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Editing Audio
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.
The other important aspect of the measuring process is the sampling
resolution. The sampling resolution determines how accurately the
amplitude of each sample is measured. At present, the music industry
has settled on a system that provides 65,536 different values to assign to
the amplitude of a waveform at any given instant. Thus, each sample
saved by your computer requires 2 bytes (16 bits) to store, since it takes
2 bytes to store a number from Ð32,768 to 32,767. The scaling of the
electrical input signal level to amplitude value is determined by your
audio hardware and by the position of your input level control.
What if the amplitude of the sampled signal gets too high, such that a
16-bit number is not large enough to represent it? What typically
happens is that the signal is clipped, cut off at the maximum value.
Here is what a clipped waveform might look like:
Clipping is not usually desirable and may have unpleasant audible
effects. Sudden irregularities in the waveform of any type can cause
clicks, pops, and distortion of the original sound.
The Decibel Scale
In acoustics, the decibel (dB) scale is a scale for measuring the relative
loudness of two sounds. For example, environmental noise is often
measured as follows:
L = 20 log (p/p0)
where L is the sound pressure level (in dB), p is the sound pressure
amplitude, and p0 is a reference amplitude of 20 micropascals (less than
one billionth of atmospheric pressure). On this scale, a barely audible
sound (p = p0) has a sound pressure level of 0 dB, normal conversation (p
= 1,000*p0) is at a level of around 60 dB, and a jet engine at close range
(p = 1,000,000*p0) is at a level of around 120 dB.
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