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Audio Specifications

4.0.1 Magnitude Response

The magnitude response

1

of any audio

device is a measure of how much its
output level deviates from the
expected level at di

erent frequencies.

In a turntable, this can be measured in
di

erent ways.

In the case of the Beogram 4000c, the
frequency response is measured from
a standard test disc with a sine wave
sweep ranging from at least 20 Hz to at
least 20 kHz. The output level of this
signal is recorded at the output of the
device, and the level is analysed to
determine how much it di

ers from the

expected output. Consequently, the
measurement includes all components
in the audio path from the stylus tip,
through the RIAA preamplifier, to the
line-level outputs.

4.0.2 Rumble

In theory, an audio playback device
only outputs the audio signal that is on
the recording without any extra
contributions. In practice, however,
every audio device adds signals to the
output for various reasons. As was
discussed above, in the specific case of
a turntable, the audio signal is initially
generated by very small movements of
the stylus in the record groove.
Therefore, in order for it to work at all,
the system must be sensitive to very
small movements in general. This
means that any additional movement
can (and probably will) be converted to
an audio signal that is added to the
recording.

This unwanted extraneous movement,
and therefore signal, is usually the
result of very low-frequency vibrations
that come from various sources. These
can include things like mechanical
vibrations of the entire turntable
transmitted through the table from the

floor, vibrations in the system caused
by the motor or imbalances in the
moving parts, warped discs which
cause a vertical movement of the
stylus, and so on. These low-frequency
signals are grouped together under the
heading of

rumble

.

A rumble measurement is performed
by playing a disc that has no signal on
it, and measuring the output signal’s
level. However, that output signal is
first filtered

2

to ensure that the level

detection is not influenced by
higher-frequency problems that may
exist.

If the standard being used for the
rumble measurement is the DIN 45 539
specification, then the resulting value
is stated as the level di

erence

between the measured filtered noise
and a the standard output level,
equivalent to the output when playing
a 1 kHz tone with a lateral modulation
velocity of 70.7 mm/sec.

4.0.3 Rotational speed

Every recording / playback system,
whether for audio or for video signals,
is based on the fundamental principle
that the recording and the playback
happen at the same rate. For example,
a film that was recorded at 24 frames
(or photos) per second (FPS) must also
be played at 24 FPS to avoid objects
and persons moving too slowly or too
quickly. It’s also necessary that neither
the recording nor the playback speed
changes over time.

A phonographic LP is mastered with
the intention that it will be played back
at a rotational speed of 33 1/3 RPM
(Revolutions Per Minute) or 45 RPM,
depending on the disc. (These
correspond to 1 revolution either every
1.8 seconds or every 1 1/3 seconds
respectively.) We assume that the
rotational speed of the lathe that was
used to cut the master was both very
accurate and very stable. Although it is

the job of the turntable to duplicate
this accuracy and stability as closely as
possible, measurable errors occur for a
number of reasons, both mechanical
and electrical. When these errors are
measured using especially-created
audio signals like pure sine tones, the
results are filtered and analyzed to
give an impression of how audible they
are when listening to music. However,
a problem arises in that a simple
specification (such as a single number
for “Wow and Flutter”, for example)
can only be correctly interpreted with
the knowledge of how the value is
produced.

Accuracy

The first issue is the simple one of
accuracy: is the turntable rotating the
disc at the correct

average

speed? In

the Beogram 4000c, this speed is
governed by a tachometer built into
the drive motor, that (like on almost all
turntables) can be adjusted by the user
using the controls shown in Figure

4.1

.

These adjustments allow for a usable
range of

±

3%, which in music terms is

equivalent to

±

half of a semitone.

Figure 4.1: The fine adjustment controls

for the 33 1/3 and 45 RPM settings.

Stability

Like any audio system, regardless of
whether it’s analogue or digital, the
playback speed of the turntable will
vary over time. As it increases and
decreases, the pitch of the music at
the output will increase and decrease
proportionally. This is unavoidable.
Therefore, there are two questions that
result:

1

This is the correct term for what is typically called the “frequency response”. The di

erence is that a magnitude response only shows output level vs. frequency, whereas the

frequency response would include both level and phase information.

2

The characteristics of the filters are defined in internal standards such as DIN 45 539 and IEC98-1964.

14

Содержание Beogram 4000c

Страница 1: ...Beogram 4000c Technical Sound Guide Bang Olufsen A S This manual is for information purposes only and is not legally binding November 27 2020...

Страница 2: ...ross 7 3 2 Signal Levels 8 3 3 Tip shape 10 3 4 Bonded vs Nude 12 3 5 Tracking force 12 3 6 E ective Tip Mass 12 3 7 Compliance 13 3 8 Soundsmith SMMC20CL 13 4 Audio Specifications 14 4 0 1 Magnitude...

Страница 3: ...Emil Berliner was awarded a patent for a sound recording and reproducing system that was based on a groove in a rotating disc rather than Edison s cylinder the original version of the system that we k...

Страница 4: ...are the same at Time 0 ms However it is also evident that when this is true they have very di erent amplitudes in fact the amplitude would have to double for every halving of frequency a drop of 1 oc...

Страница 5: ...ns of the RIAA equalisation filter define the transition points as time constants instead of frequencies So instead of 50 Hz 500 Hz and 2122 Hz as shown in the response plots the points are listed as...

Страница 6: ...down left up right for example This means that signals that are identical in both channels move the stylus laterally exactly as in earlier monophonic discs 4 Figure 2 10 An over simplified depic tion...

Страница 7: ...eophonic Transducer Cartridge In 1963 Erik R rbaek Madsen of Bang Olufsen filed a patent for a cartridge based on the Moving Iron principle In it a cross made of Mu metal is mounted on the stylus Each...

Страница 8: ...be encoded without incurring additional distortion that is inherent in the encoding system itself is when the maximum or minimum value in the audio signal reaches the highest possible signal value of...

Страница 9: ...here 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...

Страница 10: ...here are a number of options when choosing the shape of the playback stylus 3 3 Tip shape The earliest styli were the needles that were used on 78 RPM gramophone players These were typically made from...

Страница 11: ...k deeper into the groove making it more di cult for it to move independently on the two audio channels The second is that the point of contact between the stylus and the vinyl becomes smaller which ca...

Страница 12: ...pin which is in turn connected to the cantilever the long arm that connects back to the cartridge housing This bonded design is cheaper to manufacture but it results in a high mass at the stylus tip...

Страница 13: ...the compliance of the stylus suspension Compliance is the opposite of spring sti ness the more compliant a spring is the easier it is to compress and the less it pushes back Like many other stylus pa...

Страница 14: ...of 70 7 mm sec 4 0 3 Rotational speed Every recording playback system whether for audio or for video signals is based on the fundamental principle that the recording and the playback happen at the sam...

Страница 15: ...changes in the speed that are higher than 100 Hz This is typically only a problem with analogue tape decks caused by the magnetic tape sticking and slipping on components in its path and is not often...

Страница 16: ...nal relative to the average frequency however the e ect of very slow and very fast changes have been reduced by the filter Finally the standard deviation of the variation from the average is calculate...

Страница 17: ...mith SMMC20CL Stylus Nude 0 12 mm square Radius of curvature Contact Line Recommended tracking force 1 3 g Compliance 28 m mN E ective tip mass 0 32 mg General information Automatic speed selection Ye...

Страница 18: ...nditions in the Cutting and Playing of Stereo Disk C R Bastiaans Vol 11 Issue 1 Jan 1963 Factors A ecting the Stylus Groove Relationship in Phonograph Playback Systems C R Bastiaans Vol 15 Issue 4 Oct...

Страница 19: ...commendations for Lateral Cut Commercial and Transcription Disk Recordings 98 Processed Disk Records and Reproducing Equipment 386 Method of Measurement of Speed Fluctuations in Sound Recording and Re...

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