Helical Scan - Slant Azimuth Recording

Slant Azimuth Recording

Every videotape system attempts to pack as much video as possible onto a given-sized tape, but information from one recording stripe (pass of the video head) must not interfere with information on adjacent stripes. One method to provide isolation between the stripes is the use of guard bands (unrecorded areas between the stripes), but this wastes valuable tape space. All the early reel-to-reel machines and the first cassette formats, the Philips VCR and the Sony U-matic used this system.

Later helical scanning recorders instead usually use a method called slant azimuth recording, also called Symmetric Phase Recording. The head drum usually contains two heads with the magnetic gap of one head slanted slightly leftwards and the magnetic gap of the other head slanted slightly rightwards. (The slant of a magnetic head is referred to as its azimuth adjustment). Because of the alternating slants, each head will not strongly read the signal recorded by the other head and the stripes can be recorded immediately next to each other, alternating between left slant on one television field and right slant on the next television field. (In practice, it's not uncommon for the recorded stripes to overlap somewhat). Later machines including the JVC VHS and the Sony Betamax used slant azimuth recording as well as all later machines and their digital derivatives.

Using slant azimuth recording, the need for guard bands is completely eliminated, allowing more recording to be placed on a given length of tape.