Echo Audio Indigo io Bedienungshandbuch PDF herunterladen (Seite 29)

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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Echo Audio Indigo io Bedienungshandbuch Seite 29