maximum level above which distortion
will start to increase. However, many
analogue systems have a slightly
di
ff
erent characteristic, as can be seen
in the idealised model shown in Figure
. Notice that, just like in the digital
audio system, the noise floor is
constant, and as the level of the input
signal is increased, it rises above this.
However, in an analogue system, the
transition to a distorted signal is more
gradual, seen as the more gentle
slopes of the curves on the right side of
the graph.
-120
-100
-80
-60
-40
-20
0
20
40
Input level (dB)
-10
0
10
20
30
40
50
60
70
80
90
100
Level of Signal relative to Dist Noise (dB)
Figure 3.7: Two examples of the rela-
tionship between the levels of the sig-
nal and the THD+N in a simplified ana-
logue audio system, showing two di
ff
er-
ent maximum SNRs.
As a result, in a typical analogue audio
system, there is an “optimal” level that
is seen to be the best compromise
between the signal being loud enough
above the noise floor, but not
distorting too much. The question of
how much distortion is “too much” can
then be debated – or even used as an
artistic e
ff
ect (as in the case of
so-called “tape compression”).
If we limit our discussion to the stylus
tracking a groove on a vinyl disc,
converting that movement to an
electrical signal that is amplified and
filtered in a RIAA-spec preamplifier,
then a phonograph recording is an
analogue format. This means,
generally speaking, that there is an
optimal level for the audio signal,
which, in the case of vinyl, means a
modulation velocity of the stylus,
converted to an electrical voltage.
Although there are some minor
di
ff
erences of opinion, a
commonly-accepted optimum level for
the groove on a stereo recording is
35.4 mm/sec for a single audio channel
at 1,000 Hz. In a case where both
audio channels have the same 1 kHz
signal recorded in phase (as a
dual-monophonic signal), then this
means that the lateral velocity of the
stylus will be 50 mm/sec.
Of course, the higher the modulation
velocity of the stylus, the higher the
output of the turntable. However, this
would also mean that the groove on
the vinyl disc would require more
space, since it is being modulated
more. This means that there is a
relationship between the total playing
time of a vinyl disc and the modulation
velocity. In order to have 20 minutes of
music on a 12” LP spinning at 33 1/3
RPM, then it the standard method was
to cut 225 “lines per inch” or “LPI”
(about 89 lines per centimetre) on the
disc. If a mastering engineer wishes to
have a signal with a higher output,
then the price is a lower playing time
(because the grooves much be spaced
further apart to accommodate the
higher modulation velocity) however,
in well-mastered recordings, this
spacing is varied according to the
dynamic range of the audio signal. In
fact, in some classical recordings, it is
easy to see the louder passages in the
music because the grooves are
intentionally spaced further apart, as is
illustrated in Figure
.
Figure 3.8: An extreme example of a
disc in which the groove spacing has
been varied to accommodate louder
passages in the music. One conse-
quence of this is that this side of the disc
contains a single piece of music only 15
and a half minutes long.
As can be seen in the Technical
Specifications at the end of this
document, the Beogram 4000c is
factory-calibrated so that a standard
reference modulation velocity of 35.4
mm/sec on one channel at 1 kHz will
produce an output of 354 mV RMS at
the output. For a sine wave, this
corresponds to a peak level of 500 mV.
It can safely be connected to a Line
input of any audio device.
However, it should be noted that the
maximum possible output level of the
turntable is 8.0 V peak, which may, in
fact, be reached with some discs.
Consequently, it should be noted that
playing these recordings with higher
modulation velocities (and therefore
higher output levels) may result in the
Beogram 4000c clipping the Line input
stage of a device connected
“downstream” (depending on its
maximum allowed input level).
If you are connecting the Beogram
4000c to the RCA Line input of a Beolab
90 or Beolab 50, it is recommended
that you set the Maximum Input Level
of that input on the loudspeaker to 4.0
V RMS (which corresponds to 5.7 V
peak) or 6.5 V RMS (9.2 V peak) using
its Input Setup menu. This will ensure
that you maintain adequate headroom
for playback.
A large part of the performance of a
turntable is dependent on the physical
contact between the surface of the
4
(35.4*2) /
√
2
because the two channels are modulated at an angle of 45
◦
to the surface of the disc.
9
