[mythtv-users] MythTV and twin-tuner Freesat
linux at thehobsons.co.uk
Thu Nov 12 16:34:03 UTC 2009
Nick Morrott wrote:
> > (b) what do I need in terms of physical cabling from the dish? Do I need
>> two separate coax cables to plug into the card? If anyone can post some
>> photos of the back of a box with even one freesat tuner in it, that'd be
>> really helpful.
>You'll need a dual/quad LNB on your dish, and 2 (or more) cables
>running down to your machine if using 2 (or more) separate DVB-S
>tuners. Each card/input will have a female threaded F-type socket, and
>you'll need F-type connectors on each lead. You may be able to use
>'shotgun' cabling used for Sky+ installs - two cores in a single cable
>- which gives a cleaner and less cluttered install. Either way, use
>the best quality cable you can get.
The thin "shotgun" cabling Sky contractors use is mutually exclusive
with "best quality" - I'd avoid it unless you have short runs and/or
a larger than normal dish. The signal loss is a LOT higher than
standard sized cable. it isn't two cores, it's actually two (much)
smaller coax cables moulded into one "figure of 8" outer sheath.
Of course the contractors love it as it's much smaller (so they can
drill the holes faster) and lighter. Basically it means they can do
the job and be off to the next one in less time - you didn't think
you got 'quality' with your "free install" did you ?
If you go to http://www.satcure.co.uk/tech/cablespecs.htm, you'll see
that WF100 or WC100 has attenuation of 'only' 29.4dB/100m @ 2050MHz,
while CT63/WF65 (shotgun) is 48.6dB/100m. In other words, for every
metre of WF100/WC100, you'll only get to use 0.605m of CT63/WF65
cable to get the same signal. Alternatively, if you had (say) a 25m
cable run, then the larger cable would get you 4.8dB more signal -
about 3 times as much !
Sorry - seen too many crappy installations and the effects of 'cheap'
cabling to let it pass without saying. It's often the one piece of a
system that gets economised on.
Back to part one, there are 4 bands in use - high and low frequency
(selected by selecting different mixer frequencies in the LNB), and
horizontal and vertical polarisation (selected by physical antenna
selection) - and these are selected/switched in the LNB where the
incoming signal is converted into something suitable to send down a
cable. To guarantee no conflicts, you need one cable per receiver -
each one going back to a separate output of a multi-output LNB.
Within each of the 4 bands, there are then multiple frequencies, and
at a particular frequency, there may be a digital multiplex with
A few receivers are capable of slaving multiple inputs - I believe
one of the Freesat boxes can be configured to use the same cable for
both inputs. The limitation here is that whatever you tune on input 2
is always restricted to the band selected by input 1, but it can be a
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