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Kolja Waschk (kawk@Yo.COM) writes:  
"To make the DTMF detection more reliable, ZyXEL implemented an echo
cancellation filter in the DTMF detection software in the Elite series.
This filter requires a short probe to determine the channel parameters; it
takes place only once before the first sound is to be played over a phone
connection - thus the "noise".
  
The result is a very reliable DTMF detection - my modem detects tones
generated by the cheapest DTMF transmitters even while playing music.
(There should be some S-register bit to disable this, but you'll have to
look this up yourself, as it is not in my pre-release manual).

In voice mode, one of the two DSPs is used solely for DTMF detection!"

* Applicable to the Elite series only