13. A circuit for rendering an intelligible electrical speech signal unintelligible but recognizable as representing speech, characterized by means for distorting the speech signal in a first prescribed manner to produce a first distorted signal, means for distorting the first distorted signal in a second prescribed manner to produce a second distorted signal, and means for distorting the second distorted signal in a third prescribed manner, wherein said three distorting means make up the group consisting of means for sampling an analog signal at a prescribed rate to produce individual samples of the analog signal each having a prescribed duration and having a duty cycle of substantially less than 100%, means for rectifying a second analog signal, and means for attenuating individual audio frequency components of a third analog signal by prescribed amounts according to the frequency of the components.
14. The invention of claim 13 wherein the sampling means is characterized by means for generating said samples each with a said duration within the range of 100 milliseconds to 1 second and a ratio of sample-on time to the sum of sample-off time plus sample-on time within the range of 20% to 67%.