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Scatter Plot

Posted: Tue Jan 10, 2017 12:18 am
by eddy

I'm new to Kryoflux. I was wondering if someone could explain the scatter plot, or point me towards a resource where it is explained.

How is that visualization created, and what does it represent?

Apologies if this question is a little obvious, but I haven't been able to locate this info elsewhere.


Re: Scatter Plot

Posted: Tue Jan 10, 2017 7:38 am
by ZrX
The scatter plot graph shows the flux timing data from the disk over one rotation of the currently selected track.

Usually you will see two or three horizontal bands corresponding to fm (2) and mfm/gcr (3) encoded data.

You can often see slight variations as a repeating pattern there which are the sectors on the track. If the bands end up being smeared together the magnetic signal is usually damaged. If the bands are wavy there might be a speed issue with the motor or the disk isn't spinning properly.

The other graph just shows the statistical peaks of those bands.

Re: Scatter Plot

Posted: Tue Jan 10, 2017 5:15 pm
by eddy

So in this image, for instance:

I believe this was taken from a MFM encoded disc, which makes sense with the three rows of lines. Are the dotted lines in the center each sectors of the same track? Is this the clock?

I've also noticed the bottom line is usually the thickest. Why is that?

Thanks again for your help with this!

Re: Scatter Plot

Posted: Tue Jan 10, 2017 10:23 pm
by ZrX
That looks like PC/Atari DD disk with 9 sectors per track.

You can interpret it such that the bottom line would generate a bitsream of 11111... Middle one as 101010... And the top one as 100100100...

Bits on the middle line are likely the gaps between sectors and the longer bit going from the right side (over to the left) is the track gap, so the track would be aligned to the index signal.

My guess would be that those sectors are also empty and that the disk has a very slight speed problem.

Re: Scatter Plot

Posted: Tue Jan 10, 2017 10:29 pm
by Feltzkrone
Oh, just saw that ZrX already answered but anyway...

You can zoom in using the mouse wheel until you see single dots. These are actually the data. In your screenshot the bands are approx. 4, 6 and 8 µs which are for bit sequences 01 (2 bits), 001 (3 bits) and 0001 (4 bits) at 2 µs respectively. As an example if you zoom in and find dots at 4, 6, 4, 8, 4 µs in this particular order these represent the bit sequence 0100101000101 and to put it simple: this is basically how the whole track is made up. Use zooming to figure out why the bottom line is the thickest. ;)

ZrX wrote:...that the disk has a very slight speed problem.
If the bands are wavy the reason could be that the disk has been written using a belt driven floppy drive which usually introduces obvious drive speed wobble. You can use the F-keys (F1 to F5) to view the different sampled revolutions. If the waves are the same there must have been a slight speed problem when writing the disk, if the waves are changing the drive that you are using to dump the disk has a slight speed problem (which doesn't necessarily exclude that the writing drive has also had a problem). And last but not least the disk itself could be the problem.

Re: Scatter Plot

Posted: Mon Jan 16, 2017 3:14 pm
by anormal
I wonder why these things are not explained in the documentation. Whatever.

Re: Scatter Plot

Posted: Sun Jan 29, 2017 4:43 pm
by IFW
Actually, the waves can also mean that the disk is literally slightly bent... this is usually not a problem for the host software though.