Handbook for Sound Engineers

Magnetic Recording and Playback 1065

damage the respective electronics but will certainly

“peg” any analog meters in the circuit. Turn off the

power to the recorder before using the degausser.

3. Move the degausser slowly and smoothly from
   bottom to top along the gap line of each head,
   moving at a rate of approximately 1 _ in/s
   (3 mm/s). At the top of the head, smoothly withdraw
   the degausser 6 inches (15 cm) and then move
   smoothly to the next item to be degaussed.


4. To be safe, move the degausser at least 3 ft (1 m)
   away from the transport before disconnecting the
   power from the degausser.


5. Multiple degaussing passes on a component do not
   improve the quality of the results. A single slow,
   smooth pass is adequate.

The rapid collapse of the magnetic degaussing field
at turnoff can easily undo all of the benefits of
degaussing if the degausser has not been pulled away
sufficiently. (For this reason, avoid degaussers that have
momentary power switches that might be accidentally
released in the middle of the degaussing routine.)

28.3.9 Tape Components

Modern magnetic tape consists of a powder of very
small magnetic particles, which has been glued to one

surface of a plastic substrate or base film. The backside

of the substrate is coated with a very thin layer of

carbon particles to improve winding characteristics and

to reduce the buildup of static electricity.

28.3.9.1 Base Films

Although several base film materials were used in the

past, including paper and acetate film, virtually all tape

manufactured today uses polyester film (polyethylene

terephthalate) such as Dupont’s Mylar ™. Polyester is

not only extremely strong and tear-resistant, but it is

also relatively stable with respect to changes in temper-

ature and humidity.

Depending on the intended application, the nominal

base film thickness ranges from 1.4 mils (35μm) for

heavy-duty professional tapes down to a scant 0.25 mil

(6.25μm) for a C-120 cassette. To achieve reliable

performance with these very thin films, the film must be

not only very thin but also uniform in thickness from

end to end and from edge to edge.

To enhance the strength of the thin base films used

for cassettes, the polyester is prestretched or Tensilized.

Although Tensilized tapes are more resistant to

stretching than normal tapes, residual stresses that result

from the Tensilizing process can produce physical

distortion of the tape. For thin, narrow tapes these

distortions are satisfactorily flattened out at the record

and playback heads. The thicker, wider tapes used for

professional formats, which would manifest severe

contact problems due to these distortions, are consid-

ered to be strong enough without Tensilizing to provide

adequate performance.

28.3.9.2 Binders

The glue or binder that holds the magnetic particles to

the base film is a necessary evil that makes no active

contribution to the magnetic performance of the tape.

The use of new high-strength binders containing

urethanes has improved both the durability and the

recording characteristics of recent tapes.

The magnetic characteristics of the magnetic parti-

cles never wear out. The particles can be recorded

and/or reproduced an unlimited number of times

without any performance degradation.

The useful life of the tape is determined by three

factors—the inherent strength of the tape, the amount of

physical wear caused by the tape transport, and the

performance required by the application. A typical test to

measure the life of the tape would consist of many repeti-

tive cycles on the intended transport while monitoring the

Figure 28-29. A head degausser.