Why do modern TV sets have such exaggerated overscan?

Ó July 12, 1999 By David Broberg

We often assume that since broadcast display monitors and computer display monitors seem to have much tighter tolerance of overscan, it must be the fact that consumer TV's are just built cheap and sloppy. This is really not the case!

Consumer TV's have to be built to handle all the likely extremes available in content the typical consumer TV might experience. This includes broadcast TV signals, cable TV signals, VCR signals, DVD signals, home camcorder signals, and video game signals. When they can't, consumers will complain.

How do you design for such a range of signals? Simple, you have to design for the center with tolerance of the extremes.

What determines the extremes for overscan? Variances in blanking widths of the source material. In the horizontal blanking there is a very wide rage. One of the worst offenders is the residual "rabbit-ear" distortion left by so many analog cable set-top-boxes on scrambled channels. These white pulses on the front and back porch of horizontal blanking have ragged edges and must be hidden in the overscan. Home VCR recordings are also made from these premium cable programs and exchanged among friends and family further extending the problem.

Video games, and consumer camcorders also can have wide (sloppy) tolerances for horizontal blanking and will show edge distortion if visible on the screen.

Vertical blanking is usually set to hide the head-switching positions of older VCRs including the U-Matic tapes that are frequently used on Cable TV. The AGC pulses in the Macrovision signal can sometimes cause bending or black level distortion near the top of the screen, this too must be hidden by overscan.

When TV makers get trouble calls from consumers about annoying lines or edges that appear near the side of the screen, it can usually be traced to overscan that was set too close to the "standard" settings. The response that a TV makers is forced to take is to expand the overscan design setting to a point where all possible edge signals will be hidden in the overscan.

Unfortunately, the sets with the highest feature and performance packages, often generate the most vocal complaints when they don't perform as expected: "Why does my new $4,000 Miyazaki 50" TV have white lines on the edge of the screen when my old 25" Philmo never had such a problem with the same signal? " Hence, even the high end sets have exaggerated overscan.

Another unfortunate side effect of this exaggerated overscan process affects the sales floor dynamics. When two sets are compared side by side with the same screen size, the one with the greater overscan "appears" to have a bigger screen! Since screen size is the largest differentiater by price among sets, it stands to reason that people tend to pick the set with the exaggerated overscan. (Of course a savvy sales person can usually point out content that is missing on that set, but emotion often gets in the consumer's way.) This phenomenon probably tends to favor more overscan on the low-end sets.

The "overscan issue" has been raised with CEMA many times with the hope of some new recommended practice or standard, but there is probably little that can be done to tighten the expectations until the wide variety of source material itself tightens. Perhaps as analog TV becomes a diminishing percentage of viewed TV, overscan can once again be tightened up? Until then expect 6% to 8% to be typical with it sometimes reaching as much as 10%.

This article may not be duplicated, reprinted or published in whole or part without express written permission from the author. dbroberg@hotmail.com

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