[Freeswitch-users] echo cancellation on PRI cards

[Steve Underwood]

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? :-\

>
> 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 many
> channels.
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.

>
> 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".
>
> 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.
>
> 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.
>
> 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.

Regards,
Steve