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The checksum is handled by taking each track offset and summing them together, resulting in a "large number", which is then brought into the range of 0-4095 by applying mod4096.

Why 4096 and not 255 or 65536?
Short answer - Cos FFF in hex is a nice size for the checksum to be

Long answer...

DiscID checksum was based on

  • track offsets in seconds
  • mod 255 (it should have been mod256, but little difference I think)

NCDI checksum is based on

  • track offsets in frames

I then tested with mod256, mod4096 and mod65536 to see how they fared.

  • mod256 - clearly better than DiscID - this is only attributable to the difference in using frame-level offsets
  • mod4096 - a clear improvement over mod256 - more numbers for the checksum to spread over = less chance of duplication
  • mod65536 - marginal improvement on mod4096 - not enough to be worthwhile using compared to mod4096 however

The following graph shows respective counts of Unique and duplicate ID's for each type of ID. the big confusing graph

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