Sidetone
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[edit] Telephony
In telephony, sidetone is the effect of sound that is picked up by the telephone's mouthpiece and reproduced by the earpiece of the same handset, acting as feedback that the phone is really working.
Without sidetone, the user does not hear their own voice in the earpiece and the telephone user may think the phone is not working. Too much sidetone causes the phone user 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 which created sidetone in older phones, so digital phones include electronic circuitry to reproduce the sidetone. Many cell phones do not provide sidetone. Usability experts believe this causes some people to shout, or speak too loudly, when using a cell phone. [1]
Sidetone is useful for people using handsets but can cause acoustic feedback in teleconferencing systems if not treated properly.
[edit] Amateur radio
In 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 also the tone generated 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.
