[mythtv-users] Too many multiplexes (UK)

Mike Perkins mikep at randomtraveller.org.uk
Fri Sep 28 11:46:07 UTC 2007

Recently I had to replace my aerial array because it had become dangerous[1]. 
The new setup is so much better that it's now causing me problems with scanning 
that some small adjustments to the code may be of help to others besides myself.

I am situated about 8-10 miles from Hannington. My aerial points to about 150 
degrees or so (30 deg. east of south)[2]. When I do a scan I get all the 
Hannington channels, as expected, but also some from Crystal Palace (90 deg) and 
some labelled Oxford/Bexley (0 deg/100 deg) (!). As all these multiplexes 
contain the same digital channels it makes a right mess of the channel table. 
The channel numbering in particluar can be completely randomised every time I do 
a rescan (thanks MTD!) depending on the weather and atmospheric conditions at 
the time.

When I do the scan, the display is completely blocked by a dialog which shows 
virtually nothing except a progress bar and various field labels. The only other 
variable shown is "Offset 2". Behind it, I can see the listbox scrolling up with 
potentially useful information, but I can only look at this when the scan ends.

(1) One of the items shown in the listbox, when a multiplex is found, is the 
source, hence "Hannington", "Crystal Palace" and "Oxford/Bexley". It would be 
extremely useful if this item was added as a new column in the dtv_multiplex 
table and displayed in the Transport Editor dialog. It would also be useful if 
an "ORDER BY frequency" clause were added to the Transport Editor listbox 
display. At the moment the items are displayed in a seemingly random order and 
it is difficult to separate the wheat from the chaff.

It is possible that the behaviour above is UK-specific and would not be useful 
in other parts of the world.

(2) The scanning dialog box seems to serve no useful purpose. The information 
shown could just as easily be displayed as fields below the results listbox.

(3) It might be possible to do a pre-scan, just identifying the multiplexes, and 
then to ask the user to choose which ones they require before processing the 
digital channels within.

I'm using http://www.dtg.org.uk/retailer/transmitters.html and I also found this 
http://www.digitalspy.co.uk/terrestrial/tuning/ to be very useful. I am supposed 
to pick up digital muxes on channels 40, 41, 43, 44, 46 and 50 from Hannington. 
I can also receive 22, 25, 34, 48, 52 and 68. The analog channels are a similar 

Mike Perkins

[1] The old array had a 2.5m (8 foot) pole with two band IV/V aerials 
sandwiching a Band II FM aerial pointing in a different direction. The whole 
thing was attached by a bracket mounted on the bargeboard at the side of the 
house. It swayed alarmingly in high winds. The birds loved it. When we took it 
down we got a bucket and a half of water from inside the tubing. The feed cables 
were loosely draped over the roof and down the walls.

The new aerial is a wideband digital-quality item. The aerial downfeed is 
high-grade CT100 foam-filled satellite cable. The bracket supporting the two 
metre down tube is bolted to the brickwork at the side of the house, so it's 
unlikely to move. The cable goes straight into the loft space and runs to the 
opposite side of the house, straight into the top of my computer cupboard, about 
15 metres/50 feet.

When the aerial guy tested the signal after installing, he found there still 
wasn't quite enough signal at the top end of the range (where analog channel 4 
is). He added a masthead amp, but rather than putting it up the pole, it's 
mounted on the inside of the wall where the cable comes through.

The power supply for the unit is in the computer cupboard. I have a 4-way splitter


three of which outputs go to my DVB-T cards in the master backend. I also have a 
two-way splitter


which I am using as a combiner, the inputs being the fourth output from above 
and a feed from my NTL set-top box, which is currently downstairs. From there 
the output goes to a distribution amp which sends the signal to sockets all over 
the house.

[2] Although I am quite close to the transmitter, as the wood-pigeon flies, I am 
on the wrong side of a hill and there is a band of mature trees not 10 metres/30 
feet from the aerial. This means the signal strength goes from quite good in the 
winter to average in the summer. (At the end of the garden I have the A339 in a 

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