VGAtoBaseband

         

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

Description

This software generates analog I/Q Signals that can be feed into a rf-modulator. This software is intended to be used with SDRT.

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

What it does

Linux

tested with:

• Mesa 9.0.2
• fglrx 8.97.2

Windows

NOT SUPPORTED

• it is possible to add custom edids / modelines, but that's rather a hack using modified inf files
• other ways of adding modelines are unknown

Software

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 symbol (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.

vgatobaseband uses the first VGA output found. If none is found it terminates. The VGA output has to be configured first using xrandr, arandr, ....
The coordinates are saved, a new modeline is added and set. On termination the modeline is removed and the initial mode is restored.
No need to do any modesetting by hand !!!

Make sure you copy fragment.glsl into the same directory as the compiled binary.

All baseband signals contains positive as well as negative numbers. If your signal range covers -1.0f to 1.0f you have to convert them.

newval = (float2)(oldval.i/2 + 0.5f, oldval.q/2 + 0.5f)

The newval is now in range 0.0f to 1.0f and can be used with this programm.
Due to most GPU only having 8bit DACs the floats are converted into a range from -127 to 127 (using the blue channel as differential reference). A hardware low-pass filter is required.

Examples

./vgatoiqbaseband /tmp/myfifo -v -msps 56 -cutofright 0 -cutofbottom 0

This will read from the file /tmp/myfifo, pixelclk is 56Mhz, it will output 56/7 = 8 MSPS, set HSYNC to 0 and VSYNC to 0.
0 VSYNC might only work on Intel GPU.

./vgatoiqbaseband -v -t

This will generate test paterns, pixelclk is 64Mhz, it will output 64/7 = 9.142 MSPS, set HSYNC to 1 and VSYNC to 2.
It might work on all GPUs.

You should set the VGA position first ! otherwise it will overlay your default output (clone mode).

xrandr --output VGA-0 --right-of LVDS --auto

adjust VGA-0 and LVDS according to your system.

Arguments

./vgatoiqbaseband inputfile [ARGS]

if inputfile isn't specified, use -t argument

ARGS could 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. 
-direct          force direct rendering
-t               generates test patterns
-pclk <x>        use a pixelclock of x Mhz
                 default: 64

Libraries

• libglut
• libglu
• libgl
• libxrandr

You may also use

• libX11

Debian packages:

• freeglut3-dev
• libglu1-mesa-dev
• libgl1-mesa-dev
• libxrandr-dev

OFDM 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

Phase shift

As phase-shifting OFDM symbols results in malformed spektrum, the horizontal sync is padded with blanking pixel to make sure the sync has the same size as all other symbols.

Interpolation Filter

The convolutional filter is critical in this application. Using 8 Mhz Bandwidth the I and Q channel maximum frequency is 3.81 Mhz. The DAC sampling rate is 9.142 MSPS which is enough to reconstruct all frequencies, but due to the low sampling rate the baseband signal contains aliasing artefakts. To remove those artefakts a si-filter is neccessary, inclusing an interpolation DAC, which is running at much higher sampling rate.

Screen size

On fglrx the default screen size is 1600x1600. To increase this limit generate an xorg.conf using 'aticonfig --initial' and add this line:

[...]
Section "Screen"
       Identifier "aticonfig-Screen[0]-0"
       Device     "aticonfig-Device[0]-0"
       Monitor    "aticonfig-Monitor[0]-0"
       DefaultDepth     24
       SubSection "Display"
               Viewport   0 0
               Virtual   3600 1600
               Depth     24
       EndSubSection
EndSection

The line containing "Virtual" is critical. This should be no problem on Mesa as the default screen size is 8196x8196.

TODO

• Windows modesetting support
  • You may use a custom edid, see here for more details HackingVGAforFun.

Links

• http://software.intel.com/en-us/articles/custom-resolutions-on-intel-graphics
• http://bk.gnarf.org/creativity/vgasig/
• http://www.sevenforums.com/tutorials/7947-force-dvi-hdmi-resolutions-refresh-rates-4.html