helical scan

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helical scan A method of using magnetic tape, derived from video recording, in which the tape is wrapped in a helical path around a rotating drum so that one or more heads embedded in the drum record diagonal tracks on the tape. The tape is moved slowly so that a separate track is recorded at each pass of a head.

Helical scan allows data to be recorded at a very high density and hence at very low storage cost. When first introduced (using standard video cartridges) the error rates were too high for general use but powerful error-correcting codes were later introduced, and the method is suitable for backing up hard disks and for short-term archiving (long-term stability has yet to be proved). The digital audio tape (DAT) cartridge is popular for this purpose; larger cartridges with capacities of tens of gigabytes are also in use.