Re: Disk II Phase Question

[mjmahon@aol.com (Michael J. Mahon)]

Bryan Parkoff wrote:

>> The Disk II has a nominal rotation speed of 300 rpm; that causes one
>> revolution every 200 msec. The nominal read pulse spacing is 4
>> microseconds. That means approximately 50,000 bits in one revolution of
>> the disk.
>    I understand now.  The reason is that I say ($5F00) 24,320 bytes or 
>194,560 bits per track.  It is what it is found in Copy II Plus' Nibble 
>Anaylzer.  I assume that it should be 6,576 bytes for valid data including 
>address header and data header and 6-2 data per track.  I will study more.

Track analysis programs must read more than a full track--that is they
read at least part of the track more than once--so that they can find
the track start in the nibble stream.  If it is desirable that an image of
the full track be present in ascending addresses in the buffer (without
wrapping), it is necessary to read the entire track at least twice.  This
guarantees that after the start is found, searching from the start of the
buffer (which could, in the worst case, be almost an entire track offset),
there must be a little more than the full track again, to ensure that the
entire image is present without wrapping in the buffer.

If the program also plans to look for "soft" bits (bits which cannot be
read repeatably because there are, for example, too many consecutive
zeros, then it is desirable to have even more than two copies, so that
the differences in different read passes can be discerned.  Some copy
protection mechanisms used this technique to defeat even nibble copy
programs, since if a "soft" bit was always read the same (because a
nibble copier wrote it as a "hard" bit), then a copied disk was detected.

-michael

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