[Freeswitch-users] echo cancellation on PRI cards

[Steve Underwood]

Fadil Berisha wrote:
>
>     That's a simplification.  Simple IVR (record, replay, collect
>     DTMF) probably
>     doesn't need EC;
>
>
>
> Dave
>
> Collect DTMF does not need EC. I take out your word "probably" 
> because  no need for any dilemma. Interaction between DTMF detector an 
> EC *when EC exist* is different question and deserve separate  thread.
> Although  I am confirming your statement,// I can not say "I am voting 
> for you", simple because this is not political forum to express 
> believing to one or other leader or authority.
Receiving DTMF reliably needs a signal to noise ratio of about 10dB if 
the noise is Gaussian. The statistics of voice mean you need the DTMF to 
be more like 15dB above voice. Most hybrids are only required to have a 
return loss of better than 12dB. They can be *much* better, but don't 
count on it, especially at the phone's hybrid. You have multiple hybrids 
in the path (usually 2 or 4). Let's take the better case with only 2 
sitting between the outgoing exchange card and the far end phone. Put 
10km of copper between the exchange the phone (typical copper planning 
limit) and you probably have 15dB of attenuation on the line. Now your 
DTMF is actually below the level of the voice prompt for much of the 
time. Think that will work with an echo canceller?

Sure you can get reliable DTMF detection on 70%-80% of call paths with 
no echo cancellation, but if you want reliability with close to 100% of 
phone lines, you need echo cancellation to remove the voice prompt from 
the signal received at the IVR. Dialogic, NMS, and the others didn't put 
EC on their cards for nothing. The only reason the normal connection of 
a phone to a line card gets reliable detection of the first dialed digit 
in the presence of a dialing tone is that the DTMF detector heavily 
filters that dialing tone.

Forget the political forum crap. If you want to refute what I just said, 
try to back up your argument with some actual engineering.

Regards,
Steve