High & Why
by Mark Schubin
Oct 15, 2004, (From http://www.uemedia.net/CPC/videography/article_10488.shtml)
Here are two simple, if little-known, facts: It is, and will continue to be, impossible for most viewers to see the high definition of HDTV. It is, nevertheless, a good idea to shoot in HDTV for all viewers. The highest-definition form of today’s HDTV has 1920 active (image-carrying) picture elements (pixels) per scanning line on each of 1080 active scanning lines—roughly two million pixels altogether. That’s twice as many as in the 720p version of HDTV, which, itself, has roughly five times as many pixels as in U.S. analog broadcast television.
Try this simple experiment. Find something with fine print—an insurance policy, the back of a credit card, or even classified ads in a newspaper. Place the print where it can be read. Then step back. If you can still read it, step back farther. Eventually, you will reach a point where the print is unreadable. At some point, you won’t even see the print. Farther still, you won’t even see the newspaper (think aircraft window at 35,000 feet).
There is a limit to the definition we can perceive. Human vision is said to have a maximum detail resolution of 30 cycles per degree (CPD). That means that two black lines separated by a white space one arc minute wide can under the right circumstances be seen as two lines. If the space between them shrinks, they will be seen as just one line. In fact, many viewers can’t even perceive detail that fine, so 22CPD is often used as a figure for nominal maximum resolution. In television terms, that means that on an ordinary 25-inch TV set viewed at the normal home viewing distance (roughly nine feet) a viewer who can perceive 30CPD will be just able to make out the finest detail in a standard definition, 480-active-line picture. If the perception limit is 22CPD and the picture is widescreen, then 480 lines is the limit even on a 42-inch screen. So why bother with HDTV?
It’s because human beings are more sensitive to sharpness than to definition. Sharpness is a psycho-visual sensation proportional to the square of the area under a curve plotting contrast ratio against detail fineness. Such curves look like the right side of a bell shape. At the finest detail, there’s virtually no contrast, so there’s virtually no sharpness. That’s how Sony can get away with recording just 1440 active brightness samples per line in HDCAM (with JVC and Panasonic doing similar in their D-9 HD and DVCPRO HD formats): There’s a significant drop in resolution from 1920 but a much smaller reduction of sharpness. At the highest contrast, where there’s no fineness of detail, there’s still virtually no sharpness. In between, the higher the curve, the greater the sharpness.
HDTV cameras offer a higher curve than do their standard-definition counterparts. The added sharpness provided by the higher curve is visible to viewers of an HDTV screen of almost any size at almost any viewing distance. It’s also visible on a standard definition TV set. It’s visible even on a set fed from a VHS recording. So, although almost no one can see HDTV, HDTV looks better to almost everyone.
Mark Schubin is an engineering consultant with a diverse range of clients, from the Metropolitan Opera to Sesame Workshop.