I am looking for detailed info on the low-level floppy disk structure of an MSDOS-compatible 720k Double Sided Double Density disk. Stuff like GAP/SYNC patterns, ID and CRC Field structure, standard interleaving, etc. All my googling has been in vain.
All IBM floppy variants (5.25" and 3.5", DD and HD) use the same floppy controller, just the timing is different. So the bit pattern structure is the same, even though number of sectors per track etc. can differ. Interleaving is handled by software, I don't know the details.
The IBM PC floppy controller chips are descendants of the venerable NEC uPD765, which implements the IBM 34 MFM format as used in the mainframes. You can find details in the datasheet of the controller, and in IBM mainframe manuals.
Here is a transcription of the byte patterns (in hex) from the datasheet:
80x 4E Gap 4a 12x 00 Sync 3x C2 IAM FC " 50x 4E Gap 1 12x 00 Sync 3x A1 IDAM FE " ** CYL = Cylinder ** HD = Head ** SEC = Sector ** NO = ? 2x ** CRC 22x 4E Gap 2 12x 00 Sync 3x A1 DATA AM FB " ?x ** Data 2x ** CRC ? ? Gap 3 ? ? Gap 4b
CRC is calculated by hardware. Some gap lengths can differ between variants.
The A1 and C2 marker bytes are special and don't follow the standard MFM encoding: A1 is missing a clock transition between bits 4 and 5, and C2 is missing a clock transition between bits 3 and 4.
They are written by a WRITE TRACK controller command that interprets F5 and F6 to write those special bytes.
Most floppy disk controller chip datasheets have typical values, but it's really the BIOS that controls these values. As a starting point, take a look at Intel 82077 datasheet. I've successfully been able to decode sectors from a logic analyzer dump of floppy drive data pin.
The IBM AT bios listing. https://archive.org/details/bitsavers_ibmpcat618ferenceMar86_25829277/page/n325 Seems to have some of the data you're looking for.
CRC would have been implemented in hardware (there are a few variants) so probably an early FDC documentation would be your best bet.