Kryoflux with Macintosh 400K & 800K Floppy Drives

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Re: Kryoflux with Macintosh 400K & 800K Floppy Drives

Post by JDW »

spags wrote:
Sun May 17, 2020 5:44 am
...the Kryoflux handles the “variable speed” aspect because it samples at a hardware level and decodes the signals (probably in software). It doesn’t care that the drive might be running at constant velocity because it is probably capable of sampling the signals at a much higher rate than they would be written to on the disk.
How would you harmonize these conflicting pieces of information though?

Only Sony MPF920-1 works reliably

Even Sony MPF920-1 doesn't work

And then there's this detailed blog that yet again says the Sony MPF920 works.


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Re: Kryoflux with Macintosh 400K & 800K Floppy Drives

Post by brightcaster »

Different users may have different experiences! There are horses for courses...

It's not a matter of the drive you're reading with alone. If a floppy was written with a bad or missaligned drive or was stored at a bad place with high temperature or humidity it will be unreadable with any drive.

Sometimes a bad drive can read bad disks better than a good drive (if they are both missaligned to the same direction for example). There may be also differences in the timing of the AGC due to aging capacitors (which is a problem with GCR-coded floppies) making a certain floppy readable with one drive but failing on an other of the same brand and type...


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Re: Kryoflux with Macintosh 400K & 800K Floppy Drives

Post by spags »

As an end user, I don’t harmonise. I accept that different drives give different results with different disks. I have a small collection of drives and although I have a favourite or two, those same drives have failed me terribly for certain Apple disks whereas a least favourite gave an almost flawless result. Why? Who knows!

Each drive is different, even from the same model. Remember that these things (or at least the good quality ones) are generally at least 20 years old. Each head, capacitor, circuit design, AGC method, motor, alignment, etc, are all going to be different.

There is a certain “art” involved here. You try on one drive and see how it goes. If not well, then you try again on another drive. If you’re lucky, you can combine parts from two (or more) different attempts to get a good read. But you also have to know that depending on the condition of the disk, every attempt could potentially be the last. There is no real way to know - it is an art, not a science.

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Re: Kryoflux with Macintosh 400K & 800K Floppy Drives

Post by SomeGuy »

Why then are "Apple DOS 400K/800K" disks mentioned here as being compatible? As you know, the only way to get 400K or 800K onto single sided 3.5" media is to use a variable speed drive mechanism, which is exactly what the Macintosh 128K and 512K used.
There are actually THREE different ways to read/write variable bitrate data.

1: Change the rate at which the disk spins. The Apple 3.5" drives do this.

2: Change the rate at which the hardware reads/write bits. The Central Point Transcopy card does this, and uses IBM PC 3.5" floppy drives that spin at a normal constant rate.

3: Handle the rate differences in software. This is what the Kryoflux does. It just reads and writes flux transition information sampled at a very high resolution (higher than the bitrate). Once read, the software can then analyze the signal and decode data regardless of the bitrate. This also lets it do all this on normal IBM PC drives.

The issue with IBM PC style drives is that to improve reliably they filter out noise and amplify weak signals. Unfortunately, these are optimized around standard bitrate MFM encoded disks used by IBM PC compatible computers. That means GCR encoding and odd bitrates may look like "noise", and wind up getting filtered out or distorted.

Different drive models, and even the same model with different revision hardware, may implement this slightly differently and yeild different results. As mentioned, add age, usage, and other variances in the mix, and two seemingly identical drives may give different results.

Heck, even when dealing with damaged or marginal standard IBM PC floppy disks, I sometimes have to try different drives, and may get different results.

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