Mackie Digital 8 Bus Owner's Manual page 201

Figure B-2. Sampling
tures are shown at a rate of less than 20 per
second, the eye can distinguish between indi-
vidual frames. But above 20 pictures per
second they tend to blend into a continuous
stream. In the U.S., films are made at a rate of
24 frames per second. Due to the nature of
sound and the sensitivity of the ear, audio sig-
nals must be sampled at a much higher rate.
Figure B-3. Aliasing
B-2
Digital 101
One Sampling
Period
Experiments have shown that if still pic-
Signal below the Nyquist Frequency
Time
Signal at the Nyquist Frequency
Time
Signal above the Nyquist Frequency results in aliasing
Time
Time
Sampling
The sampling rate is defined as the number
of samples, or snapshots, the ADC takes of the
analog audio signal per second. The most com-
mon sampling rate used in digital audio is
44.1kHz (44,100 samples per second), because
that is what is used in making CDs. This may
seem like a lot, and it is, but remember that we
have to be able to sample frequencies up to
20kHz. With a 20kHz signal, this allows for
slightly over two samples per cycle.
A 20Hz signal, on the other hand, re-
ceives over 2000 samples per cycle!
In fact, there is a rule called the
Nyquist Theorem, which states that
in order to accurately represent a sig-
nal there must be at least two
samples per cycle. So when using a
44.1kHz sampling rate, the highest
frequency that can be accurately rep-
resented is 22.05kHz. Attempting to
convert anything higher than half the
sampling frequency results in a phe-
nomenon called aliasing. Aliasing
occurs when the digital-to-analog con-
verter is fooled into thinking that the
frequency of the signal is lower than it
actually is. Figure B-3 might help you
better understand this concept.
Quantization
Another important aspect of digi-
tal audio is quantization , or sampling
resolution. Sampling represents the
time of the measurement, and quanti-
zation represents the amplitude value
of the measurement. This goes back
to our discussion about the binary
number system, so let's define a few
more terms: