Blue LEDs, digital TV bring daylight-bright signs to masses

LEDs transform full-color sign technology from older techniques, yielding significant benefits.

By Bill Schweber, Executive Editor -- EDN, 4/13/2000

If you're in Times Square in New York to see its remarkable transformation over the past decade from a seedy, dirty, crime-ridden area to a glitzy tourist attraction, look up at the office building at 4 Times Square, which is the southeast corner of Broadway and 43rd St. You'll see an eight-story, full-color, dynamic sign that represents the triumph of LEDs over CRT- and incandescent-lamp-based outdoor signs. It shows how far LEDs have come from their humble beginnings as red indicators and simple seven-segment displays. This sign's brightness (even in daylight), color fidelity, image resolution, and 170° viewing angle are truly impressive.

The National Association of Securities Dealers owns the sign and uses it to promote Nasdaq. It shows stock information, advertisements, simple text, and full-motion video, all of which the company manages through a control room in the building on which the sign is mounted (Reference 1).

The sign uses carefully arranged red, green, and blue LEDs, supported by suitable image-transformation and electrical-drive circuitry, Saco Smartvision (www.smartvision.com) designed, built, and installed the sign, which consumes less energy, generates less heat, and costs less to maintain than the alternatives. (Note that Saco Smartvision is not the only vendor of these LED-based signs; Daktronics Inc (www.daktronics.com) is another.) In addition to the electronic and optical challenges, the vendor had mechanical, structural, and building-code issues to overcome (see "Whaddya expect? This is New York!").

It's not just the simple availability of blue LEDs that makes this type of sign practical. The primary-colored LEDs have to be bright enough to make a sign that is visible even in direct sunlight. Bright red and green LEDs have been available for years, and LED vendors have experience at manufacturing reliable, consistent devices; these LEDs have matured to the point at which they now find use in mechanically and environmentally harsh applications, such as traffic lights and vehicle brake lights and turn signals. Even with all this vendor know-how and experience, the color purity and consistency of both the red and the green LEDs had to improve to make them suitable for accurate color rendition in signs.

In contrast to the red and green LEDs, blue devices have been unavailable for most of the history of LEDs. Users waited for years for vendors to produce even dim blue LEDs, and the brightness of blue LEDs has until recently been low. Usable blue LEDs have been available only since 1996 from vendors such Nichia (www.nichia.com) in Japan and Cree (www.cree.com) and Purdy (www.purdyelectronics.com) in the United States.

Because vendors use LEDs in panels to create large areas of color that have to be "correct," Smartvision sorts each LED within each primary color so that the LED's brightness and color values match those of their mates. The final objective is to have the color spectrum of the LED-based images match the color spectrum as defined for signals using the well-established and widely used NTSC color standards, which were developed for CRTs. LEDs cannot directly match these phosphor-based colors, so the signal-processing circuitry must transform and adjust the color space of the video signal. After this adjustment, the spectrum that the LED emits will yield correct colors, according to NTSC and other standards, even though the video signal assumed it was driving a non-LED display.

Modular electronics

Saco Smartvision built the overall sign from 320-mm-sq modular panels, which each contain an array of 16×16 pixels, totaling 256 pixels. Each 30×30-mm pixel, in turn, comprises eight surface-mounted LEDs: two blue ones in the middle and two greens and a red on either side (Figure 1). The side-by-side arrangement of 8200 panels yields a 1600-pixel-wide, 2000-pixel-high screen. If you do the math, you'll see that this arrangement yields nearly 2.1 million pixels and 16.8 million LEDs. The sign measures 36.6m (120 ft, or eight stories) tall and 27.4m (90 ft) wide. If you go further with the math, you'll see a discrepancy between the area of the sign and the stated number of panels the vendor used to build it. This situation is not a case of playing with the specifications; it occurs because the sign's surface is not an unbroken solid but instead has 30 cutouts for the building's windows.

According to Saco Smartvision, the design uses a roughly equal split between custom-built circuitry and standard, off-the-shelf systems. The company configured the sign as one logical screen and as eight physical screens, so it can show as many as eight simultaneous full-motion images. Circuitry digitizes the source video signal, which can be in composite-video, RGB-video, or digital-video format if necessary. A front end then passes the signal through standard high-definition-TV-signal and "video-wall" processors for resizing and 12-bit spatial color-conversion and correction. A memory map contains the results of the processed signal, ready for display. A pixel can generate any of 16.7 million colors.

But the signal processing and transformation are only parts of the story. You still have to drive this enormous array and large number of LEDs (Figure 2). To accomplish this task, the system converts the memory-map output from a parallel bus to a 1.4-Gbps serial signal and conveys it to the LED array by a coax cable. A status cable operates in the reverse direction to report on LED and power-driver conditions. This serial signal goes to a receiver-and-distributor circuit, which strips out the appropriate section of the image. The design uses Standard Device Interface (SDI) signals and format, which are compatible with ANSI/SMPTE 259M standards, for the interconnections. Driver ICs and 800 toaster-sized switching power units with more than 90% efficiency use pulse-width modulation to drive the LEDs.

Power consumption of the Nasdaq sign is 60W/ft² (540W/m²). The LEDs themselves are approximately 30% efficient compared with 3 to 8% efficiency for incandescent bulbs. Several light-output-spectrum, eye-response, and solid-angle factors complicate determining efficiency when converting supplied electrical watts to photon power (Reference 2). Therefore, these approximate LED and bulb figures give you a sense of the relative difference.

Incandescent bulbs have 1000-hour lifetimes, whereas LEDs last 100,000 hours. This longer life results in a tremendous savings in labor and maintenance downtime. The reduced power consumption also provides a saving in energy cost and greatly simplifies thermal and dissipation problems. In addition, the electronics that drives the LEDs can be smaller and easier to package and mount than that for bulb-drive circuitry. These savings are significant for indoor signs, too, because maintenance is always an undesired expense and heat-producing displays force the arena or building to have a larger air-conditioning system to deal with the additional heat load.

Nasdaq and Saco Smartvision say that this sign is the largest LED-based sign ever built. The vendor has used the same, standard underlying electronics to build smaller signs for both outdoor and indoor arenas. For these smaller displays, they choose among LED-based pixels as small as 3×3 mm with just a trio of LEDs per pixel in the smaller pixels rather than the eight that the Nasdaq sign uses.

As for cost, Saco Smartvision quotes $16 million for this sign. (Some news reports claim a figure nearly twice as much.) That price does not include the installation costs, the operating costs, or the $2 million annual rent that Nasdaq pays to place it on the building. You have to see it for yourself to decide whether it is worth the money, but the potential audience is millions of New Yorkers and tourists who can stop and stare at it.

REFERENCE

1. Blair, Jayson, "Newest Times Square sign packs high-tech punch," The New York Times , Feb 17, 2000.
2. "LED output conversion," Photonics Spectra , October 1999, pg 92.