitter 2 the incoming video from the computer 1 is fed through a black-and-white test pattern generator 12, one implementation of which is shown in Fig. 2. This stage includes a rectangular wave generator 13 and a switching unit 14 which is operated by the transmit controller 7. The test pattern produced by the generator 13 is fed to a multiplexer 15, together with the normal incoming RGB video. The switching unit 14 operates the multiplexer to combine each of the three channels of the video input with a rectangular test waveform produced by the generator 13. When the rectangular waveform is low the multiplexer generates an output signal level corresponding to black, and when the waveform is high a signal level corresponding to white is generated (i.e. equal maximum signal levels for all three red, green and blue components). The test pattern may occupy the entire video image for the duration of a setting-up period. The rectangular waveform may or may not be synchronised to the timing of the normal video input. This depends on the particular circumstances of each application, but does not materially alter the operation of the compensation equipment.

To set up the equalisation amplifiers 10 and the skew correction delay lines 11, the receive controller 8 requests the transmitting controller 7 to temporarily generate and send the black-and-white test pattern, such that the red, green and blue signal components passing along the cable 3 switch repeatedly and simultaneously from the black level (minimum amplitude) to the white level (maximum amplitude). The receive controller 8 then examines the signals at the output of the receiver 4 and adjusts the equalisation amplifiers and the delay lines to optimise the signals as described below. When the adjustment is complete, the receive controller requests the transmitting controller to revert to the normal video source.

It is also possible to inset the test pattern into the video image so that the test pattern could be transmitted without disruption to the normal operation of the equipment. In fact, the test pattern could be transmitted continuously, inset into an inconspicuous part of the image An alternative method of generating the test pattern, which may be used where the video source I is a computer, is for the transmit controller 7 to signal the video source itself to generate a black-and-white test pattern.

No multiplexer or rectangular wave generator is required in this case, but a control connection to the video source is still necessary. The requirement for such a control connection could be removed altogether, at the expense of increased complexity in the receiving equipment, by arranging the receive end controller to automatically detect video test patterns generated by the video source.

The receive controller 8 sends control signals to each of the RGB equalisation amplifiers 10 to adjust the frequency-dependent gain (to compensate for the frequency-dependent part of the cable attenuation), the frequency-independent gain (to compensate for steady-state losses in the cable), and its d.c. offset. By way of illustration, the waveforms of test signals (R, G or B channel) that are over-compensated, under- compensated, and correctly compensated are shown in Fig. 3. Referring to Fig. 4, the receive controller 8 tests the levels of each of the red, green and blue signal components just after their rising edges, at time ti, at a predetermined distance from the rising edges, at time t2, and again following their falling edges, at time t3 within the black portion of the rectangular test signal. By comparing the amplitudes of the signals at each of these sampling times with predetermined target reference levels, indicated by the dashed lines in the drawing, the controller can adjust the gains and d.c. levels of the compensation amplifiers for the best response. The levels measured at ti, t2 and t3 are affected as follows.

The level measured at ti is dependent on the d.c. offset, the frequencyindependent gain and frequency-dependent gain of the amplifier, whilst that measured at t2 is affected by the d.c. offset and frequencyindependent gain. The level measured at t3 is solely dependent on the d.c. offset.

Fig. 5 shows an implementation of a signal level measurement stage which is incorporated in the receiver 4. The video input signal is the blackand-white test pattern, comprised of the three signal components, red, green and blue. Each component is applied to one of three identical circuits, only one of which is shown in the drawing. The controller 8 determines that it wishes to measure the level of the video signals at a time T after the rising edge of the input waveform (which might be ti, t2 or t3 shown in the previous drawing or such other time as might be necessary). The controller applies this time setting T to a timing generator 18. The rising edge of each video component triggers the timing generator 18, and after the set period T has elapsed a strobe signal is generated. Where the desired measurement point occurs after the falling edge of the waveform, such as point t3, the timing generator 18 may instead be triggered by the falling edge of the waveform. The level of the video signal is continuously converted to a digital representation by means of an analogue-to-digital converter 19. The strobe signal controls a latch 20 to capture the digital value of the signal at the desired measurement point, and informs the controller 8 that the measurement is complete so that the controller can read the captured digital value from the latch 20.

Fig. 6 shows a block diagram of one implementation of the dc offset compensation stage as incorporated in the receiver 4. The controller 8 adjusts a current source 21 so that a current I is generated, flowing through the resistor Ri into amplifier 10. This generates a dc voltage V = I x R which is added to the ac video signal, thus compensating for any dc offset which is present. As noted earlier, the d.c. offset compensation technique may equally well be applied in the transmitter 2.

Skew adjustment is achieved as follows. While the black-and-white test pattern is being sent, the receive controller 8 compares the relative timing of the rising edges of the red, green and blue components of the video signal. As shown in Fig. 7, the controller adjusts the setting of the respective delay lines Ii for the three signal components until all three edges line up with each other, under which condition the skew adjustment is correctly set.

Fig. 8 shows an implementation of the skew measurement system used in the receiver 4. The video input signal is again the black-and-white test pattern. The black level is taken as digital logic 0, and peak white level is taken to be digital logic 1. Each of the video signal components is latched in a respective flip-flop, 22r, 22g and 22b, by the rising edge of one of the other colour components. With the example timing shown in the drawing, the rising edge of the green signal latches a logic 0 in flip-flop 22r, the rising edge of the blue signal latches a logic I in flip-flop 22g, and the rising edge of the red signal latches a logic 0 in flip flop 22b. Thus the receive controller 8, using a simple algorithm, can adjust the delays added to the three signal components to align them. When the three components are aligned, all three flip-flops will contain the same logic level, either three Os or three is depending on the particular flip-flops used.

In an alternative implementation of the level measurement system described above, the analogue to digital converters 19 may be replaced by a single reference voltage derived by filtering a pulse-width-modulated signal, and three comparators (one for each of the red, green and blue signal components). The level of each signal component is measured by comparing it against the reference voltage, the latter being adjusted by the receive controller 8 as required. This avoids the need for fast multi- bit analogue-to-digital converters. The reference voltage could also be derived by a number of other digital-to-analogue conversion techniques.

In another implementation of the level measurement technique, a single timing generator may be triggered from rising or falling edges of the red signal component such that the red, green and blue signal component levels are measured at the same instant. This requires that the skew adjustment has been done first. In another possible implementation, each of the red, green and blue signal components can have their own independent timing generators with independent time settings.

Other implementations of the level measurement system that achieve -11 similar results include (a) analogue sample-and-hold techniques in which the level is latched in the analogue domain instead of in the digital domain, (b) continuous sampling techniques in which many level measurements are made and stored and the sample of interest is chosen from this array, and (C) analogue storage techniques such as chargecoupled-devices in which many samples are stored in the analogue domain, each converted in turn to digital values at a later time.

Other implementations of the skew measurement system include controllers that time the three signal components using an independent clock, common to all three. The flip-flop method described above is an economical method that avoids the need for a very fast common clock sampling system.

It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.

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Claims

1. A video display system in which a transmitting end is connected to a remote display at a receiving end via a cable medium carrying separate RGB components of a colour video signal, and the video signal path incorporates compensation means for adjusting the amplitude of the three components, in which the transmitting end is arranged to send a video test signal along the cable medium, and the receiving end includes a receive controller which is arranged to monitor the amplitudes of the RGB components during transmission of the test signal and control the compensation means to equalise the amplitudes of the three components.

2. A video display system according to Claim I in which the video test signal comprises an edge which is transmitted simultaneously in all three RGB channels, and the receiving end includes means for detecting the said edges in each of the three received channels.

3. A video display system according to Claim 2 which includes means for measuring the signal levels in all three of the RGB channels at substantially the same positions in the received video test signal following said edge.

4. A video display system according to Claim 3 in which the each of the measured signal levels is compared with a reference level and used to adjust the compensation means to equalise the amplitudes of the three RGB channels.

5. A video display system according to Claim 4 in which the signal levels are measured at two or more spaced positions in each of the RGB channels following said edge.

6. A video display system according to Claim 5 in which said measured signal levels are used to adjust the compensation means to vary the frequency dependent gain in the three RGB channels.

7. A video display system according to Claim 5 or 6 in which said measured signal levels are used to adjust the compensation means to vary the frequency-independent gain in the three RGB channels.

8. A video display system according to Claim 5, 6 or 7 in which one or more first measured signal levels is used to adjust the compensation means to vary the frequency dependent gain in the three RGB channels and one or more further measured signal levels, following said one or more first measured signal levels, is used to adjust the frequency-independent gain.

9. A video display system according to any of Claims 3 to 8 in which the signal levels are measured in a black portion of the video test signal and said measured signal level is used to adjust the compensation means to set the d.c. offset level of the three RGB channels.

10. A video display system according to Claim 9 in which the measured black signal levels are used to adjust a current source to vary the current flowing through an amplifier within each of the RGB channels.

11. A video display system according to any of Claims 3 to 10 in which said means for measuring the signal levels includes, for each of the RGB channels, sampling means for sampling the signal level in the respective channel, timing means responsive to said edge to commence a timing period and generate a timing signal when the timing period has elapsed, and means for capturing the signal level from the sampling means upon the occurrence of said timing signal.

12. A video display system according to any of Claims 3 to 10 in which said means for measuring the signal levels includes, for each of the RGB channels, an analogue-to-digital converter which is arranged to convert the waveform of the respective channel to a corresponding digital value, a timer which is triggered by a rising or falling edge of the waveform in the respective channel and generates a strobe signal after a timing period has elapsed, and respective latching means which is responsive to said strobe signal to capture the digital value of the respective channel from the analogue-to-digital converter.

13. A video display system according to any of Claims 3 to 10 in which said means for measuring the signal levels includes a timer which is triggered by a rising or falling edge of the waveform in one of the RGB channels and which generates a strobe signal after a timing period has elapsed, and, for each of the RGB channels, a respective analogue-todigital converter which is arranged to convert the waveform of the respective channel to a corresponding digital value, and latching means which is responsive to said strobe signal to capture the digital values of all three of the RGB channels.

14. A video display system according to Claim 12 or 13 in which the receive controller determines the timing period of the or each timer and reads the digital values from the latching means when the measurement is complete.

15. A video display system according to any of Claims 3 to 10 in which said means for measuring the signal levels includes, for each of the RGB channels, a comparator which is arranged to compare the waveform of the respective channel with a reference level.

16. A video display system according to Claim 15 in which the said reference level is determined by the receive controller.

17. A video display system according to any preceding claim in which the compensation means is arranged to adjust the relative timing of the three components and the receive controller is arranged to monitor the relative timing of the RGB components during transmission of the test signal and control the compensation means to align the relative timing of the three components.

18. A video display system according to Claim 17 in which, for aligning the relative timing of the three components, the compensation means includes respective delay means for each of the three RGB components, the receiving end includes a respective edge detector for each of the RGB components which latches the respective signal level in response to the occurrence of said edge in one of the other two components, and the receive controller adjusts the delay means until the three latched levels correspond.

19. A video display system according to any preceding claim in which the receive controller is arranged to send control signals to the transmitting end.

20. A video display system according to Claim 19 in which the control signals travel via an additional communication channel.

21. A video display system according to Claim 19 or 20 in which the transmitting end includes a waveform generator which is operated by the control signals from the receive controller.

22. A video display system according to Claim 21 in which the transmitting end includes a multiplexer which combines the video test signal produced by the waveform generator with a video signal which is generated at the transmitting end.

23. A video display system substantially as described with reference to the drawings.

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