Why multiple LNB or multi switch?

[spinnaker]

I am a bit of a techie guy but I have to admit that I know nothing about satellite TV.

Why do I need multiple LNBs to recieve local stations and HDTV? Is it that each of these services is broadcast on a different frequency and each LNB is tuned differently?

Why do I need a multi switch for more receivers? Why can't I just use a splitter? Does it have something to do with the digital feed?

What is an LNB anyway?


Is there a website that explains the technical aspects of satellite TV?

[scooper]

This site or www.DBSforums.com have covered your question in many threads - just do some looking in the old messages ( more than the default) .

In short -
1 - You need multiple LNB's (that thingy at the end of the arm on the dish) for different satellite SLOTS , i.e. 101, 110, 119 are the common CONUS ones in use.

2 - ALL DBS uses the same frequencies .

3 - SPLITTERS are a no-no between the LNB and the receiver - get over it - that's the way it was designed. There are Left-hand circular polarity and Right-hand circular polarity - your Receiver selects which by changing the voltage supplied to the LNB. THIS IS WHY YOU CAN'T USE SPLITTERS ! What the multiswitch does lock the LNB's into LH and the other into RH, then uses internal logic to connect each receiver to it's requested polarity. THE DBS compaies use LH for even transponders, and RH for the odd transponders (I might have that backwards, but the principle is the same)

[spinnaker]

I never said I was complaining about not using splitters. I just wanted to know why I can't use them. I suspected that the receiver controls the LNB.

Thanks.

[Mike123abc]

The LNB is controlled by the reciever. Having 2 recievers sending control votages down the same line to the same LNB would not work

[waydwolf]

Okay, to put it straight out...

DBS as done by DTV and E* use 12.2-12.7Ghz to broadcast from their satellites down to Earth, giving them only a 500Mhz wide band to work with. But open transmissions allow polarization which you may remember from high school science class. Some satellite services use vertical or horizontal polarization but DTV and E* use left hand or right hand circular polarization. This effectively gives them 1000Mhz to work with.

Transponders cut it up further, each with a slice of the 500Mhz, odd on one polarization and even on the other.

Since you can't look at both at the same time with one receiving element, you have to switch between them and that's why splitting is generally not going to work. You need to pass voltage and which receiver sending would control the LNB switch? Multiswitches essentially split the output of multiple elements and feed them to voltage controlled switches that then allow each receiver to control what it sees. This is the only place where a splitter usually happens in DBS, inside the switch itself.

The LNB downconverts the 12.2-12.7Ghz to 950-1450Mhz. The exception was with single sat systems where you could add the second output to the first by upconverting it to 1550-2050Mhz and put both on the one line. That's called stacking. Multiple satellites make a hash of that.

DTV still uses 13/18VDC switching to select polarization/transponder set and a 22Khz tone to select which sat. By binary math this limits them to four transponder sets total. 00, 01, 10, 11. Dish Network on the other hand uses an addressable switch system where multiple switches can be cascaded in complicated ways allowing many sats to be looked at. And their new Dish Pro system uses stacking on top of it, maximizing the use of coax far more than DTV.

The sad thing is that Dish is run relatively ineptly compared to DTV so while they theoretically can deal with much more bandwidth through additional birds, and DTV has no such ability yet, DTV gets all the play and sizzle while Dish doesn't. It's hard to believe that these two companies have different levels of incompetence given their abysmal failure to "kill cable" as they've been bragging they'd do for years, but one is slightly more technically competent, and the other has almost mastered marketing bull.

BTW, DTV can do three sats only because some transponders are not used on each bird and the output from the third one is mixed in with the second and they don't overlap and the receiver pretty much thinks it is seeing two sats worth of transponders. I'd get more technical and quote the fine points, but you get the picture. Reading the tech stuff from Sonora Designs will make it brutally clear.