[mythtvnz] Tinfoil madness

[Dylan Hall]

Let me begin with the mad picture, then explain how I got there (see
attached).

The only way I can get reliable reception with the TVNZ and Mediaworks
transponders is to wrap my USB sticks with tinfoil and ensure that
tinfoil is connected to the casing of the RF splitter. The tinfoil can't
cover the fly leads from the USB sticks, and it can't contact with the
shield of the USB connector.

Let me start from the beginning....

I decided to have a go with DVB-T for freeview. I got a pair of USB
sticks. I didn't want them in the lounge where I have existing RF
outlets so I installed a splitter in the basement and added 23m [1] of
good Quad shield RG6 cable to extend the aerial connections into my
computer room at the other end of the house. 

This almost worked. I had good TVNZ and Mediaworks, but the Kordia
transponder was breaking up, regular blocking etc.

I had a play with tzap to try and get an idea of what's going on.

I was getting a signal strength of 45-50%, with a low BER and no
uncorrected blocks for TVNZ and Mediaworks.
I was getting a signal strength of 35-42%, with a moderate BER and a
small number of uncorrected blocks for Kordia.

I decided it was time to audit the whole aerial/cabling/splitter setup.
The UHF aerial on the roof was small, rusty, and using the cheapest
nastiest cable. The VHF/UHF combiner was full of corrosion and a spider
colony. The cable from the combiner to the basement was a little nasty.

I replaced the UHF aerial, the combiner, the cable to the basement, and
the splitter in the basement (with a splitter with the right power pass
options should I need to add a masthead amplifier later).

At the end of this exercise the signal strength seems to have dropped
slightly, but it's now the same for all the transponders (approx 45%). 

If I plug the pair of USB sticks into my laptop they both work fine, low
BER, no uncorrected blocks.

If I plug them into the backend PC it goes nuts. Sometimes they work.
Sometimes one stick works, the other fails with a high BER and lots of
bad blocks, and sometimes both sticks fail, again with high BER and lots
of bad blocks. More confusing is the failures are with the TVNZ and
Mediaworks transponders. The Kordia one is perfect.

I went through all kinds of exercises, changed the splitter, changed the
cables, added a powered usb hub, tried different usb ports on the
backend PC. At one point during the process I convinced myself the
problem was RF interference from the backend PC so started playing with
bits of tin foil. I eventually settled on the madness described at the
beginning of this email which works rather well!

Does anyone have a sane explanation for the issues I'm seeing?

I have a number of theories...

1. The signal strength is too low and it only takes the slightest bit of
interference/noise to push it below the working threshold and fail. 
2. There is a nasty source of interference in/near my house.
3. Something electrical, ground loop, noisy power, etc... Laptop running
on battery is happy, PC running on mains sad. Added an online UPS to the
mix but this didn't help.

I see a number of things I can do now:

1. Make myself a hat from the left over tin foil and hope I get a moment
of clarity.
2. Accept the status quo, it is working after all, I just don't
understand why.
3. Futz with the antena setup further, mast head amp, new UHF aerial
closer to the PC, professional installer.
4. Buy a PCI/PCIe based tuner and hope it works better.

Any ideas/comments/etc greatly appreciated.


Thanks,

Dylan


[1] In case people don't know, most cables (RF, cat5/6) had printing on
the casing every meter. Somewhere buried in the writing is the number of
meters left on the reel. If you can find the writing closest to each end
of a cable you can subtract the two values and determine how long your
cable is :)


-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.ourshack.com/pipermail/mythtvnz/attachments/20091104/d0e4ea80/attachment-0001.htm 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: tinfoil_madness.JPG
Type: image/jpeg
Size: 53989 bytes
Desc: not available
Url : http://lists.ourshack.com/pipermail/mythtvnz/attachments/20091104/d0e4ea80/attachment-0001.jpeg