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PART
I "WHAT YOU SHOULD KNOW"
1. MAGNETIC SOUND RECORDING
A tape recorder serves to record sound on an audio tape and
to reproduce the sound thus recorded in due time, preferably
with the greatest possible fidelity.
However, before the sound can be recorded on the tape, it
should be processed to suit this purpose.
The following Chapters describe in a concise but compre-
hensible manner the process of recording sound on magnetic
tape. Each Chapter deals with part of this process, thus
following the sound through the various steps, from the air
vibrations which are recorded on the tape via the microphone
and tape recorder, to reproduction via the loudspeaker.
1.1. FROM SOUND TO ELECTRICAL VIBRATIONS
Sounds that can be percepted by the ear are caused by alter-
nations in air pressure; these are changes in air density
which propagate in space as waves. A normal air flow at
constant pressure (e.g. wind) is not yet perceived as sound.
The number of air-pressure vibrations per second determines
the pitch of a tone. The higher the number of vibrations, the
higher the pitch (frequency). The human ear can perceive
approximately 20 to 16,000 vibrations per second. One
vibration per second is called "Hertz" (after the well-known
physicist), which is the unit of frequency.
The above-mentioned sound waves may be reflected, for
instance by bare walls, or absorbed for example by curtains,
and they can also cause an object to vibrate (e.g. the eardrum).
The latter property is employed in a microphone.
A microphone is a device which converts sound into an
electric current which can be further processed by the tape
recorder. This principle is based on a physical law which,
in simplified form, states: "Moving a magnet inside a coil
(i.e. a change in magnetic field), causes a current in this coil."
The intensity of the current changes with the movement of
the magnet. It is evident that a similar effect is obtained when
the coil is moved around the magnet. Modern microphones
are usually based on this principle:
A coil is attached to a diaphragm. The sound causes the
diaphragm to vibrate. As a result, the coil is moved in the
magnetic field of a permanent magnet (the so-called electro-
dynamic microphone).
When the diaphragm and coil move in the rhythm of the
alternations in air pressure, a current is induced in the coil
whose intensity and frequency vary in accordance with the
intensity and frequency of the sound vibrations.
This "'alternating current" is a faithful representation of the
sound and can now be applied to the tape recorder.
1.2. FROM ELECTRICAL VIBRATIONS TO MAGNETIC
FIELD STRENGTH ALTERNATIONS
The alternating current produced by the microphone is ,
amplified several times in the tape recorder.
The degree of
amplification can be adjusted with the recording level
control and can be checked with the recording level indicator.
This is necessary, since with a fixed amplification the loudest
passages could be distorted and during the soft passages too
much noise could be audible. During each recording the re-
cording amplifier should be adjusted so that at the loudest
passages the recording level indicator deflects to 100 %o; this
is the limit at which just no audible distortion will occur.
When the recording level has thus been set, the soft passages
will be reproduced clearly, doing full justice to the dynamic
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properties of both the recorder and the tape.
1.3. MAGNETISING OF THE TAPE
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