Audio spectrum shift techniques

Although spectrum shift and spectrum inversion have been known for quite some time as a means of protecting voice links mainly over the telephone, little has been written on the design and operation of the electronics involved. This article aims at offering you a design base and two practical circuits to start experimenting with an interesting form of analogue audio scrambling and descrambling which may be used to, say, personalize a voice link cassette mall, telephone, wireless babysitter, etc.). Remarkably, you will not be able to tell the difference between the encoder and the decoder until you compare the two units and spot a couple of components with different values.