# Your First Module with Faust

For our first module, we will make a simple low-pass filter, with one big knob to control
the cut off frequency.
First create your new project by creating a directory and populating it with a simple Faust template.
Choose first where you want this project to live. In this example we will put this project in
our home folder directly. Then type:

```shell-session
~$ mkdir LowPass
~$ cd LowPass
~/LowPass$ erbb init --name LowPass --language faust
~/LowPass$ erbb configure
```

If you are using Xcode open `project_vcvrack.xcodeproj` in the `artifacts` folder.

```{note}
In the following source code files, we have removed all comments to
keep the file easy to read for this tutorial.
```

Let's now add the pot with a big knob, by editing the `Lowpass.erbui` file,
as highlighted in the following `erbui` source code:

```{code-block} erbui
---
emphasize-lines: 9-13
---
// LowPass.erbui

module LowPass {
   width 12hp
   board kivu12
   material aluminum
   header { label "LowPass" }

   control freq Pot {      // 1
      position 6hp, 34mm   // 2
      style rogan, 6ps     // 3
      label "FREQ"         // 4
   }

   control audio_in AudioIn {
      position 6hp, 96mm
      label "IN"
   }

   control audio_out AudioOut {
      position 6hp, 111mm
      label "OUT"
   }
}
```

1. Add a pot and give it the name `freq` (we will see how names are used later)
2. Give it a position, first is on the horizontal axis, and second is on the vertical axis.
   Note that both have units, with `hp` being the Eurorack HP for convenience.
   Since half of 12HP is 6HP, this will place the pot at the horizontal center in the module
3. Let's give it a specific style, which here is the knob style. `rogan` is the brand of the knob used
   in many Mutable Instruments and Make Noise modules, and `6ps` is the Rogan manufacturer
   part name. You could use here `xlarge` instead. The style is optional, so you can remove
   this line and the engine will select one default style for you.
4. Give it an optional label, which will be printed on the front panel aluminum.

You can already build the project using {guilabel}`⌘B` and run VCV Rack to see what it looks like.

```{image} first-add-freq.png
:align: center
```

You can connect inputs and outputs, and you can hear that the input goes on the output
but moving the FREQ knob doesn't have any effect.

Let's now take care of this. 
One big advantage of Faust over C++ is the huge standard library of already meticulously
crafted audio objects at your finger tips!
Let's update `LowPass.dsp` to add our filter:

```{code-block} faust
---
emphasize-lines: 3-6
---
// LowPass.dsp

import("stdfaust.lib");

fc = hslider("freq", 1000, 100, 10000, 1);
process = fi.resonlp(fc,1,0.8);
```

We are using a [`hslider`](https://faustdoc.grame.fr/manual/syntax/#hslider-primitive)
which delivers a continuous value, which is what we want. Note
that we could have used [`vslider`](https://faustdoc.grame.fr/manual/syntax/#vslider-primitive)
and that wouldn't change a thing:
that's because Eurorack-blocks ignores all layout and appearance information dictated from
the Faust dsp file. Instead, the `erbui` content is used.

Finally, `hslider` is using the Faust primitive label `freq` exactly like it was defined in our `erbui` file:

```{code-block} erbui
---
emphasize-lines: 1
---
   control freq Pot {
      position 6hp, 34mm
      style rogan, 6ps
      label "FREQ"
   }
```

This is how you map your control name in `erbui` to the primitive label in Faust dsp file.
The mapping system is powerful, and what you see now is just some syntactic sugar in the
simple cases.
You can already build the project using {guilabel}`⌘B` and run VCV Rack to see what it looks like.

```{important}
Don't forget to set your VCV Rack Audio Output to 48kHz!
```

Why do we need to set the audio output to 48kHz?
This is because Eurorack-blocks controls the audio sample rate to ensure that how it
sounds in VCV Rack is the same as how it sounds on the hardware platform, and also
because the hardware platform only supports some specific sample rates.

You can read more about sample rates and latency in the Target Audio Configuration section
in this manual (coming soon).

You can test your plug-in with this typical configuration. Don't forget to put a VCA as a pure
VCO output can be quite loud!
You will get the following result, and moving the "FREQ" knob will change the sound as expected!

```{image} first-add-filter.png
:align: center
```