Welcome to your ultimate assistive technology resource


Text telephones guide

A telecommunications device for the deafto deaf persons.
(TDD) is an electronic device for textMicon produced over 1,000 MCMs per month
communication via a telephone line, usedresulting in approximately 50,000 MCMs
when one or more of the parties hasbeing disseminated into the deaf
hearing or speech difficulties. Othercommunity. Before he left Micon in 1980,
names for TDD include TTY (telephoneMichael Cannon developed several
typewriter or teletypewriter), textphonecomputer compatible variations of the
(Common in Europe) and minicom (UnitedMCM and a portable, battery operated
Kingdom).printing TDD, but they were never as
About The typical TDD is a device aboutpopular as the original MCM. Newer model
the size of a small laptop computer withTDDs could communicate with selectable
a QWERTY keyboard and small screen thatcodes that allow communications at a
uses light-emitting diodes or an LCDhigher bit rate on those models
screen to display typed textsimilarly equipped.
electronically. In addition, TDDsHowever, the lack of true computer
commonly have a small spool of paper oninterface functionality spelled the
which text is also printed — olddemise of the original TTY and its
versions of the device had only aclones. During the mid-1970s other
printer and no screen. The text isso-called portable telephone devices
transmitted live, via a telephone line,were being cloned by other companies,
to a compatible device, i.e. one thatand this was the time period when the
uses a similar communication protocol.term "TDD" began being used largely by
In certain countries there arethose outside the deaf community. The
Telecommunications Relay Services, sodeaf community, interestingly, does not
that a deaf person can communicate withusually use the term "TDD" but instead
a hearing person on an ordinary voiceprefers "TTY." Protocols There are many
phone using a human relay operator.different textphone standards. The
There are also "carry-over" services,original standard used by TDDs is the
enabling people who can hear but cannotBaudot code implemented asynchronously
speak ("hearing carry-over"), or peopleat either 45.5 or 50 baud, 1 start bit,
who cannot hear but are able to speak5 data bits, and 1.5 stop bits. Baudot
("voice carry-over") to use theis a common protocol in the US. In
telephone.Europe, different states use different
History APCOM (Applied Communications)protocols. For example, V.21 is found in
located in the San Francisco Bay areathe UK and several Scandinavian
developed the acoustic coupler, orcountries. Other protocols used for text
modem. Couplers were cabled to TTYstelephony are EDT, DTMF, V.23, etc.
enabling the Bell Telephone companyThe TDD/TTY protocols are generally
standard "500 handset" to couple, orincompatible with standard
fit, into the rubber cups on theHayes-compatible modems. In 1994 the ITU
coupler, thus transmitting and receivingapproved the V.18 standard. V.18 is a
a unique set of tones generated by thedual standard. It is both an umbrella
different corresponding TTY keys.protocol that allows recognition and
The entire configuration of teletypeinteroperability of some of the most
machine, acoustic coupler, and telephonecommonly used textphone protocols, as
set became known as the TTY. Thewell as offering a native V.18 mode,
acoustic coupler modem was the inventionwhich is an ASCII full- or half-duplex
of deaf physicist Robert Weitbrecht inmodulation method.
1964. The actual mechanism for TTYComputers can, with appropriate software
communications was accomplishedand modem, emulate a V.18 TDD. Some
electromechanically through frequencyvoice modems, coupled with appropriate
shift keying (FSK) allowing only one-waysoftware, can now be converted to TDD
(simplex) communication. In 1973 the MCMmodems by using a software-based decoder
(Manual Communications Module), whichfor TDD tones.
was the world's first electronicIn the UK, a virtual V.18 network,
portable TDD (Telephone Device for thecalled TextDirect, exists as part of the
Deaf) allowing two-wayPublic Switched Telephone Network,
telecommunications, premiered at the CADthereby offering interoperability
(California Association of the Deaf)between textphones using different
convention in Sacramento, California.protocols.
Enlarge The battery-powered MCM wasThe platform also offers additional
invented and designed by Michael Cannonfunctionality like call progress and
in conjunction with physicist Art Ogawastatus information in text and automatic
and deaf interpreter Kit Patrick Corson.invocation of a relay service for
It was manufactured by Michael Cannon'sspeech-to-text calls.
company, Micon Industries, and initiallyIn addition to regular Baudot, the
marketed by Kit Corson's company, SilentUltraTec company implements another
Communications. In order to beprotocol known as Enhanced TTY, which it
compatible with the existing TTYcalls "Turbo Code," in its products.
network, the MCM was designed around theTurbo Code has some advantages over
five-bit Baudot code established by theBaudot protocols, such as a higher data
older TTY machines instead of the ASCIIrate, full ASCII compliance, and
code used by computers. The MCM was anfull-duplex capability. However, Turbo
instant success with the deaf communityCode is proprietary, and UltraTec only
despite the drawback of a $599 cost.gives its specifications to parties who
Within six months there were more MCMsare willing to license it.
in use by the deaf and hearing impairedDevices In addition to TDD, there are a
than TTY machines. After a year Miconnumber of pieces of additional equipment
took over the marketing of the MCM andthat can be coupled to telephones to
subsequently concluded a deal withimprove their utility. For those with
Pacific Bell (who coined the term "TDD")hearing difficulties the telephone ring
to purchase MCMs and rent them to deafand conversation sound level can be
telephone subscribers for $30 per month.amplified or pitch adjusted, ambient
After Micon formed an alliance withnoise can also be filtered. The
APCOM, Michael Cannon, Paul Conoveramplifier can be a simple addition or
(Micon), and Andrea Saks (APCOM)through an inductive coupler to interact
successfully petitioned the Californiawith suitable hearing aids. The ring can
Public Utilities Commission (CPUC)also be supplemented with extension
resulting in a tariff that paid for TDDbells or a visual call indicator.
devices to be distributed free of cost



1 A B C D 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114