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Aug 1, 2008 12:00 PM
Transition from monochrome to NTSC transmission
Dear editor:
I just read the interview with Tim Carroll of Linear Acoustic in the July 8 edition of the “HD Technology” e-newsletter. He made the closing comment about how after 13 years of having the ATSC standard, he is amazed that the industry is down to a few months before analog cutoff and is still dealing with so many loudness-related issues. I wonder, from a historical point of view, did we have this magnitude of issues when television went from black and white to color? If not, why? Somehow it seems the transition was easier — much easier — as long as you forget about the CBS color system versus RCA.
Gary Brefini
SBE member
John Luff responds:
The transition from monochrome to NTSC transmission was almost trivial, but the production issues were enormous. First, you can start at recording. A quad recorder that only handled monochrome needed just a rudimentary time base error corrector, while color required nanosecond accuracy. For many years, there were issues with controlling saturation of the picture, which, if not limited, created interference with the recovery of the sound subcarrier.
Similarly, production professionals had to figure out how to make a picture that was pleasing in color but also looked acceptable on legacy sets that didn't filter out the color subcarrier living smack in the middle of the video passband. DAs and other devices that clamped the video on the back porch (between sync and start of active video) had to contend with burst, which many could not. Sync generators were replaced to get genlock accuracy needed for color. Plenty of primary engineering had to be done, often as problems came up. Ask anyone who edited quad tape about color framing and editing.
Size matters
Dear editor:
Does size matter for HD transmissions? You bet it does. The satellite industry uses nonionized radiation. Although not radioactive, at higher levels it could still cook soft skin such as the cornea of the eyes.
Although fully licensed by the FCC, 2.4m C-band antennas are often incapable of providing high-grade, high-order modulation HD transmissions without using higher power levels than published by the FCC.
Link budgets and hazard studies have consistently yielded poor performance results for these antennas, and yet 2.4m platforms are being assembled as you read this. These antennas lack the gain needed and interfere with adjacent satellites while exposing everyone around them to high-level radiation.
Coupled with two 750W phase combined amplifiers, a comparison between a 2.4m and a 4.5m antenna yielded the following results: The 2.4m yielded acceptable results for a DVB-S, QPSK, 18Mb/s carrier at 130W. The Eb/No and BER were usable. For DVB-S, QPSK, 40Mb/s and 60Mb/s carriers, the antennas yielded unacceptable results.
The FCC published the maximum feedhorn flange power density allowed as -50.90dBW/Hz. With a 2.4m antenna, the total EIRP needed for the 60Mb/s carrier (for acceptable Eb/No, BER, EVM and MER) is 72dBW. The 2.4m dish gain is 41.8dBi, which leaves 30.2dBW or 1047W needed from the amps. At that power level, the feedhorn flange power density is -45.80dBW/Hz, higher than what the FCC requires.
Such a level produces a wider radiation pattern around the antenna, up to 30m in diameter. In turn, this creates a wider beam to space, which would most likely interfere with adjacent satellites using similar frequencies. This clearly is not a good outcome.
The 4.5m antenna (with 48dBi of gain) would need 251W to achieve acceptable Eb/No, BER, EVM and MER at the receive sites. The power density would then equal -52.0dBW/Hz, a lower, safer value than what the FCC requires.
An opposing argument would be that the 2.4m antenna is mounted on the top of the SNG vehicle, and, therefore, all the radiation is focused toward the satellite. This argument is valid under very low-power outputs; higher power levels are magnified by the antenna, therefore creating a wider electromagnetic field around the dish, the vehicle and passersby.
Eddie Maalouf
Gil Hanna
Electronic version of Broadcast Engineering
Dear editor:
I was wondering if Broadcast Enginnering comes in a PDF or CD form?
Mitch Kindley
F.C. Schafer Consulting
Concord, NC
Editor responds:
Currently, Broadcast Engineering is not available in a pdf or CD form. However, we do produce a monthly Ezine, an online version of our world edition. For more information, visit: http://subscribers.broadcastengineering.com/subscribe.cfm?tc=NN6031
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