Voice Inversion scrambling is an analog method of obscuring the content of a transmission. It sometimes used in public service radio, automobile racing, cordless telephones and the Family Radio Service. Without a descrambler, the transmission makes the speaker "sound like Donald Duck". It is called "voice inversion", but the technique operates on the passband of the information and so can be applied to any information being transmitted.
There are various forms of voice inversion which offer differing levels of security. Overall, voice inversion scrambling offers little true security as software and even hobbyist kits are available from kit makers for scrambling and descrambling. The cadence of the speech is not changed. It is often easy to guess what is happening in the conversation by listening for other audio cues like questions, short responses and other language cadences.
In the simplest form of voice inversion, the frequency of each component is replaced with , where is the frequency of a carrier wave. This can be done by amplitude modulating the speech signal with the carrier, then applying a low-pass filter to select the lower sideband.  This will make the low tones of the voice sound like high ones and vice versa. This process also occurs naturally if a radio receiver is tuned to a single sideband transmission but set to decode the wrong sideband.
There are more advanced forms of voice inversion which are more complex and require more effort to descramble. One method is to use a random code to choose the carrier frequency and then change this code in real time. This is called Rolling Code voice inversion and one can often hear the "ticks" in the transmission which signal the changing of the inversion point.
Another method is Split Band Voice Inversion sometimes called VSB. This is where the band is split and then each band is inverted separately. A rolling code can also be added to this method for Split Band Variable Inversion.
Common carrier frequencies are: 2.868 kHz, 2.632 kHz, 2.718 kHz, 2.868 kHz, 3.023 kHz, 3.107 kHz, 3.196 kHz, 3.333 kHz, 3.339 kHz, 3.496 kHz, 3.729 kHz and 4.096 kHz.
- Secure Communication – Scrambling and key distribution scheme for digital television
- Secure voice
- Hendrick J van der Bijl, "Method and system for radiosignalling." United States patent 1 502 889, issued 29 July 1924.
Kanjanarin, W.; Amornraksa, T. Networks, 2001. Proceedings. Ninth IEEE International Conference on Volume, Issue, 10–12 October 2001 Page(s): 140 – 145 Digital Object Identifier 10.1109/ICON.2001.962331 Summary: The scrambling scheme is a part of the conditional access system (CAS) that is used to prevent unauthorized access to Pay-TV systems. We propose a new scrambling scheme and key distribution scheme. The scrambling scheme is used in conjunction with ordinary encryption techniques, for protecting the digital video streams from unauthorized viewing. A hash function and a pseudo-random number generator are used to prepare the video stream before being encrypted. The proposed scheme helps reduce computational time and complexity while providing the same level of security as encrypting the entire video stream. In addition, the proposed secure key distribution scheme can be used with any, scrambling scheme e.g. with our scrambling scheme. By using the Chinese remainder theorem (CRT) for distributing parameters in the scrambling process, the security of the scheme can be increased. Moreover, our scheme can prevent two common problems, namely, smart card cloning and McCormac Hack.