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Understanding sound quality
When a sound card is digitizing audio, it is measuring
the voltage coming from the audio source many times per second.
These measurements are called "samples," and they are
saved to the hard drive as a list of numbers. When it is time to
play back the sound, the computer can then read the numbers from
the hard drive and reproduce the original voltage levels.
The quality of digital audio depends on two factors
- the "sample rate" and the "sample size." Sample
rates usually lie between 8 kHz and 44.1 kHz; a sample rate of 44.1
kHz means that the sound is sampled, or measured, 44,000 times per
second. 11 kHz is considered phone quality, 22.5 kHz is radio quality,
and 44.1 kHz is CD quality.
Sample size, usually 8 or 16 bits, tells how accurately
the voltage is measured. For an 8-bit sample, numbers between 0
and 255 are used to describe voltages. For a 16-bit sample, numbers
between 0 and 65,536 are used to describe 0%-100% voltages. Using
a higher sample size gives a closer approximation to the original
sound with smoother transitions between voltages.
CDs use a 44.1-kHz sample rate and a 16-bit sample
size; that is 44,000 samples per second, with each sample taking
up 16 bits (two bytes). Therefore one second of CD-quality sound
would take up 88,000 bytes, or 88K of disk space. This would be
doubled if the sound were in stereo.
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