Dumbed down consumer electronics: Adding DTV channels
in [Design]
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From: Greegor on 6 Aug 2010 12:25 Martin Brown wrote > > What broadcast frequencies are we talking about in the US? I am guessing > > upto 1GHz or thereabouts. A phased array can be of very modest size. You > > probably only need to roughly null out a narrow band of sky. Joerg write > Just the UHF band is left, with the upper 100MHz gone, auctioned off by > the FCC. VHF has largely been shut down by stations moving to UHF. I > don't think it was a good idea for them to give that up as lower > frequencies mean better range. Fabulous.
From: Tim Shoppa on 6 Aug 2010 16:27 On Aug 6, 10:08 am, Joerg <inva...(a)invalid.invalid> wrote: > Yesterday the usual happened, none of the news channels made into > into this area, they all pixelated out shortly before 10:00pm. Meaning > lots of people in a middle-class neighborhood haven't seen any of the > ads, meaning ... I seem to recall that you're on the left coast and don't get thunderstorms and the like, but I have noticed that here in the Mid-Atlantic, that very wet weather (not necessarily thunderstorms) can render even the strongest local DTV channels unwatchable. With the exact same antenna on analog stations there was no noticeable degredation (except for nearby lightning) at all in the same weather conditions. My pet theory is some sort of multipath from wet tree leaves but I have no scientific evidence at all! Tim.
From: Joerg on 6 Aug 2010 16:40 Tim Shoppa wrote: > On Aug 6, 10:08 am, Joerg <inva...(a)invalid.invalid> wrote: >> Yesterday the usual happened, none of the news channels made into >> into this area, they all pixelated out shortly before 10:00pm. Meaning >> lots of people in a middle-class neighborhood haven't seen any of the >> ads, meaning ... > > I seem to recall that you're on the left coast and don't get > thunderstorms > and the like, but I have noticed that here in the Mid-Atlantic, that > very wet weather (not necessarily thunderstorms) can render even > the strongest local DTV channels unwatchable. With the exact same > antenna on analog stations there was no noticeable degredation > (except for nearby lightning) at all in the same weather conditions. > > My pet theory is some sort of multipath from wet tree leaves but I > have no > scientific evidence at all! > Yup, you got it. When clouds roll in it's all over with DTV while analog was just fine. I guess this is called progress :-( -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Paul Keinanen on 7 Aug 2010 03:27 On Fri, 6 Aug 2010 13:27:50 -0700 (PDT), Tim Shoppa <shoppa(a)trailing-edge.com> wrote: >On Aug 6, 10:08�am, Joerg <inva...(a)invalid.invalid> wrote: >> Yesterday the usual happened, none of the news channels made into >> into this area, they all pixelated out shortly before 10:00pm. Meaning >> lots of people in a middle-class neighborhood haven't seen any of the >> ads, meaning ... > >I seem to recall that you're on the left coast and don't get >thunderstorms >and the like, but I have noticed that here in the Mid-Atlantic, that >very wet weather (not necessarily thunderstorms) can render even >the strongest local DTV channels unwatchable. Digital television is typically operated only a few dB above threshold, producing a "perfect" image. However, if the signal level drops only 2-5 dB, the signal can be below thresholds for extended period of times and the picture is useless. > With the exact same >antenna on analog stations there was no noticeable degredation >(except for nearby lightning) at all in the same weather conditions. If the normal signal to noise ratio is say 50 dB, you would hardly notice, if it drops to 45-48 dB. At least in Europe, the digital transmitter ERP power was initially 10-20 dB below the analog TV synch tip ERP power, however, in order to maintain sufficient fade margin, the -20 dB settings are now out of the question. > >My pet theory is some sort of multipath from wet tree leaves but I >have no >scientific evidence at all! The refractive index for vacuum is by definition n=1 and for air n=1.0003xx. In propagation calculations, the relative refractive index N is often used and it is the number of ppms above 1.0. At sea level N=300 is quite common. The relative refractive index depends on the "dry term", which depends of air pressure, temperature and height and the "wet term" depending on temperature and amount of humidity. The relative reflective index in general drops at higher altitudes and reaching N=0 in space. The globally annual average gradient is -40/km, while at some places it can be -30 /km and others up to -150/km (in desert areas in particular). For a particular place and a particular altitude layer, the instant gradient can vary even more than this during the year. The reflective gradient is responsible for slightly bending the radio waves towards the ground and hence, the radio horizon is slightly further away than the optical horizon. For N=-40/km, the radio horizon is about 15 % further away than the optical horizon. For N=-157/km, the radio waves exactly follow the curvature of the earth and the signals could travel thousands of kilometers as long as the gradient remains stable. In practice any layer of warm/cold and/or wet/dry masses at some altitude along the signal path will alter the gradient, sometimes bending the signal slightly upwards above the receiver and into the space. Sometimes it is bent slightly more downwards than normally and strong signals are available just behind the horizon. With a strong gradient, the transmitter signal hits the ground in front of the receiver and is reflected from ground up to the space and is lost. With very strong gradients (N<-157/km), ducting will occur, the signal reflected from ground will be bent back towards ground at some other place to be reflected again from the ground. Depending on the location of the receiving station, it might be just were the ground reflection occurs and a strong signal is obtained or it might be between the location of two ground reflections and the signal passes over the receiver station. Since the atmospheric conditions vary constantly, so will the signal strength vary with a typical rate of some hours "slow fade". This kind of fade affects all frequencies equally (at least the whole UHF TV band) as is known as "flat fade". However, if the receiving antenna also gets local reflections from ground and nearby buildings, there are also going to be multipath (fast frequency selective) fading atop of the slow fading, since even minute variation in the gradient will cause variations in the amplitude and phase of both the direct and reflected multipath signal, causing fast variations in the frequency selective fading, when those are combined in th receiver antenna. The fading that took out all channels for an hour or two was clearly slow flat fading due to variation in the refractive gradient. Failing to find all channels at once in a program scan looks very much like a fast frequency selective multipath fading. In general, the signal from a distant station are often strongest very early in the morning, when the ground is still very cold and some inversion layers have been formed. Try to scan for the channels early in the morning, when the average signal levels are higher than during a hot evening.
From: David Lesher on 7 Aug 2010 13:02
Tim Shoppa <shoppa(a)trailing-edge.com> writes: >I seem to recall that you're on the left coast and don't get >thunderstorms and the like, but I have noticed that here in >the Mid-Atlantic, that very wet weather (not necessarily >thunderstorms) can render even the strongest local DTV channels >unwatchable. With the exact same antenna on analog stations >there was no noticeable degredation (except for nearby >lightning) at all in the same weather conditions. Note that many DTV channels are a) on different frequencies b) from different transmitter sites/antennas with c) different power levels than with NTSC. That said, in DC "7" and "9" DTV are on RF 7 & 9. But there's been a major effort to understand why VHF freq's have delivered so much worse ASTC propagation than was there under NTSC. Multiple stations have gotten temporary FCC authority to raise power levels. One suspect is the cheap CECB {couponed converters} suffer from front-end desensitization by FM broadcasters. -- A host is a host from coast to coast.................wb8foz(a)nrk.com & no one will talk to a host that's close........[v].(301) 56-LINUX Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433 |