Understanding the visual floppy display

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ethump
Posts: 3
Joined: Wed Jun 29, 2022 3:10 pm

Understanding the visual floppy display

Post by ethump »

Hi all,

Kinda new to this so please bear with etc.

I'm trying to understand what the visual floppy display is showing me.

In the track view, the display gives me multiple views of the same track. What's going on there?

I see lines horizontally but at different uS. What's that all about?

If I switch to stream view, the 'fuzziness' of the lines increases as we go up the uS scale. What am I seeing there?

Why do some disks have lines at two uS levels, and others at three levels?

I'm not sure what the right question to ask is... I guess I'm just looking for guidance interpreting what the display is showing me when it comes to visualising what is on the disk...

Sorry if that's a bit vague - I guess my lines of questioning will finesse if I get answers that help my understanding of the above!

cheers!
e

Jeff
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Re: Understanding the visual floppy display

Post by Jeff »

Thanks for asking, i need to document all of this anyway. So i will do a multi posts response, slowly completed when i have some minutes of free time :wink:

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
(Post 01)

The stream view shows you the distribution of the pulses timing send by the drive during the read process. This is comparable to the FFT views used on audio and analog signals.

Low level Floppy disk data encoding

The data recorded on the magnetic support is done by changing the magnetic polarization with different time intervals.
At reading the head coil generate a voltage peak at each polarization change (also called "flux reversal" / "flux transition").
This voltage is then amplified and "reshaped" by the drive and sent to the machine floppy controller on one wire.
magnetic.png
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There are different data encoding schemes used. The most common ones are FM (Frequency Modulation) and MFM (Modified Frequency Modulation).

FM (Frequency Modulation) scheme (used on "Single density / SD" disks) :
FM.png
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For each data bit there are 2 pulses : One clock pulse and one data pulse. The data pulse is only present if you encode an '1'. '0' is encoded by a space.
You can see that there are 2 different pulses spacing with this encoding scheme.
With a 125Kbit/s data rate you will have 8uS and 4uS pulses intervals.

MFM (Modified Frequency Modulation) scheme (used on DD "Double density", HD, ED disks ) :
MFM Example.png
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With this encoding scheme the clock pulses and data pulses are multiplexed. This allows to double the data density.
As you can see there are 3 different pulses intervals. For a DD disks you will have 4uS, 6uS and 8uS pulses intervals. With an HD disk this is 2uS, 3uS and 4uS...

Others encoding scheme :
There are several others encoding scheme used by some specific machines : Apple I & II GCR (Group Code Recording), Commodore 64 / Victor 9000 GCR, Arburg...

To get more explanations about the GCR : https://en.wikipedia.org/wiki/Group_coded_recording


→ To sum up all recording schemes use pulses spaced with different duration to encode the data.

On top of this you need to define a sector format to be able to synchronize the floppy controller when reading and to get back the recorded user data.
There are many sector formats but the most commonly used is the ISO/IBM format.
track3.png
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track4.jpg
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Right now i will not go into the detail of each existing sector formats but they are generally composed by an header to identify the sector (note : some exceptions with the hard-sectored disks) and a data part. The sector integrity can be checked with a checksum or CRC.

Jeff
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Re: Understanding the visual floppy display

Post by Jeff »

(Post 02)

Now back to the HxC floppy emulator stream viewer and track viewer.

Both stream and track viewers show you the time interval between pulses on the track. The following example show you a standard MFM track. You can see the 4uS,6uS and 8uS pulses.
viewer_mfm.jpg
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About the pulses here is a short reminder if you don't have read the previous post :
When reading a disk the drive is just sending these kind of pulses to the machine on one wire (the "Read" wire). The different time intervals between pulses is used to encode the data on the disk.
drive_pulses.png
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Please note that the track and stream viewer can show you the decoded data if you zoom enough one sector :
mfm_zoom.jpg
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And in track view mode the way the pulses are drawn is changed when you increase the zoom level :
mfm_zoom2.jpg
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Jeff
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Re: Understanding the visual floppy display

Post by Jeff »

One important point about the difference between the "Track viewer" and the "Stream viewer" in the HxC Floppy Emulator software :

The "Stream view" directly shows you the pulses as they are in the stream file. It shows you all the dumped revolutions. You can see all the noises captured during the dump in this mode. This view mode is only available with stream files. (stream files : hxcstream from Pauline, *.raw files from Kryoflux, *.scp files from SuperCard Pro, ....).
stream_view.jpg
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The "Track view" shows you the tracks after the stream data analysis done by the HxC Floppy Emulator software : There is only one revolution per track reconstructed by the analyzer from the stream, weakbits/flakey bits are located on the track. The pulses time intervals and jitter noises are filtered and pulses are normalized into cells. This view mode is available with all file image supported by the HxC Floppy Emulator software.
track_view.jpg
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These views allow to check the dump quality and the state of the disk. You can see the signal jitter, the surface defects, the write splices, the disk rotation stability and the variable bitrate and weakbits protections.

ethump
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Joined: Wed Jun 29, 2022 3:10 pm

Re: Understanding the visual floppy display

Post by ethump »

Jeff, thankyou so much. This is exactly what I was looking for! It'll take me a little time to inwardly digest this! thanks again!

Jeff
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Re: Understanding the visual floppy display

Post by Jeff »

ethump wrote:
Tue Jul 05, 2022 12:23 am
Jeff, thankyou so much. This is exactly what I was looking for! It'll take me a little time to inwardly digest this! thanks again!
I still have to post some examples :wink:

ethump
Posts: 3
Joined: Wed Jun 29, 2022 3:10 pm

Re: Understanding the visual floppy display

Post by ethump »

Look forward to it. Would be good to see the context.

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