Satellite TV questions

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jdjoubert

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I have couple of questions regarding satelite TV signals etc etc...

Seeing as though a DSTV dish can receive 80 odd channels, how much of data can your dish actually receive? Can someone indicate this in terms of how many Gigs/seconds we are talking about?

Also, I was told that in order to have 'duel view', one must have 2 LNB's infront of the actual disc. Why is this since your dish can receive all 80 channels at once?

If you dish does receive all of these 80 channels at any give time, then surely it should be possible to view all 80 channels if you happen to have 80 TV's n your house? Is it not just a matter of getting a decoder that can handle the calculations and decoding of 80 channels at once and then have 80 output ports?

Also, those satllites that are sending all of these channels, how much power do they need to run? I mean, my small 2 way radio uses 5W and can only transmit up to a kilometer or 2 of very crappy quality sound only. How many Watts does those satellites consume in order to send all these streaming video signals?

Just wondering

(my desktop pc went in for repairs today and now I am left with only my laptop so I can't do any work. That's probably the reson for all the questions....... too much time on my hands....)
 
Yes the satelite is broadcasting all the channels at the same time to you, however channels are multiplexed on a specific transponder (think frequency) and that transponder can be broadcasting in a specific polarization (horizontal or vertical)

So with a single LNB, you could be tuned into a single transponder and then you could watch all the channels broadcast on the same frequency and polarization, you would just need a fast enough processor or mpeg chip to decode the signals and display them on the TV's.

This is most probably a rather simplistic view.
 
The 80 channels are all broadcast on the same transponder, or frequency. They are multiplexed, i.e. the data gets sent in sequence, each channel getting a short turn. This happens so fast that the whole process is seamless. Timing, however, is crucial. So much so that the lengths of wiring in the satellite is measured to compensate for the speed of light! The frequency used is in the Gigahertz range, around 20 GHz iirc. The atmosphere is transparent to electromagnetic waves in the Gigahertz range, this is known as the "microwave window". Signal power is in the milliwatt range. The parabolic antenna ("dish") focuses the signal to a point, this is where your feedhorn sits. This would be the thing sitting out in front of the dish on an arm. The lifetime of each satellite depends on the amount of fuel used for course corrections. Although they are in geosynchronous orbit they still need slight course corrections in order not to drift out of orbit. The actual transmission equipment is powered by solar cells charging batteries in the satellite.

The reason for two LNbs is to have two signals fed to two different decoders, or two decoder chips in one decoder.

Hope I didn't get too technical :)
 
Valis said:
The reason for two LNbs is to have two signals fed to two different decoders, or two decoder chips in one decoder.
Click to expand...

The rest all makes sense. Its just this last part that does not make sense.

I have seen a guy with a hacked DSTV decoder and a Linus pc to decode it all and record.
From what I can remember, he was able to record 20 or 30 channels simultaneously on the pc.
 
jdjoubert said:
The rest all makes sense. Its just this last part that does not make sense.

I have seen a guy with a hacked DSTV decoder and a Linus pc to decode it all and record.
From what I can remember, he was able to record 20 or 30 channels simultaneously on the pc.
Click to expand...

if memory serves me corectly on the old dual views it allowed you to use a single LNB but the limitation was both decoders could only view chanels in the same 'bouqet' so if 1 person was watching supersport the other could only watch another supersport chanel, (also you couldnt use the tv guide if installed this way), so i am guessing this is how he managed it
 
There are two signal polarities, vertical and horizontal. Both carry different channels. You can't send both polarities down the same cable. That is why you need a twin. As simple as that.
 
jdjoubert said:
Ok, it makes sense now.

So the decoder tells the LNB which channel to send down the cable?
Click to expand...

No.
The decoder does all the thinking and all the decoding in the decoder itself.
The 2x LNB are needed so that it can dedicate 1 tuner to one channel (on a specific freq) and still have the full spectrum of frequencies available to the other tuner.
Think redundancy.
In a HD PVR + Dualview setup you need 4x LNB's for the HD PVR and another 2 for the other guy. But you still only need a dual LNB receiver... the 2 cables get fed into a "LNB-Switch" which routes the frequencies into many more cables. Converting 2 LNB's into 6! MAGIC!


The amount of data (in terms of mbs/s) is restricted by the size of the dish and the receivers.
In a large building, some body corporates opt for a massive dish with a multi-LNB receiver which leads into further LNB-switches, then each unit gets 1 or 2 LNBs. So there doesn't need to be a gazillion unsightly dishes on everyone's balcony.

A simple way to describe it is the satellite is spraying the whole country with the info, like digital beer. The bigger your beer mug is, the more beer you can catch falling out of the sky.
 
The decoder tells the LNB which polarity to send down. When the decoder sends 12V up the line, it get vertical. When it sends 18V it gets horizontal. There is actually vertical high and horizontal high bands too. This gets send down by the LNB when the decoder or multiswitch sends the voltage and a 22kHz tone up the line. Multichoice only uses the low band though. In a 2-input multiswitch, one input gives the vertical and the other horizontal. The outputs then gets switched between the two polarities according to the voltage sent from the decoder to the switch.
 
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