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Sample Rate
The rate at which the ADC generates the numbers is equally important in
determining the quality of a digital recording. To get a high level of
accuracy when sampling, the sample rate must be greater than twice the
frequency being sampled. The mathematical statement of this is called the
Nyquist Theorem. When dealing with full-bandwidth sound (20Hz
−
20kHz),
you should sample at greater than 40,000 times per second (twice 20kHz).
Your Echo hardware allows you to sample at rates up to 96,000 times per
second.
If the sampling rate is lower than the frequency you are trying to record,
entire cycles of the waveform will be missed, and the result will not
resemble the proper waveform. When the sample rate is too low, the
resulting sound has diminished high frequency content.
Figure C. Increased sample rates yield a more accurate reproduction of the
source signal.
By the way, the circuits that generate the sample rate must be exceedingly
accurate. Any difference between the sample rate used for recording and the
rate used at playback will change the pitch of the recording, just as with an
analog tape playing at the wrong speed. Also, any unsteadiness, or jitter, in
the sample clock will distort the signal as it is being converted from or to
analog form.
