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

[Mike Perkins]

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