Apple Final Cut Pro 7 - Manual (Page 210)

• Instrument level: Between microphone and line level, around –20 dBV or so. Guitars
and keyboards usually output at instrument level.
• Line level (consumer): Consumer line level is output at –10 dBV.
• Line level (professional): Professional line level is output at +4 dBu (or dBm in older
equipment).
• Speaker level: This signal varies considerably depending on the amplifier used, but it is
very strong compared to the others because it is used to drive speakers.
• Headphone level: This signal is like speaker level, but much lower. The sole purpose of
this signal is to drive stereo headphones.
About Units of Analog Audio Measurement
Professional audio equipment typically uses higher voltage levels than consumer
equipment, and it also measures audio on a different scale. Keep the following points in
mind when using consumer and professional audio equipment together:
• 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 using 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.
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
cancelation to eliminate noise while maintaining the original audio signal.
210 Chapter 12 Connecting Professional Video and Audio Equipment