Philips N4450 - PART I What You Should Know; Magnetic Sound Recording

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
56
properties of both the recorder and the tape.
The
alternating
current
thus
amplified
now
reaches
the
recording
head
along
which
the
magnetic
audio
tape
is
fed.
The
actual
recording
process
is,
in
fact,
the
reverse
of
what
happens
in
the
microphone:
A
current
through
a
coil
causes
a
magnetic
field
to
develop
in
the
core
of
that
coil,
which
varies
in
accordance
with
the
current
intensity.
So,
if
the
current
is
an
alternating
current,
the
resultant
magnetic
field
will
be
a
so-called
alternating
field.
To
boost
the
magnetic
action
of
the
recording
head,
the
coil
is
wound
round
a
ring-shaped
iron
core,
which
contains
a
microscopically
small
slit,
the
so-called
air
gap.
Because
the
iron
coie
presents
only
a
very
low
resistance,
the
magnetic
lines
of
force
propagate
in
the
core
and
close
across
the
above-mentioned
air
gap,
which
they
bridge
in
the
form
of
an
arc.
These
lines
of
force
which
due
to
their
alternating
direction
and
strength
are
a
faithful
reproduction
of
the
alternating
current
and
thus
of
the
sound
to
be
recorded,
are
used
to
“write”
the
sound
onto
the
tape.
1.3. MAGNETISING OF THE TAPE
Modern
audio
tape
consists
of
a
base
of
a
synthetic
material
on
which
a
highly
uniform
layer
of
iron
oxide
is
coated.
This
coating
consists
of
many
microscopic
particles
of
iron
oxide
which
can
be
magnetised
individually.
When
the
tape
passes
the
air
gap
of
the
recording
head,
the
iron
oxide
particles
located
in
front
of
the
gap
are
magnetised
by
the
instantaneous
magnetic
field
strength.
When
the
whole
tape
has
passed,
it
contains
a
pattern
of
adjacent
iron-oxide
particles
that
are
differently
magnetised
and
have
the
same
or
opposite
polarity,
depending
on
the
strength
and
direction
of
the
magnetic
lines
of
force
by
which
they
have
been
magnetised
when
passing
the
re-
cording
head.
Since
the
magnetic
lines
of
force,
in
turn,
were
a
faithful
representation
of
the
sound
to’
be
recorded,
the
magnetisation
on
the
tape
is
also
representative
of
this
sound.
Audio
tapes
retain
the
information
once
recorded,
even
after
numerous
reproductions,
unless
they
are
exposed
to
another
strong
magnetic
field
(see
“ERASING”,
I.2.1.).
1.4.
FROM
MAGNETIC
RECORDING
BACK
TO
ELECTRICAL
VIBRATIONS
When
playing
back
a
magnetic
recording
on
a
tape,
the
tape
is
fed
past
the
playback
head
and
the
process
described
in
the
previous
Chapter
is
reversed.
When
passing
the
air
gap,
the
magnetised
particles
of
different
polarity
and
degree
of
magnetisation
induce
a
magnetic
field
in
the
iron
core
of
the
head
which
results
in
an
alternating
current
in
the
coil.
This
alternating
current,
which
is
still
representative
of
the
recorded
sound,
is
amplified
by
the
playback
amplifier
and
is
applied
to
the
loudspeaker.
‘
1.5.
FROM
ELECT
RICAL
VIBRATIONS
BACK
TO
SOUND
The
electrical
alternations
of
the
alternating
current
are
converted
into
sound
by
the
loudspeaker
which
is
in
fact
the
opposite
of
a
microphone.
The
alternating
current
from
the
playback
amplifier
is
fed
through
a
coil
attached
to
a
diaphragm,
which
is
located
in
a
strong
and
uniform
magnetic
field.
The
alternating
current
in
the
core
of
the
coil
produces
an
alternating
magnetic
field.