Dear David, Regarding the wet transformers used in Germany:
I have designed the front-end of a modem for the Deutsche Telekom some time ago. Oddly enough we have had two interface versions for the same modem: initially a wet design with a Midcom transformer (it was a V.32bis modem) and about a year later we had to add leased line operations and had to go to dry tranformers because we were running out of space. BZT was still checking for bit errors while the tax pulse was on for both interface versions (and the procurement department of Deutsche Telekom was doubling up on that test for sure). We had to use the two stage filter in both cases and couldn't find much of a performance difference. About 8000 modems were bought by Deutsche Telekom and used on synchronous dialup connections but no differences were found in performance. Since it was a synchronous application there was no way of hiding bit errors from the client's application. Deutsche Telekom guaranteed a certain buit error rate for the data connection since they provided the modems and the lines to customers as one complete package. Like your message already said: it is important to put that first filter stage in front of the line hold circuit. I think after that there is little difference between wet or dry. Best regards, Rene Debets
