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, 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 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. 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.

Types

There are several types of helical scan. These include: Many helical scan cassette formats such as VHS and Betacam use a head drum with heads that use azimuth recording, in which the heads in the head drum have a gap that is tilted at an angle, and opposing heads have their gaps tilted so as to oppose each other. This eliminates the need for guard bands between the helical tracks allowing for a higher density of information on the tape.

History

Earl Edgar Masterson from RCA patented the first helical scan method in 1950. German engineer Eduard Schüller developed a helical scan method of recording in 1953 while working at AEG. With the advent of television broadcasting in Japan in the early 1950s, they saw the need for magnetic television signal recording. Dr. Kenichi Sawazaki developed a prototype helical scan recorder in 1954. Helical scan machines were demonstrated by Toshiba in 1959 and since they recorded one field of video per track, they were the first to allow video to be paused and played back at speeds other than real time. Helical scan type B and type C videotape began to be used in 1976.

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