How do local channels on satellite TV work?

June 6, 2005 4:46 PM

During a vicious thunderstorm last night, our DirecTV satellite lost the signal for all of our channels except the local ones. This has also happened before. What I'm wondering is, how are these channels delivered?

I would think they're not part of the main signal because otherwise they'd just go out with everything else during inclement weather. Googling doesn't get me much (even the HowStuffWorks article on how DBS works sort of glosses over how local channels get beamed to your house) except mentions in DirecTV FAQs about how "federal law" allows them to offer local channels in certain areas.

Does this law require Comcast or other cable providers in the area to allow DirecTV/DISH/etc. to piggyback on their cable signals to deliver local channels? Does the dish have some sort of powerful antenna in it (because the local channels look like they're cable-quality)? I'm really puzzled as to how it gets from the source to my house, since as I understand it, satellite TV is a fairly passive medium because the signal blankets the continent and things are largely the same across the nation for subscribers. Yet somehow they're able to supply local information (though there's still no local version of the Weather Channel.)

posted by Kosh to technology (3 comments total)

Your local signals really do come in via the satellite but are delivered by spot-beaming. The antennas are aimed at particular parts of the country, allowing the broadcaster to reuse the same frequencies for more than one set of local channels. It could well be that these signals are stronger because the antennas are more directional, which would explain why you could continue picking up your locals during the storm.

In some areas you need an oval dish that can pick up two satellites to get your channels; they do use that second satellite for locals and they also use spot-beaming there.
posted by kindall at 4:51 PM on June 6, 2005

Kindall's got it for the locals. For other channels there is a different explanation. For DirecTV it's just transponders that are weak or the FEC is set wrong. Hey, you try living in space for a decade and see if parts don't start to go bad! :-D Each transponder can easily carry 5 - 20 channels, depending on the quality wanted.

Providers are able to change the FEC rate (Foward Error Correction). A low FEC (1/2) means half the data is error correction, making the signal easier to receive after correction.

A high FEC (7/8) means 1/8 the data is error correction, making the signal harder to receive after correction.

The tradeoff with FEC is data rates. A typical transponder runs at about 20 Msym after viterbi correction (another required error correction), so at 1/2 FEC the provider transmits 10 Msym of usable data. At 7/8 FEC the provider can transmit 17.5 Msym of usable data.

Satellite providers can adjust the FEC down to compensate for a weak transponder (or turn off the weak transponder, boost their other transponders and up the FEC on them for the same effect).

And this (along with other things, like calibration) is why I have to contain my laughter when someone tells me their receiver got "84" signal strength, but their neighbours' got "98". If you want someone useful to listen to you on that, you need to tell them the BER (Bit Error Rate) and signal level in dB.

There's a lot of people outside North America laughing at me for having to tell you this. North America is an EXTREME oddity in satellite TV -- the other 95% of the world population would think us crazy for buying a receiver that *only* works with one provider, ever, never mind doesn't let you adjust things like symbol rate, transponder frequencies, buy legally programming preloaded satellite cards, etc. Oh well *sigh*.
posted by shepd at 10:26 PM on June 6, 2005

There's a lot of people outside North America laughing at me for having to tell you this. North America is an EXTREME oddity in satellite TV -- the other 95% of the world population would think us crazy for buying a receiver that *only* works with one provider, ever, never mind doesn't let you adjust things like symbol rate, transponder frequencies, buy legally programming preloaded satellite cards, etc. Oh well *sigh*.

I don't know about that, we're pretty much the same in the UK and Ireland.
posted by ascullion at 8:55 AM on June 7, 2005