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.