Apple Soundtrack Pro 2 - Manual (Page 494)

494 Appendix C Working with Professional Video and Audio Equipment
About Microphone, Instrument, and Line Level
Audio equipment can output line level at –10 dBV (consumer level), +4 dBm/dBu
(professional level), or microphone level, which is around 50 or 60 dB less than line
level. When you use a microphone, the level is very low, requiring a preamplifier to raise
the signal to line level before it can be recorded or processed. Most audio mixers,
cameras, and professional portable recording devices have built-in preamplifiers.
Instrument level is between microphone and line level, around –20 dBV or so. Guitars
and keyboards usually output at instrument level.
About Balanced Audio Signals
Audio cables can be either balanced or unbalanced, depending on their intended use.
For long cable runs, especially when using relatively low microphone levels, a three-wire
balanced audio circuit reduces noise. Balanced audio cables use the principle of phase
cancellation to eliminate noise while maintaining the original audio signal.
Signal Differences Between Pro and Consumer Equipment
Professional audio equipment typically uses higher voltage levels than consumer
equipment, and it also measures audio on a different scale.
 Professional analog devices measure audio using dBu (or dBm in older equipment).
0 dB on the audio meter is usually set to +4 dBu, which means optimal levels are
4 dB greater than 0 dBu (.775 V), or 1.23 V.
 Consumer audio equipment measures audio using dBV. The optimal recording level
on a consumer device is –10 dBV, which means the levels are 10 dB less than 0 dBV
(1 V), or 0.316 V.
Therefore, the difference between an optimal professional level (+4 dBu) and
consumer level (–10 dBV) is not 14 dB, because they are referencing different signals.
This is not necessarily a problem, but you need to be aware of these level differences
when connecting consumer and professional audio equipment together.