Echo AUDIOFIRE 8 Owner's Manual

Also see for AUDIOFIRE 12: Owner's manual

Appendix B: An Introduction to Digital RecordingUnbalanced and Balanced Inputs and OutputsAn unbalanced signal, commonly used for guitars and consumer electronics,contains two components, a ground signal and a “hot” or active signal. The groundis 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 theground. These are referred to as the “plus” and “minus” signals. A balanced inputamplifier amplifies the difference between these two signals. Any extraneous noisepicked up from power lines or other sources will appear equally on both the plusand minus inputs. This is called “common mode” noise since it is common to bothsignals and the input amplifier will subtract the noise on the minus input from thenoise on the plus input.If the input amplifier is perfectly balanced and the noise on both plus and minus isprecisely equal, the noise will completely cancel out. In the real world this is notthe case and some of the common mode noise will still make it through, althoughat a much reduced level. How well an input amplifier rejects this common modenoise is called the “common mode rejection ratio” (abbreviated as CMRR) and isexpressed in dB.Balanced outputs typically drive the plus and minus components of a balancedsignal in one of two ways. A “differentially” balanced output signal is one wherethe minus output is the mirror image of the plus output. That is, if the “plus” outputis at +1 volt, then the “minus” output is at –1 volt.An “impedance” balanced output provides most of the benefits of a differentiallybalanced output at a lower cost. In this case only the plus component actuallycarries the signal. The minus component is tied to ground through a resistor so thatthe output impedance matches that of the plus component. Since the impedancesare matched, any noise added to the minus component will still be approximatelythe same as that picked up by the plus component and will be subtracted out by theinput amplifier. Your Echo product uses this type of output.46Appendix B: An Introduction to Digital Recording