[Freeswitch-users] echo cancellation on PRI cards

[David Knell]

Raymond Chandler wrote:
> What's interesting to me is.... everyone on this thread except you has 
> said that in real-world scenarios, they need the EC for reliability.  
> One of which, does signal processing programming professionally. It 
> seems to me that if you "build a better mouse trap" you must know what's 
> involved in making it work properly.  I'm not sure what your background 
> really is, but you'd be hard pressed to match up to Steve's reputation 
> and/or experience.
>   
Public willy-waving is undignified but, in brief, I've built and sold 
IVRs since
1997, wrote a CAPI-based soft IVR in 1999 (which required software for, 
inter
alia, DTMF detection), developed a software fax modem (V.29, V.27ter, T.30,
etc.) which I sold to a CTI card vendor and so on.

I've collected some data, of which it is commonly said that the plural 
of anecdote -
which is what we've had so far - is not.  The IVR collects a 16 digit 
DTMF string,
terminated by #.  TDM->IP conversion was performed by an Asterisk box with
an el-cheapo quad E1 card (no EC) for half the calls, and an AS5400 
(with EC)
for the other half.

The proportion of entries missing one or more digit was 3.1% (Asterisk) 
and 3.3%
(AS5400); this is not a statistically significant difference given the 
sample size.
The reason for looking at this criterion is (a) that it's easy to 
measure, and (b) the
most likely way that a DTMF detector will fail in the presence of excess 
noise,
which includes echo, would be to miss a digit.  This error rate is the 
sum of
human error + detector error, and I've no measurements to show how this 
might
be split; I would expect it's almost all human. Note that this is a 
digit error rate of
about 1 in 500.

This is, of course, only data from one site, but it's a start; it's only 
by collecting
data such as this that one can understand how well one's mouse trap works
and whether it needs improvement or not.
> That said, it might be a good idea to just agree to disagree as this is 
> starting to sound like the faxing over IP talks I hear a lot. (i.e. 
> "faxing over g.711u with no t.38 works fine for me") Where it might work 
> for some people by some mysterious phenomena, it won't work for the 
> general public. And telling people that they don't need EC, where so 
> many have already said that they obviously do, is just as irresponsible, 
> IMHO, as you claiming Steve was for telling them that they don't need it.
>   
That's a simplification.  Simple IVR (record, replay, collect DTMF) probably
doesn't need EC; if you're trying to do ASR with barge-in, bridge callers to
other callers or operators, etc., then you probably do.

I am interested that the recommended solution is 'buy Sangoma' - expensive
and proprietary - when Oslec, a FOSS echo cancellers which, by all accounts,
works extremely well, is out there and has been for some time.

--Dave