-If it's just one channel, chances are it's just your cable provider or some anomaly
-If it's a range of channels (say 2-13, but gets slowly better from 14 up to the 30s), then you've probably got older cable with a smaller bandwidth (like RG59), and they've "cooked up" the feed for the weak signal attenuated by the cable at the higher channels, but the channels the cable will carry well get overmodulated
Sorry guys... every time I try to put it in layman's terms I think it sounds more confusing.... reason number 48 why I didn't go into education.
Bren R.
