Applications - Audio/Visual Connectivity
Digital signs were originally low-resolution LED displays that were capable of displaying alphanumeric characters and simple graphics. These types of signs are still in use today for such applications as freeway traffic information, stock ticket, and scoreboards. Most of these signs have serial RS-232 ports, so a serial device server like Silex SX-500 is a good way to connect multiple such signs to a single computer system.
With the introduction of plasma and LCD displays, digital signs are now capable of displaying high-resolution graphics and full motion video. The dramatically falling prices of these displays has spawned a whole new range of applications, including in-store advertising, classroom instruction, courtrooms, and much more. In addition, applications that were previously done using the older LED-based digital signs can now be presented with far superior graphics, plus the addition of full motion video.
Until recently, there have been two ways to connect multiple LCD/plasma digital signs :
- Analog video distribution systems. With analog video display systems, one computer system sends out the same broadcast to multiple displays, typically using twisted pair cabling. Most analog video display systems basically convert the VGA signals on the computer to analog differential signals to allow them to go longer distances. The problem with this approach, however, is that analog signal quality degrades over long distance. In order to overcome this problem, some manufacturers have developed complex and expensive filtering systems.
- Media player systems. With media player systems, each display is connected to an intelligent medial player device, typically an embedded computer system with mass storage. Video/audio content is downloaded from a central computer via Ethernet to each of the media players, which then display the content on the attached digital sign. The central computer must use special management software for synchronizing the media players to ensure that they all display the same content simultaneously. The drawbacks of a media player system include lack of real-time capabilities (for example, a TV broadcast could not be immediately displayed on the digital signs because it would first need to be downloaded), more complex setup and management, and lower reliability (since each media player typically has a disk drive).
The latest generation of video distribution systems, such as the Silex MVDS, uses digital multicast technology to overcome the limitations of analog and media player systems. Digital multicast technology has some significant advantages vs. analog, because the image quality does not degrade with distance. It also allows users to take advantage of their existing Ethernet infrastructure and to use inexpensive Ethernet switches and hubs (note that although some analog VDS systems use Category 5 twisted pair cabling, they are not Ethernet compatible). Compared with media players, it is simpler to use (anything displayed on the computer's monitor will also appear on the digital signs without the need for special management software), has true real-time capabilities, and does not have any moving parts. Perhaps the most significant advantage of digital muliticast technology, however, is that it allows cost-effective wireless connectivity using the industry-standard 802.11 protocols to enable digital signs to be mobile and/or located in areas where cabling is not practical.