Manley SLAM! - Manual (Page 12)

VU METERS
Two toggles are used for the VU meters. One is used to select
whether the VUs display Input Level (after the Input Pot), Output
Level, or the OPTO Limiter Gain Reduction. If in BYPASS the
OPTO GR the meters drop out rather than sit on zero. A VU meter
showing OPTO GR seems to be a bit of a standard and the time
constants and ballistics are a good match, even if the VU does not
show every drop of gain reduction. The LED meter also can dis-
play Opto GR or the total gain reduction.
The second toggle is a pad or attenuator for the VUs in the Input
or Output modes and has no effect on GR mode. The 3 position
switch gives 0 (no attenuation and calibrated to +4dBu), -3 dB and
–6 dB. While it is very unusual to get an attenuator for VU meters
on standard rack processors, it is a feature we have been building
onto mastering consoles for many years. A regular VU meter would
be continually pinned in the red without the attenuator (which may
have a disturbing effect on some clients). There are several reasons
for this, some pre-dating the trends of heavy squashing. Compare
individual acoustic tracks at DFS, to a mix at DFS, and the mix
will generally look hotter on the VUs. Mixes tend to have more
average energy than individual tracks. Both mixing engineers and
mastering engineers often (usually) compress a mix, which also
increases the ratio of average to peak somewhat. For example in
recording, typical peak to average ratios are 16 to 20 dB, but in
mastering 14 dB or less.
The SLAM! is a Swiss Army Knife and is intended to be used for
much more than just mixes and mastering. Because it can severely
reduce the ratio of average to peak levels, and because most of
us now reference levels to digital full scale (a peak reading), it
follows that without the attenuator, the poor VUs would be ‘in
the red’ much of their life. We caution that the –6 position can
mislead one into a situation where the audio is really too hot, or
too squashed. On mastering consoles, the most often used VU pad
is –4dB (a hint).
METERING GENERALLY
The VU meters and the SLAM! peak meters will never agree and
maybe the SLAM! peak meters are a little ‘off’ from the peak me-
ters on an external ADC or DAC. What is going on? Which ones
do I trust? How can I calibrate them to look the same?
These questions have been around for as long as there have been
meters. VUs should match well with other VUs because there is a
comprehensive list of standards and qualifications to meet to be a
true VU meter. Part of the specification are ‘dynamic characteris-
tics’ which describes “The pointer shall reach 99 on the percent
scale on 0.3 second, overshoot less than 1.5% (.15dB)”. This es-
sentially makes the VU meter act with “approximate RMS” re-
sponse and our ears have an “approximate RMS response”. Ahh-
hhh loudness.
Peak Meters are much faster and are supposed to catch events less
than 0.0001 Sec (compared to the VU’s 0.3 Sec) which means
that transients have a much bigger influence on peak meters. One
problem is that our eyes won’t be able to adequately see a 0.1
mSec flash or even 1 or 10 mSec so the designer has to stretch the
duration of the leds or pointer. In a sense, this exagerates transient
contribution, but at least looks good. There can also be a peak hold
dot that adds a digital stretch to the duration and allows us the
luxury of blinking or looking away once in a while. We have that
in mode 2.
In pro audio there are few measurement standards (or even de-facto
standards) that exist and peak meter calibrations and digital con-
verter I/O levels are more prime examples. With digital peak me-
ters that are married to digital converters, one at least expects that
an ‘over’ in the A to D over would light the top red LED. Wrong -
the better meters use 4 samples over and the best allow that number
to be user set. 4 samples over is about the threshold of where we
hear a clip. Makes sense to me.
The SLAM! LED meter is not digital, it is an analog meter with a
micro-processor. It can be adjusted for different analog sensitivities
with an internal trim pot, but this is not guaranteed to ‘match’ other
peak meters, nor was it intended to. The SLAM displays approxi-
mately 1 dB per segment, except near the bottom which are much
bigger steps to show signal present. Other meters may show 2 dB
or 1/2 dB steps or be dB scaled on a curve.
Our solution to allow some sort of matching with other peak meters
(like the ones on your favorite A to D), is to give you a way to set
where the color changes occur. The transition from green to amber
is settable, and for mode 2, also the transition from amber to red.
This can be done from the front panel without ever removing the
unit from the rack or unscrewing the top (mode 3).
In order to provide some useful indication of A to D clip or overs,
any green LEDS turn red, which is hard to miss. What is interest-
ing, is that because the meter is looking at analog levels, one can
see how many dBs the peak went beyond A to D clipping and this
is also a good indication of how audible the clip was.
So to answer the question “Which one should I trust?” the best
answer is “all of them with some interpretation and a grain of
salt”. The VU is best to show apparent volume or apparent GR, the
SLAM! peak meters show analog peaks, headroom in the SLAM!,
and momentary GR that may or may not be audible. External digi-
tal peak meters on the recording device should accurately display
available headroom and clipping there. None of them are 100%
accurate, nor implicitly perfect, and because each may have differ-
ent response characteristics, might look different especially after
a peak and the rates that dots fall and/or hold. If this is what you
would like to calibrate, sorry.
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