User Manual

Appendix G: An Introduction to Digital Recording

level of analog audio signals. The most common are dBu and dBV decibels. Both

of these represent voltage levels and still double for every increase of 6dB. It is

only the reference point, or 0dB level that is different. A 0dBV signal has a

voltage level of 1.0 volts. A 0dBu signal has a voltage level of .775 volts. Since

.775 is approximately 2dB less than 1.0, 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.

Nominal Signal Levels and Headroom

Today’s equipment is generally referred to as +4dBu equipment (professional) or

–10dBV equipment (consumer). These levels are the typical or “nominal” signal

levels you can expect to see with professional (studio) equipment such as mixers or

with consumer equipment such as home stereos and CD players. A +4dBu signal

has a voltage level of 1.23 volts and a –10dBV signal has a voltage level of .316

volts.

The above nominal levels represent typical or average levels that are often

exceeded when recording loud signals such as drum beats. The difference between

the nominal level and the loudest signal that can be recorded without clipping is

called “headroom”. Your Echo product provides approximately 14dB of headroom

allowing an 18dBu signal to be recorded.