Eeyore wrote:

Mike J wrote:

curious wrote:
Only the phones need to have filters. The broadband modem doesn't need
one and it can cause problems if one is fitted or so I hear.

Not quite true (as kraftee says later)
The 'modem' side of a filter should (on decent units)have additional
components in it to isolate/protect the modem from the DC on the line,
the 25c/s ringing voltage and the dc changes caused by lifting/replacing
the handset on other phones.

No. Any components there may adversely affect the line balance and impedance.
All the required 'protection' is in the modem itself. It would be nuts if it
wasn't.


Not necessarily.

However I suspect the cost of the odd blown modem is a lot less than
trying to distinguish between a 1Mhz signal and a lightning transient,
without making the modem cost £1000 and therefore not be saleable.

I had USR 9600 modem that DID get literally struck by lightning: they
replaced it FOC. I queried this and they said 'its built into the price.
Its easier to simply replace blown units with no paperwork and
invoicing, than to fiddle about trying to determine if the customer is
lying, and its a heck of a lot cheaper than trying to proof them against
it. We strip out the guts and scrap any parts that don't work: In your
case sir, probably the whole thing"

But as far as the typical ADSL unit goes, part of what it does is to
have a high pass filter of probably at least second order set at about
20Khz: That's more than enough to get rid of any DC, speech and ring
tone on the line. All you ant is to reduce it so it doesn't overload the
digital filters that come later.


So you just plug em straight into BT and Robert is a relative.


If the modem has a phone port, that will have its own low pass filter in
it. Another reason why you wouldn't want to do that sort of filtering
higher up the food chain: phones plugged into a phone socket on the
modem would NOT then work.



Graham