[mythtv-users] DVB technical issues about scan

[Stephen Worthington]

On Sat, 31 Dec 2016 09:29:00 +0800, you wrote:

>Hi
>when I scan I get duplicates (and conflicts)
>
>As I understand
>
>Each station has its own frequency (where ever the transmitter is)
>Packet 1234 as an echo is indishingable from packet 1234 from another transmitter.
>Either the CRC is correct and the packet is used or it is wrong and myth or the card does something (maybe wait to see if there is a good packet, maybe uses it broken whatever)
>
>Googling gives bugs and reasons for conflicting channels
>
>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?
>
>Can I delete all conflicts?
>
>James

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.  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.  DVB-T is able to distinguish signals that are on the same
frequency but come from different transmitters, as long as the time
delay between the two sites is not too small, and that is what allows
the same frequency to be reused.  But it is possible to be equidistant
from both transmitters, and then the two signals will interfere with
each other.  Careful designers try to prevent that.

If you get good reception from one transmitter site, you should get
all your channels from that site and delete any duplicates coming from
other sites.  Signals from other sites will likely be much lower
signal level anyway, and therefore much more prone to interference and
signal loss.  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.