[MLB-WIRELESS] Free P2P phones will pressure telcos

Jason Hecker jhecker at wireless.org.au
Wed Nov 12 15:43:32 EST 2003

Mucking with the guts of an old cordless would still mean you'd need to 
be at the PC to answer or originate a call, but then you'd be free to 
roam about using a phone as you normally would.  Alternatively you could 
hook up the parallel port pins to detect the phone going off hook and to 
drive the ringer logic.  To make effective use of this you'd need to 
then modify something open source like GnomeMeeting or use Vovida's VoIP 
libraries and hack up your own DTMF detection and signalling (fairly 
trivial... he says).

I recall trying to fix a cordless phone ages ago and it had Motorola 
cordless phone chips.  Typically in these phones you have the line 
interface circuit which has a gyrator and transhybrid.  The gyrator is a 
fancy little transistor and capacitor circuit which emulates a high 
inductance inductor.  It effectively shorts the line at DC completely 
forming the current loop the exchange needs to detect the line is in use 
- 20mA is typical.  The AC (audio 300Hz to 3400Hz) passes through to a 
small isolation transformer and then to a transhybrid cicuit which 
multiplexes the transmit pair and receive pair onto the line pair.  A 
good transhybrid won't allow what you are saying to feed back into your 
speaker.  There is also a ring detect circuit which detects if a +-60V 
AC ring at a particular frequency (20Hz say) is present.  These circuits 
you can bypass in an electronic phone and hopefully be able to patch in 
your soundcard and parallel port I/O pins for ring and line-hook to fool 
the phone into thinking it's business as usual.  Hopefully then your 
impedances are OK so you don't get echo.  Does VoIP software employ echo 
cancellation?  I have no idea but I hope they do.

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