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.
If the standard is included in the
specification (DIN or AES6, for
example), then it is still di
ffi
cult to
compare wow and flutter values. This
is because, even when performing
identical measurements and applying
the same weighting curve shown in
Figure
, there are di
ff
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
ff
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
. The result shows the
instantaneous frequency variations
over the length of the captured signal,
relative to the average frequency
(however, the e
ff
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
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
