Dual Tone Multi-Frequency (Touch-Tone®) Reference

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Dual Tone Multi-Frequency, or DTMF is a method for instructing a telephone switching system of the telephone number to be dialed, or to issue commands to switching systems or related telephony equipment.

The DTMF dialing system traces its roots to a technique AT&T developed in the 1950s called MF (Multi-Frequency) which was deployed within the AT&T telephone network to direct calls between switching facilities using in-band signaling. In the early 1960s, a derivative technique was offered by AT&T through its Bell System telephone companies as a "modern" way for network customers to place calls. In AT&Ts Compatibility Bulletin No. 105, AT&T described the product as "a method for pushbutton signaling from customer stations using the voice transmission path."

The consumer product was marketed by AT&T under the registered trade name Touch-Tone®. Other vendors of compatible telephone equipment called this same system "Tone" dialing or "DTMF".

The DTMF system uses eight different frequency signals transmitted in pairs to represent sixteen different numbers, symbols and letters. This table shows how the frequencies are organized:

The frequencies used were chosen to prevent any harmonics from being incorrectly detected by the receiver as some other DTMF frequency.

The transmitter of a DTMF signal simultaneously sends one frequency from the high-group and one freqency from the low-group. This pair of signals represents the digit or symbol shown at the intersection of row and column in the table. For example, sending 1209Hz and 770Hz indicates that the "4" digit is being sent.

At the transmitter, the maximum signal strength of a pair of tones must not exceed +1 dBm, and the minimum strength is -10.5 dBm for the low-group frequencies and -8.5 dBm for the high-group frequencies. When not intentionally sending DTMF tones (including the inter-digit interval), any leakage of these tones must not exceed -55 dBm. The frequencies generated by the transmitter must be nominally within 1.5% of the stated values and the receiver must not accept signals that deviate more than 3.5% from the stated values.

The receiver is responsible for performing several checks on the incoming signal before accepting the incoming signal as a DTMF digit:

1. Energy from a low-group frequency and a high-group frequency must be detected.

2. Energy from all other low-group and all other high-group frequencies must be absent or less than -55dBm.

3. The energy from the single low-group and single high-group frequency must persist for at least 40msec*.

4. There must have been an inter-digit interval of at least 40msec* in which there is no energy detected at any of the DTMF frequencies. The minimum duty cycle (tone interval and inter-digit interval) is 85msec*.

5. The receiver should receive the DTMF digits with a signal strength of at least -25 dBm and no more than 0 dBm.

6. The energy strength of the high-group frequency must be -8 dB to +4 dB relative to the energy strength of the low-group frequency as measured at the receiver. This uneven transmission level is known as the "twist", and some receiving equipment may not correctly receive signals where the "twist" is not implemented correctly. Nearly all modern DTMF decoders receive DTMF digits correctly despite twist errors.

7. The receiver must correctly detect and decode DTMF despite the presence of dial-tone, including the extreme case of dial-tone being sent by the central office at 0 dBm (which may occur in extremely long loops). Above 600Hz, any other signals detected by the receiver must be at least -6 dB below the low-group frequency signal strength for correct digit detection.

It should be mentioned that Radio Shack, one of the worlds largest retailers of consumer telephone equipment in the 1960s-1990s, required that all telephone devices it sold generate DTMF tones of no less than 70 msec of duration. Radio Shack developed this number based on real-life use of their equipment on telephone networks throughout North America and the finding that shorter tones are more likely to cause dialing troubles. Telephones sold by other retailers usually follow the Radio Shack specifications by default since they are getting the devices from the same manufacturers that build telephones to meet the Radio Shack criteria. In addition, many manufacturers of modems also set the default DTMF dialing speed to at least 70 msec. AT-Compatible modems control DTMF digit duration via the S11 register.

Labeling of DTMF numeric digits

The letter codes affixed to digits on rotary (pulse) dial phones were considered obsolete by the 1960s by AT&T, and attempts were made to eliminate their use. Some of the earlier Touch-Tone telephones, including pay phones, lack these letters. However, the use of "named" phone numbers flourished thanks to marketing campaigns like 1-800-THE-CARD (used by American Express), and AT&Ts manufacturing division eventually resumed making Touch-Tone telephones with the letter codes on the digit buttons. (The A, B, C and D special use Touch-Tone buttons shown in the fourth column above did not appear on consumer equipment, and were typically only found at PBX and central office operator stations. These same keys were changed to FO, F, I and P in the U.S. military AUTOVON telephone network, introduced in 1963.)

Almost all telephones have no letters associated with the "1" digit, including both rotary and touch-tone telephones. This is a holdover from the days when the letters first appeared as a way to make the newly-introduced longer telephone numbers easier for the public to accept. For example, someone who had the telephone number 34597 for years was told one day that their telephone number was now ED3-4597, where ED was short for "Edison", the name of the local telephone exchange in that part of town. In reality, that person with the five digit telephone number now had a seven digit telephone number (333-4597), but probably didn't realize this, and continued to use the ED3-4597 long after AT&T tried to stop the practice.

Since no local telephone number started with a "1", there were no central offices that had a prefix starting with "1", no letters were needed for the "1" digit.

A very small number of POTS telephone sets were manufactured with the letters "Q" and "Z" shown on the "1" digit, but this must not be considered to be a standard assignment. Many cellular phones put "Q" on the "7" digit button and "Z" on the "9" digit button, but this differs from wired telephones too. All of this means that the handling of "Q" and "Z" are not consistent, and "named" telephone numbers can't be used predictably if they contain "Q" or "Z".

Other DTMF References

• AT&T Compatibility Bulletin No. 105, Issue #1, August 8, 1975.
  This is probably the best document from AT&T on the subject of DTMF, or as they prefer to call it, Touch-Tone. After all, AT&T did invent the DTMF system and held the monopoly on the product for years.

• ANSI T1.401-1988, Interface between Carriers and Customer Installations - Analog Voicegrade Switched Access Lines Using Loop-Start and Ground-Start Signaling
  If you don't trust AT&T, here is a 95 page ANSI document that largely says the same thing as AT&Ts document, but the ANSI version will cost you a lot more money. To be fair, T1.401 does cover a lot of other material and the ANSI specification differs slightly from the AT&T specification, but mainly in ways that only impact the design of equipment installed at telephone central or end offices.

• Bellcore Digit Simulation Test Tape, TR-TSY-000763, Issue 1, July 1987, (Replaces PUB 56200)
  This is a set of audio tapes that contain over 50,000 speech samples, music and other sounds. These are played into a test loop configuration with a DTMF receiver monitoring the loop and reporting any DTMF digits or symbols that the receiver thinks it has detected. A perfect DTMF receiver should report zero digits, but this will not happen unless the test is not set up correctly. Instead, a DTMF receiver passes the test if it does not falsely detect more than a certain number of digits, letters and symbols over the duration of the test.

  (As of September 2004, I am informed that the Bellcore Digit Simulation Test tape product was an asset that changed hands to another entity, as part of various divestments that have occurred at AT&T since 1984. The current owner of the asset reports that the master tapes were not archived properly by one or more of its owners, and that the tapes are now no longer playable. Subsequently, these tapes are no longer sold.)

• Telcordia DTMF Digit Simulation Immunity, FR-763-01, March 2006
  This recently-appeared product has a part number strangely similar to that of the old Bellcore Digit Simulation Test Tape that is no longer available. Telcordia (spun-out of AT&T/Western Electric), claims this to be an all-new product, and it comes on three compact discs, rather than three cassette tapes.

  Information can be found at: http://telecom-info.telcordia.com/site-cgi/ido/index.html
  (NOTE: FR-763-01 contains LP-763-01 and GR-763 Issue 2)

• MITEL DTMF Receiver Test Cassette, CM7291, Issue 3, September 1989
  Although this tape contains a "talk-off" section similar to the Bellcore Digit Simulation Test Tapes, this tape also contains DTMF tone-pairs that are supposed to be correctly detected. My personal experience is that the Bellcore tape is far superior at talk-off testing.

  (As of January 2002, I understand that this product is no longer available from the manufacturer.)

• Federal Communications Commission 47 CFR Ch. 1, Part 68
  Although the FCC rules for telephone equipment compliance would seem like a logical place to get more information on DTMF, Part 68 contains virtually no information on this subject. However, Part 68 does describe in great detail just about everything else you would want to know about how to build a device that connects to the Public Switched Telephone Network (PSTN).

  Text and PDF versions of Part 68 documents can be found at the US Federal Governments web site.

This information is provided on an "AS IS" basis and has no warranty. Most countries have their own regulatory authorities who will have the latest specifications and local compliance information.

Related Topics

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[Copyright 1995,2000-2014 Frank Durda IV, All Rights Reserved.
Mirroring of any material on this site in any form is expressly prohibited.
The official web site for this material is:  http://nemesis.lonestar.org
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