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I mostly used 3½″ floppies back in the day but I also used 5¼″ floppies to some extent on Apple II's and TRS-80's. I was never exposed to 3″ floppy disks as used mainly on Amstrad microcomputers and their Spectrum +3.

So I'm familiar with the concepts of single-sided vs double-sided floppies, and single-density vs double-density floppies.

But in my recent explorations of the Spectrum +3 and its floppy disk formats and their image file formats, I've come across various mentions of "single-track" vs "double-track" and their abbreviations "ST" and "DT". But googling for more information on these terms, I haven't been able to find anything.

Is this double-track thing something specific to the Amstrad 3″ floppy drives? And what is it and how does it work? Was it a dead-end technology? Did it require different disks? Did it double the disk capacity?

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  • 1
    Another method used for increasing disk capacity was by having variable speed drives. Commodore and Sirius used this technique on their 5.25 inch drives.
    – cup
    May 3, 2020 at 11:00
  • 1
    @cup Commodore actually varied the data rate, not the physical speed of the drive.
    – TeaRex
    May 4, 2020 at 9:19
  • I remember the sound changing but I can't remember whether it was Commodore or Sirius.
    – cup
    May 4, 2020 at 9:48

1 Answer 1

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It’s just nomenclature for 40-track (‘single’) versus 80-track (‘double’) drives.

It’s not 3”-specific, e.g. here is a reference using the same terminology for 5.25” drives; 3” drives don’t do anything differently from 3.5” or 5.25” drives, and use regular Shugart signalling.

The original PCWs, the CPC and the Spectrum all used 40-track drives; later PCWs used 80-track drives.

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    The answer is not complete. Double track meant a 80-track drive had to be instructed to advance the head with 2 pulses so that it can read and write 40 tracks on a floppy. I had a controller in the Apple II, a german product, that allowed to use more modern floppies (TEAC or NEC) with the Apple II. It could read and write on 35 track DOS 3.3 (Apple didn't use the 5 most inner tracks) but only if it was patched to double the head seek commands.If you didn't do that, disk formatted on an 80 track drive would only use half of the disk. May 5, 2020 at 6:20
  • Re @PatrickSchlüter 's comment: this could be implemented in software (OS sends twice as many step instructions), or hardware. The hardware approach could be built into the drive, or as a separate circuit that intercepts a step pulse, and sends two pulses to the drive instead. Either hardware option could be controlled with a physical switch near the drive.
    – Kaz
    Feb 8 at 13:55
  • @PatrickSchlüter: An unmodified Disk II is capable of selecting more than 140 discrete positions, but normal software would skip half of them entirely, and only stop on half of the remainder (so a move from track 3 to track 5 would be handled by requesting track 3.5, waiting for the head to move a little, then requesting track 4.0, waiting a little, then 4.5, wait a little, then track 5. I wonder why one would design an 80-track drive with 280 accessible positions, rather than continuing to use the same head movement signals as the Disk II?
    – supercat
    Feb 8 at 16:59

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