User Manual

Appendix C: An Introduction to Digital Recording

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.

Unbalanced and Balanced Inputs and Outputs

An unbalanced signal, commonly used for guitars and consumer electronics,

contains two components, a ground signal and a “hot” or active signal. The ground

is the barrel of a ¼” connector and the shell of an “RCA” style connector.

A balanced signal contains two active signals instead of one in addition to the

ground. These are referred to as the “plus” and “minus” signals. A balanced input

amplifier amplifies the difference between these two signals. Any extraneous noise

picked up from power lines or other sources will appear equally on both the plus

and minus inputs. This is called “common mode” noise since it is common to both

signals and the input amplifier will subtract the noise on the minus input from the

noise on the plus input.

If the input amplifier is perfectly balanced and the noise on both plus and minus is

precisely equal, the noise will completely cancel out. In the real world this is not

the case and some of the common mode noise will still make it through, although

at a much reduced level. How well an input amplifier rejects this common mode

noise is called the “common mode rejection ratio” (abbreviated as CMRR) and is

expressed in dB.

Balanced outputs typically drive the plus and minus components of a balanced

signal in one of two ways. A “differentially” balanced output signal is one where

the minus output is the mirror image of the plus output. That is, if the “plus” output

is at +1 volt, then the “minus” output is at –1 volt.

An “impedance” balanced output provides most of the benefits of a differentially

balanced output at a lower cost. In this case only the plus component actually

carries the signal. The minus component is tied to ground through a resistor so that

the output impedance matches that of the plus component. Since the impedances