L O N D O N A R E
N A - C O N S U L T A N C Y
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CENTRE STAGE |
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The completed gondola being lifted into the roof, where it was attached to a purpose-built platform with catwalks (44k). |
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Initial assembly of the gondola steelwork with 'triad' revolving advertising boards. |
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After constructing a decision matrix, it became apparent that the original proposals could not be compared in any meaningful way. While each document appeared to be very complete (averaging 50 pages), and the quoted prices were within a reasonable margin of one another, there was no further basis for comparison. Each video screen was a different size and resolution; one vendor was providing a turn-key installation, while another was providing supervisory personnel for the owner's own labour force; yet another included modest video source equipment, while another included none. We decided that the only fair and reasonable way to evaluate these vendors would be to produce a detailed specification for the project, and to have them requote on this basis. The spec would, then, perform four basic functions: |
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owners of the London Arena had already received several proposals to
supply and install a centre-hung, eight-sided scoreboard with jumbo
video screens at the point that we were hired as technical consultants.
Our role was to evaluate these proposals and to make a recommendation.
There are a number of essential points in the evaluation process which
will be common in many such projects.
Because of this specification process, we received a very different set of bids, and proceeded to evaluate them on a level playing field. Of the three components involved (the video screens, the scoreboard, and the gondola structure), the jumbo video displays are the most difficult to evaluate - they also represent the majority of the owner's investment. During this evaluation process, we identified several important issues involved in selecting a jumbo video display. Display Technology
Apart from this consideration, we selected LED technology due to weight and power restrictions. The roof structure in the arena would only allow 10,000kg for the total system - four CRT video displays alone would have weighed this much. CRT technology was limited to two principal vendors due to the complexity of the manufacturing process. LED technology is much more accessible - resulting in many more players. This is unfortunate in a sense, since this accessibility can result in some poorly designed product and some short-term manufacturers who may sell only a few systems before vanishing. The key is to select a manufacturer with a record of accomplishment, and who is committed to the industry. You do not want to wind up owning an unsupported 'orphan'. Video Screen Resolution
Resolution, simply stated, is the total number of vertical and horizontal pixels (dots that form the picture). The video signal that the screen will be reproducing has a native resolution of about 580/485 (PAL/NTSC) vertically and anywhere from about 240 to 700 horizontally (depending on the quality of the source). Thus, to reproduce these signals with no loss of image quality, you want an ideal screen resolution of about 580 x 773 (PAL) or 485 x 645 (NTSC). Unfortunately, we do not live in an ideal world. Jumbo video screens at these resolutions are certainly possible, but are not generally economically feasible and may pose physical problems (the display may be too large). Let us look at an example; suppose you have two video screens which each measure 3m high x 4m wide. One has a 12mm pixel pitch and the other 18mm - pixel pitch is the centre-to-centre distance between pixels on the display. The first screen will have 250 pixels vertically (3000/12) and 333 horizontally (4000/12), or a resolution of 250 x 333. The second screen will have a resolution of 166 x 222. Although both of these fall far short of our ideal resolution (particularly for a PAL image), the first is clearly preferable - it will, of course, also be more expensive. The choice of screen resolution is dictated by any physical size constraints you may have, coupled with the viewing distance, and, of course, your budget.
The eight-sided scoreboard
with Video Processing
The first process that must occur is de-interlacing (do I sense your eyes glazing over?). A video image is made up of a number of horizontally scanned lines - the visible lines on a PAL video number about 580. These do not all appear on a television screen at the same time. In the first 1/50 of a second the 'odd' lines are shown (the first field), and in the second 1/50s the 'even' lines are shown. This is called an interlaced display, and everyone's television works this way. This is not the best way to display an image, and most displays that do not need to pick up a broadcast signal use a non-interlaced system whereby everything is shown at once. To convert from an interlaced to a non-interlaced signal is not as straightforward as you might think. The simplest way to do this is to take the first set of lines (field), double it up, and show it - ignoring the second field. There are some video processors that do just this - they throw away half of the original picture resolution. A better approach is to store the first field in memory, combine it with the second field when it comes along, and then display a complete frame. While this is a superior approach, there is, of course, a problem with it. Each field represents a 1/50 of a second snapshot. If an object was moving rapidly it may be in a different position in the odd field than in the even field - if you have ever paused your VCR and seen a person's waving hand fluttering while everything else is frozen, you have seen this effect. To fix this requires some real-time digital processing whereby the two fields are interpolated before being combined - this is called motion compensation. Assuming we have a PAL signal coming in at a resolution of about 580 x 773, and a screen with a resolution of 250 x 333, we clearly need to reduce the incoming resolution of the video to match the display. This is the next job for the processing electronics. This can be done in a brute force fashion by simply discarding pixels, or, again, digital processing can do this intelligently to retain as much picture detail as possible. There are considerable differences in how display manufacturers process the video signal for display, and obtaining this information from the manufacturers is not an easy process. It is a very worthwhile endeavour, since the processing can make a dramatic difference to the quality of the displayed image. Brightness and Other Obfuscation
Brightness is always measured in nits (cd/m2) - the higher the number, the brighter the display. As a rule, you will need 1,500 to 2,500 nits for an indoor display (lean towards the upper range if you have television lighting) and 5,000 or more for outdoor displays. The problem is that, while each manufacturer will present you with a brightness figure, there is no standard method of measuring this, making comparisons difficult. To be meaningful, each manufacturer should set their LED drive current to a specified amount (preferably the current setting they use to determine the LED life), then adjust the brightness and contrast for optimum video viewing using standard test patterns, then apply a known signal (such as 100 IRE white), and finally measure the brightness on-axis with a specific type of light meter. Contrast ratio is a measure of the dynamic range of a displayed image - the difference in brightness level between the whitest white and the blackest black. This is a very important specification in judging how well an image will be produced in an environment with high ambient lighting (such as an indoor arena). When we compared figures from various manufacturers, we found they fluctuated wildly (a difference of over ten to one between the highest and lowest). Again, they were all using varying techniques. To complicate matters, a very different method must be used when measuring the contrast ratio of an indoor display versus an outdoor display. The numbers, of course, will be quite different, and can only be compared similarly (i.e. indoor with indoor, outdoor with outdoor). Angle of view is another important consideration - can every seat in the house see the display equally well. There are two factors at play here: the viewing angle of the display and maximum viewing angles for people trying to make sense of any display - this latter is a design issue for the arena and needs to take into account size, number, and location of screens. The manufacturer quotes the display's viewing angle and, again, it is not always clear how this is measured. One typical method for other types of displays is to (while taking the brightness measurement) move off-axis to a point where the brightness drops to 50% and record this angle. Unfortunately, LED displays have a problem that is unique to this technology called 'shouldering'. There is a colour shift caused by one LED blocking the view of another LED at extreme angles. Thus, for LED technology, the 50% brightness measurement is not a completely valid specification - it should also take into account colour temperature shifts. If a significant colour shift occurs before the brightness falls to 50%, then this lesser angle is truly the viewing angle. A similar effect (caused by built-in sun visors) can seriously degrade the vertical viewing angle of an outdoor display. Life expectancy is another interesting issue.
Quoted life figures for the LED's range from 20,000 to 100,000 hours.
These figures are clearly only meaningful if they are determined at
the actual drive current that will be used under real display conditions
- certainly at the drive levels used to produce the brightness measurement.
Check Those References!
Once you have narrowed the field, arrange to see exactly the product you are considering. When we came to this part of our evaluation, we found that one manufacturer did not actually have a 12mm display - although they did not mention this until we asked to see one! I was not prepared to recommend that my client become a guinea pig for the first commercial model of a manufacturer's jumbo video screen. If you find yourself unavoidably in this situation, be sure to contract-in, a free upgrade to whatever changes may be made within, say, six months, or the next four screens sold. This is reasonable, as you are essentially testing a prototype. If the manufacturer gets it right, and there are no problems, then they have nothing to lose. If things do not go right, the manufacturer will learn from your problems to the benefit of subsequent customers - you should be compensated in some way for being placed in this situation. Clearly, this is not an exhaustive description
of a rigorous evaluation process, but it does give you an idea of some
of the important items to look for. The best advice we can give you
when faced with evaluating any relatively expensive high-tech system
is to produce a detailed specification outlining system performance
and quality of workmanship - before soliciting vendor proposals. Research
the current state of the technology (for displays, it changes about
every six months), and check references. Garry Musgrave is Principal Consultant with Conceptron Associates, an audio/ video design consultancy based in Coquitlam, Canada. Their website can be found at: www.conceptron.com |
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