[Freeswitch-users] echo cancellation on PRI cards

Tue Mar 17 19:40:59 PDT 2009

Sharing my humble experience: in Brazil we usually need echo cancellation to
have reliable DTMF detection _and_ voice quality over E1 lines (be it on
MFC/R2 - r2d - or ISDN PRI lines), either for sip/tdm gateway devices or IVR
applications.

Usually there's no need for echo cancellation on links from some Telcos, in
some specific places. But we need it in the majority of cases, even when my
box is just a gateway between legacy pbxes.

This represents just a subset of the available E1s in the world and it's
just a practical experience, but it's a fact for me. If I don't have a card
with echo cancellation, I don't offer reliability to my customer; I've done
that in the past and didn't work out.

I'm not theoretically discussing anything, just sharing what I've been
through in the last 4 or 5 years.

2009/3/17 David Knell <dave at 3c.co.uk>

>  Steve Underwood wrote:
>
> David Knell wrote:
>
>
>  Steve Underwood wrote:
>
>
>  [whopping big snip]
>
>
>
>  The first bit of that's a tad patronising, isn't it,
>
>
>
>  You are the one who started out being offensive.
>
>
>
>  I'm sorry if you find disagreement offensive; you might not wish to
> read beyond this
> point if so.
>
>
>  and, in the case of the decade-old Aculab
> cards which which I'm most familiar, is also untrue.
>
>
>
>  I can't find too much about the old cards on the web now, but I found http://www.amdevcomm.com/voice-mail-products/voice-mail-components/dialogic/dti_sc.html
> which is pretty much a copy and paste from the old Dialogic web pages,
> and you'll see it says "Cut through : Local echo cancellation permits
> 100% detection with a >4.5 dB return loss line". The Aculabs did the
> same thing for sure. They just couldn't work without cancellation. There
> were some very early Dialogic cards, using DTMF receiver chips and OKI
> ADPCM chips, and had no general purpose DSPs. They performed really
> badly because of the lack of cancellation, and were quickly replaced
> with cards that put the OKI ADPCM, DTMF anf echo cancellation algorithms
> into a Motorola 56k DSP chips.
>
>
>
>  The same document, under the bit which you've quoted, says:
> "(E-1) Digital trunks use separate transmit and receive paths to network.
> Performance dependent on far end handset's match to local analog loop."
> - i.e. the card does no echo cancellation.
>
>
>  Your messages are starting to looked deranged. Why would they only apply
> echo cancellation to T1s? Its a bizarre idea, and you must realise its
> wrong. Are you so desperate to support a wrong answer you'll clutch at
> straws? :-\
>
>
>  More insults.  Answer me this: if there were echo cancellation in use, why
> would
> DTMF detection performance depend on the far-end handset's match to the
> loop?
>
> And the follow-up question (which you've already pretty much asked) - if
> the
> card doesn't echo cancel for E1s, why would it for T1s?
>
> As an aside, I'm not convinced that the document's not talking about return
> loss
> on the T1 line itself, the implication being that the T1 is being carried
> on a single
> pair, which makes the first sentence about E1s make a bit more sense.  But
> that's
> just a guess.
>
>  Aculab didn't even offer echo cancellation on Prosody for years and,
> when they did, it
> consumed prodigious amounts of DSP.  Nonetheless, the DTMF detection
> worked
> perfectly well, even across 120 channels per 40MHz SHARC - there's
> just no way
> that those DSPs had enough horsepower to do echo cancellation across
> that manychannels.
>
>
>  This page http://www.aculab.com/support/pdf_documents/v6_solaris/ting/pubdoc/an-dtmf-det-issues.html
> seems to support what you say. It also implies DTMF detection sucks
> unless you echo cancel. The statement "If the outgoing signal is a tone
> of some sort (e.g. a 'beep'), ensure that its frequency is below 600Hz"
> is telling you to keep your outgoing signal in the same frequency range
> as dial-tone where the dial-tone filter on the DTMF receiver will
> obviate the need for an echo canceller. They are freely admitting
> exactly what I have said. If you want a normal IVR with cut-through to
> work you better turn that echo canceller on.
>
> My only experience with Aculab was fitting a box designed by other
> people into a system. That one definitely echo cancelled, as it worked
> as well as the Dialogic based boxes we developed ourselves.
>
>
>  That only holds true if your premise - that you need echo cancellation for
> good
> DTMF detection - is correct, which I don't believe it is.
>
>  An Asterisk box with an el-cheapo quad E1 card in that I use for
> TDM-SIP gatewaying
> detects DTMF perfectly well with no echo cancellation.
>
>
>  You must have very low standards for "works well".
>
>
>  Nothing like a good old ad hominem attack.  Beats reasoned argument any
> day.
>
>  You just don't need echo cancellation to achieve perfectly acceptable
> DTMF detection.
>
>
>  Well, not if you expect people to wait for silence before entering DTMF,
> but who would tolerate that these days? Cut through has been de rigeur
> since the late 80s.
>
>
>  Oh, for pity's sake, you get perfectly good cut through without echo
> cancellation.
> Humour me and draw a quick mental picture of the spectrum of a random bit
> of
> speech at -20dBm; now add tones at -10dBm and -7dBm.  They stick out like
> a pair of sore thumbs.
>
> I'm sure it's quite possible to come up with a pathological case - e.g.
> cut-through
> against a 1kHz milliwatt tone, but that sort of thing just doesn't happen
> in real-
> life IVR applications.
>
>  ASR - yes, maybe, but surely only in the case where the application
> requires barge-in;
> even then, I'd be interested to see some test results, particuarly
> where the outbound prompt
> is killed the moment the ASR reports start of speech.
>
>
>  Doesn't any sane system expect barge in to be nearly as reliable as
> waiting for silence? Who would tolerate something that doesn't? It has
> been a standard expectation of any decent IVR since they began.
>
>
>  Sorry - ASR with barge-in has been a standard expectation since the first
> IVRs?
>
>  I'm afraid that your original bald claim - that "IVRs badly need echo
> cancellation" is simply
> wrong, misleading and irresponsible: those believing it will end up
> spending large sums
> of money on technology which they probably do not need.
>
>
>  You must have very low standards for what works well. If you suggest
> people leave out echo cancellation you are just asking for customer
> service issues down the line. That whole Aculab page is a clear response
> to just such issues they had, which forced them to add the necessary
> improvements.
>
>
>  Repeating you ad-hominem really doesn't make it any stronger, I'm afraid.
> And
> the Aculab page you refer to offers four solutions for problems caused by
> far-
> end echo, of which cancellation is just one; not playing a stationary tone
> above 600Hz
> is another.
>
> Do you have any real-world samples of DTMF+echo which give your DTMF
> detection code trouble?
>
> --Dave
>
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-- 
Arnaldo M Pereira
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