## A Simple Faust Program

Figure K.1 lists a minimal Faust program specifying the constant-peak-gain resonator discussed in
§B.6.4.
This appendix does not cover the Faust language itself, so the
Faust
Tutorial,
or
equivalent, should be considered prerequisite reading. We will
summarize briefly, however, the Faust operators relevant to this
example: signal processing blocks are connected in series via a colon
(`:`), and feedback is indicated by a tilde (~). The
colon and tilde operators act on ``block diagrams'' to create a larger
block diagram. There are also signal operators. For example, a
unit-sample
delay is indicated by appending a prime (`'`) after a signal
variable; thus, `x'` expands to `x : MEM` and indicates
a signal obtained by delaying the signal `x` by one sample.

Function application applies to operator symbols as well as names;
thus, for example, `+(x)` expands to `_,x : +`, and
`*(x)` expands to `_,x : *`, where `_` denotes a
simple ``wire''. There is a special unary minus in Faust (as of
release 0.9.9.4), so that `-x` is equivalent to
`0 - x`. However, `-(x)` still expands to `_,x : -`,
which is a blockdiagram that subtracts signal `x` from the
input signal on `_` .

The `with` block provides local definitions in the context of
the `process` definition.^{K.3}

Other aspects of the language used in this example should be fairly
readable to those having a typical programming background.
^{K.4}

process = firpart : + ~ feedback with { bw = 100; fr = 1000; g = 1; // parameters - see caption SR = fconstant(int fSamplingFreq, <math.h>); // Faust fn pi = 4*atan(1.0); // circumference over diameter R = exp(0-pi*bw/SR); // pole radius [0 required] A = 2*pi*fr/SR; // pole angle (radians) RR = R*R; firpart(x) = (x - x'') * g * ((1-RR)/2); // time-domain coefficients ASSUMING ONE PIPELINE DELAY: feedback(v) = 0 + 2*R*cos(A)*v - RR*v'; }; |

Constants such as `RR` in Fig.K.1 are better thought of as
*constant signals*. As a result, operators such as `*`
(multiplication) conceptually act only on signals. Thus, the
expression
`2*x` denotes the constant-signal
muliplied
pointwise by the signal `x`. The Faust compiler does a good job
of optimizing expressions so that operations are not repeated
unnecessarily at run time. In summary, a Faust expression expands
into a block diagram which processes causal signals,^{K.5} some of which may be constant signals.

**Next Section:**

Generating Faust Block Diagrams

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Frequency Response Plot: swanalmainplot.m