[asterisk-dev] Building an embedded Asterisk PBX

[Greg Boehnlein]

On Sun, 3 Sep 2006, David Rowe wrote:

> > With regards to echo cancellation, it is my opinion that it is 100% 
> > neccessary to have hardware based echo cancellation on any piece of 
> > hardware that will be deployed in a customer scenario. In all of my 
> > experiences, I have finally come to the conclusion that the Software Echo 
> > cancelling routines in Asterisk are just not acceptable enough to deal 
> > with, and eliminate all forms of echo. I.E. some echo and artifacts always 
> > remain. As a result, I have made the decision to deploy all hardware based 
> > Echo cancelling cards.
> 
> OK, thanks for the tip.  As a first pass I plan to use the Asterisk echo
> can software, but we can always try and improve from there.  I know from
> personal experience that echo cancellers are tough pieces of DSP
> software to write, so I understand the problem.  So to be fair I would
> say that the authors of the Asterisk software echo can have actually
> done a pretty good job - much better than I have to date :-)
> 
> Just to clarify "hardware echo cancellers" are actually proprietary
> software running on programmable DSP chips (just like say G729 software
> in binary form).  The reason they work so well is that they have been
> written by echo can experts (like I said - this is tough, specialised
> software).  The same software would work just as well on an x86 with
> enough MIPS available.
> 
> There is no fundamental reason that Asterisk couldn't include an open
> source, carrier grade, high performance echo can.  Its just that no one
> has written one yet.  But then again a few years ago no one had written
> an open source PBX, either.

I'm not saying it can't be done, but w/ existing DSP solutions such as 
Octasic, many have chosen to let the experts practice their black arts and 
move on to bigger and better things.
 
> > Do you think the blackfin STAMP has enough horsepower to enable carrier 
> > grade echo cancelling? 
> 
> In terms of MIPs, I have calculated that about 12 channels of 128ms
> echo-can will run comfortably on a Blackfin (or 24 ports of 64ms tail
> echo can).  Thats while running Asterisk at the same time.  You see the
> Blackfin _is_ a DSP chip, so it's optimised for this sort of thing.  It
> just happens to run uClinux (and therefore Asterisk) as well.  
> 
> > Or is there an easy way to include a DSP based Echo 
> > canceller into the system?
> 
> So what you really mean is how to get carrier-grade echo can?  You could
> (i) license some proprietary echo can software and use it on the
> Blackfin (ii) as a community we could develop one (I have talked to some
> experienced DSP/echo guys about this) and (iii) you could hang something
> like an Octasic chip off the side.  (i) and (iii) will make it cost
> more, (iii) is needs some community development work.  And would be more
> fun :-)

As I suspected. It would be nice to have a board that supported the 
addition of a socketed Octasic DSP. That way, you could load the existing 
Octasic firmware that ships w/ Zaptel into it.

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