Helical scan

Helical scan is a method of recording high-frequency signals on magnetic tape. It is 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 are placed on a rotating head drum. This allows the tape heads to move at a high rotational speed or angular velocity. The tape is wrapped tightly around the drum. Either the drum and/or the tape are tilted at an angle to allow the heads in the drum to read the tape diagonally. The drum spins at a high speed relative to the linear motion of the tape. By reading the tape diagonally, the linear speed at which the tape moves through the drum can be much lower than the speed at which the heads read the tape. The high speed allows high frequency signals, such as video, to be recorded practically without the need for high linear or longitudinal tape speeds. The diagonal tracks read or written using this method are known as helical tracks.

There are several types of helical scan. These include:

• Alpha wrap (α), in which the tape is wrapped around the drum in a full, 360 degree fashion.
• Omega wrap (Ω), in which the tape is wrapped almost fully around the drum similar to the Greek letter Omega. Used in Type C videotape recorders. The tape is wrapped 346 degrees around the drum with 270 degrees used for recording. Because of this, the vertical blanking interval of the video signal is lost and to prevent this a secondary head in a "1 1/2 head" configuration must record the interval when the video head is not reading the tape. A full frame or field of video can be recorded in a single revolution of the drum with a single head creating a single diagonal track on the tape.
• C wrap, where the tape is wrapped around the head drum in the shape of a backwards C, used in the Betacam format, uses a wrap of 200 to 300 degrees where 180 to 270 degrees are active or used for recording, similar to the U wrap which is reminiscent of an U laid on its side and is used in the U-matic format. Because the tape is not wrapped around the drum as much as with the omega wrap, two heads creating two diagonal tracks must be used to record a video frame, one field for every track and head.
• M wrap, used in VHS and the D-1 (Sony) and D-2 (video) digital videotape formats, wraps the tape around the head drum in a pattern or in a tape path reminiscent of the letter M, around the left and right side of the head drum, 250 to 300 degrees around it where 180 to 270 degrees are active or used for recording, with two heads if 180 degrees are used.
• Half wrap, used to denominate any type of wrap where the tape covers approximately 180 degrees, or half of the circumference of the drum. To record a full frame of video it requires at least two video heads, each sharing a video field.

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.