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The forces of this magnetic field now counteract the uniform
force of the magnetic field of the loudspeaker magnet, so
that the coil together with the diaphragm start moving in
the rhythm of the alternating current. The diaphragm, in
turn, causes the air to vibrate, thus producing the sound
that was previously recorded via the microphone.
According to this method, sounds can be recorded on
magnetic tape at any desired moment and reproduced un-
modified at any moment afterwards.
2. SOME MORE TAPE RECORDING THEORY
2.1. ERASING
As already stated in Chapter 1.3, the tape retains the re-
corded information until it is exposed to another strong
magnetic field.
This property is used in erasing. An existing recording is
erased from the tape, when a new recording is made on the
same part of the tape. This can be repeated indefinitely,
without a deterioration in quality of the new recording.
(However, erasing is also possible without making a new
recording, by not applying a new signal to be recorded,
Chapter IJ.3.6). For this purpose, the recorder is fitted
with a so-called erase head, past which the tape is fed
before it reaches the recording head. This erase head
operates as follows:
Through a coil wound on a ring-shaped iron core, an
alternating current is fed of a frequency far beyond the
limit of audibility. The air gap in the iron core has been
designed so that the magnetic field bridging the gap is
dome-shaped.
When a magnetised particle of the tape passes the erase
head, it is initially subject to increased magnetisation,
after which the intensity subsequently decreases slowly
to zero. (The increasing and decreasing magnetisation
is the result of the shape of the magnetic field; the
magnetisation is caused by the high frequency (RF)
alternating current.)
The relevant particle is thus initially magnetised with
increasing and then with decreasing strength and is
magnetically neutral after passing the air gap: The
recording is now "erased".
2.2. FOUR-TRACK SYSTEM
The four-track system indicates that four recordings can
be made over the full width of the tape, so that four
adjacent "tracks" of recordings are obtained on the tape.
These can be four mono recordings or two stereo recordings,
since in the case of stereo we have essentially two separate
recordings. Because the recorder operates in accordance
with the reverse principle, it incorporates two head: systems
each consisting of an erase head, recording head and play-
back head. One system is used when the tape runs to the
ieft and the other when it runs to the right. Each system
covers two tracks, which can be used separately (mono)
or in combination (stereo) both for recording and play-
back. Depending on the position of the track selector
you use track 1 and/or 3 when the tape runs to the right
and track 2 and/or 4 when the tape runs to the left.
Recordings made on another four-track recorder can be
played back on your recorder using the reverse system.
The opposite is also possible.
Stereo recordings made on a twin-track recorder can be
played back in one direction only and mono recordings
in both directions.
Recordings made on your recorder can be played back on a
twin-track recorder, if only tracks 1 and 3 are used and
tracks 4 and 2 are empty or the other way round.
2.3. STEREO
When listening to, for example, an orchestra, your left ear
perceives the sound coming from the left slightly sooner and
louder than your right ear, and for sound coming from the
right this is the other way round. The sounds from the
middle are heard simultaneously and equally loud. This
makes it possible for you even with your eyes closed, to
determine the direction in which the various instruments lie:
you can still "see" the orchestra in front of you!
When making a stereo recording of an orchestra, two micro-
phones are used which pick up the sound in a similar way as
your ears. The sound is then recorded on two separate tracks,
one for the sound picked up by the left microphone (left
channel) and one for-the sound from the right microphone
(right channel). When this stereo recording is played back
stereophonically, these tracks are separately reproduced via
two separate loudspeaker enclosures, one for the left channel
and one for the right channel. Again, you can "see"' the
orchestra in front of you.
With your stereo tape recorder you can now make your own
stereo recordings, for example of music, songs, speech, stereo
broadcasts and stereo records, and reproduce these also in
stereo. It is obvious, however, that you cannot make stereo
recordings of e.g. a mono broadcast or mono record.
2.4. MULTIPLAY
This is the recording of a synchronised programme, consisting
of several recordings on the same track, which renders it
possible for instance to record a quartet which is rendered
all by yourself.
While making e.g. the second recording, you can listen to the
first recording via the headphones, the first recording being
duplicated simultaneously and mixed with the second
recording on the same track. By repeating this several times
a combination of separate but synchronous recordings is
obtained which are reproduced together.
2.5. ECHO AND REVERBERATION
When recording songs or music in e.g. a living room, rever-
beration can be added to the recording to create the im-
pression that the recording was made in a large hall or
church.
By the addition of echo, very special effects can be achieved.
With your recorder echo and reverberation recordings can
be made in a simple manner, as the recorder is equipped
with separate, recording and playback heads which are
mounted in line. The signal just recorded is then fed back
from the playback head to the recording head and re-
recorded together with the original signal. The effect is due
to the distance between the two heads and depends on the
tape speed; at the relatively low tape speeds of 4.75 and
9.5 cm/sec., echo effects are obtained and at the higher
speed of 19 cm/sec. a reverberation effect.
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