ALESIS SR-18 Portable Drum Machine - Manual (Page 56)

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UNDERSTANDING RHYTHMIC NOTATION
Measures A piece of music is divided into smaller units called measures (also
called bars), and each measure is divided into beats. In the SR18, each beat is
further sub-divided into 96 sub-beats.
Rhythmic Values for Notes With a measure written in a 4/4 time signature,
there are 4 beats per measure, and each beat represents a quarter (1/4) note.
Thus, there are 4 quarter notes per measure of 4/4 music. With a 3/4 time
signature the "numerator" indicates that there are 3 beats per measure, while the
"denominator" indicates that each of these beats is a quarter note (1/4).
There are two
eighth notes per quarter note. Thus, there are eight eighth notes
per measure of 4/4 music.
There are four
16th notes per quarter note. Thus, there are sixteen 16th notes
per measure of 4/4 music.
There are eight
32nd notes per quarter note. Thus, there are thirty-two 32nd
notes per measure of 4/4 music.
There are also notes that span a greater number of beats than quarter notes. A
half note equals two quarter notes. Therefore, there are two half notes per
measure of 4/4 music. A whole note equals four quarter notes, so there is one
whole note per measure of 4/4 music. (We keep referring these notes in 4/4 time
because that is the most commonly used time signature in contemporary
Western music.)
Triplets The above notes divide measures by factors of two. However, there are
some cases where you want to divide a beat into thirds, giving three notes per
beat. Dividing a quarter note by three results in
eighth-note triplets . The reason
why we use the term eighth-note triplets is because the eighth note is closest to
the actual rhythmic value. Dividing an eighth note by three results in
16th-note
triplets. Dividing a 16th note by three results in 32nd note triplets.
Rests You can also specify where notes should not be played; this is indicated
by a rest, which can be the same length as any of the rhythmic values used for
notes.
Time Signatures 4/4 (and to a lesser extent 3/4) are the most common time
signatures in our culture, but they are by no means the only ones. In jazz, both
5/4 (where each measure consists of five quarter notes) and 7/4 (where each
measure consists of seven quarter notes) are often used. In practice, complex
time signatures are played like a combination of simpler time signatures; for
example, some 7/4 compositions would have you count each measure not as 1,
2, 3, 4, 5, 6, 7 but as 1, 2, 3, 4, 1, 2, 3. It's often easier to think of 7/4 as one bar
of 4/4 followed by one bar of 3/4 (or the other way around, depending on the
phrasing), since, as we mentioned, 4/4 and 3/4 are extremely common time
signatures.