Apple Final Cut Express HD - Manual (Page 574)

574 Part VIII Audio Mixing
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.
Audio Connectors
Different audio connectors are suited for different purposes. Audio connectors are
often indicative of the kind of signal they transmit. However, there are enough
exceptions that it’s important to know what kind of audio signal you are connecting, in
addition to the connector type. An important distinction is whether an audio
connector carries a balanced or an unbalanced signal.
Signal Differences Between Pro and Consumer Equipment
Professional audio equipment typically uses higher voltage levels than consumer
equipment, and 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.