[mythtv-users] DVB-T tuner appears to de-tune itself.

[Paul Gillingham]

2009/3/31 Mike Perkins <mikep at randomtraveller.org.uk>:
> Paul Gillingham wrote:
>>
>> Hi,
>>
>> I am having a problem where my DVB-T tuners seem to be de-tuning
>> themselves. I am running 0.21-fixes on Mythbuntu 8.04 (Hardy) with a
>> Happauge Nova-T 500 to pick up UK Freeview DVB broadcasts. Here's a
>> description of the problem:
>>
>> 1. In order to get good reception for all channels I have to first do
>> a full scan in the backend setup. This will pick up all channels, but
>> the signal strength is not good enough to watch. However if I now plug
>> in a signal amplifier between my Mythbox and the aerial that fixes the
>> problems and all is well. Note that if the amplifier is plugged into
>> before the channel scan I find I am missing several channels (ITV1,
>> ITV2, Channel 4 etc I think this is Multiplex 2)
>>
>> 2. At some point overnight my reception drops away (especially on
>> Multiplex 2) and recordings fail/live TV becomes unwatchable.
>>
>> 3. The next day in order to restore reception I have to repeat the
>> procedure detailed in 1. Basically, go into mythbackend-setup, delete
>> all channels, unplug the amplifier, do a full scan, plug in the
>> amplifier again, watch TV.
>>
>> It seems to me that Myth is doing some kind of housekeeping/scanning
>> during the night, as the situation in the morning is the same as the
>> situation if I were to do a full scan with the amplifier plugged in
>> (ITV1, ITV2, Channel 4 etc missing).
>>
>> It is probably also worth mentioning that this is a fairly new
>> problem.Previously my system worked well, but I believe my communal
>> aerial has been adjusted recently. I would have to contact the
>> management company to find out (I certainly can pick up several new
>> channels now). I am within range of both the Guildford and Crystal
>> place transmitters so there may be an issue where I am picking up the
>> same channel from two sources..
>>
> Since you can get both transmitters, you will have to make sure you are
> picking up the strongest signal. If the scanner picks up the strongest one
> first, and then the weaker one, it may be overwriting the stronger one and
> only storing the weaker one (since it will only choose one copy of e.g.
> channel 4).
>
> I use this table: http://www.dtg.org.uk/retailer/dtt_channels.html
> this table: http://www.ofcom.org.uk/static/reception_advice/index.asp.html
> (which leads to a downloadable 2-page pdf) and this table:
> http://www.digitalspy.co.uk/terrestrial/tuning/ which together will allow
> you to find out which muxes are broadcast on which channels by which
> transmitters. A spreadsheet helps here.
>
> Once you have figured out the strongest, go into the transport editor within
> mythtv-setup and delete the ones you don't want (indexed by frequency). Then
> you can selectively re-scan the ones you do want.
>
> If it won't let you delete the weaker ones, for example because of the scan
> order, you may have to re-orient your aerial to make them weaker still.
>
> I had to do this. I can see Hannington, Guildford, Crystal Palace and Oxford
> (!) from here in Newbury, aerial facing about 30 deg. east of south.
>
> --
>
> Mike Perkins
>
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Thanks,

That makes sense. But what's to stop all this careful configuration
being lost again overnight when the system re-scans? (if indeed that
is what it is doing). I may be wrong about myth updating the channel
data overnight, but that's the only thing I can think of which
explains why my perfect reception is being lost, re setup and then
lost again.

Anyway, I have just tried deleting all channels and the doing an
existing transport scan (rather than a full scan). This just scanned 6
transports (which is correct as I think there are 6 multiplexes in the
UK?), I believe these are good transports for me as I've already done
my unplug the amp/full scan trick today. Anyway the scan returned all
the channels including ITV. etc. so that's good.

I suppose what I have to do now is hope that Myth doesn't alter this
overnight. (i.e. if it does do any channel editing it will remember
that I only did a restricted scan and won't try looking for signals
from different transports)