Helical scan

Helical scan
Helical recording method. As the tape moves horizontally, the drum rotates and the heads in the drum create or write diagonal tracks with signals.
Media typemagnetic tape
Usagerecording high-frequency signals

Helical scan is a method of recording high-frequency signals on magnetic tape, used in open-reel video tape recorders, video cassette recorders, digital audio tape recorders, and some computer tape drives.

With this technique, magnetic tape heads (or head chips) are placed on a rotating head drum,[1] which moves the chips at high speed by due to its high angular velocity. The speed of the head chips must be higher than the linear speed of the tape. The tape is wrapped tightly around the drum. The drum[2] and/or the tape is tilted at an angle that allows the head chips to read the tape diagonally. The linear speed of the tape is slower than the speed of the head chips, allowing high frequency signals to be read or recorded, such as video. As the tape moves linearly or length-wise, the head chips move across the width of the tape in a diagonal path. Due to geometry, this allows for high head chip speeds, known as writing speeds, to be achieved in spite of the low linear speed of the tape. The high writing speed allows for high frequency signals to be recorded.[3][4][5] As each head chip enters into contact with the tape, it creates or reads long and narrow areas with information recorded magnetically known as tracks. In Helical scan, these tracks are positioned diagonally, relative to the length of the tape. The diagonal tracks read or written using this method are known as helical tracks.[2]

The head drum of a Hi-Fi NTSC VHS VCR; three of the six heads face the reader. The helical path of the tape around the drum can clearly be seen.
The same head drum with the rotating portion elevated for clarity
The rotating portion of the head drum showing the rotary transformer and three of the six tape heads used in this particular VCR
  1. ^ "Rotary magnetic head drum with fluid bearing and with head chips mounted together in parallel".
  2. ^ a b Tozer, E. P. J. (November 12, 2012). Broadcast Engineer's Reference Book. CRC Press. ISBN 9781136024184 – via Google Books.
  3. ^ Capelo, Gregory; Brenner, Robert C. (June 26, 1998). VCR Troubleshooting and Repair. Newnes. ISBN 9780750699402 – via Google Books.
  4. ^ Daniel, Eric D.; Mee, C. Denis; Clark, Mark H. (August 31, 1998). Magnetic Recording: The First 100 Years. John Wiley & Sons. ISBN 9780780347090 – via Google Books.
  5. ^ "New Scientist". Reed Business Information. December 1, 1983 – via Google Books.[permanent dead link]