sidetone
Telephony
In telephony, sidetone is the effect of sound that is picked up by the telephone's mouthpiece and introduced (at low level) into the earpiece of the same handset, acting as feedback. Sidetone in 19th century telephones varied, often being so loud as to discourage speaking loudly enough, and occasionally so loud as to throw the instrument into uncontrolled oscillation or "howling". Antisidetone circuitry incorporating the principle of the hybrid coil brought sidetone under control in the early 20th century, leaving enough to assure the user that the phone is really working, and allowing the use of a unitized telephone handset.
Without sidetone, users do not hear their own voice in the earpiece, and may think the phone is not working. Too much sidetone causes users to hear their own voice loudly. They may feel uncomfortable and lower the level of their voice.
Digital telephones lack the mechanical acoustics and circuitry that created sidetone in older landline phones, so digital phones include electronic circuitry to reproduce the sidetone. Many cell phones do not provide adequate sidetone. Usability experts believe this causes some people to shout or speak too loudly when using a cell phone. 
Similarly, in military field operations in enemy territory, operatives can be encouraged to speak quietly by increasing the volume of the sidetone.
Sidetone is useful for people using handsets but can cause audio feedback in teleconferencing systems if not treated properly.
Radiotelegraphy
In wireless telegraphy (WT) and amateur radio, sidetone is the audible indication of a CW signal as the operator sends Morse Code. Like the telephony definition of sidetone, it acts 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 BFO frequency to generate a difference frequency, which is audible over the radio receiver loudspeaker or headphones.
See also
 Telephone
Beat Frequency Oscillator
Continuous wave
Echo cancellation
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