A NEW APPROACH to video/RGB switching
Apr 1, 1999 12:00 PM, Pete Putman
Whenever professional A-V dealers get together these days, the subjectinevitably gets around to competition from resellers of PCs and theirability to move into the A-V systems integration business. Some proclaimdoom; others scoff at the possibility. Admittedly, items like automatedmixers and video/data projectors are becoming more plug-and-play with eachpassing year, and it is conceivable that some precocious PC systemsintegrators may be able to figure out how to sell a simple installation andpull it off, particularly with the slow migration of video and audiodevices to serial digital inputs and outputs. The fact remains, however,that we still live in an analog world when it comes to signal distribution.How we choose to port signals around a facility may change, but the laws ofphysics remain constant. In fact, our cabling options-particularly thosefor video-are more complex than ever, thanks to wider use of componentvideo from computers and video sources. The wild card is digital signaldistribution. It does not matter if a digital signal is composite orcomponent video; it needs only one cable to move from point A to point B.The D/A converters act as virtual switchers, decoding and processing theRGBS, RGBHV or YUV signal elements. When every large-screen display hasdirect serial digital inputs, the single-cable signal distributionenvironment will become reality, but that is some time off in the future.
Some of the major decisions we face today arise from connection andswitching options provided by the fast-growing family of desktop andboardroom LCD and DLP projection systems. These boxes can do it all,jumping from composite video to workstation graphics to notebook computersignals with surprising agility, stereo audio following in lockstep. Doesit make sense to use a projector as the switching hub? What about thedifferent component formats? How about audio-follow switching? Should it bedone separately? Are there ways to achieve smoother switching withoutglitches when moving between video and computer sources? These are alllegitimate questions.
Today, we can move video as a composite baseband signal, a two-channel(Y/C) S-Video signal, and as a three-channel (YUV) signal. If the videosource is a composite format-like VHS or laserdisc-S-Video signaldistribution has little benefit. The decoded color/luminance artifacts willbe present, no matter how you handle the signal. In these cases,single-cable distribution makes the most sense.
For sources that provide S-Video and YUV component outputs-DVD, Betacam, MII and component laserdisc-the difference between running two cables andthree is a matter of economics and consumer demand. The advantage of amulti-wire system is that both YUV and RGB video can be ported down thesame path. If you are looking for future compatibility, HDTV signals willbe moved in only two ways-as single-cable digital bitstreams or as decodedanalog (Y Pb Pr), requiring three cables.
Where do you switch all of these signals? With a stand-alone box or in aprojector or other large-screen display? One obvious problem with using aprojector or monitor for switching is that if it must be serviced, you loseyour switch and patch point. Outboard switchers capable of handling everypossible composite and component signal are reliable and inexpensive enoughthat it makes more sense to use them. Many have audio-follow connections,too.
As we sit and contemplate how to design a system that can accommodatesingle- and multi-wire video signals, along comes a new product that cansimplify the job in a way not seen before. Manufacturers are combiningconventional RGB switchers with digital video scalars, resulting in ascalar/switcher, or seamless video/RGB switcher.
Here's the method behind the madness: Scaling video-composite, S-Video, orYUV/RGB-allows it to be handled like computer video signals. A scaled videosignal can be converted to the same horizontal scan and frame refresh ratesas a computer source, thereby providing seamless hard-cut switching betweenthe two signals ahead of a projector or display, with no loss of picturesync and no "blank" screens waiting for the projector to scan and lock onto the input signal.
Like any other switcher, these products will be RS-232 compatible forremote operation, and only a single RGBHV cable needs to be run to theprojector or monitor. Best of all, the transitions between video andcomputer signals will be instantaneous and smooth with a scalar/switcher-something that rarely happens when selecting video and computerinputs from the projector. Once the projector locks up to the scalars' syncoutput, the transitions are as smooth as a genlocked video cut.
A good analogy would be converting a mix of audio lines-unbalanced Hi-Z,balanced, mic and line levels-all to three-wire balanced audio withconsistent levels. That is a much easier trick to manage with audio,though. Video and computer signals have such a dizzying variety of sync andrefresh rates that the thought of trying to achieve smooth cuts (yes, andeven dissolves) between them sends lesser engineers into fits.
Scalar/switchers also allow the best resolution and sync match to thedisplay. For flat-matrix projectors, that will be the native resolution ofthe LCD or DMD imaging device. For CRT projectors and monitors, the matchwill be to the sweet spot or point where maximum resolution and brightnessmeet, such as SVGA (7 inch/178 mm), XGA (8 inch/203 mm), and SXGA (9inch/229 mm). Not only have you done away with image instability during atransition (which your clients will love), you are always driving thedisplay at its optimum resolution.
First-generation video scalars-like high-end units made by Faroudja andSnell & Wilcox-use one RGB input and one video input, scaling and timingthe video to match the sync rates of the computer. These systems actuallyallow the user to mix video right onto a Windows desktop, resize it, andthey even stretch it anamorphically.
For fixed-install applications, sophisticated models from RGB Spectrum(SynchroMaster series) and Folsom Research (Variable Format Converter)allow the user to customize several output resolutions and scan rates forscaled video. As a bonus, both models offer seamless dissolve/cuttransitions between RGB signals and up-converted video.
With some clever programming and timing, it would not be too difficult tomix everything from videoconferencing signals, Powerpoint slides,high-resolution graphics and the latest corporate video using nothing morethan a couple of upstream RGB and video switchers.
One manufacturer has actually gone ahead and incorporated a switcher intotwo of its scaling products. Analog Way's Smart Cut will accept composite,S-Video or component video and one RGB input. Four preset output scan rates(VGA/60, SVGA/60 + /70, XGA/60) are selectable, and the result is aseamless cut between video and computer images. Analog Way's SeamlessSwitcher goes even further; it can mix, fade, or dissolve between up to sixdifferent RGB computer inputs (1,600x1,280 maximum resolution), outputtingthem as one VGA, SVGA, or XGA signal.
If the concept of an integrated scalar/switcher catches on with othermanufacturers, it will be the start of a paradigm shift in video signaldistribution. The catch, of course, will be the quality of video scalingprovided, although even the low-end scaling engines are outputting abetter-looking image than can be had from 75% of the desktop andinstallation-grade flat-matrix projectors currently available.
The next step will be an all-digital scaling/switching module, acceptingboth analog and digital video inputs and outputting a single-cable digitalbitstream, compatible with one of the new digital RGB video standards beingproposed by Intel and other companies. After that?
Economies of scale in manufacturing and lowered costs will make it feasibleto incorporate high-quality scaling/switching into future flat-matrixprojectors and monitors. Digital outputs and inputs will mean asingle-cable wiring infrastructure, which might be coax, CAT-3, or CAT-5wire - even fiber optics.