STILL MORE ON 999 A.D.

A few paragraphs into the very interesting article "Getting Ready for the (Last) Millenium" [Horizon, May 12], a sentence reads: "Without the vanished Roman legions (powered by slave labor) to maintain roads, cities and fortifications, travel had become extremely difficult." This statement is partly right and partly erroneous. It is right in noting that travel was much more difficult due to deteriorated roads, political squabbles, brigandage, etc.

But the Roman legionnaires would have been surprised to learn that they "were powered by slave labor." While slaves undoubtedly were used when available (e.g., in the siege of Masada), the skilled labor involved in building roads and fortifications was mostly done by the legionnaires themselves, who were not slaves.

The legions were generally assigned building duties when not on campaign, to keep them fit and busy and to provide good roads for military use. At no time were the legions used to maintain cities, with or without slaves, to any significant degree.

Bill Shields

Vienna

I was a little dismayed to find the article refer to "Europe in 999 A.D. -- on the eve of the last millennium." Since the subheading of Horizon is "The Learning Section," I had hoped that at least here there would be some acknowledgement that there was no year 0 and that the last year of last millennium was 1000 A.D. While no one can change the popular perception that we are quickly approaching the millennium, it is the media's duty to report the facts and not be led by popular opinion.

Kent C. Boese

Alexandria

Our article made no claims about the starting date of any millenniums. But for the record, we will state again, and for the last time, what everyone with an IQ above tree bark already knows: The current millennium began with the year 1001. The next one -- and the 21st century, for that matter -- will begin on Jan. 1, 2001. And that's all there is to it.

However, so profound is the allure of big, round numbers that most Americans still are prepared to regard the year 2000 as something special or even auspicious. Many folks in the 10th century (at least those who paid attention to such things), apparently felt the same.

That group included not a few clerics, which is odd. The now-universal Gregorian calendar, after all, was instituted by church-sponsored scholars, among other reasons, to keep the date of Easter on track over decades. Its start-up date was set by the 6th century figure Dionysius Exiguus ("Shorty" in Latin), who put it at Jan. 1, 1 A.D. So the sequence of years ran from 2 B.C. to 1 B.C. to 1 A.D. The rest is history.

-- The editors

Your article on the life and times of year 999 was fascinating, but it raises one large question, to wit: You write that "In the late 10th century, average life expectancy was about 30, largely because the infant mortality rate was 40 percent."

How come, then, that the Bible tells us that "three score and ten" are the "days of our years?" And that supposedly was written way back about 150 B.C. in Psalm 90:10?

Chalmers M. Roberts

Bethesda

SCANNING REVISITED

In the last "How Come" feature [May 12], I believe your expert made a mistake that would bother most engineers. In paragraph five, he is describing "progressive" scanning, which is found on most computer monitors, not TVs that use "interlaced" scanning.

For a TV, the first frame is scanned as he describes it, then the electron beam jumps to the top middle of the screen for the second frame, not the upper left corner, and begins scanning, filling in between each previous scan line from the first frame. Actually, there is a slight slope to the first-frame scan lines so the second-frame scan lines starting halfway across the tube, naturally filling in the space left by the first frame. Then the process for the two frames begins all over again.

Thus, the actual scanning needed to build the complete picture frame is two interlaced frames. Since the scanning of each frame takes 1160 of a second, it takes 1130 of a second to obtain the complete two frames needed to show the complete picture, not 1160 of a second as for progressive scanning.

The engineers who created the TV standard were quite smart because a refresh rate of 30 frames per second (fps) requires less bandwidth than the 60 fps refresh rate since only half of the picture information is transmitted in each frame. This allows more TV channels to be placed in the limited frequency spectrum and still provide a good picture, which is good economics -- something engineers are always concerned about.

Also, they were smart about how low the refresh rate could go and still let the eye/brain combination integrate all of the scan lines from both frames so as to produce a nice picture.

Some older computer monitors used interlace scanning, but as technology advanced, they are not very common any more. Also, the choice of scanning mode was a big issue in designing the standard for new digital TVs, but that's another story.

Harold Corbin

Gaithersburg

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