ProAVmag

Gotta Have The Pipes

I've been working on a multi-part training program that covers, the ?A to Z? of video signal interfacing and distribution. Sounds pretty mundane, doesn't it? After all, we simply select the appropria 9/22/2005 6:51 AM Eastern

Gotta Have The Pipes

I've been working on a multi-part training program that covers, the ?A to Z? of video signal interfacing and distribution. Sounds pretty mundane, doesn't it? After all, we simply select the appropriate interfaces, cable them up, turn on the power, and move on, right?

I'VE BEEN working on a multi-part training program that covers, the “A to Z” of video signal interfacing and distribution. Sounds pretty mundane, doesn't it? After all, we simply select the appropriate interfaces, cable them up, turn on the power, and move on, right?

That may have been true a decade ago, when the operative resolutions for a majority of video sources and computer displays were capped out at 800x600 or 1024x768 pixels. But we live in a different world now, one where we're pushing high-resolution computer and video through analog and digital interfaces with greater frequency.

Our displays have gotten better at showing all of this detail, too. In 1995, CRT projectors and monitors still ruled the roost for professional and consumer display applications. Looking back at the 1995 InfoComm Projection Shoot-Out program, I found that of the 80 total entries in all projection categories, 43 of them used either projection tubes or direct-view picture tubes.

In those days, the highest-resolution category was “Graphics,” which generally meant 1024x768 (XGA) resolution. XGA stands for eXtended Graphics Array, and in a day where the first 640x480 front LCD projectors were just coming to market, 1024x768 was considered a lot of pixels.

Was bandwidth an issue then? Not really, given that all of the displays entered in the Graphics category used CRT technology. Three of the front projectors were equipped with 7-inch tubes, typically good for 480p and 600p imaging, while the remaining four employed 9-inch tubes, capable of handling only SXGA (1280x1024).

So, the limiting factor in the display system was the resolving power of the device, not the bandwidth. Pumping a 1024x768 signal with a refresh rate of 60 Hz through an interface required 70 MHz of system bandwidth, but even if the bandwidth rolled off at 60 or 50 MHz, the limited resolution of a 7-inch CRT meant you wouldn't notice the lost data anyway.

Fast-forward 10 years, and it seems that most laptop and desktop computers have at least 1024x768 resolution. Sometimes it's 1400x1050 (SXGA+), or it might be a widescreen resolution such as 1280x768 or 1366x768. And on the TV set, we now have 1280x720 and 1920x1080 HDTV coming through the antenna or cable jack.

What's more, CRTs are slowly disappearing, being replaced by LCD, DLP, plasma, and other fixed-pixel technologies. So we no longer have excuses for a “clogged” pipe — one that rolls off high-frequency information before it gets to our new high-resolution displays.

While 70 MHz may have seemed exotic at one time, today it's probably the bare minimum bandwidth specification we'd want to have. Consider the bandwidth requirements for 720p HDTV (83 MHz), SXGA+ @ 60 Hz (132 MHz), WXGA @ 72 Hz (94 MHz), and 1080i (93 MHz), and you can see where we might have a bit of a problem.

Oddly enough, not all display manufacturers seem to be focused on bandwidth issues. On more than one occasion, I've tested expensive home theater front projectors and plasma monitors that can't pass enough detail in a 1280x720 HDTV signal, yet use fixed-pixel arrays that equal or slightly exceed that resolution. The rationale given by a marketing manager for one of these companies was that the HD analog component inputs on a particular projector (which retailed for more than $10,000) were primarily intended for use with DVDs (480p resolution), based on actual customer use. Additional signal enhancement circuitry used to enhance edges of 480i and 480p video also degraded the quality of HD images connected to the HD analog component inputs.



1 2 Next

Gotta Have The Pipes

I've been working on a multi-part training program that covers, the ?A to Z? of video signal interfacing and distribution. Sounds pretty mundane, doesn't it? After all, we simply select the appropriate interfaces, cable them up, turn on the power, and move on, right?

Instead, I was advised to use the DVI digital inputs to achieve higher bandwidth. Running some test signals through that connection revealed that indeed the projector was a much better performer with 720p and 1080i. However, that wouldn't help anyone with an older set-top box equipped solely with component video jacks.

I don't review projectors, monitors, and TVs as much as I used to, but when I do, the first step is to sweep the display's component video and RGB inputs for frequency response using a multiburst pattern generator. While viewing a 720p luminance multiburst pattern, which contains data out to 37.5 MHz, I inevitably find the 37.5 “burst” to be a mass of solid gray, instead of finely-delineated black and white lines. You might be surprised at how many expensive projectors and monitors can't handle the next-lowest frequency, 18.5 MHz, let alone the 37.5 MHz multiburst. Yet these displays are touted as “high-definition,” proving that pixel count alone does not an HDTV make.

As LCD and plasma manufacturers migrate away from standard definition to higher resolutions (1024x768, 1280x768, and up), bandwidth will become even more of an issue. And the new 1080p projectors, monitors, and TVs coming to market will only magnify the problem of clipped bandwidth. Toss in larger screen sizes, and you can see a lot of puzzled viewers wondering where all the picture detail went.

Hand-in-hand with the move to higher resolutions and widescreens is a trend toward digital interfaces such as DVI and HDMI. Both of these “pipes” can easily handle the flow; single-link DVI has a bandwidth of 165 MHz and dual-link is good for double that. HDMI goes even higher, promising bandwidth into the GHz range.

As long as every signal processing circuit before and after these interfaces can match those figures, things will be fine. But they rarely do. Digital TV set-top boxes often have clipped bandwidth, and if some sort of format conversion is used to match a TV's or monitor's scan rate limits, then image detail is also being tossed aside. An example would be format-converting 720p to 1080i to accommodate the 33.8 kHz horizontal scan rate commonly used in CRT direct-view HDTVs. The resulting 1080i signal itself is often converted to 540p by grabbing every other field of video, a process that saves money but compromises image quality. If the set-top box doesn't have enough bandwidth to first process the 720p signal before converting it, you'll notice reduced image detail and possibly scaling and scan-conversion artifacts.

With all the talk about 1080p displays, it might be a useful reality check to calculate the required system bandwidth to pass a 1080p/60 signal, should one ever come into existence as a transmission and distribution standard. 1920x1080 = 2,073,600 pixels; X 60 Hz = 12,441,600. Divide by 2 (6,220,800), then multiply by 3 (for a 3 dB bandwidth specification) = 186,624,000 or 186.6 MHz.

Realistically, that means a true, flat 200 MHz bandwidth for a 1080p system. In turn, that means using better components in a display, which raises its cost. In a world where projector, TV, and monitor manufacturers are trying to hack their prices as much as possible to maintain market share, I'd bet that conserving video signal bandwidth is probably the farthest thing from their minds….

Pete Putman is a contributing editor for Pro AV and president of ROAM Consulting, Doylestown, PA. Especially well known for the product testing/development services he provides manufacturers of projectors, monitors, integrated TVs, and display interfaces, he has also authored hundreds of technical articles, reviews, and columns for industry trade and consumer magazines over the last two decades. You can reach him at pete@hdtvexpert.com.



Want to read more stories like this?
Get our Free Newsletter Here!
Past Issues
June 2015

May 2015

April 2015

March 2015

February 2015

January 2015

December 2014

November 2014