Abstract: In a speech transmission system, a first privacy apparatus for making reversible alterations in speech message waves for rendering unauthorized reception of the message difficult, a second privacy apparatus for making a different kind of reversible alterations in the speech message waves for increasing the difficulty of unauthorized reception of the message, a common coding mechanism for controlling the alterations made in both privacy apparatuses, said coder supplying control potentials to the first of said privacy apparatuses, and means in said first privacy apparatus for translating said control potentials into control potentials of different character for controlling said second privacy apparatus.