Sky Q and many channels at once - how does it work?

[Commander Kinsey]

How does Sky Q get several channels through one cable? When Sky HD came out, we had to add a second cable to the dish to get two channels. But Q does not need 6 cables to get 6 channels.

[spuorgelgoog]

Universal LNBs work by the receiver selecting one quarter of the channels at any one time. It does this by alternating the voltage between 13V and 18V which will enable the LNB to switch between Horizontal and vertically polarised signals. A 22kHz tone then switches the LNB from a low and high band. Hence a quarter of the channels at any one time. Vertical Low, vertical high, horizontal low, horizontal high. It is for this reason a normal traditional splitter can't be used on your satellite dish as the box may be trying to request a channel on one quarter or the other box be requesting a channel on the other quarter.

The LNB output at Satellite intermediate frequency (IF) which sits just above the UHF band used for terrestrial TV and 4G internet signals and coax cable has a bandwidth for half the satellite signal (i.e. low band or high band).

The Sky Q system now uses a frequency band which now includes all the band previously reserved for traditional terrestrial TV signals. It is what we call a "wideband LNB" which ranges from 230Mhz just above DAB radio right the way through the terrestrial TV signal band and up to 2359Mhz just beneath the 2.4Ghz band typically used for WIFI. Because it is "nicking" space that it did not used to have it has a far greater bandwidth for all of its services so there is no longer the need for a high and low band.

The Sky Q LNB itself has two outputs which are not the same. One is a horizontal output and the other a vertical output.

Because the Sky Q system uses terrestrial space, it can't be diplexed with terrestrial tv on the same cable, it must have its own cables.

summarised from

Owain

[Andy Burns]

I can't but a fibre LNB can do that (well one fibre for signal an a coax for power)

Q needs two coaxes, one for H polarized and one for V polarised, each cable carries the entire 10.7 to 12.7GHz band, rather than selecting the upper or lower "half" like a universal LNB does.

[Commander Kinsey]

Surely the problem wasn't a lack of bandwidth (there were always hundreds of channels out there), but that the old LNBs couldn't handle the whole band at once.

Ah that makes sense, I guess they lose their polarization inside the cable.

If it uses terrestrial space (I assume you mean the same frequencies as digital terrestrial TV), why don't they interfere in the air?

[Commander Kinsey]

It does, Sky Q boxes can pick up 7 channels simultaneously, through only two cables.

So better LNBs then.

[spuorgelgoog]

Lack of bandwidth at the LNB IF output.

Because the signal is at such low level it isn't radiated outside the coax?

Perhaps in some circs it would interfere in the air?

Owain

[T i m]

<snip>

In that the LNB has converted the signal from unbound 'waves' with polarity (horizontal, vertical, circular etc) to a straight electrical signal (also waves of course) on / in a conductor (not wave guide)?

Cheers, T i m

.

[Brian Gaff (Sofa 2)]

You should only need two for all channels since there are only two possible polarisations surely?

Brian

[Brian Gaff (Sofa 2)]

If that is true though, how come you do not get leakage either from terrestrial to sat or the other way around which would screw up the one most affected? Brian

[Brian Gaff (Sofa 2)]

I think more to the point is close in where people want to use both, nasty sprogs and crossmod effects should wreak havoc, but seemingly do not, so I guess they rely on the robust error checking of the digital systems. I have a receiver that can cover from the top of dab to 2 gig, but surely this also has the military air band in it? Brian

[Brian Gaff (Sofa 2)]

Differently designed ones perhaps, with a wide band output and lower noise. Brian

[Commander Kinsey]

Yes. You can't polarize waves in a conductor, there is no up down left right.

I wonder, could we have diagonally polarized beams form the satellite? I'd guess no, as they'd be picked up at half power by horizontal and vertical receivers. There's probably countless more modulations to increase bandwidth, just look at the huge list of things they did with modem signals.

[Commander Kinsey]

What? There are several hundred channels, and I might want to record say three of them. The old Sky boxes would only receive any two channels. Q will receive 7 (12 apparently, but the other 5 are reserved for technical stuff like red button and so on).

Oh I think I know what you mean. What you're saying is why didn't we only ever need two? I think the answer is the old LNBs couldn't get the whole band at once. So, better LNBs invented.

[Commander Kinsey]

If Sky have use of the TV band, and so do terrestrial transmitters, I don't see why they don't interfere. You can't have two things on the same frequency.

[Commander Kinsey]

They should be called RLNB - for Really Low Noise Block down converter. But that's starting to sound like a cross between saving birds and drowning sailors.

[Commander Kinsey]

What I mean is I fail to see the connection between Sky getting hold of more frequencies and them being able to send more than two channels form your dish to your box. The extra channels at once is purely to do with better LNBs.

Yes I meant in the air. Yeah I guess if it's much weaker than terrestrial, then terrestrial aerials won't pick it up. And your dish won't pick up terrestrial as it's aimed into the sky.

[T i m]

Quite. ;-)

I'm sure most good techniques have already been explored. ;-)

Increase the effective bandwidth?

Yeah. Again, especially if you only have the spectrum of 300 Hz to

3.3KHz (or whatever a voice line is / was). ;-)

My first exposure to modems was 300 baud jobbys with BT (well, 'The Post Office, Datel Div). They were about the same size of an old IBM PC! and actually had a MOdulator and DE-Modulator in different modules. ;-)

Cheers, T i m

[alan_m]

It's only sky that needs two cables. Other wideband (unicable) LNBs connected to the appropriate tuners only require a single cable, and the signal can be split.

Because these unicable LNBs don't steal the DAB/UHF frequencies it's possible to combine the signals from terrestrial and satellite in the same cable.

The cynic in me says Sky have adopted a bespoke system to tie people into their Q service. Its no longer just a matter of swapping out a box at the end of the cable as can be done with a "traditional" LNB.

[Commander Kinsey]

How do they put H and V polarized channels onto the same cable?

It's not stealing, it's making use of them.

Why on earth would you want to do that? I have an aerial and a Sky dish, I have a cable from each. I'd never have thought of trying to cram both signals onto one, then trying to split them at the other end. Sounds like a lot of hassle to me.

If I wanted to stop Sky, I'd just change the LNB back to whatever was needed by the new service.

[T i m]

<snip>

Different frequencies, modulation, multiplexing and others (I suspect). [1]

Only if you don't destroy them if present?

Depends how big the block of flats you are distributing such signals to. ;-)

'Just' may not be so easy if it's on the roof or a 3rd+ floor flat and assuming your replacement LNB fitted the bracket etc?

Cheers, T i m

[1] Over 40 years ago I was bench repairing TDM and PCM equipment (BT) that put say ~30 simultaneous telephone calls down one pair of copper wire. And later (Different Co) kit that would often 'effectively' compress 4 x 48K datalinks into 1 x 48k link. ;-)