MythTV reception woes

So we finaly decided to go the Mythtv route. I bought an AMD Sempron based computer to use as the server and the front end. I used very standard components which I researched to ensure they would all work with linux, and after a few teething troubles due to a stupid supplier who couldn’t even be bothered to test the machine before despatching it to me even though I specificaly requested it, it’s up and running and so far has been solid as a rock. I also bought two Twinhan Vision Plus DVB-T tuner cards for a very reasonable $77 each delivered.
I installed Debian on the machine which turned out to be a remarkably painless process. I have been a long time fan of debian and every time I install it, it just gets better and better. Then I set up LVM so when I get a new hdd I’ll be able to add it easily. Then I found the .debs of mythtv and installed them. It pretty much worked out of the box. There was quite a bit of fiddling around to configure it all, but basicaly it worked. Reception was great on all the channels except ABC and ABC2. On those channels it was breaking up quite a lot. According to Brendan, the technical term is those channels were off the cliff.
On our TV (analogue reception) we have trouble with 2 channels. SBS and ABC. SBS is quite snowy and grainy, like the signal is too weak, but the reception is fine through the Visionplus cards. ABC is usually quite distorted, and slightly shadowy, the distortion is particulary bad when there is a lot of bright objects on the screen, and at those times the distortion often spreads into the audio.
My first thought was that maybe were not getting enough signal into the cards. I do have to split the signal three times (one for the telly, and once each for the Visionplus cards) but even trying one of the cards connected directly didn’t solve the problem. I bought a tv signal amp, plugged it in and hoped for the best. I held my breath, and tuned to ABC on the mythbox. It was just the same or possibly worse. I tried adjusting the gain of the thing, but it only made it worse. Back the the drawring board.
I had a look at the output from tzap, a command line utility for tuning the cards. I was getting slightly lower signal to noise ratio on ABC than on the other channels. This led me to believe it might be some sort of noise in the system causing the problem. So I decided to build a 1/4 dipole antenna specificaly tunned to the ABC frequency in the hopes of just getting that signal into the card and seeing if that gives me good reception. It turns out that a quater wave of the ABC frequency is 33cms, so I just built it using a bit of cardboard and some wire and a little PAL plug in the cardboard. I might put a picture up of it later. I plugged it in, tuned to ABC and… nothing… well… something, broken up signal, worse than when I was using the normal antenna. Back the the drawring board.
After researching this problem, I started to think it might be caused by the signal being too strong. We do live fairly close to the ABC Transmitter in Gore Hill, and my futher reading (pdf) also lead me to believe this was the case. That document suggests a notch filter to solve that problem, but before spending big on notch filters, I wanted to be sure that was the problem. So I went to Jaycar to buy a variable attenuator that they sell. As I work in Alexandria, I went to the Alexandria branch. I walked in, found the product, went to the counter to pay and the sales assistant says “Oh… you must live near Gore Hill” which of course I do. He goes on to say “No one who lives around here buys those but we sell loads in the Gore Hill branch because its so close to the ABC transmitter”. Anyway, I took it home, plugged it in and hey presto! Almost perfect reception on ABC.
But now I have a new problem, in order to get good reception on ABC I have to turn the attenuation up so high that the other channels start to break up. So back to the notch filter. The trick is, where do I get one and how do I configure it correctly for the frequency I’m after? I did notice that there is a product that Hills sell that might do the trick, but it looks very expensive. I also heard there is a place called Electrocraft that specialise in antenna equipment. I’ll give them a call and see what they have.

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