[mythtv-users] Digital technicalities (Was: Obama Recommends Delay in Digital TV Switch)

[Simon Hobson]

I'm in the UK, so there are bound to be differences, but from the 
sound of it, you guys are getting the same deal as we are !

On the politics side, our government (or rather OfCom which is the 
toothless so called 'regulator' covering communications areas) held a 
consultation in which the only allowable options to support were HOW 
to sell off the spectrum released - keeping some or all of it and 
having more multiplexes has been ruled out. Some consultation eh !

On the technical side, I suspect your issues are similar to ours - more below :

Brad DerManouelian wrote:

>The FCC rep explained that the analog transmitters all currently take 
>precedence over the digital. They are on top and the digital are on 
>the bottom of the tower and in February when the change-over happens, 
>the digital will replace the analog transmitters which are larger and 
>will be placed at the optimal position for transmission. I believe his 
>quote was, "Don't worry. It will be much better in February when we 
>don't have to accommodate the analog transmitters any more."
>
>While I don't fully believe him, there is hope that what he says it 
>true and digital transmissions will be better when they take down the 
>analog transmitters and move the digital ones in their place. I guess 
>the message is that what you're getting now isn't necessarily as good 
>as it will get. You should check with your local broadcasters to see 
>if this is their plan. They are usually surprisingly happy to talk to 
>their customers.

Frequency planning is a tricky art. In the TV UHF band there are 
rules based on the technology that determine what frequencies can be 
used in an area, and in adjacent areas, without causing interference. 
In practical terms, these rules limit analogue TV to about 5 channels.

When Digital is added to the mix, there are no spare frequencies that 
would fit with the 'old'rules. As a result, the digital transmissions 
are at a MUCH lower power than analogue. To give an idea, I get my 
signal from a main transmitter at Winter Hill, some 40 miles away. 
Digital is being transmitted on 6 multiplexes at 10kW each, analogue 
has four channels on 500kW and one at 12.5kW*.

* The low power analogue is our Channel Five which was the last 
analogue channel added. It's at low power as there were no 
frequencies available in most places that wouldn't cause interference 
so it's transmitted at low power to avoid upset.

Additionally, as someone mentioned, there may be issues with finding 
space on masts for the extra antennae - so there could be cases where 
temporary masts have been set up to carry the digital stuff, and 
these may well not be in such good locations, and may not be as high. 
On  my 'local', the digital antennae are only part way up the mast, 
while the analogue ones are at the top - the tower is 1035ft high.

Once analogue is turned off, the rules change. I believe the digital 
signal is such that a single frequency network is possible - ie the 
same multiplex can be transmitted on the same frequency from 
different transmitters. There won't be analogue to interfere with, 
and so the power can be turned up, and in some cases, the transmitter 
can relocate to the mast previously used for analogue.





David Brodbeck wrote:

>I remember hearing that a lot of towns that relied on translators for
>their analog TV service might lose TV coverage altogether.  Was that
>problem fixed, or is that still the case?
>
>(For those of you in urban areas, a translator is a system that receives a
>TV broadcast on one channel and retransmits it on another.  They're used
>to extend the range of TV stations into remote areas, or to fill in holes
>caused by hilly terrain.)

Over here they are called repeaters, which is possibly a more natural 
name. We have (IIRC) over a thousand of them on our little island 
over here - so that puts some perspective on the scale of the 
problem. In the town where I work, there are actually two small 
repeaters to get the coverage (one is only 3.2kW/channel !).

Even our stubborn bunch running the country have recognised that to 
turn off TV would be suicidal. Supposedly every repeater is going to 
get digital, but won't get all the channels - typically they will 
only get three multiplexes carrying the main channels. Even so, there 
are some that will lose terrestrial TV, and for them, our main 
broadcasters have launched "freesat" (not to be confused with 
"Freesat from Sky") which is really a spec for EPG and minimum 
receiver capabilities to offer the same "buy once, never pay a 
subscription" deal that people are used to with terrestrial digital.

freesat also carries our free HD channels, which won't arrive on 
terrestrial until after the analogue switch off when they plan to 
shuffle things around a bit.


Oh yes, and technically, there is nothing to prevent a repeater (such 
as a private one) being upgraded to digital. Simple ones may simply 
need retuning. But if it's  private one, it may be simpler to just 
stick a dish up and get freesat.



Hope this has managed to explain some of the issues.


-- 
Simon Hobson

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