Editing Scrambler (section)

== Purposes of scrambling ==
A scrambler (or randomizer) can be either:
# An algorithm that converts an input string into a seemingly [[Randomness|random]] output string of the same length (e.g., by [[Pseudorandomness|pseudo-randomly]] selecting bits to invert), thus avoiding long sequences of bits of the same value; in this context, a randomizer is also referred to as a scrambler.
# An analog or digital source of unpredictable (i.e., high entropy), unbiased, and usually independent (i.e., random) output bits. A "truly" random generator may be used to feed a (more practical) [[Determinism|deterministic]] [[Pseudorandomness|pseudo-random]] [[Random number generation|random number generator]], which extends the [[random seed]] value.<!--note: the MIL reference is *very* unclear on that point; does it refer to "true" randomness, statistical randomness, or cryptographically secure randomness?-->

There are two main reasons why scrambling is used:

* To enable accurate timing recovery on receiver equipment without resorting to redundant line coding.  It facilitates the work of a [[timing recovery circuit]] (see also [[clock recovery]]), an [[automatic gain control]] and other adaptive circuits of the receiver (eliminating long sequences consisting of '0' or '1' only).
* For energy dispersal on the carrier, reducing inter-carrier signal [[interference (communication)|interference]]. It eliminates the dependence of a signal's [[Spectral density|power spectrum]] upon the actual transmitted data, making it more dispersed to meet maximum power spectral density requirements (because if the power is concentrated in a narrow frequency band, it can interfere with adjacent channels due to the [[intermodulation]] (also known as cross-modulation) caused by non-linearities of the receiving tract).

Scramblers are essential components of [[physical layer]] system standards besides [[Error correction code#Interleaving|interleaved coding]] and [[modulation]]. They are usually defined based on [[linear-feedback shift register]]s (LFSRs) due to their good statistical properties and ease of implementation in hardware.

It is common for physical layer standards bodies to refer to lower-layer (physical layer and [[link layer]]) encryption as scrambling as well.<ref>EN 301 192, ''Specifications for Data Broadcasting'', [[European Telecommunications Standards Institute]] (ETSI), 2004.</ref><ref>ETR 289, ''Support for use of scrambling and Conditional Access (CA) within digital broadcast systems'', [[European Telecommunications Standards Institute]] (ETSI), 1996.</ref> This may well be because (traditional) mechanisms employed are based on feedback shift registers as well.

Some standards for [[digital television]], such as [[Conditional access|DVB-CA]] and [[Multiprotocol Encapsulation|MPE]], refer to encryption at the link layer as scrambling.