Bit encodings that cause frequent transitions between 1 and 0 solve this problem, because the disk drive knows when there is a transition from 0 to 1 or 1 to 0 that a bit boundary is found there. ASCII NULL would be 1010101010101010, which gives regular and frequent transitions. In fact, the bit encoding we have been using, which is called Manchester encoding, never puts more than two 1s or two 0s in a row.

Fig. 17.3.2 illustrates a bit stream encoded using Manchester encoding. The receiver still has a clock and it senses the signal level regularly. But Manchester encoding never allows three half-bit time intervals to have the same signal level, so if the receiver sense this condition, it knows its clock is drifting. Therefore, it takes corrective action and resets its time interval.