ZX Microdrive - Technology

Technology

Microdrives used tiny (44 × 34 × 8 mm including protective cover) cartridges containing a 5-metre (200-inch) endless loop of magnetic tape, 1.9 mm wide, driven at 76 cm/second (30 in/second); thus performing a complete circuit in approximately 8 seconds. The cartridges held a minimum of 85 kB when formatted on a ZX Microdrive (exact capacity depended on the number of "bad" sectors found and the precise speed of the Microdrive motor when formatting). The data retrieval rate was 15 kB/s, i.e., 120 kbit/s. It was possible to "expand" the capacity of a fresh microdrive cartridge by formatting it several times. This caused the tape to stretch slightly, increasing the length of the tape loop so that more sectors can be marked out on it. This procedure was widely documented in the Sinclair community magazines of the 1980s.

Unfortunately, the system acquired a reputation for unreliability. The tapes stretched during use (giving them a short life span) eventually rendering the data stored unreadable. Also the "write protection" was software-based so that a computer crash could erase the data on an entire tape in 8 seconds. The cartridges were relatively expensive (initially sold for £4.95 each, later reduced to £1.99). Similar technology was used in other devices, such as the Rotronics Wafadrive, and was sometimes known as a "stringy floppy".

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