Creating Python OOT with gr-modtool

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This tutorial was written using GNU Radio v3.10.1.1.

Creating an OOT Module

Open a terminal and navigate to an appropriate directory for writing software, such as the home directory:

$ cd $HOME

GNU Radio comes packaged with gr_modtool, software used to create out-of-tree (OOT) modules. An OOT module can be thought of as a collection of custom GNU Radio blocks. Create an OOT module named customModule using gr_modtool:

$ gr_modtool newmod customModule

The directory gr-customModule is created which contains all of the skeleton code for an OOT module, however it does not yet have any blocks. Move into the gr-customModule directory:

$ cd gr-customModule

List all of the files and directories within the OOT module:

$ ls

The directory listing will be as follows:

apps/  cmake/  CMakeLists.txt  docs/  examples/  grc/  include/  lib/  MANIFEST.md  python/

Creating an OOT Block

Now a block needs to be created within gr-customModule. The custom block will either add or subtract based on an input parameter, so the block is named addSubSelect:

$ gr_modtool add addSubSelect

The command will start a questionnaire about how to the block is to be defined: what block type, language and parameters:

GNU Radio module name identified: customModule
('sink', 'source', 'sync', 'decimator', 'interpolator', 'general', 'tagged_stream', 'hier', 'noblock')

Select the sync block, which produces an output for every input:

Enter block type: sync

Enter python as the language:

Language (python/cpp): python
Language: Python
Block/code identifier: addSubSelect

Enter the name or organization of the copyright holder:

Please specify the copyright holder: YourName

Now enter the argument list:

Enter valid argument list, including default arguments:

Enter the argument list as if writing the Python code directly. In this case the selector will determine whether or not the block performs addition or subtraction. A default argument of True is given:

selector=True

Determine whether or not you want the Python quality assurance (QA) code:

Add Python QA code? [Y/n] n

New files will be generated:

Adding file 'python/customModule/addSubSelect.py'...
Adding file 'grc/customModule_addSubSelect.block.yml'...
Editing grc/CMakeLists.txt...

Two new files were created, addSubSelect.py which defines the operation of the block and customModule_addSubSelect.block.yml which defines the interface of the block for GNU Radio Companion (GRC). The CMakeLists.txt file was modified so the two files will be installed when the module is compiled and installed.

Modifying addSubSelect.py

Move to the directory containing addSubSelect.py

$ cd python/customModule

Open the file with a text editor:

gedit addSubSelect.py &

The following code will be listed: ‎

#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# Copyright 2022 YourName.
#
# SPDX-License-Identifier: GPL-3.0-or-later
#


import numpy
from gnuradio import gr

class addSubSelect(gr.sync_block):
    """
    docstring for block addSubSelect
    """
    def __init__(self, selector=True):
        gr.sync_block.__init__(self,
            name="addSubSelect",
            in_sig=[<+numpy.float32+>, ],
            out_sig=[<+numpy.float32+>, ])


    def work(self, input_items, output_items):
        in0 = input_items[0]
        out = output_items[0]
        # <+signal processing here+>
        out[:] = in0
        return len(output_items[0])‎

Both the __init__() function and work() function need to be modified. The __init__() function is modified to define the input type. The addSubSelect block will accept two complex inputs and produce a complex output, therefore the in_sig and out_sig parameters need to be changed. The selector parameter also needs to be saved as a member variable:

    def __init__(self, selector=True):
        gr.sync_block.__init__(self,
            name="addSubSelect",
            in_sig=[np.complex,np.complex],
            out_sig=[np.complex])
        self.selector = selector

The work() function is modified to either add or subtract the two inputs based on the selector parameter:

    def work(self, input_items, output_items):
        in0 = input_items[0]
        in1 = input_items[1]

        if (self.selector):
            output_items[0] = in0 + in1
        else:
            output_items[0] = in0 - in1

        return len(output_items[0])