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Recently I've also looked inside microdrive drives of Sinclair QL and inside microdrive cartridges for it.

The tape looked like a compact cassette tape cut longitudinally to a half width (which seems to be true -- microdrive tape has width of 1.9mm, while compact cassette tape is 3.85mm wide). The head also looks like a common head in compact cassette players or recorders.

The schematics reveals that the single microdrive head has also erase coil, while the compact cassette recorders usually use separate erase head.

The question, therefore: Were the compact cassette technologies really used in microdrives, modified or not? If yes, what modifications did Sinclair implement to it?

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    One big difference is microdrives are a continuous loop, which cassettes have an end and need to be rewound. But I don't know much more than that
    – Omar and Lorraine
    Jul 17, 2020 at 12:28
  • You might find much help at blog.tynemouthsoftware.co.uk/2016/10/…
    – Robbie Goodwin
    Jul 17, 2020 at 23:33
  • @OmarL: One popular and more than a few unsuccessful formats also used continuous loops. 8-track would be the one people are most likely to be familiar with. But 8-track is not compact cassette and was not popular in the UK where Sinclair was based.
    – hippietrail
    Jul 21, 2020 at 3:18

2 Answers 2

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The tape was actually the same quality as used for VCR tape - according to the QL Service manual "high-quality video tape" was used.

Other than the rotating heads in VCRs, however, Sinclair Microdrives use standard, two-track heads, apparently from some dictaphones.

It's unlikely they deeply looked into compact cassette technology, given all these differences, like Continuous loop tape, which first showed up in the Exatron Stringy Floppy and a tape using different coating and tape width.

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    Continuous loop was used way before the Exaton.
    – Raffzahn
    Jul 17, 2020 at 13:45
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    Apart from continous loop, it seems that VCR-type tapes give more strain-resistance and probably better operation with faster recordings and frequencies, while everything else remain compact-cassette graded.
    – lvd
    Jul 17, 2020 at 14:08
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    Worth looking at broadcast 'cart' machines as well, which I suspect inspired the continuous loop of tape and which go back at least to the 1960's.
    – Dan Mills
    Jul 17, 2020 at 21:46
  • My friend's dad had an Exatron stringy floppy on a TRS-80 a couple of years before the Speccy came out from memory. But I'm sure it didn't turn up there first. 8-tracks used continuous loops, and if you watch the YouTube channel Techmoan, you'd see that quite a few unknown/unsuccessful formats also used it. I'm sure 8-track and some of the others at least predate the Exatron.
    – hippietrail
    Jul 21, 2020 at 3:21
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(No definitive answer, just some thoughts)

Tape size isn't a major indicator as custom size manufacturing isn't a big deal. Already the Exaton Stringy Floppy of 1978, which can be seen as predecessor, did use a 1.6 mm tape. Similar BSR's wafer drive, used in the Rotronics Wafadrive or the Quick Data Drive for Commodore computers (*1), which used, IIRC, as a 1.8 mm tape

I wouldn't wonder if Sinclair simply had ordered it's own tape type, as the cassettes had to be manufactured special to type anyway, but used an existing head. So I guess the best lead would be to follow the head used to see what it was originally intended for.

Such integrated heads (erase and write) were common for voice recorders were the lower quality could be tolerated. These devices often used mini/micro cassettes (standard tape width). An attempt to use a smaller tape was the Dictaphone Picocassette which came about the same time as Sinclair's microdrives and uses, IIRC, a 1.9 mm wide tape. Sony tried it again (way later, ca. 1990) with the NTC using a 2.5 mm and digital recording.

*1 - A quite advanced beast as it connected to the cassette port and booted its Kernal extension like loading from cassette. Seriously thoughtful made, worth to spend some time to understand its details.

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    Existing heads from compact cassettes use exactly half width of the 3.85mm tape, therefore if using only single direction for the tape with that head, the need using exactly half-width (1.9mm) tape results.
    – lvd
    Jul 17, 2020 at 14:05
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    @lvd true. that's why I suggest using the heads as lead for further investigation.
    – Raffzahn
    Jul 17, 2020 at 14:14

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