write on the disk however the duration differences of successive transitions vary so much also in a random fashion that they create inter symbol interference such that the read process becomes unreliable furthermore as we increase the packing density this inter symbol interference becomes a significant problem consequently several encoding schemes were used to correct the read error due to the inter symbol interference and other problems associated with data recovery during the read operation these coding schemes have the basic concept that a certain combination or pattern of 1s and 0s will help to minimize inter symbol interference the process is based on some mathematic formula such that for so many bits the maximum number of consecutive 1s and 0s must meet a predetermine pattern in most cases additional 0s will be added to the input sequence to meet this coding requirement as an example a 1000 1s and 0s of an incoming signal become 1400 1s and 0s as they are being written onto the disk this 40 increase in overhead is really not as bad as creating a redundant track to overcome the unreliable read operation prml is another improvement for increasing storage capacity and read reliability however it is still dependent upon the channel coding scheme and while the read reliability of prml is much better than the peak detect method the overhead is still.