[Steely Tom]

The original touch-tone phones contained a very clever circuit that generated the two simultaneous (DTMF) tones using only a single transistor.

This was done by using one transistor driving two separate resonant circuits in parallel. As long as everything stayed linear, this was allowed by the superposition principle.

What kept everything linear was that the inductors in each of the oscillators were designed to saturate; this is what kept one resonant circuit from "hogging all the gain" from the transistor; this would prevented the other tone from being generated.

This was during the relatively short time that transistors were more expensive than inductors.

This just one of the things about the old analog phone system that were very clever. Bell Labs, which was the source of much of this cleverness, employed literally the best scientists and engineers in the world.

When the American phone system needed something done, Bell Labs would literally invent technology that would change the world forever. They did this more than once.

As an EE, I thought the one-transistor DTMF generator showed such an impressive level of creativity and insight into the thing I liked best at that age, which was creative circuit design.

[Tell It Right]

I, too, was easily impressed, but for a slightly different tech. It was a Vicmodem getting 300 baud so I could dial up a terminal BBS with my Commodore-64. I thought that was the neatest thing as a 15-year-old. LOL

[steve86]

“this was allowed by the superposition principle.”

Nice quantum application. Is there a cat hiding in there?

[Blueflag]

DTMF = dual tone, multi frequency.

[old-ager]

> one-transistor DTMF generator

I did not know that. Your post made my day.

I was not quite old or connected enough to become a phone phreak, but I did do a little experiment to see if audio could be sent to a caller without picking up an incoming call. I put the high impedance side of an audio output transformer on the line and put audio into the low impedance side, then drove up to the gas station and called home. I could hear the audio. The high side of the transformer did not take the line off hook, but with audio going out at a voltage competitie with the ring voltage, I could hear the sound on the calling end.

Also I hope every kid who was interested in the phone system figured out how to dial out without using the dial.

[Bigg Red]

Thanks for that explanation.

[Fresh Wind]

Of course, Bell Labs invented the transistor. Change the world? Absolutely!

They called it the "crystal triode".

[cymbeline]

“This just one of the things about the old analog phone system that were very clever. “

You want “clever”, look into the original color TV design. Done initially with vacuum tubes (not thousands of transistors), and the color signal would work with the old black and white TVs. And the sets were manufactured at consumer prices.

And the picture tube was very complex with three guns that shot three beams through a mask to illuminate thousands of red, green and blue phosphors.

[Myrddin]

I was in a 12 week course at the Bell System Center for Technical Excellence in Lisle, IL in 1980. We were treated to previews of unreleased technology. One of the stranger items was a D to A audio transducer. The phone was on a digital line and the earpiece had a set of semi-circular elements whose size represented the weight of a bit in the digital data. It was a simple electro-mechanical device. Fidelity was acceptable. Not as spiffy as the integrated circuits with u-law or a-law DtoA converters used now. The switch room had an old #1 ESS with core memory and ferrite sheet memory. All discrete components. Equipment frames were 6 feet tall and 30 ft long. Loading the call store was a 3 week operation. The "code" arrived on aluminum plates with ferro-magnetic squares representing bits of the 43 bit call processor. A milk carton size box held about 30 plates. It held a rack that could hang on the "card reader" to read the data into the core call store. Really primitive. The 30 MB hard disk had two surfaces with fixed head disks. It was spun with a washing machine motor with belts.

At the time I took the course, my duties at work included removal of analog O/ON carrier systems and installation of T1/T1C repeater bays and D4 channel banks.