THE BIG PICTURE

Joel Hoekstra asks a couple of common questions about TV specs:

My question has to do with the video processor found in all TVs. What defines a good one? Some TVs have 8-bit processors, others have 10-bit. Some have a special name like LG's XD Engine and Samsung's DNIe.

I've been looking at the specs for "Walmart brand" TVs here in Canada, like Insignia, Dynex, etc. The specs seem to match up fairly well except that most of these cheaper models have an 8-bit video processor, so I wonder what kind of performance you get from a processor like that?

Also, I see really high numbers for dynamic contrast ratio—say, 10,000:1 or something like that—but the true contrast ratio is often around 2000:1, with some as low as 750:1. What would you say is an adequate true contrast ratio?

Great questions, Joel. As far as a TV's video processor is concerned, the special name given to it by some manufacturers means absolutely nothing—this is merely marketing mumbo jumbo. Processors with 10-bit resolution generally perform better because they are more precise in their calculations than 8-bit processors, but that's not the only criterion by any means.

Unfortunately, there's no way to judge a processor's quality from specs—you have to see it in action. The best way to do that is to play test material and watch the result.

In my opinion, the best video-processor test discs are Silicon Optix's HQV Benchmark DVD and Blu-ray, which are available here. If you play these discs at 480i and 1080i, respectively, the TV's processor must deinterlace and scale the signal, and the content on the disc is designed to make it easy to see how well the processor does these tasks. Other tests reveal the processor's noise-reduction capabilities. The discs include explanations of what to look for when running the tests. I realize that Walmart might not let you play your own discs on the showroom floor, but there is no other way to know how well a TV's processor works.

On the other hand, there are some OEMs (original-equipment manufacturers) who are known to make good video processors that many TV makers use in their products. The best processor OEMs include Silicon Optix HQV (recently acquired by IDT), Gennum VXP, Faroudja DCDi, Marvell Qdeo, and Anchor Bay/DVDO VRS. If a TV touts a video processor with one of these monikers, it's probably pretty good.

Regarding contrast-ratio specs, they are usually worse than meaningless—they are misleading. Most manufacturers use unrealistic methods to come up with the largest possible numbers, but these methods have nothing to do with real-world performance. If you look at the contrast–ratio measurement in any TV review on UAV, you will see that it's typically way lower than the manufacturer's spec—in fact, I don't even publish that spec because it has no relationship to reality.

Dynamic contrast is the ability of many TVs to automatically adjust the contrast according to the overall brightness of the image and/or the amount of room light. As you point out, it's usually a much higher number than the TV's "true" or "native" contrast ratio. I generally disable dynamic contrast, preferring the look of a picture that holds a steady black and white level.

One more thing about contrast ratio, which describes a system's dynamic range—that is, the difference between the lowest and highest light levels that the system can produce or sense. The human visual system is sensitive to an overall dynamic range of about a billion to one, but not all at once. At any given moment, we can discern a dynamic range of only about 1000:1, and this range shifts up and down depending on the overall amount of ambient light in the environment.

This is why your eyes hurt when you exit a darkened movie theater into bright sunlight—it takes a few moments for your brain to shift its dynamic range upward to accommodate the higher ambient light level. Similarly, when you go from a bright environment into a dark one, you can't see much at all until your brain shifts its dynamic range downward. Thus, if a TV has a native contrast ratio much greater than 1000:1, your brain must continually shift its dynamic range up and down as the picture goes from very bright to very dark and vice versa, which means you might miss some of the detail in the picture.

If you have an audio/video question for me, please send it to scott.wilkinson@sourceinterlink.com.