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

[Andrew Kohlsmith]

On Thursday 25 August 2005 08:00, Rich Adamson wrote:
> If you mean placing a transmission test set at the customer's demarc (at
> the customer's site), the -2 to -3 db is still incorrect for "analog"
> pstn circuits. That level _will be_ the 0db generator tone minus the cable
> loss from the CO to the customer's demarc. That cable loss is 100%
> predictable if you know the length and gauge of the copper wires between
> the central office and the customer's site. (That "is" exactly how the
> engineering spec is set for the less technical telephone installers
> to measure after installing a new pstn facility to a customer site.)

Ok...

> The echo cancellation problem with the x100p and TDM cards are very
> much related to the narrow operating range that the existing echo
> cancellation software can operate within.  In the above 8 db loss
> example (assuming there is an actual 8 db of copper loss between the
> CO and the customer's site), the _correct_ rxgain and txgain settings
> would be to provide approximately +6 dbm of gain within asterisk/zaptel.
> Those gain settings would fall within the stated -2 to -3 db TLP range.
> However, that 6db of gain (in both directions) will fall outside the
> operating range of the existing echo canceller, therefore smaller
> gain values are almost always required in asterisk. It doesn't make
> any difference whether the transmission level is measured with
> ztmonitor or a $10k transmission test set. Neither will make it work
> right.

Now I'm confused.

In the first quoted paragraph you seem to be saying that I should tune the 
rxgain in order to achieve a perceived 0dBm by Asterisk.  (14844 in 
ztmonitor.)  But then in the paragraph quoted above you're saying that a 
reading of 7422 (-3dBm if ztmonitor's output is linear-reading) to 14844 is 
acceptable.

I'm reading it as "if it takes an rxgain of +8 to get a 0dBm level in 
ztmonitor, you should tune rxgain to provide 6dBm of gain in order to fall 
within the -2dB to -3dB range."  

Am I misreading?

I was under the impression that the echo canceller worked with whatever came 
out of the zaptel driver *after* the rxgain was applied and *before* the 
txgain was applied.  This would mean that your rxgain setting to achieve -2dB 
to -3dB TPM would certainly fall within what the echo canceller was designed 
to expect on its input, and that the echo canceller assumes that it's output 
is at 0dBm, so whatever it sent out would be "corrected" to 0dBm by the 
transmit amp stage of the zaptel driver.

> of reflected energy (latency), etc. The companies that engineer dedicated
> echo cancellers can throw processing power at the problem or use
> dedicated chips (DSP's) to do that function. Not likely asterisk's
> canceller will ever be equivalent to dedicated hardware cancellers.
> (And, why does the new digium T1 card have it on board?)

You just described why; it's an enormously complex problem that takes lots of 
CPU time.  By putting it in hardware before the PCI bus access you eliminate 
a lot of the variables that doing it on the host CPU introduce.

> One other point that may not be obvious from previous echo postings.
> Those asterisk users that are physically located a greater distance
> from the CO _always_ have greater echo issues. Those that are relatively
> close don't have as big an issue. That _is_ due to the small operating
> range of asterisk's echo canceller. And, that is _one_ of the reasons
> why one implementor's settings don't work at another implementor's
> location. (Not to mention differences in motherboards, etc, etc.)

Agreed.

-A.