Vorpal disks: lead-in and trailing patterns

As the question was asked, here you find a visual representation of the lead-in and trailing patterns in original disks that use the later version of Vorpal:

Flux Studio: lead-in and trailing patterns for California Games by Luigi Di Fraia
Flux Studio: lead-in and trailing patterns for California Games

From left to right we see:

  • trailing sequences
  • lead-in sequences

Worth noting:

  • even tracks before track 18 have shorter lead-in sequences (and no block-syncs – although those are not visible for any Vorpal track in the image) as they pack more data than odd ones (0x2E sectors instead of 0x2D)
  • the period of repeated sequences is either 8 bits or 10 bits: a clue to which ones are filler patterns (a track is pre-filled with them) and which ones serve an actual purpose (even if only for measurement/verification of a mastered disk)
  • currently the KryoFlux toolset that produces write splice offsets for the disk as per image fails to detect an adequate splice point for track 27 (there’s no gap on this track)

That’s all for this update. Stay tuned for more!

About Luigi Di Fraia

I am a Senior DevOps Engineer so I get to work with the latest technologies and open-source software. However, in my private time I enjoy retro-computing.
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9 Responses to Vorpal disks: lead-in and trailing patterns

  1. Fritz Laufwerk says:

    The 8-bit trailing sequences and the 10-bit lead-in sequences appear to be fixed constants.
    What’s very interesting is the short pattern between the two, which seems to be different for every track. Perhaps it encodes the track number or some check value?

    The different number of blocks on even and odd tracks below 18 is a bit odd. There should be no difference in physical properties as in raw density, so they’re wasting some capacity. Wonder what the purpose is.

    • “What’s very interesting is the short pattern between the two”. I might be wrong on this one, but if you align trailers bit by bit, then lead-in patterns don’t align bit by bit, which to me suggests the pattern in between is related to either one or the other.

      “Wonder what the purpose is”. I will see if anything obvious pops up from the drive code at some point.

      • Fritz Laufwerk says:

        So far, i haven’t found any relevant code for either of these mysteries. But then this isn’t the only title to use later Vorpal. =)

        As for the patterns between trailers and lead-in, it’s suspicious that the repetitive trailer sequence is 8 bits wide (and the lead-in isn’t). Code reading the mystery patterns could detect the trailer easily by reading successive bytes from disk and comparing them. If some number of successive bytes is equal, then the pattern should follow at some point.

        Looking at that pattern on all tracks on both sides might at least give some hints as to what it might encode. Checked only a few tracks, and the pattern always seems to start with 4 1’s. And yes, it doesn’t seem to have a fixed size. Which adds to the mystery. 🙂

      • Perhaps one day we will come across the mastering program and all these questions will be answered. For now, we can guess.

      • Fritz Laufwerk says:

        After another look, it seems like the patterns between trailer and lead-in are merely artefacts of the original mastering process indeed (before duplication).

        A track would start with the repeated 10-bit sequences (1010011010) and end with the 8-bit sequences (11010110) plus ultimately a stop mark (111101).

        Anything between that stop mark and the lead-in would just be bits and pieces of all of the above from previous track-writing runs shining through.

        The scheme still allows for validation to ensure that all relevant data is readable and that the end of the track did not run into the first block.

      • “it seems like the patterns between trailer and lead-in are merely artefacts”. I agree.

        What I’d like to understand more about are the tiny gaps in the area. However, I am not sure they weren’t introduced by the workflow that created the G64s (given to me by SLC). I shall see if I can research this topic using raw KryoFlux images themselves.

      • Fritz Laufwerk says:

        So far, i assumed the gaps to be artefacts of the duplication process.

        My information might be incomplete and this is all just guesswork, but…

        The G64 images have track-data aligned to the index hole. Track data, however, appears not to have been index-hole aligned on the original disks, thus probably not on the original master itself either. The duplication process would also need to use good write splices, starting anywhere between last and first block. Using the index hole as the anchor, it might have waited a specified time, then written the track, but also made sure not to overwrite the beginning of the track. As the gaps have varying sizes and appear rather randomly in a track-trailer, i guess there was a bit of jitter and other tolerances involved. 🙂

    • Fritz Laufwerk says:

      After another idle look, there may be an explanation for the different number of blocks on even and odd tracks below track 18. The odd tracks are those with 45 blocks, the even with 46.

      The odd tracks have (at least) one hardware sync with a standard block header, occupying some of the space reserved for the extra block on the even tracks.

      This could be helpful to identify the current track (for standard DOS or perhaps Vorpal itself) when the drive has lost its footing after the occasional mis-step (or disk turn/change), without resorting to far-stepping or bumping the head (yet).

      There seem to be a number of further hardware syncs on the odd tracks, too, but with a data marker ($55, not $52). No idea what these could be for. =)

      • “The odd tracks have (at least) one hardware sync with a standard block header”. Yep, I was showing this in my latest video on the subject.

        “This could be helpful to identify the current track”. Yep, still it seems odd they didn’t do the same in tracks > 18, i.e. put block-syncs just in each other track?

        “No idea what these could be for.” Your guess is as good as mine at the moment. Perhaps somebody will be able to find & interview the original Vorpal coders and shed some light on these open questions.

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