text: In 1973, Sony introduced double-layer ferrichrome tapes having a five-micron ferric base coated with one micron of CrO pigment. The new cassettes were advertised as the best of both worlds — combining the good low-frequency MOL of microferric tapes with good high-frequency performance of chrome tapes. The novelty became part of the IEC standard, codenamed Type III; the Sony CS301 formulation became the IEC reference. However, the idea failed to attract followers. Apart from Sony, only BASF, Scotch and Agfa introduced their own ferrichrome cassette tapes.
  These expensive ferrichrome tapes never gained substantial market share, and after the release of metal tapes they lost their perceived exclusivity. Their place in the market was taken over by superior and less expensive ferricobalt formulations. By 1983, tape deck manufacturers stopped providing an option for recording Type III tapes. Ferrichrome tape remained in the BASF and Sony lineups until 1984 and 1988, respectively.
  The use of ferrichrome tapes was complicated by the conflicting rationale of the playback of these tapes. Officially, they were intended to be played back using  equalisation. The information leaflet that Sony included in each box of ferrichrome cassette tapes recommended that, "If the selector has two positions, NORMAL and CrO, set it to the NORMAL position." (which applies  equalisation). The leaflet notes that the high frequency range will be enhanced and that the tone control should be adjusted to compensate. The same leaflet recommends that if the playback machine offers a Fe-Cr selection, that this should be selected. On Sonys machines, this automatically selects  equalisation. The service manual for the Sony TC-135SD, which was one of the few cassette decks offering a Fe-Cr' position, shows the tape type selector switch paralleling the ferrichrome equalisation selection with that of chrome dioxide (). Neither Sony nor BASF cassette tapes feature the notches on the back surface that automatically select  equalisation on those machines that featured an automatic detection system.