ANALOG
RECORDING:
In analog
recording, we would set up an
analogy between parameters (variables)
of the original sound, and
parameters of the recording medium. We would
vary some parameter of the medium corresponding to
changes in the sound wave. The two (sound)
parameters usually used were time and amplitude.
Vinyl
Disk:
In phonograph
recording, changes in the amplitude of the
original sound were used to cause changes in
the width of the groove cut into the surface
of the disk. The louder the
instantaneous amplitude of the sound, the
wider the groove was cut.
Time was
represented
as distance along the spiral groove.
As the stylus moved from the outside
edge toward the centre; the diameter
decreased. Since the disk rotated a
constant speed -- 33- 1/3, 45 or 78 rpm --
the linear speed decreased near the centre.
- Scratches and dirt
on the disk surface would disrupt
the amplitude analogy; causing noise
(pops and crackle) in the
playback.
- Warping of the vinyl
material would disrupt the time
analogy; causing changes in
pitch (warble or wow).
- Irregularities in
the composition of the vinyl
would disrupt the amplitude analogy
randomly; causing surface noise.
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Film
Sound:
In analog
photographic (or
optical) film sound tracks, changes in
the amplitude of the original sound
were used to cause changes in the width of
the clear area of the optical track.
The
louder the instantaneous amplitude of the
sound, the wider the "wiggles" sound
track.
Time was
represented
as distance along the film. Film speed
was constant -- 18 in./sec
for 35 mm. film; 7.2 ips for 16 mm.
- Scratches and dirt
on the film
surface would disrupt the amplitude
analogy; causing noise (pops and
crackle) in the
playback.
- Shrinkage of the
film with age would disrupt the time
analogy;
causing changes in pitch (warble).
- Inadequacies in the
photo-chemical
process would disrupt both amplitude and
time analogies; causing distortion.
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Magnetic
Recording:
In magnetic
recording, changes in the amplitude of the
original sound were used to cause
changes in the
strength of the magnetic field stored on the
recording tape. The louder
the instantaneous amplitude of the sound,
the stronger the magnetic
field.
Time was
represented
as distance along the recording tape.
Tape speed was constant -- 7.5 or 15
in./sec for reel to reel; 1- 7/8
in./sec for cassette.
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On a
professional reel to reel recorder,
the tape moved from the left supply reel,
past the
tape guides and head block, between the
capstan and pinch roller and onto the
right take up reel.
In the recording mode,
the
erase head erased the previously
recorded signal. The record
head recorded a new signal on the
tape; and finally, the playback
(replay) head played back the
recorded signal. The replay head
served
to provide confidence playback to ensure
the quality of the recording.
Because
of the spacing between the record and play
heads, there would be a noticeable delay
in the confidence playback; dependent on
tape speed.
In the play mode, the
erase and record heads would not
be used.
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The erase,
record and playback heads were
electomagnets; coils of wire wrapped around
pieces of iron (or other magnetic
material). A space in the magnetic
material, called the gap;
concentrated the magnetic field. The
erase head had a very wide gap to
provide the maximum erasure. The
play head had a narrower gap
to maximize response to high frequencies
(short wavelengths). The width of the
record head gap was somewhere in
between.
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- Stray magnetic
fields, from loudspeakers,
electric motors or computer monitors
could demagnetise (erase) parts of the
tape;
disrupting the amplitude analogy;
causing drops in signal level.
- Stretching or
shrinking of the plastic tape disrupted
the time analogy; causing
pitch changes (wow and flutter).
- Loss of the magnetic
coating (oxide
shedding) disrupted the amplitude
analogy; causing drops in signal level
(dropouts).
- The random variation
in magnetic particle size also disrupted
the amplitude analogy; causing tape
hiss.
The tape
heads, guides, capstan and pinch roller
had to be be cleaned regularly with
alcohol
to remove glue from splicing tape, finger
print oils and wax pencil
grease.
The moving
magnetic
field of the tape could magnetise metal
parts in the tape path and these would have
to be
periodically de-magnetised; otherwise noise
would be added to the recorded signal during
each playback.
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