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Definition: helical scan

A tape recording method that uses a spinning read/write head and diagonal tracks. Although it uses a rather complex transport mechanism, it is very gentle on the tape. After the cassette is inserted into the drive, the tape is pulled out and wrapped around the read/write head. While the head rotates as much as 30 meters per second, the tape travels as little as 1 inch per second (ips), compared to linear technologies where the tape travels at more than 100 ips.

Helical scan was invented by Ampex in 1956. It was the only method that provided fast-enough transfer rate and sufficient storage capacity to record video on tape so that TV programs could be recorded. Using two-inch tape and running at 15 ips, the going rate for tape recorders of the time, the rotating head created an effective rate of 1500 ips. The helical scan method is used in many different tape technologies, including VHS videotape, DV/MiniDV (camcorders), 4mm DAT, Exabyte's 8mm and Mammoth lines, Sony's AIT and StorageTek's Redwood.

Helical Scan
The helical scan method uses a rotating head and diagonal tracks, which allows a slow-traveling tape to provide a very fast transfer rate. The tape is pulled out of the cartridge and wrapped around the read/write head.

Helical Scan Formats
As you can see from this illustration, there are numerous helical scan formats used for digital storage. The predecessors to VHS tape (top) were the reason for helical scan in the first place. Although mostly used for analog recording, there have been digital applications of VHS tape as well.

The Real Thing
Notice the slant on this helical scan head from a VHS video recorder. This precise angle of the head is used to record and play back all helical scan formats.