SuperCard Pro Flux Image format (.scp)
#5
I think there is probably a problem of terminology or a misunderstanding.
Below is my explanation on No Flux Area (or whatever you call it) and how it is detected in a game like Turrican (this is from one of my doc)

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There has been a lot of debate around the so called No Flux Area on disk. This is a protection's mechanism used on some floppies that results in absolutely no flux transitions coming from the drive read circuitry for a relatively long period of time (usually several milliseconds).
For some times it has been thought that this was obtained by doing a so called "strong erasure" on areas of a disk. However this would be very difficult to create and would not produce the wanted effect:
    For one this can’t be done with the normal recording head/circuitry of a floppy drive and therefore it would require to use modified drives.
    Secondly if such areas, with no magnetic flux transitions, existed on the floppy disk it would cause the ACG of the read chain to be set to its maximum amplification value and this would result in reading random flux transitions which is not the case.
Bit-shift occurs on any NRZ recorded medium as a normal consequence from read/write head operation. Data are written when the read/write head generates a flux change in the gap of the head, which causes a change in magnetization of the medium oxide. In reading, a current is induced into the read/write head when a flux transition on the medium is encountered. The current change is not instantaneous, since it takes a finite time to build up to peak and then to return to zero. If flux transitions are close together, the current buildup after one flux transition then declines, but it does not have time to reach zero before the second transition begins. Consequently current pulses are summed by the read/write head, which causes the peaks to be shifted.
A No Flux Area is created by writing a large number of flux transitions close enough (i.e. at a relatively high frequency). This will result in having the read current never returning to zero and consequently this will result as no data pulse generated on the read channel. Note that in this case the ACG is not set to a maximum amplification as the input circuitry receives flux transitions even if no data is coming out of the read channel.
How is-it possible to check an NFA with a standard WD1772 Floppy disk controller? Normally the WD1772 FDC can only read the data bits. Therefore a sector with NFA is read as a sector filled of bytes 0x00 but it is normally not possible to check that the clock bits are also bytes 0x00. To be able to check the clock bits the NFA protection uses an interesting trick. Another sector is written within the sector (SWS) that contains the NFA and this sector contains 3 sync marks shifted by one bit cell. Therefore when you read the data for this second sector you are actually reading the clock data from the first one!
   
I have zoomed on a NFA located in range 89-94 ms.
Of course you have to remember that normally a 4.3ms flux transition is not possible and it is not either an unformated area that would generate noise ...

see also http://bitsavers.trailing-edge.com/pdf/m...ystems.pdf
and
http://info-coach.fr/atari/hardware/FD-Hard.php#writing
and
http://info-coach.fr/atari/hardware/fd-hard/AN-413.pdf
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Messages In This Thread
SuperCard Pro Flux Image format (.scp) - by admin - 11-01-2013, 05:54 PM
RE: SuperCard Pro Flux Image format (.scp) - by DrCoolZic - 12-19-2013, 04:06 PM



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