bobmon@iuvax.cs.indiana.edu (RAMontante) (07/31/90)
Can someone please explain what "write pre-compensation" (on a hard disk) is?
grege@gold.GVG.TEK.COM (Greg Ebert) (07/31/90)
In article <52703@iuvax.cs.indiana.edu> bobmon@iuvax.cs.indiana.edu (RAMontante) writes: >Can someone please explain what "write pre-compensation" (on a hard disk) is? At higher bit densities, bits written onto the disk tend to 'move around' due to interaction. The problem also exists on floppies. Write precompensation, as the term might imply, compensates for bit-shift during the write phase. Depending upon the data pattern, a bit may tend to shift 'earlier' (during a read); when it is written, it will be delayed a small fraction of the nominal bit time so that the net result is cancellation, resulting in the bit being read when it's expected. Why fuss about when bits are written ? MFM encoding buries clock and data bits together, so a clock/data recovery circuit is required, which uses either an analog or digital phase-locked loop (PLL). The PLL 'expects' data bits at certain times so that it can remain synchronized to the data stream. If there is excessive bit jitter, the PLL will lose synchronization and you will (hopefully) get a CRC error because the data is garbled. If you don't want to hassle with this, use single-density (FM), which has separate clock and data bits. You guessed it, it only stores half as much data.