Digital Sound & Music: Concepts, Applications, & Science, Chapter 5, last updated 6/25/2013
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5.2 Applications
5.2.1 Choosing an Appropriate Sampling Rate
Before you start working on a project you should decide what sampling rate you're going to be
working with. This can be a complicated decision. One thing to consider is the final delivery of
your sound. If, for example, you plan to publish this content only as an audio CD, then you might
choose to work with a 44,100 Hz sampling rate to begin with since that’s the required sampling
rate for an audio CD. If you plan to publish your content to multiple formats, you might choose
to work at a higher sampling rate and then convert down to the rate required by each different
output format.
The sampling rate you use is directly related to the range of frequencies you can sample.
With a sampling rate of 44,100 Hz, the highest frequency you can sample is 22,050 Hz. But if
20,000 Hz is the upper limit of human hearing, why would you ever need to sample a frequency
higher than that? And why do we have digital systems able to work at sampling rates as high as
192,000 Hz?
First of all, the 20,000 Hz upper limit of human hearing is an average statistic. Some
people can hear frequencies higher than 20 kHz, and others stop hearing frequencies after 16
kHz. The people who can actually hear 22 kHz might appreciate having that frequency included
in the recording. It is, however, a fact that there isn’t a human being alive who can hear 96 kHz,
so why would you need a 192 kHz sampling rate?
Perhaps we’re not always interested in the range of human hearing. A scientist who is
studying bats, for example, may not be able to hear the high frequency sounds the bats make but
may need to capture those sounds digitally to analyze them. We also know that musical
instruments generate harmonic frequencies much higher than 20 kHz. Even though you can’t
consciously hear those harmonics, their presence may have some impact on the harmonics you
can hear. This might explain why someone with well-trained ears can hear the difference
between a recording sampled at 44.1 kHz and the same recording sampled at 192 kHz.
The catch with recording at those higher sampling rates is that you need equipment
capable of capturing frequencies that high. Most microphones don’t pick up much above 22 kHz,
so running the signal from one of those microphones into a 96 kHz ADC isn’t going to give you
any of the frequency benefits of that higher sampling rate. If you’re willing to spend more
money, you can get a microphone that can handle frequencies as high at 140 kHz. Then you need
to make sure that every further device handling the audio signal is capable of working with and
delivering frequencies that high.
If you don’t need the benefits of sampling higher frequencies, the other reason you might
choose a higher sampling rate is to reduce the latency of your digital audio system, as is
discussed in Section 5.2.3.
A disadvantage to consider is that higher sampling rates mean more audio data, and
therefore larger file sizes. Whatever your reasons for choosing a sampling rate, the important
thing to remember is that you need to stick to that sampling rate for every audio file and every
piece of equipment in your signal chain. Working with multiple sampling rates at the same time
can cause a lot of problems.
5.2.2 Input Levels, Output Levels, and Dynamic Range
In this section, we consider how to set input and output levels properly and how these settings
affect dynamic range.
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