Changes
MFM
,Each clock and data bit on disc is 2microseconds in duration which means a full byte is 32 microseconds (2 * 16 bits).
In order to read the data correctly and to ensure the bits are in the expected order a specific bit pattern is used for syncing.When this pattern is recognised then all data following is synced and will always read the same. This special value is 4489and has the following properties. This * 4489 is an encoded version of the pattern for clock data byte A1 and has bit 7 set. It doesn't matter what the value of the previous byte is, the first clock of 4489 will always be 0. Therefore the pattern 4489 can't be influenced by dataaround it.* To make the value special, with it has a "missing clock" (in reality a single clock bit . What this means is forced to 0) and assumes that the data bit proceeding this has a value one of 0 so that the first clock bit (bit 15) clocks is forced to 0. The data equivalent A1 byte value of 4489 encoded to MFM without the missing clock is 0x0a1. It different, therefore it is unique because the corresponding value computed from 0x0a1 has all the correct clocks and bits will not match this value.
