Craig wrote: > >You could transmit all three channels (and others) synchronously from > >each stick, Bert wrote: > No. And the reason is simple: the towers are too far apart to do this > reliably. > > A true SFN depends on guaranteeing that by the time the signal of > one tower , etc............. Bert, that was an excellent analysis. Dale > -----Original Message----- > From: opendtv-bounce@xxxxxxxxxxxxx > [mailto:opendtv-bounce@xxxxxxxxxxxxx]On Behalf Of Albert Manfredi > Sent: Saturday, December 23, 2006 12:49 PM > To: opendtv@xxxxxxxxxxxxx > Subject: [opendtv] Re: Bolivia TV standard mystery > > > Craig Birkmaier wrote: > > >Dale wrote: > >>I may not have explained the Santa Barbara markets (121st > >>ranking) transmitter site distribution clearly. The NBC affiliate > >>is located in the extreme north end of the market and the > >>ABC station is 100 miles to the south, with CBS being about > >>in the middle. Santa Maria is centrally located and generally > >>can receive all three, but it deteriorates rapidly as you move > >>north, south, east or west due to intervening terrain. > > > >Sounds like most of the needed infrastucture is already in > >place. In this case, it should be possible to use the three > >sticks already in place. > > I agree with this part. Use the three sticks for transmitter > antennas, and > at best you can use the same frequency channels on the two most remote > sticks, and a different set of frequencies in the centrally located stick. > > That would allow those bent on hype to claim "SFN," where in fact > all you're > doing is reusing frequencies in two towers that are far apart > enough not to > interefere with each other. At least, not to interfere where it > matters to > anyone. > > >You could transmit all three channels (and others) synchronously from > >each stick, > > No. And the reason is simple: the towers are too far apart to do this > reliably. > > A true SFN depends on guaranteeing that by the time the signal of > one tower > is lagging or leading the signals(s) from another tower(s) by too > many usec > for the given receivers, there will be a dominant signal strong enough to > overpower the long lag or long lead signal(s). Let's say, a > dominant signal > that is 15 dB louder, or more, than any potential interering signals. In > areas where the signal of multiple towers are close in strength, > *all* have > to be within the echo tolerance of the receivers. > > This is true for COFDM as for anything else. > > If towers that are spaced too far apart for this to be true, as > these three > are, then you can try to synchronize the towers to achieve that > same effect. > The problem is, you are designing this system based on some sort of > predicted average propagation contour for each tower. You don't have any > actual signal source where you can guarantee that a dominant signal will > exist. You are hoping that the north tower's signal will have been > attenuated so much before it will interfere with the central stick, for > example. > > But this stuff is very variable. Weather, time of year, and terrain, > conspire to screw up your fine-tuned calculations. So on a good day, you > might succeed. On a rainy day, you might fail miserably. And > worse, even on > good days, terrain will make the fine-tuned calcualtions fail in > some parts > of the market area anyway. > > Bottom line: for SFNs, even with COFDM, the safe bet is to locate > the towers > of the SFN within a cluster in which no two towers are further apart than > the GI allows. Check out real-world examples, in Paris and > Berlin. In these > cases, once you get out to the far field, say starting at 20 > miles from the > SFN center, the SFN will behave very much like a single stick. > Make it low > and low powered, and you'll lose the fringe areas completely, just as you > would with any small stick. And no, you can't use OCRs effectively at the > fringes. Not unless terrain offers signal blocking where it's > needed. Which > is why, in those real-world examples, they make use of translators to > provide signal to outlying areas. > > Bert > > _________________________________________________________________ > From photos to predictions, The MSN Entertainment Guide to Golden > Globes has > it all. http://tv.msn.com/tv/globes2007/ > > > > ---------------------------------------------------------------------- > 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. > > ---------------------------------------------------------------------- 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.