LOGARITHMIC COUNTER ARRAY WITH FAST ACCESS.

In data compression apparatus it is often desirable to make logarithmic sums to compute the probability of correctly predicting a data bit from the preceding history of data bits in the bit stream. A logarithmic counter composed of a random-access memory (RAM) and a programmable logic array (PLA) is described. The data bit history is accumulated as counts in the RAM. The counts are stored as binary words representing the log to the base 2 of each count. The binary words take the form XXX. XXXXX where all eight bits are used to keep a history count but only the most significant five bits are used by the data compression algorithm. Subsequent operations on the counts by the data compression algorithm are much quicker since the counts are stored as log values.