Most magnetic tapes have a mylar or polyester base with a thin coat of magnetic material, usually gamma ferric oxide or chromium dioxide, but newer tapes are double-layered which combine the good low-frequency response of ferric oxide and good high-frequency response and low noise of chromium dioxide; the oxide is cured onto the base and the tape is calendered. The metal particles have a random orientation in unmagnetized tape, but they are aligned into definite magnetic patterns by the magnetic field produced by the recording head. If all other factors are the same, the wider the track, the greater the S/N ratio: doubling the track width improves the S/N ratio by 3dB. Professional analog tape recorders are available with tape widths up to 2″ and up to 24 tracks. There is a thin guardband of uncoated base tape between the tracks to yield improved channel separation, reducing crosstalk, and providing some tolerance for differences in head/track alignment among machines.
See Barkhausen effect, back coating, MOL, bias, domain, extinction frequency, scrape-flutter filter.
Magnetic tape has historically come in a number of widths and formats (all denominated in inches):
