Creating Your First Block

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This tutorial will guide you through creating your first block with the Embedded Python Block. The previous tutorial is here: Streams and Vectors

The example block we are creating will either add or multiply the two blocks based on an input parameter.

Opening Code Editor

The Embedded Python Block is a tool to quickly prototype a block within a flowgraph. Search for the Python Block and add it to the workspace:

AddPythonBlockToWorkspace.png


Double-click the box to edit the properties. The Embedded Python Block has two properties,

  1. Code, a click-box which contains a link to the Python code for the block and
  2. Example_Param, an input parameter to the block.


Click on Open in Editor to edit the Python code:

EmbeddedPythonBlockProperties.png


You will be prompted with another choice for which editor to use to write the Python code. Click Use Default:

ClickUseDefault.png


An editor window will then display the Python code for the Embedded Python Block:

PythonCodeGedit.png


Components of a Python Block

There are three important sections in the Python block code:

  1. import statements in green
  2. __init__ function in orange
  3. work function in red

PythonBlockCodeFunctions.png


The import statement includes the NumPy and GNU Radio libraries.

The __init__ statement:

  1. Accepts the example_param parameter with a default argument of 1.0
  2. Declares the block to have a np.complex64 input and output, which is the GNU Radio Complex Float 32 data type
  3. Stores the self.example_param variable from the input parameter

The work function:

  1. Has the input input_items and output output_items parameters
  2. Applies a mathematical operation to input_items and stores the result in output_items
  3. Returns the number of samples produced


Changing Parameter Name

The first step will be to rename example_param to additionFlag to be more descriptive. From the editor menu select Find and Replace:

SelectFindAndReplace.png


Enter example_param under Find, and additionFlag under Replace with, and then click Replace All:

FindReplaceExamplePara.png


The parameter will be changed and the Python code will then look like this:

PythonBlockAdditionFlag.png


Now change the default value to be True:

AdditionFlagDefaultParam.png


Save the file:

SaveButtonGedit.png


Return back to the GRC window and it will show that the block now displays the Additionflag parameter instead of example_param:

AdditionFlagUpdatedBlock.png

Editing Block Inputs

The default block has a single input and a single output, however we need two inputs for the block. To add an input, add a second np.complex64 to the in_sig list:

AddSecondInput.png


Now change the block name to Add or Multiply Block:

EditBlockName.png


Save the file. GRC will now display the block with a second input and the block name will be updated:

SecondInputOnBlockGRC.png

Editing Work Function

Now that the variable name has been changed the work function needs to be re-written to provide the desired functionality.

The pseudo code for the Python block is:

if (additionFlag is True)
    then add the two inputs
else
    then multiply the two inputs

Modify the work function so it has the following code:

EditWorkFunction.png


Remember to indent with multiples of 4 spaces (4, 8, 12, etc.) when starting new lines in Python!

Save the the code.


Connecting the Flowgraph

Return to GRC. Double-click the Add or Multiply Block. Enter True for the Additionflag property:

SetAdditionFlagProperty.png

Click OK to save.

Drag and drop two Signal Source blocks, a Throttle block, a QT GUI Time Sink and a QT GUI Frequency Sink block into the GRC workspace and connect them according to the following flowgraph. Set the frequency of the second Signal Source to 3000:

ConnectAddMultiplyFlowgraph.png

Running the Flowgraph

Selecting True in the Add or Multiply Block will perform the addition of the two Signal Sources. Running the flowgraph gives the following two plots:

SinusoidAddition.png


The plots show the summation of the two sinusoids, one at a frequency of 1,000 and another at 3,000. The y-axis in the QT GUI Time Sink plot is partially cutting of the amplitude of the sinusoids. Click the mouse-wheel button to bring up the display menu and select Auto Scale:

SelectAutoScaleTimeSink.png


The full amplitude of the two sinusoids can then be seen:

TimeSinkFullAmplitude.png