Digital tv reception - sat and terr - is "all or nothing", either the picture's perfect or not there at all! That is, apart from a very small received signal level difference, around the "failure strength", where there's pixilation, ie you get "flashing squares" and occasional bits of picture.
This is because, going towards complete failure, as signal strength falls and digital errors rise, the error correction system just works increasingly harder, until the point where it very suddenly can't correct any greater amount of errors and completely fails.
The "quality" reading is the inverse of received bit error rate, ie, as errors rise - with falling sig level the errors go up - so "quality" goes down.
Added. Note that errors aren't just from a "bad" signal, noise is statistical, and all digital tv needs error correction to work at all, even on very high strength signals. |