converting dBV levels into dBu levels is as simple as subtracting 2dB (2.21
to be exact).
Signals are also occasionally represented with units of dBm. This is an older
unit that measures power instead of voltage levels with 0dBm representing 1
milliwatt. Earlier tube-based audio equipment used standardized input and
output impedances of 600 ohms, so a 0dBm signal was produced with a
voltage of .775 volts. Since most of today’s equipment uses impedances
other than 600 ohms, it is more useful to represent signals by voltages rather
than power and the dBu unit was introduced. A signal level of 0dBu is
identical to a level of 0dBm.
Digital signals, after they are recorded, no longer directly represent any
physical quantity such as voltage or power and 0dB is generally used to
represent a “full-scale” or maximum signal level. All other signal levels are
lower and are expressed as negative decibels. Most meters on digital
equipment have 0dB at the top and range downward from there. A signal
that is 30dB below full scale would simply be referred to as a –30dB signal.
Dynamic Range
Dynamic range represents the difference between the maximum signal that
can be recorded and the “noise floor”, or level of noise with no signal
present. A system with a high dynamic range will be quieter than one with a
lower dynamic range. Dynamic range is a very important specification, and
your echo card uses converters that have very high dynamic range.
Theoretically, a 24-bit system has a dynamic range of 144dB and a 16-bit
system has a dynamic range of 96dB. Two questions immediately come to
mind:
1) Why doesn’t my Echo card have a dynamic range of 144 dB?
2) For mastering 16-bit CDs with a dynamic range of 96dB, isn’t
anything more than 96dB just overkill?
First, today’s analog-to-digital converters typically produce a full-scale input
voltage with an input of +7dBu. If they were to have 144dB of dynamic
range, they would have to be capable of resolving signals as small as –137
dBu (7dBu – 144dBu) or approximately 10 nano-volts. That’s 10
one-
billionths
of a volt! Transistors and resistors produce noise in this range just
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