Phi-Deck drives were offered "bare", or with electronics. Since the Digital Group used them for digital saturation recording, they used the bare drive, together with their own controller card.
Phi-Deck drives are still available from Phi Technologies, although they are most commonly used for audio recording.
A search reveals that Triple-I and the Economy Company held other patents in this area, some of which may have also been relevant to the Phi-Deck drives:

Here is a product which will be of interest to many BYTE readers. This is an electronically controlled variable speed digital cassette deck which can be adapted for use by the home computer experimenter. It is made by Triple I, a division of the Economy Company, PO Box 25308, Oklahoma City OK 73125. Its price is in the $100 range and it should prove to be an excellent medium for totally automated data storage in personal computer systems.
It seems somewhat ironic that 22 years later 3.5 inch floppy disk drives with a formatted capacity of 1.44 megabytes sell for under $25 retail, less than a quarter of the price of a Phi-Deck. Technology marches on.
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Last updated February 22, 2001
Copyright 1998, 1999, 2001 Eric Smith