Copyright 2015,2016,2021 Ron Economos

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This file is part of gr-paint

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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

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any later version.

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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

the Free Software Foundation, Inc., 51 Franklin Street,

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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

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

Note: cmake ../ will install into the default directory /usr/local. If you've installed GNU Radio with a package manager like apt, then gr-paint should be installed in /usr. To accomplish this, use:

cmake -DCMAKE_INSTALL_PREFIX=/usr ../

Contributions are welcome!