DATV-Express Project – December update report

[KenW6HHC]

Charles G4GUO has been working on adding a new mode to the DATV-Express software, a Reduced-Bandwidth DVB-S Digital-ATV (RB-DATV) signal for the new 146 MHz band in UK. The goal is to start up some DATV activity on the UK’s newly opened 146 MHz band using very low Symbol Rates around 0.300 MSymb/sec to 0.333 MSymb/sec to produce a small bandwidth around 0.5 MHz centered on 146.5 MHz.


Narrow-BW DVB-S signal produced by DATV-Express exciter board on 146.5 MHz with SR = 333 KSymb/s.
The frequency span on the Spectrum Analyzer is 1 MHz wide.

The DATV-Express software is being experimentally modified to produce a clean signal spectrum with NO alias images being produced when using very low SR. This is not as easy as it sounds….but, by using SDR and some FPGA coding changes, Charles was able to add x64 frequency interpolators coding simply to get the aliases to go to frequencies outside of the 5 MHz analogue Nyquist filters cutoff (for example 0.2 MSymb/s SR x64 = 12.8 MHz and alias filtering is achieved). The current plans are to use DVB-S with H.264 video compression to produce a video frame rate that is as fast as possible. G4GUO first tried to use a Mitsubishi 2M RF amplifier brick RA06H1317M (I believe rated at 60W FM at 150 MHz) and obtained terrible spectrum-distortion results. Charles could only obtain 750 mW average power output before excessive spectrum-distortion set in. Charles is now exploring the use of a “RF amplifier distortion correction” approach (sometimes called “pre-distortion” or “pure signal”) to reduce the spectrum distortion. I hope the reader will realize that these "narrow-bandwidth" techniques are not just restricted to 146 MHz and could also be used on any crowded DATV band in any country to insert a narrow 0.5 MHz BW DATV signal into a crowded band-plan spectrum.

Ken W6HHC has started to investigate software changes needed to use the Logitech C920 web cameras to produce H.264 directly for the DATV-Express board and eliminate the need for Hauppauge video-capture units. Not an easy task…looks like Ken will need to do some software coding and learn how to compile in a linux world. Fortunately, Alex OZ9AEC has been working with the C920 web camera on a Raspberry PI and has supplied plenty of direction and suggestions.

Art WA8RMC has received a new production batch of new DATV-Express hardware boards just before Christmas. So there is no shortage of boards that can be ordered directly from http://www.DATV-Express.com. Here is a report on where boards have shipped in 2014:
Australia 9%
Belgium 3%
Brazil 1%
Chile 1%
Denmark 1%
France 2%
Germany 6%
Italy 1%
Japan 7%
Netherlands 4%
Switzerland 4%
UK 31%
USA 28%

"project is set to cruise speed"....de Ken W6HHC

[KenW6HHC]

I had a chance to read the blog of Rob MØDTS concerning his current experiments using very low Symbol Rates (aka narrow DATV bandwidth) on the new 146 MHz band allocations. Rob is using the DATV-Express board as the transmitting exciter with two experimental software modifications from Charles G4GUO…the x64 frequency interpolators coding to eliminate alias images in the spectrum when using very low symbol rates…and also some software coding for sending transport streams to the DATV-Express.

Current status is: tests with DVB-S 1 W average power output from the RF amp stages to Terry G1LPS over a 28 KM path have resulted in a reasonable constellation being seen by G1LPS. But, the weak QPSK signal is not fully decoded yet with Tutioune DVB-S software and a TechnoTrend TT-1600 PCI tuner card being used as the DATV receiver.

The URL for Rob’s blog on 146 MHz efforts is http://m0dts.co.uk/index.php?tag=146MHz

“Bravo to Rob on his 146 MHz efforts”

73…de Ken W6HHC