4.4. Using Compression to De-Skew

Reversible compression techniques also provide a crude method of de-skewing a skewed bit stream. This follows directly from the definition of reversible compression and the formula in Section 2 for the amount of information in a sequence. Since the compression is reversible, the same amount of information must be present in the shorter output as was present in the longer input. By the Shannon information equation, this is only possible if, on average, the probabilities of the different shorter sequences are more uniformly distributed than were the probabilities of the longer sequences. Therefore, the shorter sequences must be de-skewed relative to the input.

However, many compression techniques add a somewhat predictable preface to their output stream and may insert a similar sequence periodically in their output or otherwise introduce subtle patterns of their own. They should be considered only rough techniques compared to those described in Section 5.2. At a minimum, the beginning of the compressed sequence should be skipped and only later bits should used for applications requiring roughly-random bits.