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78 Appendix A Audio and MIDI Basics
Digital Conversions
When an analog audio signal arrives at the inputs of your audio interface, it must be
converted into digital information before the computer can deal with it. This process is
called analog to digital conversion—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 heard through analog audio playback systems—an
amplifier and speakers. This process is called digital to analog conversion, which is
performed 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 are
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.
The sampling resolution—expressed as bit values—determines the precision of the
measuring scale used to store (the numbers of) each sample. Remember that
computers store audio files as digital information—a series of ones and zeroes. There
are three audio file resolutions commonly in use: 8, 16, and 24 bits. 24 bit systems are
commonly used these days.
To give you an idea of the relevance of the sampling resolution, imagine two people
are building a house. One is using a tape measure marked to the nearest foot. The
other has a tape measure marked to the nearest inch. Although the house built with
the tape measure accurate to the nearest foot may not fall down, the person using the
finer scale will build a more accurate house.