Odd O'blography

Call Me

My recent post Unsocial Media described how I used an old-fashioned rotary phone to connect an ASR-33 teletype machine to our college mainframe computer. The teletype served as a primitive terminal for real-time interaction with the mainframe. At the time, it was considered quite an improvement over the punch card. Punch card programs had to be submitted to a technician who ran the deck of cards through a card reader and then returned the printed output to the user. It took time to run any deck, and there was usually a queue. It wasn't unusual to wait overnight to see the results of a programming change. The teletype, however, sent commands directly to the computer and printed the results immediately.

Few of us realized it back then, but just placing a call with a rotary phone was a rudimentary form of digital programming. When you picked up a handset, that ubiquitous dial tone was the phone switch ‘computer’ saying ‘I’m listening’ (and it was far more useful than a radio-show psychiatrist). But the phone computer couldn’t understand analog human speech, so the dialing mechanism had to generate a series of electrical pulses similar to the ones and zeros that underlie all computer apps and programs. Those pulses were counted by mechanical relays at the phone company offices to direct your call to the desired person. I toured the phone company in Missoula, Montana in the early 1970s. Even that relatively small town had a warehouse size building with racks and racks of ten-position relays that clattered noisily as they switched calls from line to line.

The mechanism inside a rotary phone includes a spring, a speed regulator, and an electrical switch. Oddly enough, nothing really happens when you rotate the dial to a specific number. All you’ve done at that point is wind up the spring. The action occurs when you let go and the dial rotates back to the start position. An electrical circuit opens and closes as the dial moves to create pulses on the phone line. If you select 7 with your finger, then seven pulses are sent down the line. Timing is critical to the operation of the telephone switching network, so the speed regulator is used to eliminate the human factor. And when the first pulse is received, a timer starts that defines the period for counting the complete number. If seven pulses are counted during the timer window, then the phone computer knows the first digit is 7. And you can’t wind the spring for the second number fast enough to interfere with the first number. Dialing a sequence of numbers was equivalent to programming a computer to connect you to your desired party. One-finger programming was pretty simple, but dialing seven digits could connect you to any one of ten million local telephones. Ten digits could reach out to ten billion lines throughout North America.

If a single pulse was received as the first digit, that meant it was a long-distance call and the next three digits would be interpreted as the area code rather than the local prefix. For example, if my dad dialed 1-206, then the relays at his local phone company would mechanically switch positions to connect his line to a long-distance line for Washington state. Three more digits, the prefix, would be counted by a phone computer in Washington to move more relays and connect him to the desired local phone office. And then the local phone computer would count the last four digits to make the final connection to the phone in my house. And only after all of that digital programming was completed could the analog voice signal travel over a continuous wire from his mouth to my ear.

“Hello?”