Microdrive Unit Design

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tcat
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Re: Microdrive Unit Design

Postby tcat » Mon Oct 30, 2017 11:14 am

Hi,

I agree with `Byteman', the measuring current using ordinary Multimeter will add up some remanent magnetism to the head corpus material. It is questionable if we changed the polarity during measurement may correct it in one way, and worsen it in the other.

I have consulted Louis Seidelmann (fellow country QL'er) about our discussion, also studied a student's thesis on magnetic recording principles. As we have MD technology here, using direct currents for read/erase/write operations this will cause the head be magnetised just by regular use. Measuring current used in numeric MM seems negligible and will not make it any different.

This remanent magnetism may or may not with enough many passes through the already magnetised head's gap decrease the recording's intensity. But this may correspond to the life of media, so adhering to MD usage guidelines, to keep master backups, should be no issue.

Magnetised heads may cause audible noise in playback of Hi-Fi recordings, frequently discussed among audiophils. Audio Hi-Fi systems overcome this by using alternating currents for erase operations.

Having no exact means to measure remanent values, we can just probe that it is present, with a tiny needle hanged on a length of a very thin thread, the needle is attracted to the head by the magnetism.

Even if we demagnetised the head, (a simple DIY solution could be a coil for transformer pistol iron, see picture), the head will become magnetised again just by ordinary use.

Tomas
demag-coil.JPG
DIY Demag Coil
Last edited by tcat on Tue Oct 31, 2017 11:13 am, edited 1 time in total.


Byteman
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Re: Microdrive Unit Design

Postby Byteman » Mon Oct 30, 2017 12:37 pm

I'm sorry, I cannot write detauled post just now about that, I will describe some things some days later. I'm tightly working with analog tape recorders and will tell some things about it.


tcat
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Re: Microdrive Unit Design

Postby tcat » Mon Oct 30, 2017 1:33 pm

Hi Byteman,

I am not the one in the know here, I am just learning about it, so will be glad to also learn from your own experience.

The reason I suggested measuring the head is to exclude faulty erase coil, very common MD problem I have learnt. Here 3 of 4 MDs faulty! So one does not have to mount it into QL to find that out. Another common fault could be ULA.
Most of the mechanical aspects can be rectified if not correct from prime production.

You may see my quest here, thanks for your help.
viewtopic.php?f=2&t=1381&start=50
viewtopic.php?f=2&t=1511&start=30

P.S. I am now at magnetic hysteresis, and linear pre-magnetic correction, my next post to come.
P.P.S. I should possibly apologise for stealing this thread from RWAP :-)

Tomas


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Cristian
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Re: Microdrive Unit Design

Postby Cristian » Mon Oct 30, 2017 5:57 pm

Byteman wrote:It is more correct to measure not only resistance, but inductance too.


Code: Select all

MAGNETIC HEAD COILS INDUCTANCE (mH)
=========================================================

       | STRAIGHT-EDGED |   STANDARD 1   |  STANDARD 2   |
_______|________________|________________|_______________|
       |  MDV1 :  MDV2  |  MDV1  : MDV2  |  MDV1 : MDV2  |
       |-------:--------|--------:-------|-------:-------|
       |       :        |        :       |       :       |
ERASE  | 12.2  :  12    |  11.5  :  12   | 11.3  : 12.4  |
       |       :        |        :       |       :       |
       |       :        |        :       |       :       |
UPPER  | 0.8   :  0.8   |  0.9   :   1   |   1   :   1   |
       |       :        |        :       |       :       |
       |       :        |        :       |       :       |
LOWER  | 0.76  :  0.8   |  0.85  :  0.94 |   1   :   1   |
_______|_______:________|________:_______|_______:_______|


tcat
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Re: Microdrive Unit Design

Postby tcat » Tue Oct 31, 2017 10:07 am

Hi,

I am not able to interpret inductance measured, the values seem in spectrum of +/- 0.2 mH (mili henry?), which may be the precision of the inductance-meter used?

My toys here are that of a hobbyist, transformer pistol iron and a simple multimeter (not very precise).

I am now learning about magnetic hysteresis, it seems a welcome quality of magnetic materials, absorbing some inaccuracies we see here.
magnetic-hysteris.png
Magnetic Hysteresis Curve


EDIT
Some pre-magnetisation current is needed during writing. It's purpose is to build up some magnetics in the material in the linear (green) part above. The pre-magnetisation current is typically set to frequency between 60-120Khz not to interfere with the recording signal carrying the actual information. The theory has it, that without premagnetics, it won't be quite possible to read recorded information back.

Now looking in the QL Service Manual, chapter 4 on MD shows some `waveforms', I can see 100Khz frequencies. Can someone explain the process of writing? What frequency carries "1", and what "0", and what frequency is the actual premagnetics?

It would be nice to capture `waveforms' using numeric oscilloscope, and post here, but I have none so far, and I would not know how to use it anyway.

Tomas


tcat
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Re: Microdrive Unit Design

Postby tcat » Wed Nov 01, 2017 12:06 pm

Hi,

I have read some posts from Gert, also looking further at MD `waveforms' in the QL Service Manual.
MD uses direct current for premagnetisation, and FM modulation for data, 50kHz ~ "0", 100kHz ~ "1".
I do not know what value the premag current is, in the picture shown as 30mA.

premagnetics.png
premagnetising tape
premagnetics.png (9.46 KiB) Viewed 69 times


Tomas
Last edited by tcat on Sat Nov 04, 2017 9:37 am, edited 3 times in total.


martyn_hill
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Re: Microdrive Unit Design

Postby martyn_hill » Wed Nov 01, 2017 12:58 pm

Looking at the Iss6 schematic, looks like a 180 ohm resistor in series switched by transistor at 5V, so naively, I'd say 5V/180 = 28mA


tcat
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Re: Microdrive Unit Design

Postby tcat » Wed Nov 01, 2017 2:05 pm

Hi,

Thank you for the encouragement, now comes difficult part for me, that is `Frequency Modulation' (FM), I do not quite understand.
Consider `1 0 0 0 1 1 1 1' byte to be stored to one tape track.

fm-modulation.png
frequency modulation
fm-modulation.png (4.56 KiB) Viewed 228 times

How does ULA amplify and decode FM signal to TTL "0", and "1" we read in MD h/w data register?

EDIT
I borrow this definition from Gert's, he has managed to reenginer MD data line format.

To read a bit I have to wait for an edge, then wait 1 usec to make sure the data level is stable after the edge, sample the level of DATA1, wait 6 usecs to sample the level half a bit further and compare that level with the level sampled before.
If it has changed it is a '1' else it is a zero.


This possibly explains that, except how ULA is synchronised for half square waves not having the clock? See block diagram.
reading-tracks.png
MD ULA Block Diagram


Tomas
Last edited by tcat on Fri Nov 03, 2017 7:43 am, edited 4 times in total.


tcat
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Re: Microdrive Unit Design

Postby tcat » Fri Nov 03, 2017 7:17 am

Hi,

ULA block diagram above shows, in my opinion, only reading data from two tracks, outputting two channels DATA 1-2 (A/B).
But how is data written to tape, we know we need two frequencies 50/100kHz and some direct current?

tape-recording.png
erase / write polarity on tape

Tomas
Last edited by tcat on Sat Nov 04, 2017 9:38 am, edited 1 time in total.


martyn_hill
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Re: Microdrive Unit Design

Postby martyn_hill » Fri Nov 03, 2017 11:12 am

Hi Tomas

As I understand it, writing involves the ULA 'switching' its inputs to outputs and driving the same (stereo) pair of coils with the 50/100KHz signals.

The direct current is entirely independent and flows just through the single erase coil - which has a larger 'gap' - and which is offset physically to effectively de-polarise the tape shortly _before_ that same section of tape reaches the RD/WR gaps. This de-polarisation is effectively placing white-noise on the tape, which the ULA (or the 8302?) can distinguish from the polarised data and is used for the sector GAPs in the MDV timing protocol.

Also, if my understanding is correct, whilst the ULA outputs serial binary when reading, what it actually reads off the tape (during the DATA sections) are peaks in current that represent the transitions between low and high. The actual 'levels' that were recorded during writing become irrelevant - only the timing between these current surges at each transition. Thus, it's not quite pure FM, but more like (but not quite) 'Manchester' encoding - where the relative timings between transitions in a given 'bit-cell' time-window determines the bit-state. In such encoding schemes, the 'CLOCK' is actually encoded within the data stream. I may be wrong here...

I've found a good analogy described in the DIYTK 'FlexiNET' replacement driver for QLAN.



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