A digital TV trap

I’m documenting this, because it seems to me to be an easy trap for digital set top box users to fall into. Being the geek that I am, I a little embarrased that I didn’t work this out earlier. But if fell for it, my guess is lots of people will.

About five weeks ago I bought a Topfield TF5000PVRTMPB SD set top box, with hard drive.
I’ve rapidly become a digital TV – and Topfield – fan. Where I live in Sydney we have virtual line-of-sight access to the main TV transmitters, and so we get nearly perfect reception. But we quickly found one problem:

When most digital set top boxes are first set up, they normally scan all the relavent frequencies, and load up a list of all the available ‘services’ (channels). For me, that meant that all the main TV stations in Sydney (ABC, 7,9,10 and SBS), plus some program guide channels, Channels dedicated to broadcasting Federal parliament proceedings(!) and even a Christian TV channel.
For me, confusion arose when the Toppy presented two or more channels that appeared identical. In my case, there were what looked like two identical channels for ABC TV Sydney. They both used ‘2’ as a numeric identifier (in common with the analogue TV channels) and both had the same description.
So I selected the first one, and went on to select the other channels.

We quickly realised though. that we had a problem with ABC TV. Unlike analogue TV signals which degrade gradually in quality as the signal strength drops, digital TV has a clear threshold for clear reception. Over that threshold, and generally you have a perfect picture. Under that threshold and you immediately have quite severe problems that show as pixelation and audio drop outs.
And that’s what we had with ABC – the channel we probably watch the most.

Cutting a long story short, I consulted with a fellow Toppy owner, Richard, who finally came up with the solution:
The ABC uses two transmitters for digital TV signals in Sydney. The main transmitter at Gore Hill on 226.5MHz, and a repeater transmitter located at Manly on 543.625MHz. The repeater is used to get a good signal to some hilly areas in the east.
The problem was, that in the text information used to identify stations, the difference between the main and repeater isn’t noted. So I had ‘locked in’ on the repeater transmitter for ABC rather than the main transmitter.
If you look at the coverage maps for the two transmitters (Manly, Gore Hill), it’s no wonder I had signal strength issues!


  1. It would have been the tx from the Elan building at Kings Cross, not North Head (Manly) that you would have been receiving. North Head is more fill in for the Northern beaches etc. Don’t believe the ABC coverage maps – they are very optimistic at the best of times!!!

    AndrewR · Jul 3, 04:37 PM · #

  2. Well, I was going by the frequency and channel designation, rather than the coverage Andrew, which are quite different for the Kings Cross tx… ;-)

    Neil · Jul 3, 11:38 PM · #

  3. No worries – both Kings Cross & North Head ABCDTV being on a SFN (single frequency network) cause issues for us believe me – but it’s worse still for SBSDTV when you have to take the main (higher power) transmitter at Artarmon/Gore Hill (whichever one is on) into account as well. Can make trying to do field strength surveys a bit interesting, especially if you are in a “mush” (overlap) area.

    AndrewR · Jul 5, 12:55 AM · #

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