The physics
2.1 Amplitude vs. Velocity
It is this second interaction that is at
the heart of almost every modern
turntable. As the stylus (or “needle”
is pulled through the grove in the vinyl
surface, it moves from side-to-side at a
varying speed called the
modulation
velocity
or just the
velocity
. An
example of this wavy groove can be
seen in the photo in Figure
. Inside
the housing of most cartridges are
small magnets and coils of wire, either
of which is being moved by the stylus
as it vibrates. That movement
generates an electrical current that is
analogous to the shape of the groove:
the higher the velocity of the stylus,
the higher the electrical signal from
the cartridge.
Figure 2.1: The groove in a late-1980’s
pop tune on a 33 1/3 RPM stereo LP. The
white dots in the groove are dirt that
should be removed before playing the
disc.
However, this introduces a problem
because if the amplitude remains the
same at all frequencies the modulation
velocity of the stylus decreases with
frequency; in other words, the lower
the note, the lower the output level,
and therefore the less bass. This is
illustrated in the graph in Figure
in
which three sine waves are shown with
di
ff
erent frequencies. The blue line
shows the lowest frequency and the
orange line is the highest. Notice that
all three have the same amplitude (the
same maximum “height”). However, if
you look at the slopes of the three
curves when they pass Time = 0 ms,
you’ll see that the higher the
frequency, the higher the slope of the
line, and therefore the higher the
velocity of the stylus.
-10.0
-5.0
0
5.0
10.0
Time (ms)
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Displacement (mm)
Figure 2.2: Three sine waves of di
ff
erent
frequencies (from low to high: blue, red
and orange curves), but with the same
amplitude.
In order to achieve a naturally flat
frequency response from the cartridge,
where all frequencies have the same
electrical output level, it is necessary
to ensure that they have the same
modulation velocity, as shown in Figure
. In that plot, it can be seen that the
slopes of the three waves are the same
at Time = 0 ms. However, it is also
evident that, when this is true, they
have very di
ff
erent amplitudes: in fact,
the amplitude would have to double for
every halving of frequency (a drop of 1
octave). This is not feasible, since it
would mean that the stylus would have
to move left and right by (relatively)
huge distances in order to deliver the
desired output. For example, if the
stylus were moving sideways by
±
0.1
mm at 1,000 Hz to deliver a signal,
then it would have to move
±
1 mm at
100 Hz, and
±
10 mm at 10 Hz to
deliver the same output level. This is
not possible (or at least it’s very
impractical).
-10.0
-5.0
0
5.0
10.0
Time (ms)
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Displacement (mm)
Figure 2.3: Three sine waves of di
ff
erent
frequencies (from low to high: blue, red
and orange curves), but with the same
modulation velocity.
The solution for this limitation was to
use low-frequency audio compensation
filters, both at the recording and the
playback stages. When a recording is
mastered to be cut on a disc, the low
frequency level is decreased; the lower
the frequency, the lower the level. This
results in a signal recorded on disc with
a constant
amplitude
for signals below
approximately 1 kHz.
Of course, if this signal were played
back directly, there would be an
increasing loss of level at lower and
lower frequencies. So, to counteract
this, a filter is applied to the output
signal of the turntable that boosts the
low frequencies signals to their original
levels.
2.2 Surface noise
A second problem that exists with vinyl
records is that of dust and dirt. If you
look again at the photo in Figure
,
you can see white specks lodged in the
groove. These look very small to us,
however, to the stylus, they are very
large bumps that cause the tip to move
abruptly, and therefore quickly. Since
the output signal is still proportional to
the modulation velocity, then this
makes the resulting cracks and pops
quite loud in relation to the audio
signal.
In order to overcome this problem, a
second filter is used, this time for
1
Some authors reserve the term “stylus” for the device that is used to cut the groove during mastering, and the term “needle” for the device used to play a phonographic record.
However, the two terms are used interchangeably in this document.
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