What is the purpose of using a scrambler in communication systems? The link wiki_for_scrambling says that it is used to randomize data. But the data is already random (like speech,audio,video,etc..), what is the purpose of randomizing again?

Even if we ex-or the sequence for randomizing? does it guarantee a non-repeating pattern?

And regarding the statement "Scrambler removes dependency of a signal's power spectrum on the actual transmitted data." in the article, what is the use of that?.

regards, phani tej


Scramblers are indeed used to "randomize" the data, i.e. to "whiten" the input sequence. The reason why this is necessary is that a transmission system has no control over the data the user is going to transmit. This causes problems because it violates the assumptions that are usually made when designing transmission systems, such as having independent data symbols. Note that usually the data are not random, as you seem to believe. A frequently occurring problem are long strings of zeros in the data, which can cause difficulties in timing recovery and adaptive equalization. These problematic sequences are removed (or, actually, made much less likely) by the scrambler.

  • $\begingroup$ Then how about scrambler removing the dependency of a signal's power spectrum on the actual transmitted data? How is it useful? $\endgroup$
    – phanitej
    Apr 20 '15 at 7:56
  • 1
    $\begingroup$ @phanitej: A scramber will result in a flat power spectrum, regardless of the data. This cannot be guaranteed with arbitrary user-supplied data. $\endgroup$
    – Matt L.
    Apr 20 '15 at 8:47

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