driving magnetic heads ( floppy, auudio etc)...

Nagging hardware related question? Post here!
Post Reply
Nasta
Gold Card
Posts: 443
Joined: Sun Feb 12, 2012 2:02 am
Location: Zapresic, Croatia

Re: driving magnetic heads ( floppy, auudio etc)...

Post by Nasta »

If one would to attempt to recreate media of the time to see where the theoretical maximum is and what part of that could be actually used: WHere would find info on driving floppy drive head ?
I looked into this 20+ years ago and even then it was difficult to get any good data.
How about realistic characteristics of audio head ?
Could audio head be used without separate erasing head, by erasing the track by read-write head ( with an appropriate drive )
Analog audio recording is not possible without an erase head. Both the erase head and the rec/play head are actually driven by AC, in the case of recording, AM modulated. This is how the initial magnetisation error of the ferromagnetic material is defeated, and the amount of default AC is known as the 'bias'. In order for this to work, the erase head has to produce an AC field that diminishes as the tape goes past it resulting in a demagnetized tape.
The situation is completely different for a digital recording, where an erase head is not needed. There is some data available on how DCC (long defunct Philips digital audio tape format close to compact cassette) works, the head it uses squeezes multiple track within one standard audio track and does not use the erase head to record.
Looking at the net resources for floppy head assembly, it looks like it has 6 signals and 3 heads, central one for R/W and two side magnets for erasing wider track.
Or am I missing something ?
If one would go for using central head for track erase, would that increase possible number of tracks ?
How about using both heads at the same time ?
The outer tracks are used to produce an erased 'guard' track between the data tracks to prevent so called magnetic domain migration. Both disc side heads could be used at the same time but this was never intended by the standard, which started with single sided media only.
There are a number of improvements made since the standard was created, notably increasing the density by using better mechanisms and heads, which made it possible to have two sides and cram more on a smaller diameter disc. It also solved the resonant wow and flutter problems the large diameter discs had on the outer tracks, due to the warping of the disc as the head was dragging along the surface.
The second element was a change in the modulation used, from FM to MFM, producing a doubling in recorded bit density. Strangely enough, RLL encoding that was popular on MFM hard discs, never made it to floppy. Instead, 'perpendicular' recording was used, which basically magnetizes the track perpendicular to the track, using some differential trickery using a slightly different head. This produces the jump from HD to ED.
Another thing which did not get used on floppies is zoned recording, i.e. constant linear density recording. This requires a different number of sectors on each track (or track group) and a variable recording or rotating speed. It was used on the original Mac... but not after that. On a 3.5" drive it increases the amount of data by 50%.
However, it's the floptical and similar technology that ended that because the leap of capacity was almost two orders of magnitude.
Speaking of disc recording techniques - I always wondered why MD (minidisc) never got a true computer version...
Last edited by Nasta on Thu Jul 29, 2021 6:22 pm, edited 1 time in total.


User avatar
Pr0f
QL Wafer Drive
Posts: 1298
Joined: Thu Oct 12, 2017 9:54 am

Re: driving magnetic heads ( floppy, auudio etc)...

Post by Pr0f »

VHS cassettes did make an appearance in the backup world - for large capacity long(ish) term archival - where you are not overly worried about recovering a few files from the backup, but would more likely bring the whole backup image back if it was ever needed. Capacities for such medium are very large - easily in the GB regions - which when compared to the 100's of MB for contemporary tape technologies like QIC (quarter in cartridge) was very large.

https://en.wikipedia.org/wiki/ArVid

The problem with a lot of these backup media is the degradation of the recording surface medium, the stretching of the tape itself and self demagnetizing as the tape is stored in a reel so their is proximity to a different part of the tape. Audio cassettes that are played at regular intervals actually retain their recordings better than those that are recorded and never used.

Early estimates were for about 7 years retention, with later and better tapes achieving 20 years. CD's are still seen as the best long term archival for removable media - but RW devices also suffer from loss of data through degradation of the optically excited media that's used to record data. Write only CD masters can easily achieve 100 years+ whereas record-able technology only guarantees for 20-50 years tops.

I suspect magnetic degradation is why people with 20 year old floppies are having problems now with data loss and unreadable disks.


User avatar
bwinkel67
QL Wafer Drive
Posts: 1187
Joined: Thu Oct 03, 2019 2:09 am

Re: driving magnetic heads ( floppy, auudio etc)...

Post by bwinkel67 »

I found this resource from the Council on Library & Information Resources (CLIR) useful for magnetic tape information:

https://www.clir.org/pubs/reports/pub54/index/

Especially this subsection on what can go wrong:

https://www.clir.org/pubs/reports/pub54/2what_wrong/


Post Reply