[GreenKeys] Teletype Devices for the Deaf (TDD)

[jhaynes@alumni.uark.edu]

The whole story of TDD is told in a book "A Phone of Our Own" by
Harry G. Lang, Gallaudet University Press.  I won't repeat it here,
except to note that the modem was done the way it was for simplicity.
Originally it was a one-tone modem (space tone only), both for simplicity
and to allow the receiving party to break in.  The mark tone was added
not to make the signal FSK but to mask echoes from the phone line.
These showed up when Bob tried medium distance phone calls, such as
Redwood City to Pasadena.  He got a patent on the modem 3,507,997.
The mark tone cuts off when one is not transmitting to make it unnecessary
to have a send-receive switch and to allow the receiving party to
break in during a pause in transmission.