I think this used a spinning head, similar to video? The trick would be to use a static head and still record 4 or More digital tracks on a 4mm tape, with enough headroom for robust ECC. I’m pretty sure it could be done at ~kbps with contemporary recording head litho at 8 tracks per mm. That makes 16 tracks per side, 32 for the full 4mm (both sides). It would even be possible to convert existing tapes to digital in one play/pass using a 2 head system, or a read-store-record setup.