1974-01-02
1976-03-02
Cooper, William C.
H04M 150, H03K 1302
Patent
active
039419428
ABSTRACT:
A two of eight tone encoder utilizing digital techniques to synthesize the dual tones of a Touch Tone telephone operator system is provided. One of sixteen switches from the telephone keyboard selects one of four desired row tones and one of four desired column tones. The two of eight encoder circuit includes, for the row encoding section, a counter which is programmable by the row input signals. The counter output drives a plurality of exclusive OR gates and a one of eight decoder which switches one of eight resistors into a resistive divider network. The exclusive OR gates and the one of eight decoder cooperate to perform an up-down counting function. The eight resistors are chosen in value so that a synthesized stepped sine wave is generated at the output of the resistive divider network, the synthesized stepped sine wave corresponding to the desired selected row frequency. Similar circuitry generates a stepped sine wave corresponding to the desired selected column frequency. The two stepped sine waves are filtered, if desired, and provided as inputs to a summation amplifier, the output of which produces the desired tone having the frequency components for both the selected row input and the selected column input.
REFERENCES:
patent: 3538256 (1970-11-01), Lucas
patent: 3778556 (1973-12-01), Mees et al.
patent: 3778557 (1973-12-01), Frederiksen
patent: 3787836 (1974-01-01), Hagelbarger
Cooper William C.
Hoffman Charles R.
Motorola Inc.
Popek Joseph
Weiss Harry M.
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