VGAtoBaseband

VGAtoBaseband Release status: experimental [box doku]
Description	use the VGA port to generate I/Q baseband signals
Author(s)	siro
Last Version	0.1 ⁽⁾
Platform	Evil Hack
License	GPL
Download	[1]

Requirements

PC with VGA adapter
OpenGL with Direct Redering
Vsync enabled
Software-defined Radio Transmitter that generates I/Q Signals
GL_ARB_fragment_program to use the low-pass filter
GL_ARB_pixel_buffer_object for efficient CPU to GPU transfers

Linux

Mesa 9.0 (other drivers are untested)
X-server

Windows

Windows XP doesnt work out of the box
other OS are untested
it is possible to add custom edids / modelines, but that's rather a hack

Software

This software generates analog I/Q Signals that can be feed into a rf-modulator. The current version runs on any symbolrate, recommended are 5MSymbols/s or more.
You need to set the DAC clock to 7*desired MSymbols per second,because low pixel clocks might not work and high pixel clocks can be scaled down by using multiple pixel per Msymbol (here 7).

Every symbol is a complex number, that contains an I (real) and Q (imag) value.
By default the signal is lowpass filtered using convolutional sinc function. To edit the impulse response edit the file fragment.glsl. The I/Q values are feed using a unix fifo / a file. 8 byte form two 32bit floats, the first float maps to I, the second to Q. The values have to be between 0.0f and 1.0f. The real part (I) is assigned to the RED and the imag part (Q) to the GREEN channel, while the BLUE channel is set to 0.5f. 8bit and 10bit VGA Graphic cards are supported.

xrandr

To do so use xrandr:

xrandr --newmode "MODE1"   64.00  1400 1400 1401 1401 700 700 702 702 -hsync -vsync
xrandr --addmode VGA-0 "MODE1"
xrandr --output VGA-0 --mode "MODE1"
xrandr --output VGA-0 --right-of LVDS

note: VGA-0 may not apply, it could be VGA1 or VGA too, depending on the driver.
note: the last command assumes LVDS to be your primary active connector, change depending on your system.

Xorg.conf

Add a ModeLine to your Xorg.conf.

Section "Monitor"
       Identifier      "SyncMaster"
       Option          "VGA-0"
       HorizSync       30-96
       VertRefresh     50-160
       ModeLine        "MODE1" 64.00  1400 1400 1401 1401 700 700 702 702
EndSection

EDID

You may use a custom edid, see here for more details HackingVGAforFun.

TODO

Linux: add support for xrandr auto mode setting.

Bandwidth

OFDM useful carriers to total carriers ratio: 1705 / 2048 = 0.832

DAC clock [Mhz]	Msymbols/s	carrier ratio	Bandwidth
64	9.14285	0.832	7.61
56	8.000	0.832	6.66
48	6.8571	0.832	5.71

Arguments

./vgatoiqbaseband inputfile [OPTIONS]

OPTIONS can be:

-nofilter        disable fragmentshader
-v               be verbose
-verbose         be verbose
-cutofright <n>  cutof n rightmost pixels (hsync)
                 default:  1
-cutofbottom <m> cutof m bottommost pixels (vsync)
                 default:  2
-display DISPLAY Specify the X server to connect to. If not specified, the value of the DISPLAY environment variable is used. 
-geometry        W x H + X + Y 
-direct          force direct rendering

Libraries

freeglut3-dev
libglu1-mesa-dev
libgl1-mesa-dev

Links