Video Processing and Signal Handling
This part of our benchmark suite is unique to HDTV reviews. We use a series of pass/fail tests to determine the ability of a display to process different kinds of video signals. Most of the time, you want your source components handling this because they're more capable. If you own an Oppo Blu-ray player, for example, it will exceed the capabilities of pretty much any TV. Set your player to output 1080p video, and the display does no video processing whatsoever. An example of the reverse would be a cable or satellite receiver, which is usually poor for scaling and deinterlacing.
The first tests consist of a group of video clips from the Spears & Munsil HD Benchmark Blu-ray Edition, which is available to anyone online for about thirty bucks. Here’s a quick rundown of what's covered:
2:2 pulldown: This is the cadence most commonly found in content shot on video cameras (at concerts and sporting events, for example). The original image is interlaced, two fields per frame, and the display must integrate them into a single progressive frame.
3:2 pulldown: The cadence most often used to convert 24p film to 60i video, its order is two fields of the first frame and then three fields of the next, in alternating sequence. If the display doesn’t integrate the extra field properly, there is a very obvious artifact that shows in our test clip and results in a failure.
Accepts 24p: Film content on Blu-ray is encoded at 24 frames per second, and all current players can output the signal at that rate. Most displays can accept this signal and process it to a refresh rate that’s a multiple of 24 by using repeated frames.
Very few displays of any type or price can pass the 2:2 test. Where would you find this in actual content? It’s most common in high-def broadcasts, which are usually 1080i. A notable exception is Fox, which sends its signal out at 720p.
- Tests performed: 2:2 Pulldown, 3:2 Pulldown, 24p
- Pass/Fail result
The second group of tests covers an HDTV’s ability to show signals below black and above white. Unlike PC signals, which range from 0 to 255, a video signal truncates that to 16-235. The areas above and below those values are considered head and toe room, and are not used in correctly-encoded content. It is desirable, however, for a display to at least be able to show the levels between 0-34 and 236-255. It makes calibration easier, and occasionally content does stray outside the limits.
The Chroma Burst pattern shows a series of single-pixel lines, in color, to determine if a display actually achieves its maximum native resolution. Most HDTVs return different results for RGB signals than for component (YPbPr) video. 4:2:2 is the minimum bit depth output from a source; 4:4:4 is more common. Some players can output RGB, which usually eliminates a conversion step in the display. Our test shows which signal mode provides the best resolution performance.
- Patterns used: Black and White PLUGE, Chroma Burst
We hope this gives you a clear understanding of our testing methods and why the results are important. Which tests have more meaning will depend on your particular application. If you’re a photographer, color accuracy and gamut volume will matter more than input lag or viewing angles. For gamers, contrast and panel speed are likely to be the deciding factors in a purchase decision rather than color accuracy.
As always if you have questions, please let us know in the comments section.