09.09.2005 02:29 PM
Bugs in the System - TELEVISION: Bigger Isn’t Better For Broadcasters
There are issues and difficulties associated with the U.S. H/DTV transitions. And then there are the bugs.
As recently as last month, Sinclair Broadcast Group Vice President of New Technology Nat Ostroff wrote in Broadcasting & Cable, “Here we are in 2005, and there is still no demonstration of reception of the ATSC 8-VSB transmitted signal that shows that it has the same ability to be received with simple antennas as today’s analog system.” It may also be astonishing that, in late 2005, there are virtually no motion-compensating HDTV standards converters. When converting 25-frame 1080i HDTV to 29.97-frame, therefore, the signals are sometimes downconverted, standards converted, and then upconverted or are slowed to 24 frames per second, deinterlaced, and then frame-rate converted using a film-style “3-2 pulldown” process.
Fortunately, those flaws are being addressed. LG has demonstrated a digital-TV demodulator that can work with simple antennas (though long after their first demonstration they have yet to sell the system in a set-top digital-TV adapter). And Digital Vision and Vistek plan to introduce an affordable motion-compensating HDTV standards converter next year. The bugs, however, are something else.
In common television parlance, a network’s or television station’s identifying logo is called a bug. Bugs are commonly placed at the lower right corner of the picture. Unfortunately, in an era of multiple aspect ratios and limited HDTV standards, no one seems entirely certain of where the lower right corner of the picture is located.
A 4:3 television display that chops the sides off a 16:9 image theoretically truncates 25% of the width of the original—12.5% on each side. But SMPTE Recommended Practice (RP) 218, dating back to the early days of film being seen on television, says that the only safe area in which to place a title is 10% from the edge of the picture. Ten percent of a 4:3 picture extracted from a 16:9 video signal is actually 13.33% of the original. Add that to the original 12.5%, and a viewer watching the show in 16:9 might see the bug more than a quarter of the way into the picture.
So perhaps a broadcaster or network will forego the bug’s visibility to viewers of 4:3 displays. But where should it be placed for viewers of 16:9 displays? If it is placed according to SMPTE RP 218, it might be clearly visible on 16:9 screens and partially visible on 4:3 screens, leading to complaints. If RP 218 is ignored, however, bugs might be truncated on 16:9 screens.
Why? RP 218 was originally created to account for variations in film transports, telecine framing and the picture-size circuits in early television sets and loose specifications of broadcast television signal blanking widths in government regulations. In the digital television era, there should be no such slop.
The 1080-line version of HDTV is said to be 1920 x 1080, not 1920 plus or minus 20 samples x 1080 plus or minus 10 scanning lines. A projection TV or TV projector using one or more small imaging devices (D-ILA, DLP, LCD or LCoS) or any flat-panel TV (LCD, plasma or such promised technologies as OLED or SED) has a fixed image structure. Whether it’s 1920 x 1080, 1366 x 768, 1280 x 720 or 853 x 480, it is what it is, and no voltage variations, heat, aging or adjustments by a disgruntled factory employee can change any of those specifications. But a 1920 x 1080 HDTV signal will normally be expanded for presentation on even a 1920 x 1080 display.
Why? There are many reasons. Cinematographers and videographers have become accustomed to a difference between the camera aperture and the display aperture. Picture edges can cause problems, especially in compression systems. And then there’s the idea that bigger is better.
If a prospective buyer sees side by side 42-inch TVs, and the picture on one of them seems bigger, will that influence the purchase decision? Whatever the reason, display manufacturers are blowing up imagery, leaving bug positioners in a quandary. The expansion ratio varies wildly, sometimes exceeding even the safe-action area of SMPTE RP 218.
Will a new standard help? Fox Senior Vice President of Television Engineering Jim DeFilippis hopes so, and he’s working on one. All types of image framing are affected by the issue, but only one makes viewers say, “Stop bugging me!”
Mark Schubin is an engineering consultant with a diverse range of clients, from the Metropolitan Opera to Sesame Workshop.