Often, we run into pieces of equipment that either accept or output
consumer levels (-10dBV) or so-called pro levels (+4dBu). If
you're like me, you've never really understood the difference,
other than pro level is hotter, i.e. louder. Also, pro usually
means balanced 1/4" or XLR connectors, whereas "consumer" means RCA.
Well, here's some more information on this, thanks to several
people who were kind enough to answer my question in great detail.
Try this link for auto-conversions:
http://www.sengpielaudio.com/calculator-db-volt.htm
Now, for the curious, here's the difference between these two
measurements. Warning: math ahead!
Generally speaking, decibels are a unit of the relative power
between two signals, or between one signal and another one that's
known to be at a standard level of power. In the first half of the
20th century or so, the standard signal power used to be 1
milliwatt (mW) in a 600 Ohm transmission line--thus the "m" in dBm.
But audio technology isn't dominated by the telephone company any
more, and we no longer use transmission lines of any kind for audio
frequency signals (despite what you may see in the ads for
high-priced cables--or excuse me, "interconnects"--in audiophile
magazines). As a result, the normal load impedance for nearly all
analog audio connections is simply "high" (or "high enough to avoid
loading down the source significantly," whatever "significantly"
might mean to you), and most audio level measurements have become
voltage measurements.
The decibel was such a useful unit that it got carried forward and
transplanted into the brave, new voltage-dominated world, by
assuming that whenever you compare the levels of any two signals,
the load impedance on the two of them will be the same.
Computationally this allows you the shortcut of leaving
current out of the equation; you simply take the logarithm of the
voltage ratio without computing the power levels explicitly.
But then you have to multiply by 20 rather than 10 (the "deci" in
decibels) because decibels are still a unit of relative power. By
implication we assumed that the currents will always be
proportional to the voltages when we assumed that the load
impedances were some constant "high enough" value. It's a step into
virtual reality in a way, but it's been the standard practice for
decades now.
The u in dBu usually means "unloaded" or "undetermined" and refers
to impedance. The V in dBV refers to voltage.
What is dBu? A logarithmic voltage ratio with a reference voltage
of V0 = 0.7746 volt ? 0 dBu
What is dBV? A logarithmic voltage ratio with a reference voltage
of V0 = 1.0000 volt ? 0 dBV
The home recording level consumer audio of -10 dBV means 0.3162
volts, that is -7.78 dBu. The studio recording level pro audio of
+4 dBu means 1.228 volts.
The short answer to the question is approximately 11.8 dB
difference, that is, lower the volume of your pro +4dBu source by
approximately 11.8dB and you're at approximately consumer levels,
-10dBV.
Thanks to a poster over on www.taperssection.com for the following
math.
-10 dBV is ca. 316 mV while +4 dBu is ca. 1.228 V. The ratio
between them is ca. 3.886. The log (to the base 10) of that ratio
is ca. 0.59; 20 times that
is ca. 11.79; voilà.
My brain hurts!!!
Chris, who doesn't even remember logarithms from high school math class!
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