Abstract: A modem having a transmitter portion including a first modulator connected to receive audio frequency signals and supplying an output signal through a single sideband filter to a second modulator providing an output signal at a unique channel frequency, a receiver portion including a first demodulator connected to receive transmitted signals and supplying an output signal to a second demodulator through a single sideband filter with the second demodulator providing audio frequency output signals, a carrier switch supplying a small amount of the carrier to the second modulator during operation of the transmitter and a carrier level switch associated with the carrier switch for supplying the carrier at high and low levels, and a crystal controlled channel oscillator connected to the second modulator of the transmitter portion and the first demodulator of the receiver portion with removable, plugin crystals connected thereto for controlling the channel frequency of the modem.

Abstract: A transistor switch having a square pulse with distorted leading and lagging edges and a distorted amplitude applied to the base thereof and the same pulse applied through two semiconductor junctions and a voltage divider to provide a voltage equal to the voltage applied to the base divided by one half with the voltage across a semiconductor junction subtracted therefrom and trigger the switch at one half the amplitude of the pulses applied thereto.