Each Touch-Tone digit is represented by a unique pair of tones as shown in left image above. One tone from the low tone group (represented by the blue lines and blue frequency numbers), and one tone from the high tone group (represented by the red lines and red frequency numbers), are mixed together to form a "Dual-Tone Multi-Frequency" signal.
 

Type 66 Autovon Dial (shown at top of this page; right image):

[Photo and button designations above are the courtesy of Stephen (Steph) Kerman]

• The Autovon legends are:
  • FO = Flash Override
  • F = Flash
  • I = Immediate
  • P = Priority

   

• AUTOVON - History and description - From http://public.afca.scott.af.mil/ (link was dead when last checked):

A major development in long-haul communications in the early 1960s was the Automatic Voice Network, commonly called AUTOVON. Activated in December 1963, AUTOVON, derived from the Army’s Switched Circuit Automatic Network, was designed to provide the Department of Defense with an internal telephone capability to replace toll and Wide Area Telephone Service (WATS) calls, while also allowing precedence preemption for high priority users. Development of the AUTOVON system represented one of the most significant and comprehensive telecommunications programs ever undertaken by the DOD. While the dedicated circuits used in earlier networks provided good response time, weaknesses in survivability and reliability were significant problems. The loss of a single circuit between two points disrupted communications between subscribers, and each termination placed on the dedicated circuit required a separate instrument. AUTOVON did much to correct these deficiencies.

AUTOVON became the principal long-haul, nonsecure voice communications network within the Defense Communications System. The network served the entire Department of Defense and handled essential communications concerning command, operations, and administration. It was a global network comprised of interconnected automatic switching centers and thousands of subscriber terminals throughout the world. Over the next 25 years, the network would be continually modernized and expanded to provide more service and capabilities to the users. Finally, it became a part of the new Defense Switched Network (DSN), the replacement system activated in 1990 to provide long-distance telephone service to the military.

If you are a technical history buff, check out the Bell Labs Technical Journal article from 1960 on Touch-Tone development!  Click HERE to view it or download it.  You must have Microsoft Word or OpenOffice.org to view this file.

"Regarding your story on the original numbering layout for DTMF pads, I heard another story that may or may not be true but I'll pass it along.  The reason given to me for not adopting a "ten-key" calculator (aka, adding machine) layout was that ten-key users would dial so fast that it would trip up the decoders and result in wrong numbers or incomplete calls. Reversing the order would slow those folks down and rest of the world wouldn't care either way." - Bruce Robin

"A curious counterpoint to this corporate history are the independent statements made to me by two members of the Bell Telephone Laboratories staff at that time. One is a personal friend who is a physicist that participated in the human factors design effort and the other was one of the Noble prize recipients for the invention of the transistor. They both reported to me that there was fear that some equipment couldn't keep up with good adding machine layout inputers. Such admissions were conceivably not included in the official corporate write-ups." - J. Marshall Reber

Some historical trivia -

"A pushbutton telephone that may eventually replace the conventional [rotary] telephone dial is under development by the Bell Telephone Laboratories.  Instead of twirling a dial, users will press numbered buttons to make a call - a faster process than dialing.  These new sets were placed on trial last summer [summer of 1959], then taken out of service for study at the [Bell Telephone] Laboratories. Much research and testing remains to be done before the sets can be made in quantity and placed in service, but these may be the telephones of the future." - Telephone Almanac for 1960, Southern Bell Telephone and Telegraph Company/Bell Telephone System.

"In the late 60’s and early 70’s the "#" symbol was standardized as a preface to indicate that the call was to be a Picturephone (video) call. Picturephone service was offered in a few markets (Chicago, NY) until it was killed about 1973. For business callers with a multiline keyset (2565), the line lamp would change from a white light to a red light (special polarity sensitive "dual-color" lamps) once the "#" key was pressed." - Courtesy of Rick Walsh.

• Some photos of the Western Electric type 35 series dial:

>>> ATTENTION ELECTRONIC HOBBYIST and STUDENTS <<< We get requests for DTMF (Touch-Tone) decoder projects from students and hobbyist periodically. Although we don't have a schematic and parts list for building a complete project, we did find two manufactures of DTMF decoder integrated circuits that can be the basis of this project.  Download the PDF data files by clicking on the following links: Motorola MC145436A - a 14 pin chip HOLTEK HT9170 - Available from Digikey for under $3.00 (USA)

To view schematics and information on the Touch-Tone dials used in Western Electric phones,click on the two links below:

• Photo of one of the first touch tone dials (the 25B3).  Note the missing "*" and "#" keys.
   

• Western Electric type 35 Dial Schematic
   

• Western Electric 35Y3A <---- Direct scan of dial face
   

• Western Electric 35Y3D <---- Direct scan dial face

• Western Electric 72H3A <---- Direct scan of dial face
   

• Type 72 Touch Tone Dial schematics:

  • Page 1
  • Page 2
  • Page 3
  • Page 4
  • Page 5

In order for the central-office receiver to register the digit properly, the tone-address signals must meet the following requirements:

1) Signal Levels:
Nominal level per frequency: -6 to -4 dBm. Minimum level per frequency: Low Group, -10 dBm; High Group, -8 dBm. Maximum level per frequency pair: +2 dBm. Maximum difference in levels between frequencies: 4 dB.

2) Frequency Deviation: +/- 1.5 percent of the values given above.

3) Extraneous frequency components: The total power of all extraneous frequencies accompanying the signal should be a t least 20 dB below the signal power, in the voice band above 500 Hz.

4) Voice Suppression: Voice energy from any source should be suppressed at least 45 dB during tone signal transmission. In the case of automatic dialing, the suppression should be maintained continuously until pulsing is completed.

5) Rise Time: Each of the two frequencies of the signal should attain at least 90 percent of full amplitude within 5 ms, and preferably within 3 ms for automatic dialers, from the time that the first frequency begins.

6) Pulsing Rate: Minimum duration of two-frequency tone signal: 50 ms normally; 90 ms if transmitted by radio. Minimum inter-digital time: 45 ms.

7) Tone leak during signal off time should be less than -55 dBm.

8) Transient Voltages: Peak transient voltages generated during tone signaling should be no greater than 12 dB above the zero-to-peak voltage of the composite two-frequency tone signal.