This post is a reply to the question is it possible to find if a "sector has been overwritten" with Aufit
The new version of Aufit has new features that can help finding if a disk is an original or not.
When a disk is produced on a mastering machine the formatting and sector content data of a complete track are written in one pass. The write gate is turned on at a specific point (usually the index pulse) and turn off at the same point one revolution later. Due to speed variation it is not possible to close the write gate at exactly the same point as when the gate was open. This result in some transitions being shorter or longer than normal and these transitions are called the track write splices.
When a disk is produced on an Atari machine the formatting data of a complete track are written in one pass. The write gate can only be turned on at the index pulse and turn off at the next index pulse one revolution later. For same reasons explained before we also have track write splices located at the index pulse. In order to write data in a sector a write sector command need to be used. In that case the FDC read the address header and when the right header is found it switches from reading to writing the sector data and when it finishes it turn off the writing gate. Due to speed variation the start and end of the sector write is not aligned with the formatting data. This results in two sector write splices at the beginning and the end of the sector. If we find these sector write splices (as well as possible sector speed variation) this is a good indication that the sector has been overwritten.
It is normally difficult to detect the write splices on Aufit track view. However if you switch the disk view the write splice are seen as white marks on the track. The white marks are out of band transitions and in most cases (unless done on purpose) they correspond to write splices.
For example let’s format a floppy disk on an Atari. First we can see the write splices (in white) for the tracks that are aligned close to the index line (in blue).
At the end of the format command several information is written on the diskette. For example the boot sector, the FAT, the directory information, etc. As we have explained this information is written using a write sector command and result in two write splices on a specific sector (one at beginning, one at end of the sector). So here we see the sync marks in black followed by the address header in yellow followed by the sector gap in green. Inside this gap we see a write splice in White. After we have the sync marks in black followed by the data segment in blue. At the end of the sector we have the gap between sectors and at the beginning of this gap we see the second write splice. So we can be almost assure that this sector has been overwritten on an Atari machine.
Note that detecting the duplication of a diskette with a board like SCP is possible but more difficult.
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