'Vorpal (later) analysis'
Author:IFWSPS (guest: search)
Date: Fri, Dec 02nd, 2011 @ 13:23 ( . )

The nosync Vorpal format has 46/47, 41, 39 and 37 sectors per track depending on speed zone. The sectors are 128 bytes long.

The 46 sector tracks have a single dos header to enable track seek/verify (for the 1541 that is).

CBM tracks 5 and 21 have a wrong track number recorded on purpose.

The encoding is a continuous bitstream with syncs added to allow sector level reading.

The encoding is a maximum of 4 consecutive 1 bits, and 2 consecutive 0 bits (hw limit).
This has been achieved by an adaptive encoding scheme that treats the bitcells as a continuous stream rather than simple 5 bit entities. Some nybbles can be encoded in two different ways (the encoding scheme is somewhat similar to how MFM encoding works in this regards), and the combination of those always guarantee that the recording rules of this scheme is never violated.

Syncing is performed by finding 5 consecutive 1 bits, as the encoding won't produce that otherwise. This, however is recorded on the disk as 8x1 bits.

There is no sector header, just the sync, 7 bitcells as 0101010 and the sector data 128x10 bits, plus checksum 10 bits.

The following 9 bit contains the actual sector number, using a different encoding.

The next sector sync immediately follows that.
If the sector number encoded ends with 1s, the sector sync count is decreased to ensure that always 8x1 bits are recorded.
e.g. sector number encoded ends with ...11 the sync following it will be 6 bits instead of 8.

These tracks are by design not modifiable. It is easy to see that getting the track gap position wrong, one sector is guaranteed to get damaged and the stream will be broken. This is on purpose of course.

The KryoFlux host software can fully verify this as well as other Vorpal variants.


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Replies:
--* Vorpal (later) analysis
12/05/2011 @ 11:38--Markus Benko
12/06/2011 @ 11:38----IFWSPS
3/28/2013 @ 02:42------The Doctor
5/07/2013 @ 19:53--------bluebirdpod
5/28/2013 @ 20:47----------SaxxonPike

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