Guitar tuner

Before you start coding, you'll need to set up a new project in Anjuta. This will create all of the files you need to build and run the code later on. It's also useful for keeping everything together.

Most of the code in the file is template code. It loads an (empty) window from the user interface description file and shows it. More details are given below; skip this list if you understand the basics:

This code is ready to be used, so you can run it by clicking Run ▸ Execute.

A description of the user interface (UI) is contained in the GtkBuilder file. To edit the user interface, open src/guitar_tuner.ui. This will switch to the interface designer. The design window is in the center; widgets and widgets' properties are on the right, and the palette of available widgets is on the left.

The layout of every UI in GTK+ is organized using boxes and tables. Let's use a vertical GtkButtonBox here to assign six GtkButtons, one for each of the six guitar strings.

In the UI designer, you made it so that all of the buttons will call the same function, on_button_clicked, when they are clicked. We need to add that function in the source file.

To do this, open guitar_tuner.py while the user interface file is still open. Switch to the Signals tab, which you already used to set the signal name. Now take the row where you set the clicked signal and drag it into to the source file inside the class. The following code will be added to your source file:

def on_button_clicked (self, button):

This signal handler has two arguments: the usual Python class pointer, and the Gtk.Button that called the function.

For now, we'll leave the signal handler empty while we work on writing the code to produce sounds.

GStreamer is GNOME's multimedia framework — you can use it for playing, recording, and processing video, audio, webcam streams and the like. Here, we'll be using it to produce single-frequency tones.

Conceptually, GStreamer works as follows: You create a pipeline containing several processing elements going from the source to the sink (output). The source can be an image file, a video, or a music file, for example, and the output could be a widget or the soundcard.

Between source and sink, you can apply various filters and converters to handle effects, format conversions and so on. Each element of the pipeline has properties which can be used to change its behaviour.

In this simple example we will use a tone generator source called audiotestsrc and send the output to the default system sound device, autoaudiosink. We only need to configure the frequency of the tone generator; this is accessible through the freq property of audiotestsrc.

Change the import line in guitar_tuner.py, just at the beginning to :

from gi.repository import Gtk, Gst, GObject 

The Gst includes the GStreamer library. You also need to initialise GStreamer properly which is done in the main() method with this call added above the app = GUI() line:

Gst.init_check(sys.argv)

Then, copy the following function into the class in guitar_tuner.py somewhere:

def play_sound(self, frequency):
	pipeline = Gst.Pipeline(name='note')
	source = Gst.ElementFactory.make('audiotestsrc', 'src')
	sink = Gst.ElementFactory.make('autoaudiosink', 'output')

	source.set_property('freq', frequency)
	pipeline.add(source)
	pipeline.add(sink)
	source.link(sink)
	pipeline.set_state(Gst.State.PLAYING)

	GObject.timeout_add(self.LENGTH, self.pipeline_stop, pipeline)

We don't want to play an annoying tone forever, so the last thing play_sound does is to call GObject.timeout_add. This sets a timeout for stopping the sound; it waits for LENGTH milliseconds before calling the function pipeline_stop, and will keep calling it until pipeline_stop returns False.

Now, we'll write the pipeline_stop function which is called by GObject.timeout_add. Insert the following code above the play_sound function:

def pipeline_stop(self, pipeline):
	pipeline.set_state(Gst.State.NULL)
	return False

You need to define the LENGTH constant inside the class, so add this code at the beginning of the main class:

LENGTH = 500

The call to pipeline.set_state stops the playback of the pipeline.

We want to play the correct sound when the user clicks a button. First of all, we need to know the frequencies for the six guitar strings, which are defined (at the beginning of the main class) inside a dictionary so we can easily map them to the names of the strings:

# Frequencies of the strings
frequencies = {
	'E': 329.63,
	'A': 440,
	'D': 587.33,
	'G': 783.99,
	'B': 987.77,
	'e': 1318.5
}

Now to flesh out the signal handler that we defined earlier, on_button_clicked. We could have connected every button to a different signal handler, but that would lead to a lot of code duplication. Instead, we can use the label of the button to figure out which button was clicked:

def on_button_clicked(self, button):
	label = button.get_child()
	text = label.get_label()

	self.play_sound (self.frequencies[text])

The button that was clicked is passed as an argument (button) to on_button_clicked. We can get the label of that button by using button.get_child, and then get the text from that label using label.get_label.

The label text is then used as a key for the dictionary and play_sound is called with the frequency appropriate for that note. This plays the tone; we have a working guitar tuner!

All of the code should now be ready to go. Click Run ▸ Execute to start the application. Enjoy!

If you run into problems with the tutorial, compare your code with this reference code.

Here are some ideas for how you can extend this simple demonstration: