ON-LINE METHODS

A disadvantage of the off-line technique above is that it is off-line: The predictor does not adapt to the specific text file it sees. This limitation is not essential, however. It is straight-forward to construct an on-line variant of the approach. With the on-line variant, the predictor continues to learn during compression. The on-line variant proceeds like this: Both the sender and the receiver start with exactly the same initial predictor. Whenever the sender sees a new character, it encodes it using its current predictor. The code is sent to the receiver who decodes it. Both the sender and the receiver use exactly the same learning protocol to modify their weights. This implies that the modified weights need not be sent from the sender to the receiver and do not have to be taken into account to compute the average compression ratio. Of course, the on-line method promises much higher compression ratios than the off-line method.