On 2016-02-07, Robert Nichols wrote:

[ ... ]

AT&T's original Touch Tone telephones had a clever circuit that
allowed a single transistor to oscillate on two frequencies at
once, with both frequencies closely controlled in both frequency
and relative amplitude. Yes, transistors were relatively
expensive back then.

And -- as a result of that design Tw0 tapped inductors with
associated capacitors), and the mechanical design of the keypad
switches, if you pressed two buttons in a single row or column, you
would get a single tone, shorting out the other tapped inductor, and
thus preventing it oscillating.

Enjoy,
DoN.

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