LED It Be

In Uncategorized by tfwm

It has been the case in the past, and remains the prevailing assumption that LED screens are only for baseball stadiums, and beyond the means of all but the wealthiest of organizations.

Maybe this was the case 4 or 5 years ago, but not anymore. With falling component prices and increased design efficiency, the LED screen is very much within the realms of possibility for all but the most humble of budgets.

The unique advantages that LED screens are now offering are drawing the crowds back to venues that have been losing audiences in huge numbers. This is the driving factor, and key point. Is your event an attention grabber, or sleep inducer?

In the following article, we’ll explain some of the terminology, along with a few good rules of thumb, which should help you make an informed choice about LED.

Defining large screen video displays
The true purpose and reason for having a large-format video display is to show full-color moving images to large crowds.

In the past, successfully building very large or very bright video displays has been difficult, as the only real solution, projection, suffers from a lack of brightness and contrast in high brightness environments.

If you think back, a very famous consumer electronics company offered the first real alternative using CRT-based solutions. They were heavy and power hungry and suffered limitations on the resolution they could achieve, and all this for a king’s ransom.

The relatively recent (since 1996 onwards) full-color LED video display technology has at last made large, bright video displays practical and affordable.

Generally though, large displays are used in locations where a group of people must be able to see the display, such as sports arenas, pop concerts and shopping malls. These displays are generally large (from 1.5m to 30m wide or bigger) and naturally very bright – to overcome high ambient light such as television lighting or direct sunlight.

Let us look at the major large-screen display technologies in some detail.

Rear Projection & Video Cubes
The limitation of video projectors is brightness. Even with the latest DLP, or LCOS type of projectors, ideally the environment needs to be low-brightness to achieve an acceptable viewing experience. High gain projection screens can help in low ambient brightness environments, but in most naturally lit, or bright artificially lit environments, a projector may not be the best alternative.

A video-wall cube system, is made up of a small projector, with the image bounced off an internal mirror to appear on the rear of the video cube screen. This method keeps the profile of the units to a minimum, and allows them to be used as in-store large displays. The image is chopped up into parts by a special video scaler, and distributed to the cube projectors. A thin, but visible dividing line, can be seen between the cubes.

These displays also have low brightness (average only about 400 to 600 Nits). It is very hard to adjust each cube to exactly the same color balance, and even harder to keep them this way.

CRT Technology
This was the original big full color video screen system, and due to the costs from a difficulty in the manufacturing process, few companies ever made CRT. Now they are virtually extinct.

The best known examples of CRT screens were made by Mitsubishi (Diamond Vision), Panasonic (Astro Vision), and Sony (JumboTron).

The image from the video feed is divided up into a number of dots called pixels. Each pixel is made up of at least three tiny CRT’s (cathode ray tubes) – one red, one green, and one blue.

By varying the brightness of each of these, any color can be created. Each of these CRT’s is like a tiny television picture tube, except that it is producing only the intensity of one picture dot, and the entire picture is made up of hundreds of thousands of CRT’s, rather than one. The end result is a large, bright video image. The horizontal viewing angles on these systems were poor in comparison to current LED screens, not to mention the fact that were very power hungry.

LED Technology
In the late 1990’s, with the discovery of the stable and bright blue LED, video screen technology using LEDs was born. These LED video screens rapidly advanced beyond CRT’s for resolution and brightness, for significantly less cost.

Add this to the fact that LED displays consume far less power, and are considerably lighter than their CRT-based counterparts, and they occupy less volume (they are less than half as deep). All of these aspects made them very attractive in the AV rental and fixed installation market.

CRT technology had been limited to three principal vendors primarily due to the complexity of the manufacturing process. LED technology is much more accessible – resulting in many more manufacturers (one industry report cites as many as 50 manufacturers although there may be this many in China alone!).

An unfortunate side effect of this diverse manufacturing base is the proliferation of some poorly designed product and some short-term manufacturers who may sell only a few systems before vanishing.

One key to choosing an LED screen is to select a manufacturer with a long track in full color LED screen, and who is committed to the industry.

If you wind up owning an unsupported “orphan” product, you can’t say you weren’t warned!

In particular, beware of a new LED video product from a company that has only previously done animated LED signs, and has no video experience. Video demands very high speed processing, currently beyond the capabilities of a PC based processor, and the results are often very disappointing.

Screen Resolution
The key to a quality image on a large-format video display is to buy the highest resolution you can afford.

A screen’s resolution, is defined by it’s 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 486/576 (NTSC/PAL) vertically and anywhere from about 240 to 720 horizontally (depending on the quality of the source).

To reproduce these signals with no loss of image resolution, you want a minimum screen resolution of about 648 x 486 (NTSC) or 768 x 576 (PAL).

If you use a screen with fewer pixels than the input source, the images will have less resolution than the source. However if the display is designed properly it can still give an acceptable appearance for video images.

Screens of approx 1/3 of VGA resolution can provide a very acceptable video image, so around 200×150 pixels are OK.

For example, to achieve a 640×480 (VGA) resolution on a medium-size (3m by 2.25m) screen, we would need pixels spaced around 4.5mm apart.

(This distance between pixels is called pixel pitch, and is usually measured in millimetres.)

Typically, pixel pitches for indoor screens are 6mm, 10mm, 15mm and 20mm. Pixel pitches for outdoor screens are 15mm, 20mm, 25mm and 30mm- outdoor screens tend to be larger than indoor screens, as the viewing distance is often greater.

We could, for example, use a 6mm pitch and a slightly bigger screen to achieve a full VGA resolution, or a more cost effective solution would be to use a bigger pitch with a lower resolution screen.

The finer the pixel pitch, the more expensive the display, so the ideal solution will always be a combination of cost and resolution.

As most LED screen products come in fixed blocks of pixels built onto circuit boards (e.g.: 16×16 or 32×16), you may have to adjust your overall screen size slightly to accommodate a whole number of blocks.

A consideration is that the larger the pixel pitch, the more prone the image is to pixelization – you can start to see the pixel structure, much like looking at a newspaper photograph with a magnifying glass. This is a function of the distance between the viewer and the screen, and needs to form part of the design calculation.

Surface-mount LED (SMD) packages are now available to allow 10mm and 6mm and possibly smaller pitches – a 12mm pitch is about the limit for conventional lamp type LED packaging. At 12mm spacing the conventional lamps have poor contrast, and produce a lot of heat.

The choice of pixel pitch and screen resolution is dictated by: any physical size constraints you may have; viewing distance and sight lines; and, of course, budget – these displays are costed by area.

The color compound distance
When pixels are seen at close range, the RGB light emitting diodes appear as independent dots. The distance from the screen, where these LED’s mix to from a single color, is known as the “color compound distance”. Superior color compound ability allows images to appear clear and sharp at close range, which is a vital factor in indoor displays. For outdoor lamp-type individual LED screens, the color compound distance can be calculated by the pixel pitch multiplied by 500. For indoor surface mounted three-in-one RGB LED devices, this figure is 250, as the LEDs are very close together. Therefore for the LVP1010 this value is 2.5m. This is also sometimes mistakenly called the minimum viewing distance.

E.g. LVP 1650 16mm (pixel pitch) x 500 = 8m

The minimum viewing distance
This is calculated by the pixel pitch multiplied by 750 to 1000. This value will produce a smooth image. Closer viewing will produce an image with individual LEDs appearing as dots.
E.g. LVP 1650 16mm (pixel pitch) x 1000 = 16m

The maximum viewing distance
This is generally 20-30 times the screen height.
E.g. A 4.8 meters high screen: 30 x 4.57m = 137m

Video Processing
Once you have two competitive screens of similar technology, the same size, and the same resolution, the differences in LED manufacturers, drive electronics, and LED mounting methods can be evaluated.

A standard video signal cannot be directly displayed on an LED screen without first being processed. It is the quality of this processing, that is most often overlooked by prospective buyers of this technology. The primary rule of all broadcasting should apply, garbage in = garbage out.

Video images are made up of a number of horizontally scanned lines, but these don’t all appear on a television screen at the same time.

In the first 1/60th of a second (1/50th for PAL) the odd lines are shown, and in the second 60th the even lines are shown.

Everyone’s television works this way, and we call this an interlaced display. As most displays don’t use this broadcast signal directly, we must first de-interlace the video. 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.

Some low-end video processors do this, and throw away half of the original picture information. More sophisticated approaches involve storing the first line information, and then combine it with the second line information 1/60th of a second later. You can then display a complete frame.

However if an object was moving rapidly it may be in a different position in the second line than in the first line and this can lead to unacceptable video effects (flickering). Resolving this requires interpolation of the two sets of lines, done in real-time and then we have to scale the image to fit the output screen (this is normally a different resolution than the source).

The combination of these processes, especially the scaling, requires a lot of powerful processing to generate clean, artifact free, and fast moving flicker free video. Generally this is done by dedicated video processing equipment, and will be relatively expensive if you want a good result.

There are considerable differences in how display manufacturers process the video signal for display, and obtaining this information from the manufacturers is sometimes not an easy process. It is a very worthwhile endeavor, since the processing can make a dramatic difference to the quality of the displayed image.

Brightness & Contrast

The unit of measurement for LED screen brightness is the nit (cd/m2) with higher numbers meaning a brighter display. As a general rule, you will need no less than 1,000 nits for an indoor display and 5,000 or more for outdoor displays.

The way to measure this is at a normal angle to the screen (i.e. in front) using a light meter. The color temperature of the screen should normally be set to 5000K for indoor screens, and 6500K for outdoor screens.

If set this way, a full white signal should be measured at several points (usually 12, being the centre and the then evenly spaced around the screen) from the normal minimum viewing distance. The screen should be set to black, and then re-measured for the ambient reflected light (one measurement at the centre is OK). The brightness is an average of the 12 points of white, minus the measured ambient when the screen is black.

The viewing angle is normally defined at the point when the brightness is 50% of the maximum. If you walk around the screen you will see the brightness change, and it is advisable to review the three primary colors (and white) when walking around the screen to see if the color remains uniform at all angles.

LED displays have a problem that is unique to this technology called “shouldering”, where a color shift is caused by one LED blocking the view of another LED at extreme angles. The viewing angles should really include color shifts, and if a significant color shift occurs before the brightness falls to 50%, then this is the viewing angle.

Adding a louver between the pixels or rows of LEDs reduces the effect of glare from other light sources, and enhances contrast. It also reduces the vertical viewing angle, but usually this is not a problem for most applications.

If the screen manufacturers drive the LEDs using high currents, they can quote brightness figures in excess of 8000 nits. The problem with this is that high drive currents lead to faster degradation in the LEDs, and the screen uniformity can shift dramatically in short periods of time.

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 – and certainly at the drive levels used to produce the brightness measurement.

When evaluating any large-format video display system, always ask for references (for both the manufacturer and the installer). Try to ensure that these previous customers are in a similar situation to your own (e.g.: if you have an indoor arena, a comparison with an outdoor venue may not be completely useful).

Before You Buy

• Estimate the size, and location of your screens
• Work out the minimum viewing distances
• How many and what type of sources will you use
• Will the screen be permanent or will it need to move

Determining these factors beforehand will allow a manufacturer/supplier to estimate the pixel pitch and nature of the screen you may need. When you budget for any display, and especially an LED display, you should be aware that without consideration for the content, the final effect will be poor. It is a better idea to determine the content first and use a sample of the content in the format you plan to demo your display. In particular, scenes with a lot of motion and camera pans will create certain LED processor problems. Look for objectionable noise artifacts in large expanses of a single color (especially black or very dark areas).

Always remember, the manufacturer’s demo is designed to emphasize the best points about the display, and it is up to you to make them prove how good it really is. Test patterns, for color, greyscale and motion are essential. Take these with you to a demo, in the format you will use with your final screen.

When you have the final 2 or 3 companies defined, you should give serious consideration to a “shoot out,” with competing screens set up side by side- fed from the same source video. While this can be expensive, it is the only way to truly compare the competitor’s products.

Suppliers reluctant to engage in this kind of event may give you some insight into how confident they are of their product, and what their after sales support will be like!