[asterisk-users] Poor VoIP voice quality in one direction from three providers

[Robert L Mathews]

We currently use asterisk 1.4.x with two Zaptel cards connected to POTS 
lines. So we make "outbound" calls from their softphones (using ulaw 
format), which go over a dedicated DSL line to the asterisk server in 
our office, which then converts the calls to POTS.

This all works fine, assuming there aren't any unusual problems. It 
sounds as good as POTS on both ends.

However, we don't want to maintain the DSL line or deal with the hassles 
of analog/digital conversion any more. So we want to switch to a 
reliable VoIP provider and move the asterisk server to one of our 
colocation data centers.

We've tried getting test accounts with three VoIP providers: FlowRoute, 
CallCentric, and Vitelity. In our tests, outbound calls now go from 
softphones -> asterisk -> VoIP provider -> outside world. We use ulaw 
all the way through.

But with all three providers, we see a curious thing: The audio quality 
in the direction from our softphones to the outside world still sounds 
as good as POTS, but the audio quality in the inbound direction (outside 
world -> VoIP Provider -> asterisk -> softphone) is noticeably worse. It 
sounds "overcompressed" or "slightly robotic" somehow, with a decrease 
in dynamic range. It's not lagged or echoey; it just sounds like it's 
maybe using a crappier codec than ulaw, in that direction only.

I'm baffled by this. Both legs of the calls show as "Format: 
      0x4 (ulaw)" in "sip show channel". Testing the first provider, I 
just assumed that their analog->digital conversion was inferior to what 
the Zaptel cards offer (i.e., that they were injecting inferior sound 
quality into their ulaw connection)... but we're getting exactly the 
same results with all three providers, which makes me think it's us.

Why might this happen? Is there any possible reason other than "all 
three of the VoIP providers are decreasing the audio quality before 
injecting it into the ulaw stream"?

-- 
Robert L Mathews