The basics of routing audio

Get ready because we’re going to start using the word node a lot. I hope that’s okay. A node is what it sounds like: it’s a node. Or a point of connection.

Effect audio nodes

Each effect has one or more nodes that can pipe audio into it or out of it. All effects that process audio have both an input node and an output node. Things like an envelope tracker that are just measuring an audio signal may just have an audio input node but no audio output node. Also, some effects have additional nodes beyond input and output and this is where shit gets real. Did you see the movie Inception? That question will make sense eventually.

Details on the nodes that each effect has can be found in Appendix A.

System audio nodes

And the system has nodes for input from instrument and output to amp.

  • pedal.instr_in is the input jack of the pedal. This might blow your mind, but this is actually an output jack in the sense that it is outputting audio that we can send to the inputs of other effects.

  • pedal.amp_out is the output hack of the pedal. This might blow your mind again, but this is actually an input jack in the sense that it is receiving audio from other effects (and then sending to the amp).

Connecting nodes

As we just saw in the echo example, there is a function called route_audio that we use to connect our effects to the input and output jacks of the pedal and also to each other. The first argument of this function is an output node and the second argument is an input node.

Let’s use it in a (programming) sentence. In this example, we’re going to have a tremelo that then feeds into a delay. It’ll be like having your guitar plugged into a tremelo pedal that then plugs into a delay pedal that then plugs into your amp.

(note: if it’s not yet obvious, you can call each effect you create just about whatever you want).

// Create objects for these effects
fx_amplitude_mod  happy_tremelo(1.0, 0.5);      // 1Hz rate, 0.5 depth
fx_delay          sweet_baby_echo(1000.0, 0.7); // 1000ms, 0.7 feedback  

void setup() {
  pedal.init();   // Initialize the system

  // Route tremelo through echo/delay effect
  pedal.route_audio(pedal.instr_in, happy_tremelo.input);
  pedal.route_audio(happy_tremelo.output, sweet_baby_echo.input);
  pedal.route_audio(sweet_baby_echo.output, pedal.amp_out);;    // Run the effect

Or let’s get more crazy. Let’s say we have a delay pedal and each time through the delay, we’re going to pitch shift up. So it would sound like this: ECHO Echo echo echo (where each time you say ‘echo’ you say it in a lower pitch voice).

The fx_delay has two additional nodes called fx_send and fx_receive. We’re going to run these through our handy-dandy pitch shifter. For this, we’re going to use the more advanced delay setup function that allows us to pass a few additional parameters (more info in Appendix A on this):

// Create objects for these effects
fx_delay          echoey_snail(1000.0,     // Delay length: 1000ms
                              1000.0,     // Max delay length: 1000ms
                              0.7,        // Feedback: 0.7
                              1.0,        // Feedthrough: 1.0
                              true);      // Enable delay fx loop
fx_pitch_shift    shift_down(0.85);       // Pitch shift down 0.85 x current pitch

void setup() {
  pedal.init();   // Initialize the system

  // input -> delay -> output
  pedal.route_audio(pedal.instr_in, echoey_snail.input);
  pedal.route_audio(echoey_snail.output, pedal.amp_out);

  // Now patch in pitch shifter into delay fx loop
  pedal.route_audio(echoey_snail.fx_send, shift_down.input);
  pedal.route_audio(shift_down.output, echoey_snail.fx_receive);;    // Run the effect

Pretty cool, right?

A few routing rules

Obey these rules to avoid humiliation and sadness:

  1. An output node can be routed to multiple input nodes

    pedal.route_audio(pedal.instr_in, delay_1.input);
    pedal.route_audio(pedal.instr_in, delay_2.input);

  2. An input node can only have one input. However, you can use the fx_mixer nodes if you want to send multiple outputs to an input.

    pedal.route_audio(delay_1.output, my_mixer_2.input_1);
    pedal.route_audio(delay_2.output, my_mixer_2.input_2);
    pedal.route_audio(my_mixer_2, pedal.amp_out);

  3. And you can’t route input nodes to other input nodes, or output nodes to other output nodes. It’s always output->input.