[Freeswitch-users] echo cancellation on PRI cards

[David Knell]

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. 

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.

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 just don't need echo cancellation to achieve perfectly acceptable 
DTMF detection.

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.

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.

--Dave

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