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Osmo-fl2k » History » Revision 3

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steve-m, 04/07/2018 09:46 PM


osmo-fl2k

Introduction

Since rtl-sdr has been released a couple of years ago, cheap SDR receivers are ubiquitous. SDRs with transmission capability have become cheaper as well, but are still more expensive. osmo-fl2k allows to use USB 3.0 to VGA adapters based on the Fresco Logic FL2000 chip, which are available for around $5, as general purpose DACs and SDR transmitter.

The idea of using VGA adapters as transmitter has a long history. In 2001 Erik Thiele released (Tempest for Eliza), which transmits a signal that can be received with an AM radio. In 2005 Fabrice Bellard used an ATI Radeon 9200SE to transmit (DVB-T, PAL and NTSC video signals).

However, a major downside with regular graphics cards is that the generated signal of the RAMDAC is not entirely user controllable, as VGA uses (horizontal and vertical blanking), thus interrupting the signal. In comparison to competitor devices from DisplayLink, which resemble a classic graphics card with USB 2.0/3.0 interface, Fresco Logic took a different approach, which they've also (patented). Instead of having the framebuffer for the VGA DAC in the device itself, they use the memory of the host computer and continously stream the display content via USB. This makes such adapters very cheap, as it essentialy reduces the adapter to a signgle chip without requiring framebuffer memory. The main drawback is that if the host CPU or USB bus is busy or congested, this results dropouts and flickering of the screen.

After reverse-engineering the USB protocol of the FL2000 in 2016, Steve Markgraf discovered through experimentation that it is possible to operate the FL2000 in a way that both horizontal and vertical synchronization are disabled, thus creating a continous stream of samples.
This work resulted in osmo-fl2k, which so far was used to transmit low-power FM, DAB, DVB-T, GSM, UMTS and GPS signals.

fl2k_devices.jpg

Updated by steve-m almost 5 years ago · 3 revisions

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