Digital Video for AV Integrators
When you transform AV into digital bits and bytes, it opens up a world of possibilities?and a few challenges. Here is everything you need to know about moving digital video around an AV installation.
THE OLD SOFT SWITCHEROO
Now here’s why digital video presents such an extreme paradigm shift to the pro AV industry: When you switch between digital video signals, such as changing channels on a TV or signals on a routing switcher, you are merely switching from one stream of data to another, or from one cluster of packets to others in the same stream. You are not making or breaking physical contact, nor are you tuning in a different RF channel.
That’s a departure from the more familiar analog video switching process, which must physically make and break contacts for up to five discrete signals (red, green, blue, and horizontal and vertical sync), compensating for any changes in signal amplitude and phase that might occur along the way. Digital video switching is more like dipping into a stream and grabbing the packets you need. All the packets are present in the stream, but the video receiver only looks for those that apply and conform to the MPEG tables that it’s working with. In InfoComm classes, we refer to this process as software-based video switching. There are no issues with sync-pulse degradation or signal phase to deal withthat’s all handled in the display during the digital-to-analog conversion process.
What is critical in any digital signaling system is the signal-to-noise ratio. If the level of the digital signal drops too low relative to any noise in the distribution system, it will abruptly disappear. This phenomenon is well known to satellite and terrestrial DTV viewers as the “cliff effect.” It’s as if the signal suddenly fell off a cliff!
Fortunately, digital video systems include some degree of protection against signal dropout, usually in the form of forward error correction. A well designed digital video transport system assumes that not all bits will make it through intact and therefore adds in some redundant bits to ensure a high quality of service (QoS).
There are many ways that digital video can be implemented, one being a private video network using Internet Protocol (IP) packet headers to feed digital signage displays. In that case, all MPEG programs are present in the stream (or can be called up on demand) and sent to any or all connected displays by using their discrete IP addresses.
Need to add more monitors? Plug them in and connect them to the network. No need to pull additional cabling other than the connection to a router. AT&T’s U-Verse system works as a pure IPTV network, and their set-top boxes switch between TV channels by using MPEG program numbers and IP headers. This is just another example of software-based video switching. And U-Verse can work over wired and wireless networks, too.
Sounds intriguing, right? So when are you going to start using digital video?
Senior contributing editor Pete Putman was InfoComm’s 2008 Educator of the Year. His 2010 InfoComm Academy sessions included “Digital Video 201” and “Practical RF for System Integrators.”