How did a PC 5.25 High Density Floppy Differentiate Disks?

[Joey Z]

No. The track pitch is unrelated to the density and the write current. There were 80 tracks DD drives, both as 3.5 (ST, Amiga, PS/2) and 5.25 (not very common). And in theory, you could have HD on 40 tracks, if you wanted to.

no, head/track width is related to the write current. And when I said 'DD' tracks, I meant 40 track drives, and when I said HD, I meant 80 track. I was referring to the PC DD and HD drives which, respectively, were 40 and 80 track drives.

 

Anyway, with a wider track, you still need the same magnetic field density to magnetize HD disk material, but now you have to magnetize more material. Since you have a larger area, and same magnetic flux density, you need more magnetic flux. Since you need more magnetic flux, you need a stronger magnetic field. In order to get that, you need a higher head current.

[Bryan]

I don't know the difference in coercivity, so I don't know how much more head current we're talking about. Obviously there's going to be limits. Sometime I'll stick a scope on there and see how much difference in level we're talking about.

[Joey Z]

I don't know the difference in coercivity, so I don't know how much more head current we're talking about. Obviously there's going to be limits. Sometime I'll stick a scope on there and see how much difference in level we're talking about.

just make sure you make both measurements on the same drive, once in HD mode, and again in DD mode. Then, from there, you can get a ratio of the write current, and a good starting point for the increase you would need on the 1050. That said, I still think you're going to need to increase more than that, since you're also writing a wider track, and longer bits.

 

And I just remembered another issue with HD media, I think. Because of the nature of magnetics, there's something called 'write precompensation' which needs to be done or else you get timing skew depending on the pattern of bits written to the disk. This might be different between DD and HD media.

[ijor]

I don't know the difference in coercivity, so I don't know how much more head current we're talking about. Obviously there's going to be limits. Sometime I'll stick a scope on there and see how much difference in level we're talking about.

 

Coercivity for HD and DD 5.25 disks is 600 and 300 oersteds, respectively. But I'm not sure there is a one to one proportional relation between write current and oersteds. Not sure that double the coercivity requires double the write current. And I'm not sure the current you will measure on a PC drive would be the same required by a 1050. All we really know for sure is that you need higher current.

 

Write precompensation is the same in both densities (125 ns). This is a controller issue anyway.