Helical scan

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 heads at high speed by due to its high angular velocity. The speed of the heads 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 heads to travel across the tape diagonally. The linear speed of the tape is slower than the speed of the heads, allowing high frequency signals to be read or recorded, such as video. As the tape moves linearly or length-wise, the heads move across the width of the tape in a diagonal path. Due to geometry, this allows for high head 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 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.