The input transducer converts the message to an electrical signal say a voltage or current. Another transducer at the destination converts the output signal to the desired message form.
The transducers in a voice communication system could be microphone at the input and a loudspeaker at the output.
The terms "signal" and "message" will be used interchangeably. This is because the signal, like the message, is a physical embodiment of information.
