The methods for ensuring the privacy of voice, data, and video transmissions. Various techniques are commonly utilized to perform such functions.
Analog voice-scrambling methods typically involve splitting the voice frequency spectrum into a number of sections by means of a filter bank and then shifting or reversing the sections for transmission in a manner determined by switch settings similar to those of a combination lock; the reverse process takes places at the receive end. Digital methods first convert the analog voice to digital form and then scramble or encrypt the digital voice data by one of the methods discussed below. See Analog-to-digital converter
A simple data-scrambling method involves the addition of a pseudorandom number sequence to the data at the transmit end. Devices using this method are known as stream ciphers. A second method partitions the data into blocks. Data within a block may be permutated bit by bit or substituted by some other data in a manner determined by the switch setting, which is often called a key. Devices using this method are known as block ciphers. See Cryptography
Typical video scrambling devices used for cable television applications involve modifying the amplitude or polarity of the synchronization signals, thereby preventing the normal receiver from detecting the synchronization signals. A more sophisticated technique, used in satellite transmission, introduces a random delay to the active video signal on a line-by-line basis. An even more advanced technique called cut-and-rotate has been proposed. Video signals can also be digitized by a number of coding techniques and then scrambled by any of the data-scrambling techniques discussed above to achieve high security. See Television