[Surreywinter]

Hi

We've recently moved into a new house. The satellite system is quite complex and I'm just starting to get the hang on it.

The dish is on the roof and feeds a box in the roof. This box is some sort of booster and has five output cables that lead to different rooms. There's no decoding done by this box - each room has to have its own standard sky or free view box - it seems to be a signal booster of some sort. It was described to us as being a "small hotel" system. I can't actually even understand what the writing on the box is - its all in German.

While I can understand the concept of a signal booster I've been puzzled by the fact that there are five cables from the dish into the booster. I'd have expected (but not without any technical understanding) a single cable that would then be boosted into five cables a little like a conventional TV booster.

Does anyone know what sort of thing I'm looking at?

[fernandez]

Sound like it could be a multiswitch as per the examples here
Multiswitch. Multiswitches to be used with a quatro LNB. Satellite Superstore UK. Big Discounts.

The dish is probably larger than the usual Sky mini-dish and will be fitted with a quattro LNB

[Ghostleader]

Its not a booster, it's a multiswitch used for distribution to multiple room's or houses. One of the cables is from a aerial, the other four are from the dish via a special LNB for the purpose of feeding this multiswitch.

[Surreywinter]

If I'm understanding what I've read right having four cables from the dish means that different rooms can have different channels - is that correct?

[Ghostleader]

Yes, each receiver would have the full range of channels available to it from whatever sat the dish is trained on.

[Surreywinter]

Originally Posted by Ghostleader

Yes, each receiver would have the full range of channels available to it from whatever sat the dish is trained on.

At the risk of this becoming a tutorial does that mean that my naive assumption that all channels are coming down the cable for selection by the decoder is incorrect and that the LNB has some part to play in the channel selection? And a quatro LNB can do this for four different channels simultaneously?

[logiciel]

Originally Posted by Surreywinter

I'd have expected (but not without any technical understanding) a single cable that would then be boosted into five cables a little like a conventional TV booster.

That's the bit you got wrong - satellite installations don't work the way terrestrial aerials do.
There are four separate connections to the dish - to the four outputs of its LNB - so that each receiver gets a connection of its own and can use it to select a separate channel - the LNB doesn't do the selecting.

[MartinPickering]

This might help: Installing a multiswitch

[MartinPickering]

And this: LNB mysteries explained

[grahamlthompson]

My explanation

Digital Spy Forums - View Single Post - Will the new Humax PVR work with 1 LNB feed?

[MartinPickering]

An excellent explanation. In fact I may even steal it, correct the typos, and claim it as my own!
(But not without permission.)

[logiciel]

Yes, it's good, but links to other forums are not allowed.

[MartinPickering]

Ah, my apologies! I didn't even notice it was on a different forum.
Well, with everyone's permission, I can copy it to a web page and put the link here?

[logiciel]

Sure can but why not post the content here anyway - "with everyone's permission".

[MartinPickering]

OK, that didn't occur to me! Try this slightly-corrected version.

How an LNB works - by Graham Thomson

First of all the difference between satellite digital and Freeview terrestrial:

In Freeview, all the available TV signals come down a single cable and you can split the signal so that everything you connect can receive any channel. The aerial does nothing but gather all the available signals and squirt them down a single coaxial cable.

Satellite is very different: it comes from satellites which orbit the earth some 22000 miles away. Imagine being able to get TV via aerial from a transmitter that far away. About 50 miles is the maximum if you are lucky. The satellite signals are transmitted at a very very high frequency, known as microwaves, and are extremely weak. Microwaves are too high in frequency to send through a cable and behave in many ways like light. To use these very weak signals they have to be concentrated (collected) and this is done by a reflective dish which has a special surface shape known as a parabola; this bounces the parallel microwaves from space and concentrates them into a small space at the dish focal point. (In the same way light can be concentrated using a magnifying glass or parabolic mirror.)

Now that a concentrated source of microwaves is available, the frequency has to be reduced to allow them to be sent down a coaxial cable to a satellite TV tuner. The gadget that does this is called a LNB (low noise block-downconverter) and this is the equivalent of your normal TV aerial.

Hope you are following this so far as it's about to get a little more technical!

Firstly, unlike a TV aerial, you can fit more than one LNB in a single small housing. Four is common (quad-output LNB) and, as it happens, four is a very significant number.

For technical reasons, each LNB can only handle around one quarter of all the available signals from a satellite and the receiver tells the LNB which quarter it needs to watch the channel you request. Here comes the major difference to your normal aerial: the receiver has to be able to communicate with the LNB. How does it do that? Well, simply, it has to send a signal up the cable to the LNB, and provide power. (That's why you should turn off a satellite TV receiver to connect or disconnect the coax connection.)

It should now be obvious that the satellite signal from a single LNB can contain only around one quarter of all the available signals at any one time but that the receiver can get any one by simply telling the LNB to switch to a different quarter.

Here we have the explanation why you can't simply split the signal to more than one tuner. Imagine one tuner wanting the first quarter of available channels and the second a different quarter - result electronic chaos - remember the LNB can do only one quarter at once. You can split the signal but, as only one tuner can control the LNB, the second tuner will have only the same quarter of channels available as the first tuner is requesting.

So how can a load of people in a block of flats get access to all the channels when there is only one dish. The obvious answer is to provide each flat with a separate LNB. Sadly, there is a limit to how many LNBs you can fit on a single dish. So how is it done? Remember I said four was a significant number. Here's why: if the dish is fitted with a "quattro" LNB, that again is simply four in one housing. What's the difference between a quad-output and a quattro LNB? Well, a quattro LNB does not need telling which quarter to work with; it's permanently set to one quarter; LNB output one the first quarter, LNB output two the second etc etc. In this way, four coax cables come from the dish with a different quarter of the available channels.

All four cables are connected into a very special "multiswitch", which has lots of outputs (at least one for each flat). Suppose you are sitting in flat one and want to watch a channel in quarter one. Your sat reciever sends the signal to what it thinks is an LNB (but is actually a "multiswitch"). The switch intercepts the signal and simply connects your box to LNB output 1. Next suppose your noisy neighbour in flat 2 ( or maybe the glamorous blonde - we can all dream can't we ) wants to watch quarter 2, well you guessed it the switch connects flat 2 to LNB output 2. So all your neighbours can watch any channel at once as each receiver selects one of the four available LNB outputs, via the "multiswitch".

Now we come to the twin-tuner PVR, which needs full access to all channels to fully work for both tuners. It should now be obvious that, unless you can persuade the glamorous blonde next door to move in with you and bring her precious sat cable with her, you need another cable back to the fancy switch. Is that possible? Maybe you would have to ask your landlord.

It all sounds very restrictive, but, thanks to a happy coincidence, by far the majority of Freesat channels are concentrated in the first 2 quarters making many more available than might be thought.

For a full list see here:

Operating a PVR with one input | Join Freesat

In conclusion, if you go for the Freesat PVR, remember that there is nothing to stop you keeping Freeview as well; the two are complementary, not exclusive.

[Surreywinter]

Thats great - thanks guys - even I nearly understand!

The final part of the equation that's stumping me is where does the Sky+ cable go? We definitely have Sky+ but I can only see four cables coming off the bottom of the LNB. I even went to the extreme end of photographing the dish last night with a zoon lens and counting the cables on the screen & unless one is cunningly hidden behind another there's only four.

[logiciel]

There ARE only four from the dish - there are a maximum of four from ANY normal Sky dish.
Go back to post #3: "One of the cables is from a aerial, the other four are from the dish".

[Surreywinter]

Originally Posted by logiciel

There ARE only four from the dish - there are a maximum of four from ANY normal Sky dish.
Go back to post #3: "One of the cables is from a aerial, the other four are from the dish".

Understood - that's consistent with what I'm seeing. However I'm also aware that there's a Sky+ cable in the mix. Does the Sky+ cable feed from the multiswitch rather than the LNB?

[fernandez]

Originally Posted by Surreywinter

Understood - that's consistent with what I'm seeing. However I'm also aware that there's a Sky+ cable in the mix. Does the Sky+ cable feed from the multiswitch rather than the LNB?

The quattro LNB fitted has 4 outputs namely: High band V, High band H, Low band V & Low band H. They are designed to operate with a Multiswitch to give access to 8, 12, 18 etc receivers

So, yes, the feed to the Sky+ box (requiring two feeds) comes from the multiswitch

[Surreywinter]

Many thanks all - much appreciated!