asterisk users - May 2006 - Are my expectations too high?

I'm trying to use Asterisk with a TDM400P and 2 analog lines, but I'm
having a hard time getting the kind of audio quality that I'd like.

I'm hoping to be able to use SIP phones to make calls through Asterisk
and have the same quality as a regular analog phone connected to the
PSTN.  Are my expectations too high?  Is it that once I'm using
Asterisk as a gateway to the PSTN, I have to expect a certain amount
of voice quality degradation?  I've never used Asterisk before so I
really don't know what is truly possible and what isn't.

I'm getting a slight echo...sometimes...it varies from call-to-call,
but the biggest problem I have is a constant hiss in the background.
Again, this varies from call-to-call.  I know my SIP phones are fine
as SIP-to-SIP calls on my LAN work perfectly.  I only have problems
going out to the PSTN.

So for those of you isng a TDM400P, would you say your voice quality
is as good as a pure analog/PSTN call?

I have done the normal troubleshooting that I've seen recommended like
changing SIP phones, ensuring that the IRQ isn't shared, tweaking the
tx and rx gain and the echo cancellation values.  I haven't tried
EVERY possible combination of values, but if what I want isn't
achievable, then I don't want to waste my time.

> I'm getting a slight echo...sometimes...it varies from call-to-call,
> but the biggest problem I have is a constant hiss in the background.
> Again, this varies from call-to-call.  I know my SIP phones are fine
> as SIP-to-SIP calls on my LAN work perfectly.  I only have problems
> going out to the PSTN.

Get an FXO card with hardware echo cancellation. I use the Sangoma 
A20002D (four FXO ports with echo cancellation). It definitely costs 
more, but the hardware echo cancellation makes a huge difference in call 
quality! Software echo cancellation doesn't really work...

I don't know what to suggest about your hiss problem though.

- Mike

>So for those of you isng a TDM400P, would you say your voice quality
>is as good as a pure analog/PSTN call?
No it's not. There will be artifacts in any TDMXXX TigerJet Digium analog
card, IMO. These artifacts are mitigated through the black art and dumb luck
of different chassis, local RF interference, different handsets, different
Asterisk version, etc. But you will most likely never get the exact same
quality as plugging a handset into the line. 

On my cards, I find that handset selection using the FXS modules makes a big
difference. The best I have found is the Vista 350 and just about any
Panasonic cordless phone.