TOUCH-TONE TELEPHONE (Inventions)

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 possible 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 “multi-frequency 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 “multi-frequency 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. Multi-frequency 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. A customer 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.

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