text: In the middle of 1960s, DuPont created and patented an industrial process for making fine ferromagnetic particles of chromium dioxide (CrO). The first CrO tapes for data and video appeared in 1968. In 1970, BASF, who would become the main proponent of CrO, launched its chrome cassette production; in the same year Advent introduced the first cassette deck with chrome capability and Dolby noise reduction. The combination of low noise CrO tape with companding noise reduction brought a revolutionary improvement to compact-cassette sound reproduction, almost reaching the high fidelity level. However, CrO tape required redesign of the bias and replay equalization circuitry. This problem was resolved during the 1970s, but three unsolved issues remained: the cost of making CrO powder, the cost of royalties charged by DuPont, and the pollution effects of hexavalent chromium waste.
  The reference CrO tape, approved by the IEC in 1981, is characterized by coercivity of  (high bias) and remanence of . Retail CrO cassettes had coercivity in the range from 400 to . Owing to the very clean, uniform shape of the particles, chrome tapes easily attain an almost perfect squareness ratio of 0.90. True chromes, not modified by the addition of ferric additives or coatings, have very low and euphonic hiss (bias noise), and very low modulation noise at high frequencies. Double-layer CrO cassettes have the lowest absolute noise among all the audio formulations; these cassettes generate less noise at  than a ferric tape at . Their sensitivity is usually also very high, but MOL is low, on par with basic Type I tapes. CrO tape does not tolerate overload very well: the onset of distortion is sharp and dissonant, so recording levels should be set conservatively, well below MOL. At low frequencies, the MOL of CrO tapes rolls off faster than in ferric or metal tapes, hence the reputation of bass shyness. CrO cassettes are best fit for recording dynamic music with rich harmonic content and relatively low bass levels; their dynamic range is a good fit for recording from uncompressed digital sources and for music with extended quiet passages. Good ferric tapes may have the same or higher treble SOL, but CrO tapes still sound subjectively better owing to lower hiss and modulation noise.