DVB-T Narrow Band Testing between G4NZV & G4FRE

[G4NZV]

Main objective of these tests is to gain experience of a Portsdown4 Narrowband DVB-T transmission on 146.5MHz over a 17.5Km path and to undertake on-air testing of a homebrew GNU NBTV demodulator and a Knucker. The outcome was video and audio transmissions were received in both directions.

G4NZV Transmit setup: C920 camera + Portsdown4 + Pluto (Fitted with high stability TCXO) + Driver amp + Band Pass Filter + Power Amplifier (5W) + 9 element horizontal yagi
G4NZV Receive setup: 9 element yagi + Pre-amp + Band Pass Filter + HackRFOne + Raspberry Pi4 + GRC DVBT NB software demodulator + VLC + HDMI monitor

DVB-T setup on Portsdown4: DVB-T + 146.5MHz + Bandwidth 333KHz + FEC 2/3 + Guard band 1/32 + Mode 2K + Pluto Power 0

Lessons Identified:
- Software demodulator needs to be tuned to within +/-0.5% of the centre carrier frequency for successful demodulation.
- Occasionally a centre spike was observed on the received spectrum which was used to tune the receiver to the exact frequency of the transmission (in this case 146.497400MHz).
- During reception interference across the band appeared, however demodulation continued. This shows the resilience of OFDM to an interfering comb of frequencies. (see attached images).
- Coding and decoding delays total around 3 seconds which makes for an enjoyable QSO.
- The video is much smoother and the demodulator lock time is almost instant if tuned to the centre of the transmission.
- Lip sync is excellent.
- There is a tendency for VLC to lose lock and need restarting even though the demodulator looks as if it is working.
- Rapidly changing images (e.g. swinging the camera around the shack) can cause problems with the system, it was not possible to determine if that was caused on transmit, receive or both.
- The received SNR was 12dB (about the same level as the interference). When shack testing with no interference the demodulator remains locked with an SNR of 8dB.

Inspiration for the software DVB-T demodulator came from: HB9DUG, F4FDW, github gr-dvbt

Thank you to G4FRE for giving up his Sunday afternoon.

Steve G4NZV

[radiogareth]

Looks very promising - any likelihood of it supporting other good SDRs (like RSP-1) or even a lowly RTL dongle?
Gareth

[HB9DUG]

I have done a few SDR's : LimeSDR, Adalm-Pluto, RTL dongles and Ettus B200. You will find the graphes on the http://home.swissatv.ch web site.

73 de michel, hb9dug

[G4NZV]

It is worth mentioning that PiSDR package has recently been updated. This release is the first 64-bits image of PiSDR. This enables the software to use modern vector instructions (NEON) that help programs run faster. Some programs will run up to 60% faster than the 32-bits version! It contains many amateur radio related programs including SDRangel, GNU3.9.5 & VLC which is ideally suited to run NBDATV DVB-T RX software on a Pi4 using a RTL, HackRF, Pluto are the only SDR receivers I have tried which work. There are other SDR radios available in the package.

It is easy to install on a new SD card. Use https://www.balena.io/etcher/ to flash the .img.xz file into a MicroSD Card which needs to be 8GB or bigger. The image is ~2GB can be found here https://github.com/luigifcruz/pisdr-ima ... s/tag/v6.1

Place the SD card in your Pi4 and answer all the questions on location etc. after downloading any updates and rebooting open a terminal window CTRL-ALT-T and type volk_profile on the terminal to optimize GNU Radio to run faster on your system.

The GNU radio icon is on the desktop after a minute or so it will run.

Its worth mentioning that the whole install process takes a couple of hours so if it gives the impression of having stopped installing it probably hasn't!

PS For Windows users I also use GNU3.9.5 in a Radioconda environment. It can be downloaded as a package for Windows 10 https://wiki.gnuradio.org/index.php?title=InstallingGR

73 Steve G4NZV

[radiogareth]

Looks promising, I have installed the Win64 version on my desktop and have GQRX running with a RTL820. I can see my DATV spectrum in e.g. WBFM
...which now leave me wondering..... How can I decode it??
Loosely familiar with SDRAngel...any pointers??
Thanks
Gareth

[G4NZV]

I am making assumptions about the hardware available however here goes for some options to decode some reduced bandwidth DATV signals.

To decode DVBT or DVBS using a RTL820 and a windows 10 PC. For DVBS use SDRangel (there is an option in the menu for DVBS2 however I have found it extremely unreliable). Two screenshots show how to setup SDRangel to decode DVBS using a RTL dongle. SDRangel can be downloaded from: https://github.com/f4exb/sdrangel/releases

To decode DVBS2 reliably a minituner is required along with the minituner software or a Ryde receiver or Portsdown 4.

To decode reduced DVBT a knucker and a Ryde receiver will work except at a bandwidth of 125KHz. An alternative is to use GNU radio version 3.9.5 and Radioconda on a windows 10 PC. Incidentally GNU3.9.5 can also be installed with the PiSDR package on a Pi4.(see my previous post to install and setup). I am happy to share the gnu .grc and python .py files for use DVBT with a RTL820 plus how to use. (Just waiting for guidance from Dave G8GKQ on where to place them on BATC for download.) I have attached a screenshot of DVBT being decoded. VLC is used to display live video and audio.

I am sure there are many other alternatives however these work for me.

73 Steve G4NZV

[K0ZAK]

Steve

Have you posted your gnu .grc and python .py files for use DVBT with a RTL820 plus how to use anywhere yet? I'm be interested in checking them out. I have a Knucker, but would like to see if I could get my RTL820 to work with narrow bandwidth DVB-T

John
K0ZAK

[G4NZV]

John,
The short answer to your question is no, the files have not been posted, I am awaiting a response from BATC asking if the two files .gnu and .py could be made available via BATC.

Its also worth mentioning that apart from the tests detailed in this post there has been no further on-air testing due to lack of NBDVBT transmissions. Testing across the shack is good to check basic functionality but it does not exercise NBDVBT with real-world multipath impairments. That work needs to be done.

There is a difference in the way sample rates are implemented for RTL between GNU3.9.5 on PiSDR running on a Pi4 and Radioconda running on windows 10 PC. Which version is needed?

Also I would be interested to know which software you will use to generate NBDVBT.

73 Steve G4NZV

[g8gtz]

Hi Steve,

No problem hosting the files - they can either go on the forum, the wiki or Github but would need to check where would host the file format although if they were zipped the forum would definetely accept them.

73
Noel

[K0ZAK]

Hi Steve,

To answer your question, I am currently using an Adalm-Pluto with DATVExpress on a Windows11 PC for transmit, and a Knucker receiver with Portsdown4 for receive. I have only been able to test narrow band DVB-T in house thus far as no one else locally is set up to use DVB-T below 2 Mhz bandwidth.

John
K0ZAK