knudsen@ihnss.UUCP (12/01/84)
<bleep bleep>
Since there's been lots of concertn expressed about homebrew direct-connect
modems without expensive DAA circuits being illegal (ho-hum) and capable
of crashing your phone line (exchange, etc?) (uh-oh),
here is a simple and fairly safe way to hack a DAA.
Simply take one of your old telephones, with a separate handset ("receiver").
The hadset has a 4-wire interface to the phone's body -- one pair of wires
for receive (to the earpiece), and another pair for transmit (to the carbon
mike). The hybrid transformer and protective stuff is already present in
the body.
Now take your modem's receive analog input from across the earpiece,
and feed the transmit analog across the mike. Use DC -isolating capacitors
in all 4 wires, about 1.0 microfarad at 200 volts.
This takes care of everything except excessive transmit level, which will
cause annoying crosstalk and get the phone co. mad at you. Adjust that carefully
by calling a friend, or listening on another extension. If you drive it too
hard it's going to distort and degrade your reception at the other end anyway,
so don't push it.
Of course you can still use the dial or buttons normally, and even
talk thru the thing. Substitue a 100-ohm or so resistor for the carbon mike
if room noise pickup is a problem, and likewise for the earpiece when your
nerves get shot. mike k
PS: I don't know if this is "boy scout legal" to the FCC. The intent is
to avoid causing any damage or trouble to the phone system, while eliminating
your need for specialized hardware.