Analog Videotape Recording Technology
Recorders
Videotape recording enables the practical recording and immediate or future playback of a high-quality video image. Video recording imprints magnetic signal patterns onto a specially prepared tape. Two of the key aspects of this process are the tape itself and the recording head.
Videotape
The foundation of videotape is a strong plastic ribbon. On the back of the tape is a slick surface that helps the tape move through a mechanical transport smoothly. On the front side of the tape are metal oxides mixed with a binding compound that secures the oxides to the tape. If you pass a magnet close to these oxides, they will be left with their own, much weaker, magnetic field. The stronger the magnet that arranges these oxide particles, the stronger the induced magnetic field.
Recording Heads
In order to record the desired information, a special electromagnet called a recording head has to be used. For video, the head is very small, made of very thin metal (about the thickness of a fingernail). The head is hollow, like a tube. Thin wire coiled around the other side of the head connects the head to the rest of the recorder.
In the second figure at the right, the head has been enlarged many many times. Although the figure shows the head by itself, in reality it would be mounted in a small nonmetallic enclosure. The head and enclosure are often shaped something like a piece of bread. The curved surface is the one through which electrical information is exchanged with tape. As the changing analog voltage (the composite video signal) from the camera electronics is processed and flows through the head, it causes corresponding changes in the electromagnetic field that the video recording head produces. This leaves varied magnetic fields in the oxides on the tape. This is the basic recording process.
The playback process is just the reverse. The magnetically encoded tape is passed across a video head that has no signal flowing through it. The magnetic field on the tape induces a signal into the head corresponding to the varied magnetic fields on the tape. Thus, you reproduce the same analog signal from the tape that was induced onto the tape from the camera source.
1. Cross-section of magnetic tape.
2. Recording on tape.
3. Videotape playback.