[asterisk-users] Detecting answer with an analogue card

[Andrew Kohlsmith]

On Monday 05 February 2007 1:17 pm, Stefano Corsi wrote:
> Uhm... but wait: I remember old days modem to have also a VOICE
> return code... again, why should a 20$ box be able to detect VOICE
> while a 2000$ card shouldn't?

Eric's wrong here; you're right.  The modems of yore used inband audio 
detection, just like asterisk does.  They stopped trying to detect inband 
progress once a carrier was detected though.  :-)

The TDM400/2400 does not have onboard DSP for detecting progress tones; this 
is done in asterisk itself (or the zaptel driver, I forget where the actual 
DSP work for progressinband is done).  The algorithms and hardware used in 
the old modems probably did a much better job than the algorithms in 
Asterisk/Zaptel.  This is just an area that needs some work.

I don't think there is any attempt made in the code to STOP trying to detect 
tones once an ANSWER/BUSY/etc is detected.  This, and the ability to turn OFF 
the hangup detection in the softdsp, and also perhaps detect an answer by way 
of "no longer ringing", would make the asterisk/zaptel algorithm a lot more 
reliable.  That introduces some interesting billing problems, but for most 
SOHO users, nobody cares whether the far end actually ANSWERED to say "the 
number you have reached has been disconnected" or not.  Anyone who does care 
about that is using digital lines anyway, or rather they will once they get 
hosed on billing the first time.  :-)

In short: inband progress detection is an itch that nobody really seems to 
have.

-A.