[mythtv-users] DVB technical issues about scan

[Philip Brady]

________________________________
From: mythtv-users <mythtv-users-bounces at mythtv.org> on behalf of Simon Hobson <linux at thehobsons.co.uk>
Sent: 31 December 2016 11:21
To: Discussion about MythTV
Subject: Re: [mythtv-users] DVB technical issues about scan

Stephen Worthington <stephen_agent at jsw.gen.nz> wrote:

> James Linder <jam at tigger.ws> wrote:
>> As I understand deleting all the conflicts is the correct behaviour as each channel receives all the packets so signal strength is not an issue. As I see channel 7 - freq n mux 1 and channel 7 freq n (from far away) mux 2 is the only case where one ‘7’ is better than another ‘7’. Does such a thing ever happen?

Sorry, but I didn't follow a fair bit of that post - do you actually mean two txs using the same frequency for a mux ?
You also need to be careful to differentiate between "channel" as in RF frequency, and channel as in logical channel number (LCN), and channel as in "broadcaster using a particular name" ! Really confusing but I doubt we'll be able to change history now :-(

BTW - it would also be a good idea to explicitly state where you are - as noted below, practices vary considerably with location.


> Here in New Zealand, the designers of our DVB-T services make sure
> that adjoining regions use opposite polarisation, so that if you are
> using a proper aerial on the correct polarisation, you should not get
> out-of-region stations.  I imagine that is standard practice
> everywhere DVB-T and similar transmissions are used.

It isn't.
Here in the UK, the standard practice (there are exceptions) is that main txs are horizontal and infill repeaters are vertical polarisation. As pointed out, this does mean that a correctly aimed & polarised aerial will pick up either the main tx OR a repeater, but it gives no discrimination between different main txs or between different repeaters.

At my parents house, when we put a new aerial up - I chose the biggest "grouped" Yagi I could get on the basis that it's easier to throw excess signal away than to make up for a poor signal, and we live a fair distance from the main tx. As it happens, I needed to put some attenuators in as we were picking up Moel-y-parc in North Wales as well as Winter Hill in Lancashire (the one we wanted). The old trick of unplugging the aerial during part of the scan doesn't work as the muxes are interleaved. At home, using an attenuator was sufficient to get rid of the signals from Moel-y-Parc during a scan, but a friend needed to manually go through the list (with an awful UI) deleting the duplicated whenever she retuned the TV - great TV, really good quality, sensitive tuner, just an awful UI.

>  However, within
> one region, there are often in-fill transmitters or translators for
> getting signal into tricky places, like deep valleys.  Those are
> likely the duplicates that you are seeing.  Due to the way DVB-T
> transmissions work, you may find that the same channels are broadcast
> on the same frequency from more than one site that you can receive, or
> if there is enough spectrum, they may be broadcast on a different
> frequency.

Here in the UK they've been really careful in planning to avoid the issue of getting more than one signal on the same RF frequency. Whether that will change when more of the spectrum gets sold off remains to be seen.

> However, if you are in one of those bad signal areas,
> such as a deep valley, then you may actually need multiple aerials
> pointing to different transmitters to get all the channels
> satisfactorily.

I think we're lucky here in the UK - since all our TV transmitters are run/managed by one outfit. So we don't have the situation where there are lots of small transmitters carrying different combinations of stations. It tends to be as simple as :
Most main txs carry the full set of (IIRC) 8 freeview muxes - some using DVB-T, some using DVB-T2.
A small number of main txs, and some repeaters carry all but the two newest muxes - ie what used to be the "full service" of 6 muxes.
Many of the repeaters carry only 3 muxes - which is the main ones carrying the main channels.

The only time you might want 2 aerials is if you want to try and get 3 muxes from another tx to augment the 3 you get from the local repeater - but TBH I doubt it's worth it since if you can get a usable signal from the full service tx then there's little need to use the subset from another tx; if you can't get a usable signal then it's probably not worth the hassle of having channels you can only watch when the weather is just right.
And we do have a fairly good free satellite service as well - which is an option if your terrestrial service is poor.
-------------------------------


Hi James,
that error recovery you suggest is true of the Internet but not of adjacent TV transmitters.

A single transmitter will typically transmit a number of frequencies = multiplexes.  Each frequency/multiplex will carry a number of channels.  It will send out packets of data 'BBC1 picture', BBC2 picture', BBC1 sound' etc.

Here in the UK all BBC channels tend to be on one multiplex, all ITV on another, High definition on a third, and odd-balls on a fourth.

Now if you can see two transmitters, then you will see two copies of channels; one from each transmitter, but on different frequencies/multioplexes.  One might be good, another patchy reception.   Your tuner will only be tuned to one of those, but I have seen reports of Mythtv choosing channels at random from either transmitter so giving inconsistent recording quality (some recordings good, others bad).

You will need to establish which transmitter is which and hide or delete the channels from the weaker one.
The perl channel editor https://www.mythtv.org/wiki/Channel_Editor may help you with this.  It will identify your multiplexes and frequencies.  From that (and googling information) you can identify the rogue multiplexes and then hide all channels from them at one fell swoop.
hth
Phil


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