| 摘要 |
A dual-tone multiple frequency signal generator is provided for use with telecommunications systems, data transfer systems and other applications. The tone encoding system utilizes MOS/LSI integrated circuitry on a single chip powered directly by telephone line voltages. An electronic keyboard circuit provides synchronized pulses to decode single-pole, single-throw keyboard switches by row and column. A crystal-controlled oscillator generates a reference frequency which is divided according to the row and column of an activated keyboard switch to obtain two pulse signals having frequencies representative of the activated switch. The outputs of the divider circuitry are fed to programmed logic array which generates two digitally coded signals each representing a sinusoidal waveform. A digital-to-analog ladder network converts the digitally coded signals to continuous sine waves, and an operational amplifier combines the sinusoidal waveforms to provide a dual-tone output. The integrated circuitry also utilizes electronic switches for the common functions of tone transmission, including applying power to the oscillator, disconnecting the audio transmitter and attenuating the input to the receiver. Complementary-symmetry, metal-oxide semi-conductor elements implement the circuitry design with bi-polar transistors on the same chip performing some of the common function switching. |
