FREQUENTLY ASKED QUESTIONS
1. WHY HAVE MOST NETWORKS SELECTED PROGRESSIVE SCANNING FORMATS?
Experts agree that progressive scanning is the best format for capture, transmission and display.
The inventors of the FCCs DTV Standard, the Grand Alliance, agreed that interlace in the transmission path should be eliminated and that they would work towards that goal. Selection of 720P HDTV reflects an effort to reach that goal quickly for the benefit of viewers and the networks' abilities to better compete in a digital era.
2. HOW DO NETWORK SELECTIONS OF PROGRESSIVE RELATE TO THE FUTURE OF HDTV SETS FOR CONSUMERS?
These networks believe that new flat panel displays, which are just now being introduced, represent the future of true mass consumer adoption of HDTV. Those flat panel displays are inherently progressive. These networks have elected to aim for the second generation of consumer sets in selecting the progressive HDTV format. In the meantime, the first generation glass tube and projection digital television sets are designed to decode and display all ATSC formats, including 720P.
3. WILL PROGRESSIVE SCANNING BE COMPATIBLE WITH DECISIONS MADE BY OTHER BROADCASTERS?
Absolutely. This decision is completely in accordance with the FCCs mandated DTV Standard adopted in 1996. All broadcasters must comply with this standard. As a result, ALL digital televisions being designed for introduction this fall will receive and display either the progressive or interlaced formats. There is no need for concern from consumers that they might have to pick amongst their favorite sources of entertainment.
4. WHAT IS THE DIFFERENCE BETWEEN INTERLACED AND PROGRESSIVE?
Progressive formats represent the most advanced digital broadcasting technology available. Progressive scanning (used in all computer monitors and the advanced flat panel displays) "paints" every line of the picture sequentially every time the screen is scanned. It takes only 1/60th of a second to scan one complete picture. By contrast, interlace scanning, invented over 60 years ago, first paints the odd numbered lines every time the picture is scanned, followed by the even lines.
With interlaced formats, any horizontal motion in the original source between frames results in blurring. Similarly vertical motion results in flicker (the "venetian blind" effect). These defects are eliminated in progressive scanned systems. For example in 1080I, a moving football quarterbacks image is dissected into 1080 lines ONLY HALF OF WHICH are painted each time the screen is scanned. The portions of the quarterbacks body represented by the other half of the lines remain back where he was before he began to move. By contrast, progressive scanning re-paints the quarterbacks entire image EACH time the screen is scanned. As a result, progressive formats do a better job of rendering moving pictures.
5. IS 720P "REAL" HDTV?
The Advanced Television Standards Committee (ATSC), the group that established the standards for digital television, defined HDTV to include both 1080I and 720P. The Consumer Electronics Manufacturers Association (CEMA) has likewise defined HDTV to include both 1080I and 720P.
6. ARE 720P AND 1080I EQUIVALENT HDTV?
Actually, these networks think 720P is better. To focus solely on the "number of lines" greatly oversimplifies the complexities of the human visual system and threatens to mislead and confuse consumers. Contrast and brightness have a greater impact on the human visual system than does resolution. The 720P picture is brighter and has greater contrast than the 1080I picture.
The number of lines of resolution in progressive and interlace pictures are not an "apples-to-apples" comparison. In the time it takes 720P to paint 720 lines, 1080I paints only 540 lines. And, by the time 1080I does paint 1080 lines, 720p has painted 1440 lines.
In side-by-side subjective testing performed by the Advanced Television Test Center under the auspices of the FCCs Advisory Committee on Advanced Television Services (ACATS), it was shown that 720P had "no artifacts" under a variety of conditions, while 1080I, under the same conditions, showed "increasing quantization noise and blockiness " Nevertheless, these distinctions are slight, and the ACATS took pains to note that there was no substantive difference in picture quality between the two formats.
7. WHICH SCANNING FORMAT IS BEST SUITED TO THE FUTURE?
Whereas interlace scanning is analog friendly, progressive scanning is digital friendly. Progressive scanning greatly simplifies conversion to all other scanning formats. It is the preferred scanning format for digital signal processing. Even today, standards conversion, digital video effects such as squeeze, zoom, flip, etc., are all done in progressive.
Progressive is also better suited to multimedia applications, creating a rich, new environment for creative storytelling in the converged world of computing and television.
The 1080 x 1920 (1080I) interlace format specified in the ATSC standard CANNOT be compressed to fit in a 6MHz channel without creating objectional artifacts and it has been recommended that the 1920 pixels be sub-sampled to 1440 to reduce compression artifacts. Therefore, encoder manufacturers have elected to discard approximately 25% of the picture for over-the-air transmission. This compromise is not required for 720P. More of the original picture information remains through the transmission chain.
8. WILL THERE BE EQUIPMENT AVAILABLE TO BROADCASTERS TO ADOPT 720P?
Until very recently, based on engineering investments made many years ago before the advent of digital signal processing, most professional broadcast equipment was based on interlace scanning. However, in the past few years, there has been an explosion of interest in progressive format equipment. The networks have identified manufacturers for each of the critical components required to introduce HDTV broadcast inside the networks and therefore to consumers. Panasonic and other equipment manufacturers have accelerated their 720P production schedules based on these networks' decisions.
9. WHAT ABOUT CABLE COMPATIBILITY?
The progressive formats selected by the networks are compatible with the digital set-top device being developed by Cable Labs and the cable industry. The goal of the networks is to seek voluntary cooperative arrangements with cable operators for the benefit of consumers.