Microcontroller
Speaker
The desired speaker input was a sine wave, generated between 0 and 5 volts. The DAC system on the PIC had limitations, as the signal was produced in discrete time-steps. The signal was first amplified through an op-amp, then fed through two low-pass filters to remove any discrete step-like features from the signal. The amplifier input was nearly a pure sine wave, as observed on a oscilloscope.
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Microphone
The initial signal from the microphone was first fed through two stages of low-pass filters to attenuate any high frequency resonance that may be generated in the signal path between another station's speaker and the microphone. The signal was then passed through a comparator with a hysteresis band to effectively square-up the signal. The input to the PIC was a digital square wave with the same frequency as the audio tone.
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