Apple Logic (1.1) - Manual (Page 91)

Chapter A Audio and MIDI Basics 91
On a more practical level, digital audio files stored on hard disk have the following
advantages over analog audio stored on tape.
• Digital audio can be cut, copied, and pasted freely from location to location. Could
you imagine physically cutting a stereo analog recording stored on tape into even
four or five segments and splicing them back together? Not much fun!
• You can move to any spot in a digital audio file almost instantly rather than waiting
for the tedious mechanical rewinding and fast forwarding of an analog tape
machine.
• You can process digital audio in ways that are simply impossible with analog audio.
As one example, it is possible to adjust the pitch of a digital audio file without
changing its speed.
Digital Conversions
When an analog audio signal arrives at the inputs of your computer’s audio interface, it
must be converted into digital information before the computer can deal with it. This
process is called analog to digital conversion and is handled by the Analog to Digital
Converter (ADC) of your audio interface.
At the other end of the signal path, the digital signal needs to be reconverted into an
analog signal, so that it can be listened to through analog audio playback systems—an
amplifier and speakers. This process is called digital to analog conversion and is dealt
with by the Digital to Analog Converter (DAC) of your audio interface.
About Bits and Sampling Rates
When recording a sound into your computer, the ADC of your audio interface takes a
“sample” of the sound source, a specific number of times per second. This is known as
the “sample rate”, which is measured in kHz (kilohertz—kilo means thousand) or “x”
thousand samples per second. The higher the rate, the more samples the AD converter
takes, and the more accurate the digital representation of the sound will be.
Put another way, imagine each sample to be a photograph of a concert that you were
trying to describe to a friend. If you had taken one photo every 10 minutes, you’d find it
hard to describe the great light show. If you took a thousand photos during that 10
minutes, your friend could easily see what happened. This increased “rate”, or number of
photos (samples), delivers a far more accurate overall picture of what happened. This is
how sampling works. The more “sonic photos” taken, the more accurately the sound is
represented over time. A videotape recording of the concert—running at 25 frames
(samples) a second, would give an even more accurate idea of the light show.
Common sample rates used in audio production include: 44.1 kHz or 48 kHz, and high-
end systems may use rates from 96 kHz up to 192 kHz.