In a recent test, a half-dozen of my engineering colleagues and I couldn’t identify a high-definition (HD) image from a good standard-definition (DVD) picture on two identically sized TVs any more than 50 percent of the time. You’d have as much luck just flipping a coin. These guys aren’t average consumers, either. They’re hard-bitten video engineers trained to spot video processing artifacts at 10 paces. HDTV can be as much as six times sharper than regular-definition television signals, and we’ve all seen the improvement this offers, so how come we couldn’t see it in this test? And more importantly, will you see it on any of the newest HD displays entering the market?
What the heck is 1080p anyway?
One of this year’s hottest buzzwords has been “1080p” (read: ten-eighty-pee). It refers to the vertical screen resolution of many high-definition televisions that are hitting the market this year. General industry consensus is that a high-definition TV is one that has a vertical resolution of 720p or greater, and this has been the case with any number of plasma, LCD, and digital-projection TVs that have been introduced so far. Generally speaking, a 1080p television is the flagship of the line. And as the top dog they usually command a stiff premium.
So how can a charlatan (that’s me) fool innocent, hard-working HDTV development engineers into believing they are looking at SD instead of HD? Easy, I stack the deck. Usually, there are two things that can go wrong with a 1080p system. First, the content may not be real HD. Second, the viewing environment may not be designed to handle the display. The unfortunate thing is that many well-meaning HD enthusiasts can make these mistakes.
Content
Before we get too far lets cut to the chase: where to
find 1080p content. There is no pre-recorded 1080p content available to the
average consumer, and probably won’t be for a while. But there are two sources
of 1080i content (1080p’s kissing cousin): broadcast and prerecorded media. In
broadcast some of the major networks and several cable and satellite networks
transmit 1080i high-definition content. There’s not much pre-recorded content.
There are several pre-recorded HD formats, none of which have wide industry
acceptance. For the moment, let’s discuss broadcast HD.
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The higher the display’s resolution, the closer you can sit without noticing the pixel structure. With a 1080p TV, you can sit approximately 6.5 feet away, instead of 9 feet for a TV with a 768p resolution. |
It should come as no surprise that any chain is only as good as its weakest link. In looking at the broadcast chain there are a number of strengths and weaknesses. For an HDTV signal to look good the source material must be recorded in a format that is at least as good as the transmission medium. That means either something shot on film and then transferred to HD, or shot on HD video directly with an HD camera. From there, the raw content is distributed to the broadcast stations and finally transmitted over the air, or through cable or satellite systems. If any of those links don’t maintain the HD signal’s quality level then the broadcast image will suffer. Signal degradation can happen in numerous ways. Currently there are broadcast systems that cannot meet the full criteria. Instead of distributing at a more standard 45Mb/s rate, for example, PBS currently transmits at 19Mb/s. In some markets the local broadcasters decode this stream for local branding then re-encode the HD stream to about 15Mb/s. These up and down conversions degrade signal quality as the various decoding and encoding modules operate.
Let’s not forget about pre-recorded HD content. Currently, there is quite a stir about HD-DVD and Blu-ray. These formats are really the future of HD content. The present is bleak at best. There are a few hundred titles on Digital-VHS but this is tiny compared to the number of titles released on standard-definition DVD each day. There are also a few titles in the WMV (Windows Media Video) format but again, nothing like the number of DVDs that are available. All three of these formats require a different native playback device: HD-DVD and Blu-ray require a new DVD player, D-VHS is a videotape player, and WMV really is at home on a high-end PC. Right now, there is no stable platform for significant HD content distribution. Hollywood is also very concerned about releasing high-definition digital content. Last, and really most disturbing, there is even evidence that film transferred to video using current technology may not be capable of producing true 1920 by 1080 resolution.
While most broadcast
television shows and even local news remote units shoot in 1080p (typically with
24 or 30 frame per second refresh rates), the highest resolution high-definition
broadcasts and prerecorded content is 1080i. But nearly all new displays are
progressive, including our new friend 1080p. To convert interlaced video to a
progressive format you must deinterlace it. Why is this important? Deinterlacing
1080i into 1080p creates a valid source of 1080p content.
Reassembly of those
original progressively scanned frames isn’t all that difficult, at least on
paper. The TV’s video processor takes the incoming signal and detects if the
signal originally had a 24 or 30 frame sequence. It then takes the first and
second fields (stored in memory), reassembles the progressive image, and outputs
that frame. To keep the timing right the frame is output twice (and sometimes
three times). Even from this short discussion it should be clear that
deinterlacing (reassembly) is tricky and is best avoided.
Viewing Conditions
But even if you do have good, clean 1080p content, there is an optimal viewing distance for watching your new HDTV, and it’s probably closer than you’d think. The basic idea is to sit back enough so that the line structure (distance between pixels and scan lines) is not visible, but not so far as to not be able to resolve details in the image. The rule of thumb is to be 3,400 times the distance between the scan lines. This distance is easy to calculate. Let’s say you have a 50-inch TV that has a resolution of 1280 by 768 and the vertical height of the display is 24.5 inches. To get the viewing distance, divide the height (24.5-inches) by 768, and then multiply by 3,400. For this example we get a viewing distance of 108.8 inches. So, for that 50-inch, 768p resolution TV, we should sit about 9 feet away. Sit farther than that, though, and we won’t resolve the detail. Sit 9 feet away from the 1080p display and it won’t look any better than the 768p display.
One potential problem that occurs when you sit close to a bright TV is flicker. This is the physical perception of the actual flashing of the light source. Normally, TV images flash fast enough for us not to see the flicker. As the image gets brighter, like with newer technologies, our ability to see flicker increases. It turns out the peripheral vision is also more sensitive to flicker than the central vision area. So, if you rearrange your room so that you can sit 6.5 feet away from your new 50-inch 1080p panel, you will find that the screen covers a greater portion of your central and peripheral field of vision.
Many people can detect the 60Hz refresh that is used for both HD and analog NTSC TV. Some people can even detect the flicker of a fluorescent light bulb (120Hz) in their peripheral vision. Flicker can be reduced by any one of three things: moving the viewer back from the TV (placing the image more in the foveal (center) of the eye), increasing the refresh rate (difficult to do with most TVs), or reducing the overall brightness of the display.
So is that flagship 1080p TV really worth it? Let’s consider:
• Six times as many pixels: Generally speaking this is a weak claim. When considering how much content is encoded in NTSC, VHS tape, and DVD there still isn’t that much HD content available. While there’s enough true HD content out there to make it interesting to even a moderate enthusiast, you won’t be able to pick what you want to see. To use the capability of the TV in the near term, you will be watching what is being produced today. It’s going to be a while before you’ll be able to see the Godfather trilogy in 1080i—let alone 1080p. In addition, many HDTV shows that are supposed to be HD may not take advantage of the system’s full capabilities. Last, if your home theater isn’t correctly set up, you are missing a great deal of the experience. Over time these issues will be resolved as more HD content becomes available.
• Wide format: This overlooked aspect of HD is really more significant than you might imagine. Nearly all film-based DVDs released today are anamorphic (enhanced for widescreen TVs).
• CD quality sound: Audio is a significant element of any video presentation and can really make or break a home theater setup. Ask any serious home video enthusiast how much they spent on each part of their system. Chances are they actually spent more on the audio than the video.
Ultimately, it comes down to how you use your HDTV. If you buy a small screen and sit too far away, then there is no practical advantage over standard, low-resolution CRT. A large-screen HDTV with a close viewing distance (with brightness adjusted to minimize peripheral flicker), access to existing content and a good video processor will create a better-than-theater experience.