Basically: VoIP (sip or iax) plus an fxo equals echo and jitters.
Here is the situation I am most worried about:
VOIP PARTY <dsl> *ASTERISK* <fxo> PSTN <pots> ANALOG PARTY ;Echo and Jitters
VOIP PARTY <dsl> *ASTERISK* <fxs> ANALOG PARTY ;Beautiful Sound
My theory is that this celeron can't handle the combination of decoding the voip stream and digitizing the analog stream (i'm sure someone will tell me the hard work is done on the TDM400P, so that's not it)
Herre are two other interesting cases, which may mean more to someone else
ANALOG PARTY <fxo> *ASTERISK* <sip_NAT> WEB <sip_NAT> REMOTE DIGITAL PARTY ;When using a remote client there is echo, no jitter
;Haven't tried local client
ANALOG PARTY <fxo> *ASTERISK* <fxo> POTS <pstn> ANALOG PARTY ;Beautiful Sound Again
So, again only the combination of voip and fxo creates a problem, and fxo alone or with fxs sound fine.
The only non default echo cancel i'm using with asterisk is echocancel=yes in zapata.conf . Eyebeam is my client which also has some echo cancelation enabled.