Numerical telephone keyboard terminal - has numerical vocal frequency generator and filter to obtain sinusoidal signals

摘要

A keyboard terminal has a voice frequency generator controlled by a time base and has a filter to transform the logic signals into sinusoidal current at voice frequencies. Switching circuits are controlled by signals (AS, AS) from the frequency generator (GF). The first switching circuit has a transistor (T2) in parallel with another transistor (T1) of a current generator. The second switching circuit has a transistor (T4) in parallel with the primary of a transformer (T). The third switching circuit has a transistor (T3) in parallel with a microphone. The first switching circuit disconnects the generator (GF) during normal conversation, the second circuit short-circuits the transformer to reject keyboard frequencies and allow good listening and the third switching circuit short-circuits the microphone during the numbering phase and avoid false numberS.