Transmitting signals with VGA
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Use the harmonics of VGA DAC to transmit RF
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Has a rather long history
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2001: Erik Thiele "Tempest for Eliza", AM Radio
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2005: Fabrice Bellard transmitted DVB-T and analog TV
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2009: Bartek Kania transmitted WBFM (VGASIG)
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2013: siro at "das Labor": I/Q modulator attached to VGA card
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Similar idea: RPitx, but VGA has a real DAC instead of PWM
VGA Synchronization
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Main issue with all VGA cards: HSYNC/VSYNC
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Basically "lost samples", not user controllable
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Very bad for analog modulation types
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Some OFDM-based systems can cope with it (like DVB-T)
USB VGA adapters
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Two manufacturers:
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Displaylink
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classical graphics card with framebuffer and USB interface
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Fresco Logic
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Software defined approach
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Framebuffer in host memory
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Image constantly being streamed via USB 3.0
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Makes adapters very cheap (5-10$)
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Sounds very interesting… ;)
Reverse Engineering
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Same approach as with rtl-sdr
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VirtualBox VM with Windows and original driver
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USB 3.0 device forwarded to VM
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Sniffing USB traffic with Wireshark on Linux
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Replaying commands in libusb-based application
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Remove stuff until it doesn’t work anymore
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Play with register contents to figure out what does what
Reverse Engineering cont.
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Figure out data format used in USB buffer
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Experiment with PLL register to figure out how clocking works
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At some point after lots of experimentation end up with this:
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VSYNC and HSYNC completely disabled, but still some lost samples
Goal: Continuous stream of samples
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Look again at Wireshark traces
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Last URB of USB transfer was smaller than other URBs
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Choose a new resolution that is a multiple of URB size (61440)
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1280 * 1024 * 3 colors = 61440 * 64
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⇒ Result: 150 MHz, 3 channel 8-Bit DAC with USB 3.0 interface
libosmo-fl2k
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Initializes the device, sets the sample rate
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Can be fed with 8 bit signed or unsigned samples
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Performs conversion of buffer format
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Can use zero-copy buffers to reduce CPU load
Applications
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fl2k_file
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Streams a file with samples to device, repeats
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fl2k_tcp
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Streams samples from TCP to device, e.g. GNU Radio
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fl2k_fm
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FM Modulator with stereo and RDS support
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Can be used together with SoX to transmit WBFM
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based on VGASIG code for FM modulation and PiFmRds
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fl2k_test
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Determines PPM offset to system clock
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Useful for calibration of device
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Still needed: fl2k_upsample
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Upsample I/Q baseband samples in real time
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Hoernchen started with it, more work needed
VGA SDR transmitter
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Transmission of several signals was tested successfully:
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WBFM
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DAB
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DVB-T
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GSM
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UMTS
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LTE
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GPS (11th harmonic!)
Example: GSM transmission
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Upsample using GNU Radio
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Synthesized carrier frequency: 40.6 MHz
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DAC sample rate: 138 MHz ± 40.6 MHz
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3rd harmonic: 414 MHz ± 40.6 MHz
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5th harmonic: 690 MHz ± 40.6 MHz
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7th harmonic: 966 MHz ± 40.6 MHz
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⇒ Images: 925.4 MHz (ARFCN 976), 1006.6 MHz
Example: GSM transmission
Further ideas
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Connect I/Q modulator to do "proper" transmission
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Add reconstruction filter and use as lab signal generator (0-75 MHz)
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Synchronize clock with rtl-sdr to use both as very cheap transceiver
Quality differences
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Device with two LDOs for DAC reference and digital supply
Quality differences
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Device with two switching regulators
