Video Introduction

Magnetic Tape, Registration and TV-systems


Magnetic tape
Magnetic tape consists of a long, thin strip of material (the base), usually of polyester, coated on one side with a binder layer containing very small metal particles that can be magnetised.

To make a magnetic tape, very small metal particles are mixed with a binder, a lubricant and some other ingredients. This mixture is applied as a very thin coating to the tape – which at that point is still a broad tape- and is then rolled flat under high pressure. When dry, the tape is cut into strips of to the right width and wound on pancakes and later on spools or in cassettes.
An anti-static coating containing carbon powder and a binder may be applied to the back of the tape. Because carbon powder conducts electricity, it removes static charges built up by the friction of the tape against the metal roller guides in the recorder and playback machine.


Registration
For analogue audio the maximum frequency which can be recorded is 20 KHz. For analogue video frequencies up to 14 MHz are used. With linear recording, as used for analogue audio, a video tape would have to run very quickly in order to record all the information required to form images. To record the video signal on tape a helical scan system is used. With helical scan the video signal is recorded as one or two diagonal lines across the tape which moves diagonally along the high speed rotating drum with one or two video heads.

The video signal forces the tiny metal particles to be lined up magnetically (north-south) when the tape moves along the heads. Any change in the video signal is converted into a magnetic charge and the magnetic orientation of the metal particles is fixed on the tape.

When the tape is played back, the fluctuations in the magnetic field cause small electrical charges in the video heads. Via an amplifier these differences in electrical currency are transferred to the viewing equipment to produce an image.

There are also linear magnetic heads mounted in video recorders and playback machines which are used to erase magnetic information before recording and for audio channels and a control track.
The control track is used by playback to synchronise the speed of the rotating drum with the video heads.