Touch-tone telephone

The invention: 

A push-button dialing system for telephones that
replaced the earlier rotary-dial phone.

The person behind the invention:

Bell Labs, the research and development arm of the American
Telephone and Telegraph Company



Dialing Systems

A person who wishes to make a telephone call must inform the
telephone switching office which number he or she wishes to reach.
A telephone call begins with the customer picking up the receiver
and listening for a dial tone. The action of picking up the telephone
causes a switch in the telephone to close, allowing electric current to
flow between the telephone and the switching office. This signals
the telephone office that the user is preparing to dial a number. To
acknowledge its readiness to receive the digits of the desired number,
the telephone office sends a dial tone to the user. Two methods
have been used to send telephone numbers to the telephone office:
dial pulsing and touch-tone dialing.
“Dial pulsing” is the method used by telephones that have rotary
dials. In this method, the dial is turned until it stops, after which it is
released and allowed to return to its resting position. When the dial
is returning to its resting position, the telephone breaks the current
between the telephone and the switching office. The switching office
counts the number of times that current flow is interrupted,
which indicates the number that had been dialed.

Introduction of Touch-tone Dialing

The dial-pulsing technique was particularly appropriate for use
in the first electromechanical telephone switching offices, because
the dial pulses actually moved mechanical switches in the switching
office to set up the telephone connection. The introduction of
touch-tone dialing into electromechanical systems was made possi-
ble by a special device that converted the touch-tones into rotary
dial pulses that controlled the switches. At the American Telephone
and Telegraph Company’s Bell Labs, experimental studies were
pursued that explored the use of “multifrequency key pulsing” (in
other words, using keys that emitted tones of various frequencies)
by both operators and customers. Initially, plucked tuned reeds
were proposed. These were, however, replaced with “electronic
transistor oscillators,” which produced the required signals electronically.
The introduction of “crossbar switching” made dial pulse signaling
of the desired number obsolete. The dial pulses of the telephone
were no longer needed to control the mechanical switching process
at the switching office. When electronic control was introduced into
switching offices, telephone numbers could be assigned by computer
rather than set up mechanically. This meant that a single
touch-tone receiver at the switching office could be shared by a
large number of telephone customers.
Before 1963, telephone switching offices relied upon rotary dial
pulses to move electromechanical switching elements. Touch-tone
dialing was difficult to use in systems that were not computer controlled,
such as the electromechanical step-by-step method. In about
1963, however, it became economically feasible to implement centralized
computer control and touch-tone dialing in switching offices.
Computerized switching offices use a central touch-tone receiver
to detect dialed numbers, after which the receiver sends the
number to a call processor so that a voice connection can be established.
Touch-tone dialing transmits two tones simultaneously to represent
a digit. The tones that are transmitted are divided into two
groups: a high-band group and a low-band group. For each digit
that is dialed, one tone from the low-frequency (low-band) group
and one tone from the high-frequency (high-band) group are transmitted.
The two frequencies of a tone are selected so that they are
not too closely related harmonically. In addition, touch-tone receivers
must be designed so that false digits cannot be generated when
people are speaking into the telephone.
For a call to be completed, the first digit dialed must be detected
in the presence of a dial tone, and the receiver must not interpret
background noise or speech as valid digits. In order to avoid such
misinterpretation, the touch-tone receiver uses both the relative and
the absolute strength of the two simultaneous tones of the first digit
dialed to determine what that digit is.
A system similar to the touch-tone system is used to send telephone
numbers between telephone switching offices. This system,
which is called “multifrequency signaling,” also uses two tones to
indicate a single digit, but the frequencies used are not the same frequencies
that are used in the touch-tone system. Multifrequency
signaling is currently being phased out; new computer-based systems
are being introduced to replace it.

Impact

Touch-tone dialing has made new caller features available. The
touch-tone system can be used not only to signal the desired number
to the switching office but also to interact with voice-response
systems. This means that touch-tone dialing can be used in conjunction
with such devices as bank teller machines. Acustomer can also
dial many more digits per second with a touch-tone telephone than
with a rotary dial telephone.
Touch-tone dialing has not been implemented in Europe, and
one reason may be that the economics of touch-tone dialing change
as a function of technology. In the most modern electronic switching
offices, rotary signaling can be performed at no additional cost,
whereas the addition of touch-tone dialing requires a centralized
touch-tone receiver at the switching office. Touch-tone signaling
was developed in an era of analog telephone switching offices, and
since that time, switching offices have become overwhelmingly digital.
When the switching network becomes entirely digital, as will
be the case when the integrated services digital network (ISDN) is
implemented, touch-tone dialing will become unnecessary. In the
future, ISDN telephone lines will use digital signaling methods exclusively.

See also: Cell phone; Rotary dial telephone; Telephone switching.