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Analog Magnetic Tape
Basic Principles of Magnetic Tape Recording
Magnetic tape was first used for recording sound by Pfleumer in Germany in 1940. Previously a metal strip or wire had been used for magnetic recording.
The basic principles of tape recording are relatively simple. The tape is a piece of thin plastic that has been coated with a substance that can be easily magnetised. A pinch roller holds the tape tight against a spinning capstan which drags it across a set of three heads.
The tape itself was originally made of acetate as it was strong, not too prone to stretching and the coating adhered well to it. Moisture and age can make it brittle or wrinkled, however, so acetate was discarded in favour of newer plastics such as PVC and polyester. These are more resilient to age and moisture but tend to stretch more easily, resulting in pitch changes.
The magnetic substance started as a coating of small, spherical particles of ferric oxide but these tapes had to run at quite some speed to get a reasonable frequency response (about 30 ins/sec). The coating was later modified so that the oxide particles were in a long needle-like form - this produced tapes with better magnetic properties. More recently chromium dioxide has been used to further improve the magnetic properties (coercivity and remanence) of the tapes.
There are three heads used in magnetic tape recording (erase, record and play), all of which are of similar design, consisting of a C-shaped piece of metal with the gap of the C near the tape. A coil of wire is used to either detect or produce magnetic signals at the gap.
If the current is sufficient a magnetic spot will result on the tape and its size will be proportional to the current. A moving tape and a variable current creates a track of magnetic data that can be read again by a play head.
The third head is an erase head which serves to remove magnetic data or noise on the tape prior to it reaching the record head. In fact it works just like the record head but using a super high frequency that is not recorded but obliterates exisiting data.
The signal that is applied to the record heads is equalized in a similar manner to when recording vinyl but this can usually be adjusted to suit the speed and type of tape being used.
Magnetic Spots and Tracks
The distance between magnetic spots on a tape track represents the wavelength of the signal, which depends on the signal frequency and the speed of the tape. Obviously there is a limit to how small the distance can be between spots and in practical terms this means that the speed of the tape determines the highest frequency that can be recorded.
A stereo tape will have four magnetic tracks on it, split into two stereo pairs running in opposite directions. One pair sounds when the tape is played one way and the other when it is reversed. The older format 8 Track tapes used wider tape to accommodate more tracks and multitrack recorders can use all four tracks of a standard stereo tape, but only in one direction.
Issues Affecting Magnetic Tape
This is the tendency for the tape to develop a concave curvature across its width with the coating inside.
Any deviation from straightness along the edges of the tape. These can introduce variations in output speed resulting in flutter.
Fraying due to the tape being cut poorly to width in the manufacturing process can result in tears or the loss of pieces of coating.
The density of the particles should be as high and as even as possible and the surface should be polished to reduce wear and friction. The thickness of the coating should be consistent and the its adhesion to the base should be reliable.
Tape will tend to stick to itself on the spool either because of electrostatic charges or the gumminess of the tape material itself. This can cause snatching and wow as well as vibration and squealing. Care should be taken that the correct non-sticky splicing tape is used for jointing.
Incorrectly spooled tapes, excessive moisture or damaged equipment can all cause stretching. The breaking point of tape is about 4lb but stretching can occur at half this value. During normal play the tension should not exceed 6oz.
This is where ghosting and corruption occurs as a result of the tape sitting still on the spool for too long. Weak copies of the magnetic data become imprinted on adjacent layers. Moisture, magnetic fields, high temperature and tight-spooling can all increase print through.
The possible noise levels for magnetic tape are not generally realized in pratice due to variation in particle size and distribution, accidental DC magnetization of head or tape or modulator noise that is introduced to the signal at the record stage by oscillators or other circuitry.