TUTORIAL: YOU GOTTA HAVE THE PIPES!
Bandwidth and Its Place in the Video Signal Chain
As this is being written, I've been working on a multi-part training program that covers, among other things, the "A to Z" of video signal interfacing and distribution. It 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 computers displays were capped out at 800x600 or 1024x768 pixels. But we live in a different world now, one where we are 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. I pulled out a copy of the 1995 InfoComm Projection Shoot-Out program and 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 was generally interpreted to mean 1024x768 (XGA) resolution. XGA itself 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" tubes, typically good for 480p and perhaps 600p imaging, while the remaining four employed 9" tubes, capable of handling SXGA (1280x1024), but not a lot more.
In other words, the limiting factor in the display system back then was the resolving power of the projector or monitor, not the bandwidth. To pump 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" CRT meant you wouldn't notice the lost data anyway.
Fast-forward ten years, and it seems that every laptop computer and a majority of desktop computers have at least 1024x768 resolution. Sometimes it's 1400x1050 (SXGA+), or it might be a widescreen resolution such as 1280x768 or 1366x768. Over at the TV set, we've now got 1280x720 and 1920x1080 HDTV coming through the antenna or cable jack.
What's more, CRTs are slowly disappearing from our desktops and living rooms, replaced by LCD, DLP, plasma, and other fixed-pixel technologies. So we have no excuses anymore for a "clogged" pipe, one that rolls off high-frequency information before it gets to our new high-resolution displays.
70 MHz might have seemed exotic at one time, but it is probably the bare minimum bandwidth specification we'd want to have in our cables, interfaces, video processors, and displays. 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 have tested several expensive home theater front projectors and plasma monitors which cannot pass enough detail in a 1280x720 HDTV signal, yet used fixed-pixel arrays that equal or slightly exceed that resolution.
The rationale given by a marketing manufacturer for one of these companies was that the HD analog component inputs on a particular projector (which retailed for well over $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 so connected.
I was advised to use instead 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 any poor soul with an older set-top box equipped solely with the customary component video jacks colored green, blue, and red.
I don't review projectors, monitor, 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.
BW Figure 1. This multiburst pattern shows high-frequency detail out to the last pattern, and is a good choice for showing HD content.
BW Figure 2. This multiburst pattern is soft, showing evidence of clipped high-frequency bandwidth. That's barely adequate for viewing 480p!
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 display are touted as being "high definition", proving that a pixel count alone does not an HDTV make!
As flat-panel LCD and plasma manufacturers migrate away from standard definition to higher resolutions (1024x768, 1280x768, and up from there), bandwidth will become even more of an issue. And the new crop of 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 wide screens is a trend towards 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 cool. 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 on image quality.
If the STB 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, after which point you may wonder why you don't just watch 480p video instead? (Considering the limited resolving power of a CRT picture tube, you may never know what signal information got choked off in the pipeline.)
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. Here's a formula you can use:
BW = [(TP x Vt) /2] x 3
Where BW = Bandwidth in Megahertz (MHz)
TP = total pixels (horizontal x vertical)
Vt = picture refresh rate in Hertz (Hz)
Using this formula, we can quickly calculate the minimum bandwidth as follows:
1920x1080 = 2,073,600 pixels x 60 Hz = 124,416,000. Divide by 2 (62,208,000), then multiply by 3 (for a 3dB 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 furthest thing from their minds....
