Telephone handset howto.


This document pertains to interfacing
telepone audio with computers, which
arouse from my "WearTel" (wearable
telephone) project of the 1970s.
At the time, telephones were usually
hard-wired to the wall, and the handsets
were also usually hard-wired to the
phone.

However, more modern phones are
ordinarily connected to the wall by
a "modular" 4/6 (4 conductors used
out of 6 positions) connector and the
handsets are connected to the phones
by a slightly smaller "modular" 4/4
connector.  (Note that both these are
similar to the even larger modular
8/8 RJ45 connectors used for 10baseT
or 100baseT ethernet.)

The situation I will describe here
pertains to connecting a modern
telephone to a NETWINDER computer.
Since the NETWINDER computer has a
"HANDSET" connector on the back, it
is very easy to connect to a telephone.

The NETWINDER also has a "PHONE"
connector, but it doesn't work very
well and you will probably get terrible
results using it.  (You will likely
notice a lot of 60Hz hum.)

You can plug a normal handset into the
NETWINDER and record and play back
to/from it.  However, if you want to
connect a telephone to the "HANDSET"
connector (which is a quick hack way to
make a WearTel system) you will need to
make a crossover adapter.

Most modern telephone handsets use an
electret condensor mic rather than a
carbon mic like older handsets did.
Modern handsets usually use a raw
electret condenser mic element
(e.g. one with no FET or other
transistor or amplifier mounted to the
back), and this mic is usually connected
to the yellow and black wires.  The red
and green wires go to the earpiece
(typically balanced with no preference
of red or black to ground).

NOTE THAT UNLIKE 4/6 CABLES WHICH ARE
REVERSED, THE 4/4 CABLES ARE NOT
REVERSED IN SENSE, SO IT DOES NOT
MATTER WHETHER YOU HAVE AN EVEN NUMBER
OR AN ODD NUMBER OF CABLES WHEN YOU
ARE MAKING AN EXTENSION CABLE FOR A
HANDSET.

Almost always, the handset microphones
get their power from the phone, and the
yellow wire is typically provided with
about 1.5 to 5.0 volts.

Looking into the female connector:
              ________
 mic- Black  |        |
 ear  Red    |        -- locking
 ear  Green           -- tab
 mic+ Yellow |        |
              --------
there is often a 1N47 diode (banded
end to yellow mic+) in the handset.

The yellow and black wires are an output
of the handset, and thus are an input
to the phone.  Therefore to connect
the input of the phone to the output
of the NETWINDER, you will need to
get the signal from the NETWINDER's
red and green wires to the yellow and
black wires of the phone.  However,
because the phone supplies power through
these yellow and black wires (expecting
them to be connected to a microphone)
you need to isolate the DC and
preferably isolate any ground loops.

Likewise for the other direction (to
make your phone heard by the NETWINDER's
inputs.  This can be done with isolation
transformers (one for input and one for
output) and series capacitors to keep
DC out of the transformer windings.

Once you've got the transformers and
capacitors set up, it's a simple matter
of programming to make the NETWINDER
work as your phone.  Plug your headset
into the NETWINDER and your NETWINDER
into your phone, and then your computer
will be able to mediate the
communication path.

Later you may want to run iphone or
a similar internet based phone instead
of running through a traditional phone.

mixer -source=7
sets the input to either the built in
mic or the handset mic.  Note that the
handset mic will be active when plugged
in (replaces built in mic as input 7).