When helical scanning , helical scan or helical scan , a is from Eduard Schüller invented method for recording signals in a video recorder or tape drive referred to. In contrast to linear recording, the track is recorded at an angle to the tape.
The magnetic tape wraps around the head drum , which is arranged at an angle to the tape and rotates in the same direction as the tape runs. The aim is a higher relative speed between the magnetic heads and the tape: this allows higher frequencies to be recorded. However, this also increases the mechanical stress on the tape and the tracks are much narrower compared to the width of the tape.The exact nominal data of track width, track angle, track spacing (lawn) are defined in the underlying standard. For example, VHS works without a lawn, i.e. with overlapping tracks; Crosstalk between adjacent tracks is different azimuth angle prevents the original two video heads, edit each other alternately adjacent tracks ( English Slant azimuth recording ).
The head drum is the most important and at the same time the most complex part of helical track recording. It consists of magnetic heads, a synchronous motor and a rotary transformer for data transmission. In the case of a video recorder, the tape wraps around the head drum a little more than 180 °, so that two heads can deliver a continuous signal. The tape drive and the speed and angular position of the drum must be adapted to each other by the control of the device in order to hit the track (tracking) and to enable seamless playback at the correct speed.
- Example VHS
- v k : peripheral speed of the video heads
- v a : scanning speed of the tape
- d k : diameter of the head drum. With VHS this is 62 mm.
- n k : The speed of the video head wheel is at 25 full frames per second (PAL standard) 1500 revolutions per minute.
- v B : With VHS the tape speed is 0.0234 m / s
With VHS the relative speed is about 4.85 m / s. If one were to use the longitudinal track recording (scanning speed = tape speed) instead of the helical tracking method, one would use 4.85 m of tape per second when recording high frequencies, up to 3 MHz for VHS. With the helical track method this is only 0.0234 m / s for VHS.
The process made possible video recorders with compact magnetic tape cassettes. The so-called HiFi sound track, which was subsequently introduced in various video systems to improve the quality of the conventionally longitudinally recorded sound signal, is recorded at an angle together with the image information after frequency modulation .
According to the same principle, digital data can be recorded in high density. While classic streamers record the data on parallel longitudinal tracks, devices with helical track recording sometimes use formats from devices from the end consumer sector ( Digital Data Storage (DDS): Digital Audio Tape (DAT), Exabyte Corporation (Exabyte): Video-8 ).
- Book of tables of radio and television technology / radio electronics - Frankfurter Fachverlag
- Electrical engineering Specialized training in communication electronics 2 Radio / television / radio technology - Westermann