Voice inversion
scrambling
Scrambling is a mountaineering term for ascending steep terrain using one's hands to assist in holds and balance.''New Oxford American Dictionary''. It can be described as being between hiking and climbing, rock climbing. "A scramble" is a relat ...
is an analog method of obscuring the content of a transmission. It is 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
Donald Fauntleroy Duck is a cartoon character created by the Walt Disney Company. Donald is an Anthropomorphism, anthropomorphic white duck with a yellow-orange bill, legs, and feet. He typically wears a sailor suit, sailor shirt and cap with ...
". Despite the term, the technique operates on the passband of the information and so can be applied to any information being transmitted.
Forms and details
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
In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or freq ...
. This can be done by
amplitude modulating the speech signal with the carrier, then applying a
low-pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filt ...
to select the lower
sideband
In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands c ...
.
[Hendrick J van der Bijl, "Method and system for radiosignalling." United States patent 1 502 889, issued 29 July 1924.] 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
In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or fre ...
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''. 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 variable split band inversion (VSB).
Common carrier frequencies are: 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.
Voice inversion offers no security at all and software is available to restore the original voice,
which is why it is no longer used to protect conversations today. However, voice inversion is still found in low-end Chinese walkie talkies.
See also
*
Encryption
In Cryptography law, cryptography, encryption (more specifically, Code, encoding) is the process of transforming information in a way that, ideally, only authorized parties can decode. This process converts the original representation of the inf ...
*
Scrambler
*
Secure communication – Scrambling and key distribution scheme for digital television
*
Secure voice
References
{{reflist
External links
Audio clips of various radio scrambling and signaling types*http://www.receptionunlimited.com
Cryptography