[Asterisk-Users] Are my expectations too high?

[Andrew Kohlsmith]

On Tuesday 23 May 2006 11:00, Derek Lee-Wo wrote:
> This is what I'm beginning to suspect.  I guess someone else in the
> theard stated my question more accurately and what I really want to
> know is if the degradation is noticeable.

Take the time to set it up and tune it correctly.  I use TDM400s without these 
issues.  FXO is trickier to set up than FXS yes, but the problems you are 
experiencing are not guaranteed because of the card, as someone else 
declared.

Find the miliwatt test line for your local exchange.  Use ztmonitor and 
fxotune to tune the card properly.  Use the default MG2 echo canceller to 
start and experiment with KB1 and maybe MARK2, although it's generally 
accepted that MG2 is the best to date.  Make sure you're using the card in a 
system that doesn't have an iffy (IRQ-sharing and/or jittery) PCI bus 
implementation.  This is a realtime system and requires a decent-ish box to 
make it work correctly.  Not so much decent in terms of CPU horsepower but 
rather in terms of repeatable and consistent resource access.

The reason you don't run into these problems on the more expensive cards is 
that the hardware in them can do a lot of this tuning automatically, and the 
echo canceller's are (much) more tested and standards-compliant, but it's a 
function you definitely pay for.  The reason you don't generally have echo 
problems with a regular old phone plugged into the telco jack is because the 
latency is very low you simply do not perceive it as echo.  This is 
well-documented and heavily researched.  The delay in transporting the audio 
across the PCI bus, mangling it with software and shooting it out the network 
card (and the converse) is simply not there in a regular telephone or 
application-specific hardware (regular PBX).

So no, your expectations are not too high, but you need to spend some more 
time reading, tuning and making sure that the system you're using is doing 
what you think it's doing, and that what you are manipulating in terms of the 
PSTN signal is what the system is designed to work with.

Welcome to computer telephony.  :-)

-A.