Frequencies Used in Communication

Audio Frequencies. As explained on page 38, the average human ear functions on frequencies from about 20 to 20,000 cycles. The frequencies used in ordinary telegraphy, including teletypewriter operation, are below a few hundred cycles. The audio frequencies for ordinary commercial telephony extend from about 200 to 3000 cycles. For amplitude-modulation radio broadcasting the audio-frequency band usually extends from about 100 to 5000 cycles when network programs are used, and from perhaps 50 to 8000 cycles for local programs. For frequency-modulation broadcasting the audio band may extend from about 30 to 15,000 cycles.

Carrier Frequencies. In wire telephony and telegraphy the low-frequency signals are used to modulate a high-frequency carrier wave. This translates, or shifts, the information to be transmitted to the vicinity of the carrier frequency. Carrier frequencies range from about 4000 to 150,000 cycles, but in special systems frequencies as high as 500,000 cycles are used.

Radio Frequencies. Early radio systems operated at frequencies as low as about 10,000 cycles. The radio-frequency band extends from 10,000 cycles to many billions of cycles. The upper limit is being advanced as progress is made in radio. Radio frequencies are classified in section Radio Frequencies Used in Communication.