Radiotelegraphy
In wireless telegraphy (WT) and amateur radio, sidetone is the audible indication of a continuous wave (CW) signal as the operator sends Morse Code. As in telephony, sidetone serves as feedback to the operator that what they are sending is what is intended.
It is designed to mimic the tone generated by a typical radio receiver when a CW signal is converted to the intermediate frequency (IF), then mixed with the Beat frequency oscillator (BFO) frequency to generate a difference frequency, which is audible over the radio receiver loudspeaker or headphones.
Sidetone is also used on voice radio equipment to give the radio operator confidence that they are transmitting over the radio. The sidetone audio is typically derived from the transmit audio circuitry.
Read more about this topic: Sidetone