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LimeSDR Mini/RPi Zero mashup may be world’s smallest DVB transmitter

Jan 24, 2018 — by Eric Brown
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Lime Microsystems anounced an open source prototype hack that combines a LimeSDR Mini board, a Raspberry Pi Zero, and a PiCam, and is billed as one of the world’s smallest DVB transmitters.

Lime Microsystems is promoting an open source prototype developed by Evariste Okcestbon that it speculates is the world’s smallest DVB (digital video broadcasting) transmitter. A YouTube demo video (see farther below) demonstrates the hardware/software hack, which streams video from a 65 x 30mm Raspberry Pi Zero equipped with the official Raspberry Pi Camera (PiCam) to the 69 x 31mm LimeSDR Mini board via USB.

LimeSDR/Zero based DVB transmitter prototype (right) and Raspberry Pi 2 based receiver (left), both of which are equipped with RTL-SDR DTB-T dongles
(click image to enlarge)

“As far as we’re aware it’s the smallest DVB transmitter announced so far,” stated Danny Webster, Lime’s Principal RF design engineer. “It’s certainly the most compact one assembled from off-the-shelf components and powered by open source software!”

The prototype encodes and transmits H.264-encoded 720p video on the 1.2GHz amateur TV band using a low-cost RTL-SDR DVB-T TV tuner dongle based on the RTL2832U chipset. The project sends the video stream through the following chain of open source tools: avc2ts (H264 encoder and transport stream encapsulator), dvb2iq (IQ modulator), and limetx (tool for sending IQ to the LimeSDR).

DVB transmitter video stream flow showing software packages
(click image to enlarge)

The demo uses a Raspberry Pi 2 on the receiver end with its own RTL-SDR dongle and a software chain that starts with RTL_SDR (SDR receiver software at 1-megasample). The signal then moves to LeanDVB (DVB-S demodulator), and then to a ts2es tool that prepares the transport stream for conversion to H.264, which occurs on the RPi’s hardware decoder. The image is then sent out via HDMI using hello_video software.

DVB receiver stream flow
(click image to enlarge)

While RTL_SDR sends data to LeanDVB, it also sends a signal to KissSpectrum, a spectrum analyzer for the Raspberry Pi. This syncs up with the processed H.264 encoded video to display the spectrum on an attached monitor that can also be switched over to show the video.

DVB transmitter close-up (left) and LimeSDR Mini
(click images to enlarge)

Lime Microsystems launched the open source, $139 LimeSDR Mini on Crowd Supply in September. This size- and cost-reduced sibling of Lime’s Ubuntu Core-enabled LimeSDR board has a reduced frequency range, RF bandwidth, and sample rate. It also has half as many receive and transmit channels, and significantly fewer programmable logic gates than the original.

Yet the Mini similarly runs the LimeSuite host-side software, which supports Software Defined Radio technologies for protocols including UMTS, LTE, GSM, WiFi, LoRa, Bluetooth, Zigbee, RFID, and DTB. As in the case of its predecessor, the LimeSDR Mini’s FPGA manages DSP and interfacing tasks, while a USB 3.0-connected host system running Ubuntu Core provides user interface and various high-level supervisory functions.

Okcestbon’s DVB transmitter is still in the prototype phase, and we did not see any documentation posted beyond what was shown in the video. Okcestbon is principally known as the developer of the RPiTX RF transmitter software for the Raspberry Pi, which debuted in 2015.

Video demo by Evariste Okcestbon of his prototype DVB transmitter design

Further information

No details were provided on when the full DVB transmitter design might be posted, or whether a commercial product will emerge from the project. More details may eventually appear at the Lime Microsystems website or the Evariste Okcestbon Google+ page.

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