[Freeswitch-users] echo cancellation on PRI cards

Sat Mar 28 15:35:35 PDT 2009

Well,

>From my experience, an AS5300 thinks nearly *anthing* is dtmf.
The one I have as a PSTN onramp to our conference bridge drives me bonkers
with false positives
since every key on the pad means something in our default conference.

Jim Dixon designed the el-cheapo (tormenta 2) to put as many resources on
the host as possible, that was the goal behind the initiative.
The driver for this card was the template on which the entire Zaptel (now
Dahdi) was based.

What's interesting is that early zapata library (a user space abstraction
lib) was completely consumed by chan_zap and from there
may of the featues were gravitated towards the linux kernel then eventually
into the hardware as new cards were developed.

The typical reason for this kind of evolution is customers.  When there was
no TDM to be had at all, el-cheapo and software was the bees knees.
As they started getting more greedy and anxious for higher quality, they
started asking for improvements that lead to more stuff onboard in the new
cards.
I am guessing this will continue until the cards are too expensive and we
will go full circle back to all-on-host just in time for the 16 core CPU box
being standard issue.

It all depends on the strategy employed, if you want to use a tor2 and
oslec, (a software echo canceller that, in fact has none other than Steve
Underwood from this thread as a collaberator) then do it.  If it ain't
broke, don't fix it.....If you prefer to have hardware EC, then buy a card
that supports it.

------
see http://www.rowetel.com/ucasterisk/oslec.html
Background and Credits

Oslec started life as a prototype echo canceller and G168 test framework
from Steve Underwood's Spandsp <http://www.soft-switch.org/> library. Steve
wrote much of the DSP code used in Asterisk, and the Zaptel echo
cancellation code is heavily based on his work.
------

Bottom line: There is no real correct answer because it depends on what your
goals are and what you personally prefer.

I personally have used both, I am annoyed with hardware EC because it breaks
software dtmf but now the sangoma drivers have hardware dtmf to use together
with hardware EC so that solves the problem.

I would prefer not to take any sides in this debate since everyone on this
thread has contributed greatly to our project and I respect them all.
I would, however, like to ask that maybe we can channel all of this
intelligence into some common goal and do somethng great rather than
spend our energy doing the techno-geek version of m&m freestyle rapping.

On Sat, Mar 28, 2009 at 8:44 AM, David Knell <dave at 3c.co.uk> wrote:

> 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
>
>
>
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-- 
Anthony Minessale II

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