Underwater telephone
The underwater telephone also known as UQC , or Gertrude was
developed by the U.S. Navy in 1945 , the UQC underwater telephone is used on all manned submersible
A submersible is a small vehicle designed to operate underwater. The term submersible is often used to differentiate from other underwater vehicles known as submarines, in that a submarine is a fully autonomous craft, capable of renewing its own power and breathing air, whereas a submersible is...

s in operation. Voices communicated through the UQC are heterodyne
Heterodyning is a radio signal processing technique invented in 1901 by Canadian inventor-engineer Reginald Fessenden where high frequency signals are converted to lower frequencies by combining two frequencies. Heterodyning is useful for frequency shifting information of interest into a useful...

d to a high pitch for acoustic transmission through water.
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