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<H4>
A Beginning Tutorial</H4>
<H2>
More about the Orchestra</H2>
Suppose we next wished to introduce a small vibrato, whose rate is 1/50
the frequency of the note (i.e. A440 is to have a vibrato rate of 8.8 Hz.).
To do this we will generate a control signal using a second oscillator,
then add this signal to the basic frequency derived from p5. This might
result in the instrument
<PRE><TT>instr 1
kamp <B>line </B> 0, p3, 10000
kvib <B>oscil </B> 2.75, cpspch(p5)/50, 1
a1 <B>oscil</B> kamp, cpspch(p5)+kvib, 1
<B>out</B> a1
endin</TT></PRE>
Here there are two control signals, one controlling the amplitude and the
other modifying the basic pitch of the audio oscillator. For small vibratos,
this instrument is quite practical; however it does contain a misconception
worth noting. This scheme has added a sine wave deviation to the cps value
of an audio oscillator. The value 2.75 determines the <I>width</I> of vibrato
in cps, and will cause an A440 to be modified about one-tenth of one semitone
in each direction (1/160 of the frequency in cps). In reality, a cps deviation
produces a different musical interval above than it does below. To see
this, consider an exaggerated deviation of 220 cps, which would extend
a perfect 5th above A440 but a whole octave below. To be more correct,
we should first convert p5 into a <I>true decimal octave</I> (not cps),
so that an <I>interval</I> deviation above is equivalent to that below.
In general, pitch modification is best done in true octave units rather
than pitch-class or cps units, and there exists a group of pitch converters
to make this task easier. The modified instrument would be
<PRE><TT> instr 1
ioct <B>=</B> octpch(p5)
kamp <B>line </B> 0, p3, 10000
kvib <B>oscil</B> 1/120, cpspch(p5)/50, 1
asig <B>oscil</B> kamp, cpsoct(ioct+kvib), 1
<B>out</B> asig
endin</TT></PRE>
This instrument is seen to use a third type of orchestra variable, an <I>i-</I>variable.
The variable <I>ioct</I> receives its value at an <I>initialization</I>
pass through the instrument, and does not change during the lifespan of
this note. There may be many such <I>init time</I> calculations in an instrument.
As each note in a score is encountered, the event space is allocated and
the instrument is initialized by a special pre-performance pass. <I>i-variables</I>
receive their values at this time, and any other expressions involving
just constants and <I>i-variables</I> are evaluated. At this time also,
modules such as <B><A HREF="../Generate/line.html">line</A></B> will set
up their target values (such as beginning and end points of the line),
and units such as <B><A HREF="../Generate/oscil.html">oscil</A></B> will
do phase setup and other bookkeeping in preparation for performance. A
full description of init-time and performance-time activities, however,
must be deferred to a <A HREF="../Syntax/SYNTAX.html">general consideration
of the orchestra syntax</A>.
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