In-Band Signaling: Dual-Tone Multifrequency Dialing

One late night many decades ago, I chanced upon a technical description of the Touch-Tone system. The book I was reading had an explanation of how each key on a telephone sends a combination of two tones down the wire, and what’s more, it listed the seven audio frequencies needed for the standard 12-key dial pad. I gazed over at my Commodore 64, and inspiration hit — if I can use two of the C64’s three audio channels to generate the dual tones, I bet I can dial the phone! I sprang out of bed and started pecking out a Basic program, and in the wee hours I finally had it generating the recognizable Touch-Tones of my girlfriend’s phone number. I held the mouthpiece of my phone handset up to the speaker of my monitor, started the program, and put the receiver to my ear to hear her phone ringing! Her parents were none too impressed with my accomplishment since it came at 4:00 AM, but I was pretty jazzed about it.

Since that fateful night I’ve always wondered about how the Touch-Tone system worked, and in delving into the topic I discovered that it’s part of a much broader field of control technology called in-band signaling, or the use of audible or sub-audible signals to control an audio or video transmission. It’s pretty interesting stuff, even when it’s not used to inadvertently prank call someone in the middle of the night.

Two Tones, No Waiting

The Touch-Tone system that I was exploring that night was AT&T’s attempt to update the pulse dialing system that had dominated the phone network pretty much from its inception. Pulse dialing interrupts the loop continuity to create a stream of pulses that encodes each digit dialed. While robust and reliable, pulses weren’t terribly useful for much other than stepping electromechanical contacts in the central office and making a direct connection between two subscribers. Long distance calls required human operators to connect calls, and none of the features we now alternately take for granted or curse out loud, like voice mail or automated voice response systems, were even possible.

DTMF was developed to cure these ills and open up a wide range of potentially lucrative services. AT&T had been working on push button dialing since the early 1940s, first using mechanical systems that used vibrating reeds. World War II interrupted much of what Bell Labs had been working on, but the invention of the transistor once the war was over paved the way for the compact electronics needed to produce oscillators small enough to fit into a telephone set.

The 35-Type station dial. Source: The Beatrice Company

Bell Labs conducted exhaustive research to determine the best methods for tone dialing. They settled on a DTMF system for the control tones, and human factors research led the engineers to a 12-key arrangement for the 10 needed digits. The two extra keys were labeled with the now familiar asterisk (“star”) and hash mark (“pound sign”). Each key would be encoded by two audio tones, one for the row in which the key was located, and one for the column. The four row tones were the lower-pitched tones, from 697 Hz to 941 Hz, while the three columns ranged from 1209 Hz to 1477 Hz. A fourth column tone of 1633 Hz was also described by the standard, allowing for a 16-key dial pad. Few of these were ever fielded for the consumer market, but having the ability to encode 16 characters would prove important to the fledgling computer industry.

As with all Bell Labs projects, the DTMF spec was carefully crafted. The audio tones for the low-group and high-group were carefully chosen to avoid harmonics that might interfere with decoding the tones. The total acoustic energy between the two tones was carefully balanced, and strict minimum timing for digit and inter-digit periods were defined, especially important since as an in-band signaling scheme, DTMF decoding can’t be triggered by a human voice on the same channel and has to be tolerant of the other tones that might be present, like busy tones or dial tones.

Encoding DTMF

35-Type DTMF keypad. Click for larger image. Source: Telephone Collectors International

The original Touch-Tone dial pad was a robustly engineered affair, built to be as simple and reliable as possible. While we’ve got DTMF chips to take care of generating tones today, Western Electric, the manufacturing arm of AT&T, had only the simplest of components to choose from. They built their classic 35-Type station dial around a single-transistor oscillator; each key pressed would switch two different tapped inductors into the circuit to produce the dual-tone audio signal. Other switches controlled power to the oscillator circuit as well as attenuating the “side tone” heard in the telephone earpiece, since the tones sent down the line had to be uncomfortably loud for the receiver to decode them.

Many versions of the 35-Type station dial were fielded. The classic 12-key pad with the square gray keys familiar to readers of a certain vintage is perhaps the most recognizable, as is the hardened version deployed in pay phones — remember pay phones? As the technology progressed 35-type dials were even hooked to magnetic card stripe readers, so that callers could simply swipe a credit card to pay for a call. But the basic encoding circuit remained very much the same.

Decoding DTMF

An original Bell System DTMF decoder. From Ciarcia’s Circuit Cellar, Vol. 3 copyright 1982

On the receiving side, DTMF tones are easy to decode — at least now. We’ve got DSP chips to do the job all in one package, or we could roll a simple circuit from phase-locked loops. But how did the phone company do it back in the 1960s?

Surprisingly, finding solid information on the original DTMF decoding circuits is difficult. Most of the original Bell Labs journal articles are behind paywalls, so digging into the primary literature is tough. I did manage to find a few references to the original decoding circuitry being bulky and expensive arrays of bandpass filters, which the indispensable Steve Ciarcia discussed briefly in Ciarcia’s Circuit Cellar, Volume 3 from 1982. In the same article, which was the construction of a home automation system using DTMF control, Ciarcia uses over 100 separate components to build a full-featured decoder based on the 567 tone decoder chip. Clearly, before the advent of DSP chips and hardware implementations of the Goertzel algorithm, decoding DTMF was a much harder task than encoding, but it only needed to be done at the switchboard.

DTMF Is Everywhere

It’s hard to know if the original engineers that designed the DTMF system for in-band signaling really knew what they were starting. They were searching for a more flexible user interface to the telephone network, one that would simplify and automate the business of making connections and enable features like Call Waiting and Call Forwarding that people would pay dearly for. True, their system gave us the hated “voice mail jail,” but DTMF sees in-band duty far beyond the original intentions of Bell Labs, from the control of amateur radio repeaters to signaling local TV stations when to switch on and off their affiliate network feeds.

We’ll be exploring more in-band signaling techniques in coming installments of this series.

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