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.