Going Shopping for HDTV
OK, so you're finally ready to take the plunge and buy an HDTV. I'll leave the decisions regarding technology, screen size, and other matters for another time. For now, I'll concentrate on how to judge the performance of the display's video processor, which is a primary factor in determining the quality of the image you see on the screen.
When you go shopping in a store, take along several high-quality DVDs that you are very familiar with. Include movies that originated on film, titles that originated as video, and possibly some animation, all of which will let you test the processor's ability to handle different frame cadences. Make sure that some of the material has scenes of fast motion, as well as scenes with large areas of flat color, such as a blue sky or mostly dark scenes. This will let you observe the processor's noise-reduction capabilities. Finally, take a test DVD, such as Silicon Optix's HQV Benchmark, which includes images designed specifically to expose the strengths and weaknesses of a display's video processor.
When you get to the store, start with standard-definition material, since that will comprise the majority of broadcast images for some time to come. This includes the realm of DVD, which is why it's important to bring a few along. The video processor in any HDTV must deinterlace the signal and scale it to the display's resolution, adding as much as six times the number of pixels in the original image. In many ways, this is the true acid test of a video processor, because it must generate lots of information that isn't in the source signal and make the image look good on a large screen, which magnifies any flaws in the picture.
Jaggies on the flag (bottom) are caused by poor deinterlacing and diagonal filtering.
Better video processors quickly detect the 3:2 cadence. Not detecting 3:2 causes artifacts like the moiré pattern in the stands (bottom).
Play the test disc and some DVDs and observe how the image looks on the screen. Is it sharp and crisp, or soft, perhaps appearing slightly out of focus? Is there ringing (white lines) along distinct boundaries that separate areas of significantly different colors or shades? You may want to adjust the display's sharpness control to see what effect it has on the image; usually, it should be set to a minimal value to prevent ringing.
With film-based DVDs, look for shimmering in moving areas with lots of small details, like panning long shots of stadium bleachers. If you see this artifact, the processor's 3:2-pulldown detection and compensation isn't working very well. If you can, also check for this artifact with animation discs, which use a different frame cadence than film. Some processors perform 3:2 pulldown just fine with one cadence but not others.
Most high-budget TV shows are shot on film, so the same cadence issues apply to them. But low-budget programs, such as reality shows and soap operas, as well as time-sensitive programming, such as news, sports, and concerts, are shot directly on video. If you own such shows on DVD, play them and look for jagged edges (jaggies) along moving diagonal or curved lines. Many test discs have a waving American flag, which is great for spotting jaggies between the red and white stripes. Other possible artifacts include soft or even blurry images and resolution pumping (going in and out of focus depending on how much motion there is). You can avoid these problems if you use a high-quality, motion-adaptive processor.
Now it's time to look at some real HDTV images. (If the store can't show you a true HD feed, go somewhere else.) Evaluating a processor's performance with genuine HD material is more difficult, since you don't know what images you're going to see from a broadcast source. But you can see if the image looks sharp or soft, which reveals how well the processor deinterlaces 1080i signals. If the source is 720p, such as HDTV programs from ABC, the processor doesn't have to do any deinterlacing, so try to watch some programs that are broadcast in 1080i, such as those from CBS and NBC.
The difference a video processor can make with noise (top). Poor video processors (bottom) eliminate all high-frequency data.
If your video processor can't detect all of the different cadences, you lose half the resolution (bottom), resulting in jagged edges and judder.
If the display is one of the new 1080p models (1,920 by 1,080), the deinterlacing process should result in a supersharp picture with minimal motion artifacts and jaggies. Even 720p and 768p sets must deinterlace a 1080i signal before scaling it to the display's resolution, so the quality of the picture depends on the processor's deinterlacing capability. Unfortunately, many processors take a cost-saving shortcut with 1080i signals that degrades the image considerably. Instead of knitting together both the odd and even lines of each frame, these processors discard one set of lines (either the odd or even lines), thus using only 540 lines of actual picture information. Then, the processor artificially generates the other 540 lines to form each complete 1,080-line frame. This yields an effective resolution of 540p, not 1080p. The picture can look quite soft, and you often see motion artifacts, especially on a large screen.
Better processors use both sets of 540 lines (called fields) to produce a 1,080-line picture that is much sharper because it has most of the information contained in the original signal. The only way to be sure that a processor is or isn't doing this is to send an image such as the SMPTE RP133 test pattern (encoded as MPEG-2) from a source device that outputs 1080i signals, such as a PC with a MyHD PCI video card or one of the latest NVIDIA or ATI graphics cards capable of outputting 1080i RGB. Some signal generators, such as those from Astro Systems, can also send this pattern directly. Such devices can be fairly impractical to lug around, but, if you really want to be sure of what you're getting, this is what you have to do.
I must note that it is mathematically impossible to avoid discarding some pixels in motion during deinterlacing. If an object is in motion, it occupies different spatial positions from one field to the next; if every pixel in both fields is used to construct a frame, the moving object would appear as a double image. In this case, a good processor discards pixels in motion from field 1 and interpolates the required pixels from field 2. Visual artifacts such as jaggies can be avoided with sophisticated diagonal filtering that reconstructs some of the lost data at the edges of moving objects. In addition, the human visual system is less sensitive to detail in fast-moving objects, so a slight loss of detail in these objects is not generally noticeable.
In both SD and HD images, there is bound to be some noise, which can be introduced at many points in the signal path, from the original photography, to the encoding, broadcast, and decoding in the display. One type of noise is nothing but random bits of information that appear as grain in the picture, especially in images of large areas of dark, uniform color, such as those I mentioned earlier. MPEG encoding can sometimes introduce so-called block artifacts that appear to be solid blocks in the image. Then there's mosquito noise, which is caused by excessive digital compression and looks like a swarm of tiny insects flying around the edges of objects.
A good video processor can reduce or eliminate much of these pesky noise problems, while lower-quality processors can actually add new artifacts and motion blur to the picture as they try to remove the noise. Processors that use pixel-based, motion-adaptive, and noise-adaptive temporal filters can prevent additional artifacts while applying different levels of noise reduction to different parts of the image. Look for a display with little or no sign of these types of noise. Now that you know what to look for, go with confidence to your local video retailer and find the HDTV of your dreams. If you choose wisely, it won't give you nightmares after you get it home.
* Jed Deame is co-founder and general manager of Teranex, a division of Silicon Optix.