Thanks for the reply mikerj, I presume you are talking about threshold voltage? What I'm really interested in is how a noisy (ie flickering between 0 and 1) single bit would be arbitrated.
I've attached I diagram to better (hopefully) explain what I'm talking about. If you only sample the very centre of a bit, bit 1 would look like a 0, despite it looking very much like a 1. bit 2 would be fine, but bit 3 looks like it might be either. Alternatively if you sampled each bit 10 times and went with a majority to determine the bit, bit 1 would bit a 1, bit 2 0 and bit 3... who knows lol.
Is there a standard way of doing it on BERTs, especially if you were more interested on the effects of say emi on a transmission line? i suppose for that you'd want the simplest way of arbitrating a 1 or 0, rather than adding any filters etc?
EDIT: Realise I forgot the attachment!