Mackie 8-BUS Series, Owner'S Manual

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B-2
D i g i t a l 8 • B u s
Digital 101
Experiments have shown that if still pic-
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.
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:
Amplitude
Time
One Sampling
Period
Figure B-2. Sampling
Amplitude
Time
Amplitude
Time
Amplitude
Time
Signal below the Nyquist Frequency
Signal at the Nyquist Frequency
Signal above the Nyquist Frequency results in aliasing
Figure B-3. Aliasing