What does a two-wire circuit on a sound-powered telephone imply about power and signaling?

• A. It carries the audio signal with no separate power supply; signaling is achieved entirely by the operator's voice
• B. It uses a separate signaling line
• C. It requires external power
• D. It carries digital data

Answer: (A)

Two-wire sound-powered circuits rely entirely on the operator’s own voice to power and carry the signal. There is no external power source and no separate signaling line. The microphone converts the spoken voice into an electrical signal, and the earphone on the other end converts it back to sound, all on the same pair of conductors. In this setup, power and signaling are provided by the operator’s vocal energy, so the system can operate without any external supply or dedicated signaling line.