[Asterisk-Users] FXO impedance matching

[Michael Welter]

Jason A. Pattie wrote:
> Robert Hajime Lanning wrote:
> 
>> Echo echo ech ech ec ec e e . .
>>
>> :)
>>
>> <quote who="miguel at amplanet.com.br">
>>
>>> What's the importance of the impedance matching in a FXO interface ?
> 
> 
> If impedance matching is that important, then how is it accomplished? 
> I'm fairly sure our X101P is not impedance matching properly.  I've 
> never not had echo, and I've followed all the procedures in the wiki and 
> from help that I've gotten from IRC.  The only way to hear anything is 
> by turning up the gain in /etc/asterisk/zapata.conf, and I've been told 
> this is a big no-no, as it increases echo problems.  But without it, the 
> line is so low that it's useless.  However, if you plug a handset 
> directly into the line coming from the CO, it's at a decent volume 
> level, etc.  No echo, of course.
> 
My experience is with excessive buzz and hum on the line.  When I plug a 
vintage Western Electric phone into the line, there is no buzz or hum 
because the phone has its own impedance matching circuitry.  When I plug 
my AT&T 954 set into the line, I hear a lot of hum.  I'm told the X100P 
does not have impedance matching.

Rich Adamson is the fellow to talk with about impedance.  Apparently the 
hum on my lines is caused by a partial ground on either the tip or ring 
(or both) wire.  If both leads have the same resistance to ground 
(matched) then there is no hum.

I don't experience echo with the buzz and hum.  I've been told that echo 
is caused when the circuit goes from four wire to two wire.

I'm trying to locate a schematic of an impedance matching circuit so I 
can breadboard a device but haven't found one so far.  I anyone has 
experience with this I invite him to reply.

Mike

-- 
Michael Welter
Introspect Telephony Corp.
Denver, Colorado
+1 303 674 2575
mike at introspect.com
www.introspect.com