Cassette bias & type   

Cassettes are manufactured with several types of magnetic tape. The difference between these types of tape lies mostly in the composition of the magnetic particles that coat the tape allowing storage and retrieval of the signal. Some types of magnetic material allow a higher bias setting on the cassette deck with the advantage of being able to record a hotter signal to tape, thus improving the quality of the result. Most cassette decks have provisions for setting bias, usually a switch between low and high bias.
Standard quality cassette tapes are designed to operate at low bias.  Chrome and Metal tapes operate at higher bias settings and the cassette deck should be adjusted accordingly. 
When setting recording levels on the cassette deck a standard cassette at a low bias setting should not peak higher than 0 VU. Chrome tapes at high bias setting can record a signal +3 db above 0 VU, while metal tapes can peak as high as +6 db resulting in the best signal to noise characteristics.