Editing Scrambler (section)

=== Additive (synchronous) scramblers ===
[[File:Scrambler randomizer additive.png|right|thumb|350px|An additive scrambler (descrambler) used in [[Digital Video Broadcasting|DVB]]]]

''Additive scramblers'' (they are also referred to as ''synchronous'') transform the input data stream by applying a [[Pseudorandom binary sequence|pseudo-random binary sequence]] (PRBS) (by modulo-two addition). Sometimes a pre-calculated PRBS stored in the [[read-only memory]] is used, but more often it is generated by a [[linear-feedback shift register]] (LFSR).

In order to assure a synchronous operation of the transmitting and receiving LFSR (that is, ''scrambler'' and ''descrambler''), a ''[[Syncword|sync-word]]'' must be used.

A sync-word is a pattern that is placed in the data stream through equal intervals (that is, in each [[Frame (networking)|frame]]). A receiver searches for a few sync-words in adjacent frames and hence determines the place when its LFSR must be reloaded with a pre-defined ''initial state''.

The ''additive descrambler'' is just the same device as the additive scrambler.

Additive scrambler/descrambler is defined by the polynomial of its LFSR (for the scrambler on the picture above, it is <math>1+z^{-14}+z^{-15}</math>) and its ''initial state''.