kansas city demodulator

" (frequency-shift The most popular form of keying) modulator used by computers nowadays is the "Kansas City Standard" where digital infor- mation is transmitted sequentially by means of car- rier pulses of constant amplitude and two different frequencies (1200 Hz and 2400 Hz). A logic low level generates a frequency of 1200 Hz whilst 2400 Hz is generated by a logic high level. Ob- viously, for every modulator there has to be a demodulator. As can be seen from the circuit diagram, the Kansas City demodulator described here is quite straight- forward. Opamp Al acts as a Schmitt trigger and simply converts the output of the tape recorder into I s mmetrical squarewave signal. Opamp a per e A2 is configured as a "charge pu p conditions, Kansas City demodulator the incoming frequency into a low or high logic level. The output of A2 is fed to a low pass filter constructed around opamP A3 and the decoded, filtered signal is then fed to a second Schmitt trigger (A4) to "clean up" the fina...