Apple Soundtrack Pro 3 - Manual (Page 493)

• dBu: This reference level measures voltage instead of power, using a reference level of
0.775 volts. dBu has mostly replaced dBm on professional audio equipment. The u
stands for unloaded, because the electrical load in an audio circuit is no longer as
relevant as it was in the early days of audio equipment.
• dBV: This also uses a reference voltage like dBu, but in this case the reference level is
1 volt, which is more convenient than 0.775 volts in dBu. dBV is often used on consumer
and semiprofessional devices.
• dBFS: This scale is very different from the others because it is used for measuring digital
audio levels. FS stands for full-scale, which is used because, unlike analog audio signals
that have an optimum signal voltage, the entire range of digital values is equally
acceptable when using digital audio. 0 dBFS is the highest-possible digital audio signal
you can record without distortion. Unlike analog audio scales like dBV and dBu, there
is no headroom past 0 dBFS.
Signal-to-Noise Ratio
Every electrical system produces a certain amount of low-level electrical activity called
noise. The noise floor is the level of noise inherent in a system. It is nearly impossible to
eliminate all the noise in an electrical system, but you don’t have to worry about the noise
if you record your signals significantly higher than the noise floor. If you record audio too
low, you raise the volume to hear it, which also raises the volume of the noise floor,
causing a noticeable hiss.
The more a signal is amplified, the louder the noise becomes. Therefore, it is important
to record most audio around the nominal (ideal) level of the device, which is labeled 0 dB
on an analog audio meter.
The signal-to-noise ratio, typically measured in dB, is the difference between the nominal
recording level and the noise floor of the device. For example, the signal-to-noise ratio
of an analog tape deck may be 60 dB, which means the inherent noise in the system is
60 dB lower than the ideal recording level.
Headroom and Distortion
If an audio signal is too strong, it will overdrive the audio circuit, causing the shape of the
signal to distort. In analog equipment, distortion increases gradually the more the audio
signal overdrives the circuit. For some audio recordings, this kind of distortion can add a
unique “warmth” to the recording that is difficult to achieve with digital equipment.
However, for audio post-production, the goal is to keep the signal clean and undistorted.
493 Appendix B Audio Fundamentals