[Asterisk-Users] Will Echo problems EVER be solved, I'm scared

[Rich Adamson]

> > The -2 to -3 db is not correct for analog circuits. Copper wires have
> > a loss that is directly related to the length of the cable. (I don't
> > have the chart right here, but a 7,000 foot cable pair will have lets
> > say 6db of loss and a 3,000 foot pair will be a 3db loss. You can't
> > engineer something into a copper pair to compensate for that loss.)
> > 
> > The only thing that I can think of that you might be talking about is
> > using an old analog carrier system on a copper pair. If that's what
> > you're thinking, then yes -2 to -3 db is very reasonable.
> 
> 
> Analog carrier was(is?) engineered for -16/+7 tx/rx TLP. That was 
> because microwave links liked it that way and it kept spurious emmisions 
> down.

No, -16/+7 is associated with setting up analog Toll and CO-to-CO 
trunks, not pbx trunks.
 
> What can be engineered into a loop is an odd little device called a pad. 
>    Most often today they're found in the channel module in a channel 
> bank coming off of a digital switch.  The milliwatt are accessed right 
> at the card and measured.  the pad(s) on the card are adjusted for the 
> calculated values.
> 
> Analog switches were similar in that banks of pad sockets were wired to 
> the ports on the switch and plug-in pad/amp units installed and adjusted.

Okay, now I understand how this got way off into incorrect gain settings.

The OP was talking about pure copper analog wires from the Central Office
to the client's pbx. No carrier involved, no pads, no repeaters, no 
amplifiers, no microwave, no nothing. Purely a pair of copper wires 
terminated in a TDM04b card from the CO.

Given that, the entire discussion of undertanding/measuring the loop
loss and setting the rxgain and txgain to about 2 or 3 db below that
_is_ the realistic starting point. Then adjust those same gain settings
to balance the echo with lack of volume (for the TDM and X100P cards).

So, for those not understanding the above:
 - dial into the central office milliwatt generator and using a
   transmission test set (lots of them available), measure the cable
   loss. Can be anywhere from -1 or 2 db up to more then 12 db.
 - set the rxgain and txgain to about 2db less. (If the loss is 
   measured at -8db, set the gain at +6db.)
 - make test calls to another local CO telephone number and eval the
   echo and audio levels. (Stop and Start * after each change as a reload
   won't cut it. The gain settings should be reduced by 1 or 2 db for
   each test call.)
 - Repeat the manual calls and eval until some reasonable balance is
   achieved. (ztmonitor won't help at all.)
 - Repeat the last two steps for a LD call to ensure gain settings are
   reasonable.

Again, the above is oriented to "copper" pstn circuits from the CO to
the TDM card.

If the combination is other then a TDM or X100P card (eg, SIP gateway
with an echo canceler), then there isn't any need for mucking around
with the gain settings manually. Just set the gain in whatever 
external gateway device one is using to about 1 or 2 db less then
the cable loss and use it.