[Freeswitch-users] echo cancellation on PRI cards

[Ognjen Seslija]

To share my experience: I had issues with echo with many E1 trunks in
Serbia, especially when voice in between telco's network went to well known
bad analog lines. I used OSLEC and I was fortunate to have Steve to complain
to, he helped patching it further after my beta testing. Not many people
would do that imho.

I now switched to Sangoma cards with Octasic chips and occasionally would
still hear certain echo.
My view is that here some echo cancelling solution is very necessary,
otherwise whole VoIP business comes up to bad reputation People would just
not listen to themselves speaking, even using $400 phone.

Regards,
Ognjen
2009/3/18 David Knell <dave at 3c.co.uk>

> 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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