DVB-T Narrow Band Testing between G4NZV & G4FRE

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DVB-T Narrow Band Testing between G4NZV & G4FRE

Post by G4NZV » Mon Mar 29, 2021 3:12 pm

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
DVB-T sw demod of G4FRE 2021-03-28 15-14-48.png
DVB-T sw demod of G4FRE 2021-03-28 15-14-48.png (356.94 KiB) Viewed 350 times
DVB-T G4FRE-2.PNG (714.59 KiB) Viewed 350 times
DVBT G4FRE-1.PNG (763.96 KiB) Viewed 350 times
GNU GRC DVB-T demod.png
GNU GRC DVB-T demod.png (227.57 KiB) Viewed 350 times

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Re: DVB-T Narrow Band Testing between G4NZV & G4FRE

Post by radiogareth » Tue Mar 30, 2021 8:30 am

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

Posts: 20
Joined: Wed Jun 18, 2008 8:46 pm

Re: DVB-T Narrow Band Testing between G4NZV & G4FRE

Post by HB9DUG » Wed Mar 31, 2021 12:07 pm

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

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