It goes by many names — player to screen, the last mile, video extension — but when it comes down to it, back-end connectivity for digital signage networks can be one of the most confusing pieces of the puzzle, especially for first-time deployers.
Although there are endless custom ways and configurations to extend video from PCs and media players to screens, content is usually sent through analog and digital signals over Cat5 cable or by wireless transmission. Depending on the application, there are advantages and disadvantages for each option.
ANALOG AND DIGITAL OVER UTP
This solution consists of video distribution over UTP (unshielded twisted pair) cables. These include Cat5, 5e, 6, 7, etc., and are also called copper cabling. A PC or media player outputs VGA video signals to a UTP transmitter. The transmitter converts the signals into higher voltage signals for extended transmission over UTP cabling. Typically, red, green, and blue video signals are each sent over separate twisted pairs of wires within the UTP cable. (The remaining twisted pair is used for ancillary signals, such as serial or audio.) At the opposite end of the cable, UTP receivers convert the signals back into VGA signals, which are sent to the screen for play.
The difference between analog and digital is the form in which the video is sent. Analog video is sent as small electrical pulses through the cable, while with digital the receivers convert the video into ones and zeros which are sent over the cable, and converted back to video by the receiver at the other end.
The main advantage of wired solutions, especially with analog, is that they are relatively inexpensive. With Cat5 cable being especially plentiful and the cost of receivers and transmitters being relatively low, wired presents an affordable solution.
Typical UTP extension system
Click image to enlarge
"In terms of reduced cost of PC hardware and maintenance, these extenders provide a very real return on investment," said Chuck Pheterson, vice president of product marketing for Avocent.
Wired also allows users to send high-definition video over very long distances. There are products on the market that can transmit full 1080i and 1080p (high-definition) video up to 2,000 feet.
"You can also support multiple signals over a single Cat5 cable," said Dave Haar, managing director North America for Minicom Digital Signage. "This means you can do video, audio and bi-directional RS-232 which controls the screen on one cable."
While wired solutions remain very popular, there are still some limitations that need to be considered before installing them.
For one, using analog over Cat5 opens up the possibility of skewing or "ghosting." This can happen with standard UTP cables as the lengths differ among the four twisted pairs of wires inside the cable. The different lengths cause the color signals to arrive at the receiver at different times, making the video appear blurry. Fortunately, there are solutions for this, such as skew-free cables and skew-free receivers. For an added cost, these products can delay the signals from the twisted pairs during the transmission so they can arrive at the screen at the same time.
It also isn't recommended to send analog signals between two ground sources, such as two buildings.
"If you connected them between two different buildings and therefore two different ground sources, you will find a difference in the grounding that will cause a current to run through the Cat5 cable," Pheterson said. "The difference between the two grounds can be more than 100 volts. This causes the extenders to run hot and eventually they will burn out like popcorn."
The bottom line
For most cases, analog over UTP still remains the go-to choice for connecting digital signage. It seems to work best for confined networks, like single-location retail stores, where there is a PC in the back room controlling several screens on the floor.
"Single-location use solves the problems with analog — you don't have skew issues because you're not really going that far. And since everything is plugged into the same ground frame, nothing ends up running hot," Pheterson said.
Digital signal over UTP solves some of the problems with analog such as grounding and skewing, but the higher price of digital transmitters and receivers has kept it from taking over analog.
"Sometimes our digital solutions end up helping sell the analog ones because digital means more capital expense versus analog," said Pete Gallagher, marketing manager for Magenta Research. "Analog works, it has worked for years and it will continue to work for years."
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Wireless solutions distribute video over wireless Ethernet radios. Here a PC sends the video signal to a wireless transmitter, which converts and encodes the video signals into digital data. The data may also be encrypted for security and then sent to wireless receivers at each screen. The receivers convert the wireless data back into video signals, which are then output to the displays. There are several ways of doing wireless, including streaming content over the wireless transmission in real-time and "store-and-forward," where the content is sent and saved to a media player at the screen for future play.
The main advantage with using wireless signal is that it can go where cables can't. Users of this transmission method usually have video signals that need to go between buildings or over long distances such as from one side of a parking lot to the other. Also, wireless doesn't run into any issues with electrical ground charges as analog over Cat5 does.
Without the need to run wires, the cost of installing a wireless network is usually lower than installing a wired solution.
"A lot of customers are looking to wireless when going long range because that is where the high cable costs come in, if you have to trench through parking lots, etc.," said Mike Derby, CTO of AvaLAN Wireless.
Typical wireless extension solution
Click image to enlarge
"With Cat5 cables, the cost of installation, in most cases, is more than the cost of the cable and the Cat5 extenders," Pheterson said. "Also, consider the installers. Union labor requirements, city permits and job inspections can increase costs and bring about scheduling conflicts. In retail, hospitality, restaurants and other public venues, business disruption is a factor and downtime is required."
Wireless can also be beneficial for unique challenges, such as sending video to screens on elevators or through older buildings where asbestos can be an issue.
"Wireless is ideal for a gas station if you're using pump-top screens, because in most cases you can't get the wire underneath the concrete without having to dig it up," Haar said.
Gallagher says that in a perfect world, wireless would be a good solution for everyone. But the technology is subject to the same issues that can plague wireless routers at home, such as interference of the RF signal.
"Say a gas station goes wireless, a big truck could pull in and block half of the network," Gallagher said.
The hardware for wireless is also more expensive than the transmitters, receivers and cabling used for wired solutions, according to both Haar and Gallagher.
The bottom line
Deciding between wired and wireless is an ROI balancing act between the cost of the hardware and the cost of the installation, and will be unique for each project. For example, the cost of Cat5 extension hardware is relatively low compared to wireless, but its installation costs can be much more depending on the variables at the location.
"The cost breakpoint happens pretty quickly when you have to trench through concrete to run wires," Derby said. "For short range applications, I think wired makes a lot more sense than streaming wireless solutions. If it's that short range, just run a cable. Where the real value proposition for wireless is in the longer range situations where the cost of running cabling is prohibited or just impossible to do."
Editor's note: We understand that there are many ways to extend video to digital signage and that this article only covers two possible solutions. We plan on exploring this technology further in future articles.