Software Implementation in Faust

The Faust language for signal processing is introduced in Appendix K. Figure 3.4 shows a Faust program for implementing our example comb filter. As illustrated in Appendix K, such programs can be compiled to produce LADSPA or VST audio plugins, or a Pure Data (PD) plugin, among others.

Figure 3.4: Faust main program implementing the example digital filter. (Tested in Faust version 0.9.9.2a2.)

 

/* GUI Controls */
g1  = hslider("feedforward gain", 0.125, 0, 1, 0.01);
g2  = hslider("feedback gain", 0.59049, 0, 1, 0.01);

/* Signal Processing */
process = firpart : + ~ feedback
with {
  firpart(x) = x + g1 * x''';
  feedback(v) = 0 - g2 * v'''';
};

As discussed in Appendix K, a prime (') denotes delaying a signal by one sample, and a tilde (~) denotes feedback. A colon (:) simply indicates a connection in series. The feedback signal v is delayed only four samples instead of five because there is a free ``pipeline delay'' associated with the feedback loop itself.

Faust's -svg option causes a block-diagram to be written to disk for each Faust expression (as further discussed in Appendix K). The block diagram for our example comb filter is shown in Fig.3.5.

Figure 3.5: Block diagram generated by the Faust -svg option.

Compiling the Faust code in Fig.3.4 for LADSPA plugin format produces a plugin that can be loaded into ``JACK Rack'' as depicted in Fig.3.6.

Figure 3.6: JACK Rack screenshot for the example comb filter.

At the risk of belaboring this mini-tour of filter embodiments in common use, Fig.3.7 shows a screenshot of a PD test patch for the PD plugin generated from the Faust code in Fig.3.4.

Figure: Pure Data (PD) screenshot for a test patch exercising a PD plugin named cf.pd that was generated automatically from the Faust code in Fig.3.4 using the faust program and faust2pd script (see Appendix K for details).

By the way, to change the Faust example of Fig.3.4 to include its own driving noise, as in the STK example of Fig.3.3, we need only add the line

  import("music.lib");

at the top to define the noise signal (itself only two lines of Faust code), and change the process definition as follows:

  process = noise : firpart : + ~ feedback

In summary, the Faust language provides a compact representation for many digital filters, as well as more general digital signal processing, and it is especially useful for quickly generating real-time implementations in various alternative formats. Appendix K gives a number of examples.