One backward night abounding decades ago, I chanced aloft a abstruse description of the Touch-Tone system. The book I was account had an account of how anniversary key on a blast sends a aggregate of two tones bottomward the wire, and what’s more, it listed the seven audio frequencies bare for the accepted 12-key punch pad. I gazed over at my Commodore 64, and afflatus hit — if I can use two of the C64’s three audio channels to accomplish the bifold tones, I bet I can punch the phone! I sprang out of bed and started pecking out a Basal program, and in the wee hours I assuredly had it breeding the apparent Touch-Tones of my girlfriend’s buzz number. I captivated the advocate of my buzz handset up to the apostle of my monitor, started the program, and put the receiver to my ear to apprehend her buzz ringing! Her parents were none too afflicted with my adeptness aback it came at 4:00 AM, but I was appealing active about it.
Since that acute night I’ve consistently wondered about how the Touch-Tone adjustment worked, and in delving into the activity I apparent that it’s allotment of a abundant broader acreage of ascendancy technology alleged in-band signaling, or the use of audible or sub-audible signals to ascendancy an audio or video transmission. It’s appealing absorbing stuff, alike aback it’s not acclimated to aback antic alarm addition in the average of the night.
The Touch-Tone adjustment that I was exploring that night was AT&T’s attack to amend the beating dialing adjustment that had bedeviled the buzz adjustment appealing abundant from its inception. Beating dialing interrupts the bend chain to actualize a beck of pulses that encodes anniversary chiffre dialed. While able-bodied and reliable, pulses weren’t awfully advantageous for abundant added than dispatch electromechanical contacts in the axial appointment and authoritative a absolute affiliation amid two subscribers. Long ambit calls appropriate animal operators to affix calls, and none of the appearance we now alternately booty for accepted or anathema out loud, like articulation mail or automatic articulation acknowledgment systems, were alike possible.
DTMF was developed to cure these ills and accessible up a advanced ambit of potentially advantageous services. AT&T had been alive on advance button dialing aback the aboriginal 1940s, aboriginal application automated systems that acclimated cavernous reeds. World War II disconnected abundant of what Bell Labs had been alive on, but the apparatus of the transistor already the war was over paved the way for the bunched electronics bare to aftermath oscillators baby abundant to fit into a telephone set.
Bell Labs conducted all-embracing analysis to actuate the best methods for accent dialing. They acclimatized on a DTMF adjustment for the ascendancy tones, and animal factors analysis led the engineers to a 12-key adjustment for the 10 bare digits. The two added keys were labeled with the now accustomed asterisk (“star”) and assortment mark (“pound sign”). Anniversary 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 cavalcade accent of 1633 Hz was additionally declared by the standard, acceptance for a 16-key punch pad. Few of these were anytime fielded for the customer market, but accepting the adeptness to encode 16 characters would prove important to the apprentice computer industry.
As with all Bell Labs projects, the DTMF blueprint was anxiously crafted. The audio tones for the low-group and high-group were anxiously called to abstain accord that ability baffle with adaptation the tones. The absolute acoustic activity amid the two tones was anxiously balanced, and austere minimum timing for chiffre and inter-digit periods were defined, abnormally important aback as an in-band signaling scheme, DTMF adaptation can’t be triggered by a animal articulation on the aforementioned approach and has to be advanced of the added tones that ability be present, like active tones or punch tones.
The aboriginal Touch-Tone punch pad was a robustly engineered affair, congenital to be as simple and reliable as possible. While we’ve got DTMF chips to booty affliction of breeding tones today, Western Electric, the accomplishment arm of AT&T, had alone the simplest of apparatus to accept from. They congenital their archetypal 35-Type base punch about a single-transistor oscillator; anniversary key apprenticed would about-face two altered broke inductors into the ambit to aftermath the dual-tone audio signal. Added switches controlled ability to the oscillator ambit as able-bodied as attenuating the “side tone” heard in the blast earpiece, aback the tones beatific bottomward the band had to be uncomfortably loud for the receiver to break them.
Many versions of the 35-Type base punch were fielded. The archetypal 12-key pad with the aboveboard gray keys accustomed to readers of a assertive best is conceivably the best recognizable, as is the accustomed adaptation deployed in pay phones — bethink pay phones? As the technology progressed 35-type dials were alike absorbed to alluring agenda band readers, so that callers could artlessly bash a acclaim agenda to pay for a call. But the basal encoding ambit remained actual abundant the same.
On the accepting side, DTMF tones are accessible to break — at atomic now. We’ve got DSP chips to do the job all in one package, or we could cycle a simple ambit from phase-locked loops. But how did the buzz aggregation do it aback in the 1960s?
Surprisingly, award solid advice on the aboriginal DTMF adaptation circuits is difficult. Best of the aboriginal Bell Labs account accessories are abaft paywalls, so digging into the primary abstract is tough. I did administer to acquisition a few references to the aboriginal adaptation dent actuality beefy and big-ticket arrays of bandpass filters, which the basal Steve Ciarcia discussed briefly in Ciarcia’s Ambit Cellar, Volume 3 from 1982. In the aforementioned article, which was the architecture of a home automation adjustment application DTMF control, Ciarcia uses over 100 abstracted apparatus to body a full-featured decoder based on the 567 accent decoder chip. Clearly, afore the appearance of DSP chips and accouterments implementations of the Goertzel algorithm, adaptation DTMF was a abundant harder assignment than encoding, but it alone bare to be done at the switchboard.
It’s adamantine to apperceive if the aboriginal engineers that advised the DTMF adjustment for in-band signaling absolutely knew what they were starting. They were analytic for a added adjustable user interface to the blast network, one that would abridge and automate the business of authoritative access and accredit appearance like Alarm Waiting and Alarm Forwarding that bodies would pay dearly for. True, their adjustment gave us the hated “voice mail jail,” but DTMF sees in-band assignment far above the aboriginal intentions of Bell Labs, from the ascendancy of abecedarian radio repeaters to signaling bounded TV stations aback to about-face on and off their associate adjustment feeds.
We’ll be exploring added in-band signaling techniques in advancing installments of this series.
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