[Asterisk-Users] Anyone using * in a live production environment?

[Steve Underwood]

Stephen R. Besch wrote:

> God, I wish I could be so sure of that.  I've looked at the circuitry 
> on some high end channel bank active hybrids. SPICE modeling predicts 
> a maximum 26 dB attenuation of the returned echo, even with a balanced 
> line.  A simple circuitry change which adds a balance adjustment 
> should permit cancellation to approach the parctical limit of a good 
> quality op-amp (60-100 dB).  I don't know what the Digium stuff looks 
> like, so I can't comment on it.

The spice modeling is rather idealised if you get those figures. Few 
hybrids ever give more than 20dB. If they achieve more than 12dB across 
most of the audio band they pass the approvals. Most *only just* pass 
the approvals. What would be the point in getting great results at the 
channel bank, when the phone's own hybrid still gives you a -12dB echo? 
The phone tends to be worse than the other end. This is why VoIP systems 
have to use echo cancellation. Hybrids are for little more than keeping 
the feedback below the oscillation point.

Regards,
Steve