A scrambler (or speech masking device) is designed for encoding and decoding an audio signal transmitted over the air. To listen to an encoded signal, the receiving radio must have the same scrambler module as the transmitting one.
Scramblers are generally produced as replaceable modules. Using this function may require authorization from a national communications agency.
Scramblers are used in radio communications, public telephone networks, satellite and microwave links, and digital television. In mobile radio systems, frequency and time transformations of the signal, as well as their combinations, are the most commonly applied methods. Possible interference in the radio channel makes it significantly harder to accurately restore the amplitude of a speech signal, which is why amplitude transformations are rarely used in scrambling.
Frequency Scrambling
In analog radios, inexpensive spectrum-conversion scramblers are most often used — inversion of the entire spectrum or certain bands of the speech signal with subsequent rearrangement of these bands (band-shift inverters), or frequency inversion within each band. On the receiving side, similar operations are performed with reverse frequency rearrangement or inversion.
Time Scrambling
The simplest type of time transformation is time inversion, in which the original signal is divided into a sequence of time segments, each transmitted in reverse order — from end to beginning.
In a masker with time rearrangements, the speech signal is divided into time intervals, each further subdivided into segments, and then the speech signal segments are rearranged.
Digital Scrambler
Modern radio signal encoding systems are far more advanced than their predecessors. These are complex digitizing devices combined with encryption units. In such systems, the original signal is digitized, the resulting data is encrypted, and then transmitted. Asymmetric-key scramblers are considerably more resistant to cryptographic attacks. Such systems are considered sufficiently secure for handling sensitive information.