The difference between voice scramblers and voice encipherment is the scrambler works with the analog signal to transform that signal in some
way so it is unintelligible to the eavesdropper. Voice encipherment means converting the analog voice signal to digital, then applying a
cipher algorithm to that digital signal.
The earliest voice scramblers were invented before WW2 and used synchronized recordings
to add noise to a voice signal, then the decoder would subtract that same noise. Another option was to invert the frequency, changing that
frequency inversion over time. These methods of voice scrambling were not very secure and it was an inexact science. With practice an expert
listener learned to decipher much of the conversation without the aid of any mechanical devices. Also, the signal was studied and manipulated
to reverse the frequency inversion and time splicing.
The most advanced voice scramblers available in WW2 were used for conversations between Winston Churchill and
Franklin D. Roosevelt, which the Germans were able to quickly decipher. This advanced voice scrambler was a room-size device developed
by AT&T using synchronized recordings on long-playing records to add noise to the message. The receiving location would have to have
their recording precisely timed to subtract that same noise. Unfortunately, a German engineer worked on this at AT&T before the war and
then went to Germany and used his knowledge of this scrambler to help the Nazis decode the messages.
Use of radios and trench phones were notoriously unsecure for voice messages, so they were often used for enciphered communications using morse
code. A notable exception is the US using Navajo Indians in WW2, who spoke in their native language and were never deciphered by
the Japanese. By the early 1960's voice encipherment used digital technology, providing a high level of encryption.
US KY-57 Voice Encoder
US KY-28 Aircraft Voice Encoder
UK MSC-2001 Voice Encoder
See the Entire Collection of Cipher Machines
See detailed pictures of the voice scramblers.
Here are three examples of voice scramblers: the KY-57 for ground forces and the KY-28 for aircraft and the MSC-2001 voice encoders.