Apple Final Cut Express HD - Manual (Page 584)

584 Part VIII Audio Mixing
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 is the difference between the nominal recording level and the
noise floor of the device, and is typically measured in dB. 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.
0 dB on an analog meter refers to the ideal recording level, but there is some allowance
for stronger signals before distortion occurs. This safety margin is known as headroom,
meaning that the signal can occasionally go higher than the ideal recording level without
distorting. Having headroom is critical when recording, especially when the audio level is
very dynamic and unpredictable. Even though you can adjust the recording level while
you record, you can’t always anticipate quick, loud sounds. The extra headroom above
0 dB on the meter is there in case the audio abruptly becomes loud.