background image

comparing one Wow and Flutter
measurement value should be
interpreted very cautiously...

Expressing the result

When looking at a Wow and Flutter
specification, one will see something
like

<

0.1%,

<

0.05% (DIN), or

±

0.1%

(AES6). Like any audio specification, if
the details of the measurement type
are not included, then the value is
useless. For example, “W&F:

<

0.1%”

means nothing, since there is no way
to know which method was used to
arrive at this value.

4

If the standard is included in the
specification (DIN or AES6, for
example), then it is still di

cult to

compare wow and flutter values. This
is because, even when performing
identical measurements and applying
the same weighting curve shown in
Figure

4.2

, there are di

erent methods

for arriving at the final value. The value
that you see may be a peak value (the
maximum deviation from the average
speed), the peak-to-peak value (the
di

erence between the minimum and

the maximum speeds), the RMS (a
version of the average deviation from
the average speed), or something else.

The AES6-2008 standard, which is the
currently accepted method of
measuring and expressing the wow
and flutter specification, uses a “2

σ

” or

“2-Sigma” method, which is a way of

looking at the peak deviation to give a
kind of “worst-case” scenario. In this
method, the 3150 Hz tone is played
from a disc and captured for as long a
time as is possible or feasible. Firstly,
the average value of the actual
frequency of the output is found (in
theory, it’s fixed at 3150 Hz, but this is
never true). Next, the short-term
variation of the actual frequency over
time is compared to the average, and
weighted using the filter shown in
Figure

4.2

. The result shows the

instantaneous frequency variations
over the length of the captured signal,
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 calculated, and multiplied
by 2 (hence “2-Sigma”, or “two times
the standard deviation”), resulting in
the value that is shown as the
specification. The reason two standard
deviations is chosen is that (in the
typical case where the deviation has a
Gaussian distribution) the actual Wow
& Flutter value should exceed this
value no more than 5% of the time.

The reason this method is preferred
today is that it uses a single number to
express not only the wow and flutter,
but the probability of the device
reaching that value. For example, if a
device is stated to have a Wow and
Flutter of “1% (AES6)”, then the actual
deviation from the average speed will

be less than 1% for 95% of the time
you are listening to music. The
principal reason this method was not
used in the 1970s when the Beogram
4002 turntable was released is that it
requires statistical calculations applied
to a signal that was captured from the
output of the turntable, an option that
was not available 45 years ago. The
older DIN method that was used
showed a long-term average level that
was being measured in real-time using
analogue equipment such as the
device shown in Figure

4.3

.

Figure 4.3: Bang & Olufsen WM1, ana-

logue wow and flutter meter.

Unfortunately, however, it is still
impossible to know whether a
specification that reads “Wow and
Flutter: 1% (AES6)” means 1%
deviation with a modulation frequency
of 4 Hz or 10% deviation with a
modulation frequency of 140 Hz – or
something else. It is also impossible to
compare this value to a measurement
done with one of the older standards
such as the DIN method, for example.

4

Similarly, a specification like “Frequency Range: 20 Hz to 20 kHz” means nothing, since there is no information about the levels used to define the range.

16

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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