Reference Guide

550
Editing Audio
Digital Audio Fundamentals
Back and forth motions of the vibrating object translate to upward (positive) and downward
(negative) excursions of waveform amplitude. For example, a close-up of a portion of the guitar
waveform might look like this:
The waveform crosses the zero line twice during each complete vibration. These zero-crossings
are important in digital audio processing; they are good places to cut waveforms apart and splice
them together. If waveforms are cut or spliced at other locations, clicks and pops can occur. The
maximum amplitude of the waveform in each vibration is also important: it determines the strength of
the vibration, and thus the loudness of the sound.
See:
Digital Audio Fundamentals
Recording a Sound
To record digital audio, your computer monitors the electrical signal generated by a microphone (or
some other electroacoustical device). Because the signal is caused by a sound, the signal strength
varies in direct proportion to the sound’s waveform. The computer measures and saves the strength
of the electrical signal from the microphone, thus recording the waveform.
There are two important aspects of this measuring process. First is the sampling rate, the rate at
which the computer saves measurements of the signal strength. It is a known fact of physics that
you must measure, or sample, the signal at a rate at least twice that of the highest frequency you
wish to capture. For example, suppose you want to record a moderately high note on a violin—say
the A whose fundamental frequency is 440 Hz and all overtones up to five times the fundamental.
The highest frequency you want to capture is 2,200 Hz, so you need to measure the electrical signal
from the microphone at least 4,400 times per second.
Since humans can hear frequencies well above 10 kHz, most sound cards and digital recording
systems are capable of sampling at much higher rates than that. Typical sampling rates used by
modern musicians and audio engineers are 22 kHz, 44.1 kHz, and 48 kHz. The 44.1 kHz rate is
called CD-quality, since it is the rate used by audio compact discs.