Aliasing/Foldover (see info below)
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Nyquist Frequency (see info below)
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Tweaking
Be able to describe how you would extend the
range of any synthesizer beyond the confines of the given number of
keys on the keyboard. For example, if you needed all 88 keys of
a traditional piano and your synthesizer only had 60 or so keys, what
would you do to be able to record the entire range of the piano?
Be
able to describe the easiest way to sequence record a transposed part
found in an orchestra score.
Be able to describe how
you would correct a wrong note that you recorded in a particular
measure.
Be able to describe how you would make volume
adjustments for: an individual note, an entire track.
Be
able to describe the shortcut for recording a repeated passage
identical to the first.
Be
able to describe the difference between Sound on Sound (a.k.a.
overdub) and Overwrite modes.
Be able to list
the number of individual channels in the standard MIDI network.
Above
all, know what the acronym MIDI stands for!
Re Nyquist Frequency and Aliasing/Foldover ...
SAMPLE RATE / NYQUIST
THEOREM--the sample rate must be at least twice the frequency
of the highest sound frequency to accurately reproduce a waveform
(sound). E.g., if the highest frequency is the fundamental tone
A440, the sample rate must be at least 880 Hz to capture the
fundamental tone without distortion. To capture the faster
frequencies of overtones, the sampling rate must be increased (double
the rate of the overtone frequencies).
-
The Nyquist frequency is the frequency
whose value is half that of the sampling rate. Recording any
frequencies higher than this pitch will cause unwanted distortion
(aliasing/foldover). In other words, the Nyquist frequency
indicates the highest pitch which may be accurately sampled, given
the sampling rate.
| A good illustration of this principle can be
found in weather. To represent the annual fluctuation of
the temperature outdoors, for example, it is necessary to
record a thermometer reading once each January or so, and
again exactly six months later, in July. At this
sampling rate of two "samples" per year, however,
the daily fluctuation of temperature would not be
detected. To represent this additional fluctuation of
temperature, it is necessary to record the thermometer level
twice each day--once at midday and again exactly 12 hours
later, in the middle of the night. To represent both the
annual and the daily temperature frequencies, then, requires a
sampling rate of at least 730 temperature readings per year. |
Example: A waveform includes
a fundamental frequency of 100 Hz and an overtone with a frequency of
400 Hz. A sampling rate of 800 samples per second is more than
adequate to represent the fundamental frequency, and is also
sufficient to represent the frequency of the overtone.
Example: A waveform includes
a fundamental frequency of 150 Hz and an overtone with a frequency of
600 Hz. A sampling rate of 800 samples per second is more than
adequate to represent the fundamental frequency, but is not sufficient
to represent the frequency of the overtone. Instead, the pattern
formed from the samples of the overtone is that of a sound with a
frequency of 200 Hz. This is called an alias, or foldover,
frequency. As is usually the case, this alias frequency is not a
harmonic overtone of the given fundamental.
