[Asterisk-Users] Huge Echo

[Rich Adamson]

> > Pull the clone card out of the system and look for the chipset numbers
> > on the card. Go to the chip manufacturers web site and find the specs
> > for that chip set. The specs will likely tell you the chipset was designed
> > for the US 600 ohm impedance telephone network, and if your country's
> > telco specs are different (which I'm very sure they are), through away
> > the clone card. Without proper impedance matching there isn't anything
> > your going to be do to fix the problem.
> 
> I can check the chip on monday, but local telco impedance requirements are 600 
> ohms - just like US.
> 
> > >
> > > I'm thinking about playing around with increasing/decreasing resistance
> > > by placing additional resistors in the circut. Messy, but if it could
> > > help... What do you think?
> >
> > Adding resistance has nothing at all to with impedance matching. Resistance
> > will impact the DC loop, but not the AC impendance. The AC impedance is a
> > function of how the chipset was designed.
> >
> AFAIR, the impedance is not a simple factor, but a combination of passive 
> resistance plus reactance - which usually varies within measured frequency 
> range. Therefore channging the device resistance WILL change its impedance. I 
> know the result will not be perfect, but at least i hope for better load 
> match than it is now. I assume the specified impedance is required within 
> PSTN frequencies which will be roughly 100-4khz, right?

That's not exactly true if you think about it. Since pstn lines are
typically copper based and have a varying resistance based on the length 
and guage of copper, the copper resistance will vary from almost zero
(close to CO) to values over 1000 ohms. That certainly does _not_ change
the AC impedance of the line (regardless of its length); its still 600 ohms.

I don't recall for sure, but I'm fairly certain the impedance is relative
to 1,000 hz tone.