An OFDM Spectrum Painter for GNU Radio
CMake C++ Python C

README

# Copyright 2015,2016 Ron Economos
#
# This file is part of gr-paint
#
# gr-paint is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 3, or (at your option)
# any later version.
#
# gr-paint is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
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# You should have received a copy of the GNU General Public License
# along with gr-paint; see the file COPYING.  If not, write to
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gr-paint
======

Author: Ron Economos
Email: <w6rz@comcast.net>

The goal of this project is to build a software-defined OFDM transmitter
that "paints" monochrome images into the waterfall of a receiver.
It is based on https://github.com/polygon/spectrum_painter

After installation, a GNU Radio block called "Spectrum Painter" will
be available in the "Paint" category. The block converts a byte stream
of image data into a 4K IFFT OFDM IQ sequence for transmission.

There are two block parameters.

1) Image Width. This is the horizontal size of the image to be transmitted.

2) Line Repeats. This is the number of times a line is repeated. This is
used to adjust the aspect ratio of the image to be more or less correct.
Aspect ratio is also affected by sample rate, waterfall update time and
the size of the waterfall window.

A GNU Radio Companion flow graph (paint_tx.grc) is provided in the apps
directory. It transmits a test file with a UHD sink setup for an Ettus
B2X0 SDR. Be sure to select the correct sink for your device.

An image conversion utility (tgatoluma) is included to create image
files that work with GNU Radio (raw binary). The usage is:

tgatoluma <infile> <outfile>

For example:

tgatoluma image.tga image.bin

The utility will print the horizontal size, and this must be entered into
the "Image Width" parameter of the block.

The popular program ImageMagick can be used to convert any image to TGA
format. For example:

convert image.png image.tga

For waterfalls that start at the top, the image needs to be flipped.

convert -flip image.jpg image.tga

Three test images are included.

1) marcy.bin This is a 1920x1080 frame from a cartoon.

2) ramp.bin. This is a 1920x1080 test pattern with a grayscale. Used
to set the correct contrast with the minimum and maximum levels of the
QT GUI Waterfall Sink.

3) xhatch.bin This is a 1920x1080 test pattern with circles. Used to set
the aspect ratio correctly.

A tutorial for creating high quality images with Gqrx is available here:

https://gist.github.com/drmpeg/31a9a7dd6918856aeb60

UPDATE: Chris Kuethe KJ6GVE has submitted an "Image File Source" block
that allows any image format to be used and avoids the extra steps of
using ImageMagick and tgatoluma. The flow graph has been updated to include
the new block and a test PNG image is provided. You still have to enter
the correct image width into the Spectrum Painter block (the Image File
Source block will print the image width for you).

UPDATE 11/09/2015: I've added the OFDM Cyclic Prefixer block to the test
flow graph. This can be used to fine tune the aspect ratio of the image
in much finer granularity than line repeats. The CP Length can be any value,
it doesn't have to be a power of two like you see in regular OFDM systems.

I've also added sin(x)/x correction for bladeRF and other SDR transmitters
that don't compensate for DAC zero-order hold. Note that the Ettus B2X0
does have sin(x)/x compensation, so it's not turned on in the test flow
graph.

Build instructions:

    mkdir build
    cd build
    cmake ../
    make
    sudo make install
    sudo ldconfig

Contributions are welcome!