Recording - February 2015 - 59

much of our early studio gear. Back then,
everything had an input and output
impedance of 600 ohms. Matching output and input impedance maximized the
power passed from one device to the
next. Today's gear is designed with low
output impedance and moderately high
input impedance. That maximizes the voltage passed from one device to another.
dBm is correctly used when, for example,
you're specifying the radio-frequency (RF)
power output of a wireless mic transmitter,
but it's nearly always wrong when used in
specs for a mixer or preamp. dBA or dBC
are frequency-weighted measurements
most often associated with sound level or
noise, a topic for another installment.
Level measurements
A product's specifications often originate
in the Marketing Department. They want
their product to look better than the competition. Even though the final published spec
sheet is taken from laboratory testing of the
product, tests will sometimes be conducted
under conditions that don't represent actual
usage, making the numbers look better
than you might see in practice.
Let's take a look at maximum output
level (MOL). A test signal, nearly always
a very pure sine wave, is applied to the
input of the device being tested. When
it's cranked up to maximum, the RMS output is measured with a voltmeter.
"Maximum output" is defined as the level
where the total harmonic distortion (THD)
of the output reaches a specific amount.
Fair enough, but what amount? There's
no standard; it could be 1%, 3%, or hard
clipping. Sometimes they tell you, sometimes they don't. Furthermore, MOL is typically specified at 1 kHz, but distortion,
hence MOL, can vary with frequency.
Beware of the unspecified specification!
MOL measurements are also affected
by electrical load on the device's output.
Modern solid-state gear is happiest when
driving an input impedance of 5 kilohms
or greater, and that's how it's tested. Feed
a modern mic preamp into a vintage compressor with a 600 ohm input impedance,
though, and the preamp may distort at a
level lower than the spec sheet's MOL.
A proper MOL specification reads something like this: "+24 dBu, balanced, 20
Hz to 20 kHz, 5 kΩ load, THD < 1%." If
all it says is "24 dB" you don't have a clue.
Nominal operating level is commonly
stated as one of the two de facto standards,
+4 dBu or -10 dBV. (These two choices
have an interesting history; see page 8.) A
mic preamp with a nominal operating level
of +4 dBu will need be turned down by
about 12 dB when connected to a nominal
-10 dBV computer interface to prevent the
interface clipping on peaks. Conversely, if
you connect the output of a "-10" mixer to
a "+4" interface, you may not be able to
reach full record level.

Good-to-know levels and their relatives
While level specs are useful on their
own, they relate to two important characteristics, headroom and signal-to-noise
ratio (S/N).
Headroom is the ratio in dB between
the maximum and nominal output levels.
A preamp with +4 dBu nominal operating level and MOL of +24 dBu has 20 dB
of headroom. Another "+4" preamp with
a maximum output level of +16 dBu offers
only 12 dB of headroom. Learning how
to use headroom to best advantage is the
topic for another article.
Most devices also have a maximum
input level. The record volume control on
your grandfather's tape recorder
allowed you to get a proper recording
level from nearly any source, but your
computer's audio interface may not have
such a control. The maximum input level
of a computer audio interface is usually
the level that produces the maximum (0
dBFS) recording level. If its maximum
input level is +16 dBu, it will clip well
before your +24 dBu MOL preamp does.
Take care!
There are minimum input levels, too,
but they're more usefully specified as sensitivity, which I'll explain in detail in a
future article.
Signal-to-noise ratio (S/N) is the ratio
in dB between the nominal operating
level and the level of noise in the device's
output with no input signal. A device with
a +4 dBu nominal operating level and a
noise level of -88 dBu has a S/N of 92
dB. There's room for some specsmanship
here since noise level can be measured in
different ways - frequency-weighted or
broadband, and with the input shorted or
terminated... but never with an open
(unplugged) input since that's always the
worst case. Dynamic Range, often confused with S/N, is the ratio of MOL to the
noise floor. See Figure 1 for a diagram of
how these levels relate to one another in
a typical setup.
Good to know
Level specs can help you choose compatible gear, avoid weak links in your system, and diagnose "too hot" and "not hot
enough" problems. When comparing
specs for similar devices, differences may
appear relatively insignificant, though a
spec sheet that's clearly and completely
written is a good indication that the unit is
competently designed and the manufacturer has nothing to hide. Finally, since
level measurements are the basis for other
specifications, it's important to understand their relationships, as we'll see in
forthcoming articles.
Mike Rivers (rivers@recordingmag.com)
is a studio and live sound engineer, gear
design guru, and educator. Learn more (lots
more!) at mikeriversaudio.wordpress.com.
RECORDING February 2015

http://www.mojaveaudio.com http://mikeriversaudio.wordpress.com

Recording - February 2015

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