12-22-2021, 01:17 PM
One other thing, if you have something like 100 invalid flux reversals in a row (not uncommon for protections and unformatted tracks) when you read that data it is going to change every single time. In fact, the first bitcell time could be 0x00A0 and the next could be 0x340, and the next time you read it they are reversed or some other value based on the bitcells around them. "Weakbit" copy protection uses this fact that the data changes on every single read. It is very common to see valid data placed among weakbit data and a check is made to make sure that the valid data is there and that also the invalid data changes on every single read. When you capture tracks like this you are going to see that the number of total number of bitcells on the track changes every read. Everything depends on the exact model drive you are using. Some disk drives simply return a fixed value when the clocking window is exceeded. Most drives will increase the AGC (automatic gain control) of the read amplifier to try to read the next bitcell period and continue to do that until valid data appears. This causes the over-saturation of the signal and the PLL circuit has to reset itself, which can mean losing several bitcells. NFA is particularly nasty because there is no flux transition that occurs at all for a period of time, up to a full revolution (166.6667 for a 360 RPM drive or 200ms for a 300 RPM drive). SuperCard Pro handles this as a special case using a 25ns synchronous clock.
I think you are believing that bitcell times are always perfect and consistent. This is not the case. The best case scenario is when a disk is produced on a commercial duplication machine with a master script so only a single write splice occurs. This is done by writing more than one full revolution so that when the write is turned off the smearing that occurs when the head's energy collapses only occurs one time during the revolution. When sectors are written individually, you have write splices (invalid flux) that occurs for every single header and data block, so you will have dozens of these on a track. The big dilemma in copying a disk is determining where to start/end the track. There are MANY cases where when you turn off the writing it just so happens to work out that the write splice (smear) ends up creating a bitcell value that is valid, so you can't see an obvious write splice. SuperCard Pro's SPLICE mode looks at the 2nd revolution (regardless of how many revolutions are captured), looking for an invalid flux time and uses that for stopping the writing when making a copy of the track. If there is no invalid value found, then SPLICE mode falls back to just writing the complete revolution, which is fine for anything that uses the index pulse to start/stop tracks.
I think you are believing that bitcell times are always perfect and consistent. This is not the case. The best case scenario is when a disk is produced on a commercial duplication machine with a master script so only a single write splice occurs. This is done by writing more than one full revolution so that when the write is turned off the smearing that occurs when the head's energy collapses only occurs one time during the revolution. When sectors are written individually, you have write splices (invalid flux) that occurs for every single header and data block, so you will have dozens of these on a track. The big dilemma in copying a disk is determining where to start/end the track. There are MANY cases where when you turn off the writing it just so happens to work out that the write splice (smear) ends up creating a bitcell value that is valid, so you can't see an obvious write splice. SuperCard Pro's SPLICE mode looks at the 2nd revolution (regardless of how many revolutions are captured), looking for an invalid flux time and uses that for stopping the writing when making a copy of the track. If there is no invalid value found, then SPLICE mode falls back to just writing the complete revolution, which is fine for anything that uses the index pulse to start/stop tracks.