[mythtv-users] Strange DVB-T Interference Any Ideas

Charles Mason charlie.mas at gmail.com
Mon Dec 15 18:18:07 UTC 2008

Hi All,

I have been using MythTV for a few years now. My backend currently has
two tuners a PCI Nova-T Hauppauge card and a USB DVB-T from hauppauge.
I use both to receive Freeview from a large roof top aerial. One of
the multiplexes has always been a bit patchy and so have never really
bothered about it, however the rest seem really good & reliable.

We live out in the country on the side of a hill and have a very clear
line of site north towards Emley Moor. This is a fairly powerful
transmitter but its a long way away. We can get two other signals from
Sutton Coalfield and one near Nottingham (the hill virtually kills
that one).

My problem is without warning a recording will go from no interference
at all, to completely unwatchable instantly. Its never so bad to
completely stop the decoding but its just constantly blocky, with odd
colours, droped frames and nearly intermitent audio. This can last to
the end of the recording or some times only a few minutes. When it
does clear up its instant and the video goes back to being 100%
perfect again. I have checked its definitely on the recording not the
playback as if its played again it happens at exactly the same point.

Our nearest neighbour is half a mile away so I don't think its caused
by anyone else. I have checked its doesn't seem to be related to how
many recordings are going on. I have seen it recording 3 or even 4
shows at the same time with out any issues.

It did seem to start some time after I added the USB tuner, I am not
sure if that's to blame. Is there anyway to tell which tuner a
particular recording came from?

It seems to happen on certain multiplxes. The one that carries five
seems to be the worst effected. However it also seems to happen on the
one that carries BBC1. The one that carries Channel 4 seems to be
fine. It screwing up a recording of Spooks was the final straw
(watching it on iPlayer is just not the same :).

The only other possibility is that its my NAS. All the recordings are
stored on my FreeBSD based NAS which is on a 100 meg wired network
with the backend. I suppose it could be related to that some how. The
recordings are stored in the ZFS equivalent of RAID 5 so I should get
errors if there was a disk problem. Although I suppose if the NFS
packets never make it to the NAS I wouldn't see ZFS errors. All the
other machines on the network access the NAS and MythTV server via
WiFi so I can't see how any other use of the NAS could saturate it
stopping the MythTV server accessing it.

Does anyone have any suggestions to try and track this down. Its been
happening for months and affects about 35% of the recordings so I
really want to solve it.

Charlie M

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