Reduced bandwidth DATV aka RB-TV

Transcription

1 Reduced bandwidth DATV aka RB-TV Noel Matthews G8GTZ RSGB convention 2015

2 Topics UK ATV overview What is Reduced bandwidth or RB-TV Why do we need RB-TV The RB-TV challenges Transmitting RB-TV Receiving RB-TV RB-TV On air

3 UK ATV overview 71 & 146 MHz The new ATV bands! 70cms Digital only on 437MHz 23cms Analogue and digital Activity on repeaters and simplex 13cms Currently analogue and digital Repeaters and simplex 3.4 GHz Digital only 10 GHz Repeaters and simplex

4 New ATV band at 146 MHz From October 2014 UK amateurs had access to MHz Also access to 1 MHz at 70 MHz However, Ofcom have said new technologies only Not more of the same FM etc watts erp maximum RSGB has allocated middle 500 KHz for digital modes including reduced bandwith DATV Not enough space for normal DATV so we needed something new = RB-TV

5 What is Reduced Bandwidth (RB-TV)? RB-TV is normal fast scan DATV below 1 Msymbol / sec Bandwidth = <1 MHz wide Agreed standard is 333kSym/s 7/8 FEC = ~450Khz bandwidth (450 Kbit/s video) Based on DVB-S standard BUT Needs MPEG-4 encoding for transmit Normal satellite rx won t work below 1 MHz So we needed to do something different!

6 RB-TV tx

7 Video encoding MPEG-2 is not useable below 500 Kbit/s MPEG-4 is currently the codec of choice Brian G4EWJ and F6DZP have evaluated the performance of H264 at very low bit rates. n/aacaa02tshp6y2cvyu9pwhrka?dl=0 Audio is the next BIG problem Looking at various options

8 MPEG-4 encoder choices Raspberry Pi has native MPEG4 encoder and camera F5OEO s/w produces I&Q data Audio via external USB No external video capture... Linux based SBC (eg Odroid) with capture cards Flexible but complex PC based using ffmpeg Not portable

9 Modulator choices DATVexpress (SDR hardware) wide range of symbol rates 70 MHz to 2.45 GHz Raspberry Pi Digithin plug in card Low SR modulator (fixed 330 ks) Si570 LO = 50MHz to 1.3 GHz BATC DTX 1 - works at low SR 300 2GHz But needs external MPEG-4 encoder

10 Rpi based RB-TV Tx MPEG 4 encoder & F5OEO s/w Digithin QPSK modulator Local oscillator (si570) Filter Power amp

11 DTX1 based RB-TV Tx MPEG 4 encoder & F5OEO s/w TS dock DTX 1 QPSK modulator Filter Power amp

12 Filters RB-TV on 146 MHz demands tight filtering on tx and rx!! Minimise spectral re-growth Satellite tuners are wideband Need bandpass and notch filters on rx There s lot of loud signals on 2 mts! Blowing the dust off the VHF/UHF manual!

13 DATV Power amplifiers Minimal spectral regrowth is very hard to achieve Professional satellite uplinks = -30 db 146MHz target is -60 db Requires very linear PAs Pre-correction has been experimented with on DATVexpress Average home constructor has to back off the PA Typically 6 10 db

14 146MHz PA M0DTS has proven Mitsubishi RD series MOSFET work well when backed off! RD06 driver > RD15 PA Rated at 15 watts = 3 watts RD70 rated at 70watts 5 watts with -55dB IMD3

15 RB-TV recieve The hard part! Consumer STB do not go below 1 Msymbol Only solution was to design one! F6DZP developed Tutioune s/w for Pci card Optimised DATV receive solution.125 to 45 Msymbols (not just RB-TV) And a really valuable diagnostic tool Taught us to forget the S meter! It s the MER that counts

16 Tutioune software

17 Forget the S meter! MER 18dB -----RF level -36dBm ----VBER 0% not bit corrected TS OK MER 5.8dB -----RF level -36dBm ----VBER 4% bits corrected by Viterbi TS OK MER 4dB -----RF level -36dBm ----VBER 18% bits corrected by Viterbi and Reed Solomon TS OK MER 2.7dB -----RF level -36dBm % bits corrected by Viterbi and Reed Solomon TS NOT OK

18 I&Q inbalance

19 RB-TV rx hardware Tutioune software is a great DATV receive tool But it only works with specific chipsets And the Technotrend cards went obsolete So F6DZP developed USB tuner hardware G4EWJ designed the PCB And the BATC stocks all the parts Still a problem rxing 146 MHz

20 RB-TV Rx system Pre-amp And Filter L band up converter USB Tuner PC

21 RB-TV on air MHz 25 watts erp on 146 MHz is tough! Yes we are sticking to it! But at least we can use existing aerials / pre-amps Portable operation is needed to work DX 146 MHz is inter G only First ATV QSO on 146 MHz 30/12/14 G4CPE & G0WFT G1LPS and M0DTS worked 11 Km G8GTZ and G4CPE worked 95 Km G1LPS and M0DTS worked 117 Km

22 RB-TV on air - 70cms 70cms is easier No erp limit and more ant gain F9ZG tests over 200+ Km with high success rate 125 Ks is significantly better than 250 Ks Proven 6 db improvement over 2Msymbol DVB-S 70cms Dx mode RB-TV has the potential to sit between PU signals

23 RB-TV innovation A real community effort to overcome all the challenges And Ofcom approves! Seen as a great example of amateur innovation BATC set up a series of awards to encourage the project

24 BATC RB-TV awards to encourage innovation

25 RB-TV awards First QSO over 10 Kms on 146 MHz G1LPS and M0DTS First QSO over 100 Kms on 146 MHz M0DTS and G1LPS First RB-TV QSO over 250 Kms No submissions - yet

26 RB-TV awards Grant award for development of RB-TV receiver technologies F6DZP G4EWJ G4KLB

27 RB-TV summary Enables DATV operation in bands where not room for normal DATV It s easy to get on air Everyone has a Rpi in the cupboard! Requires some level of home construction Great support by the ATV community Shows the hobby can still innovate For more details go to

28 Thank you