Several weeks back, mono-note Dermot, while continuing his consistent praise of OFDM modulation over VSB, spilled the beans on why OFDM was inappropriate as a replacement for NTSC in the U.S. I don't know if my friend Dermot fully realized the upshot of his comments, and I suspect that only a few list members caught the import of his comments. It took me a week or two of pondering to realize the implications. What the heck am I talking about? Well, we were discussing probablity eg, f(50,50) of reception with various schemes. Rember, in this context, that 8-VSB is supposed to be the equivalent of NTSC, not the better of it. Dermot bragged how goold fashioned COFDM routinely offered F(50,90) reception probability over entire coverage areas, and with -H, it was F(50,99) (or was it F(99,99)? These figures sound great. For the uninitiated, that means that at fifty percent of the reception locations, at least 90% of the time, a sufficient signal level was received. (Analog also imposes a grade of service figure before these reception levels are relevant, but I'll table that here.) Unfortunately, as any any good and many bad engineers know, NTSC reception specifies a contour signal level and probability level. Unfortunate because the best (at the city-grade contour) that NTSC has to deliver is F(50,80). At the grade A and grade B points, the probability is F(50,50). And, the inferference contour is F(50,10), wich a desired to undesired dB figure specified. Upshot: for all it's superiority, permitting COFDM would have provided a superior signal level to that of NTSC, and -- aside from narrow technical modulation issues -- would have unwound the carelfully (politically) negotiated replacement of NTSC with an equivalent service level from 8-VSB. To permit COFDM, broadcasters would have had to pay a heavy price (or prices). At least, back then. I don't recall anybody offering to pay the government money to permit COFDM operations. Might make sense now, if it's so superior. John ("i'm glad to see Dermot supporting my 'political' position on COFDM in the U.S. with empirical data) Willkie P.S.: I saw a posting requesting information about the origins of 30-foot reception criteria. I'm suprised M. Schubin didn't respond; the 30-foot rule, as I've pointed out here several times) comes from the FCC's R-6602 (Longley-Rice) study, which, if memory serves me correctly, was first published in 1946 or so. P.P.S. I've also seen postings about Ibiquity/IBOC. Sounds great, an engineer of long tenure in the industry told me, except for it to work, Shannon's law has to be repealed, and he pointed out, the folks at Ibiquity are "real pearls" to work with. ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.