Pro Tools 101_ An Introduction to Pro Tools 11 ( PDFDrive )

If this continuous analog signal is captured on traditional recording media,
such as magnetic tape, it can be played back by directly translating the
electrical waveform, frequency, and amplitude back into analogous
variations in air pressure through the means of an amplifier and a
loudspeaker.
Analog-to-Digital Conversion
Before you can record or edit with Pro Tools, the analog audio signals
relayed by a microphone, guitar pickup, or other input device must be
digitized, or translated into digital (binary) numerical information that can be
stored, read, and subsequently manipulated by a computer. This process is
referred to as analog-to-digital conversion, commonly abbreviated as A/D
conversion. Two essential factors affect the A/D process: sample rate and
bit depth.
How Sample Rate Affects Frequency Resolution
Sampling is the process of taking discrete readings of a signal at various
moments in time. Each reading, or sample, is a digital “snapshot” of the
signal at that particular instant. Played back in succession, these samples
approximate the original signal, much like a series of photographs played
back in succession approximates movement in a film or video.
The sample rate is the frequency with which these digital snapshots are
collected. The sample rate required for digital audio is driven by a
fundamental law of analog-to-digital conversion, referred to as the Sampling
Theorem or the Nyquist Theorem.
The Nyquist Theorem states that in order to produce an accurate
representation of a given frequency of sound, each cycle of the sound’s
vibration must be sampled a minimum of two times. If the sample rate is any
lower, the system will read the incoming frequencies inaccurately and
produce the wrong tones. (In concept, this is much like the effect seen in
early motion pictures, where a wagon wheel will appear to rotate backward
due to the low frame rates being used.) In digital audio, the false tones
produced by this type of frequency distortion are known as alias tones.
Because the range of human hearing is generally accepted to be 20 Hz to 20
kHz, this law indicates that a sampling rate of at least 40 kHz (twice the
upper range of human hearing) is required to capture full-frequency audio.