Tuesday, 11 November 2014

Exercising my 146Mhz NoV

After buying a 'gaming' USB headset with boom microphone last week I set up my K3, with USA 144-148Mhz internal transverter, for use with FreeDV digital voice.

Set up was simple but getting the transmit level right proved a bit more of a problem than I expected.
At around 14:30 today, in tests, I managed to hear G4BAO on 146.900MHz, using one of the recommended digital channels at the top end of the new band. 

I immediately saw the characteristic 16 carriers of the FDMDV transmission from John. The SNR was not that high, but John's voice was unmistakeable, if a little Stephen Hawking like!
When it came to my turn to transmit I hit the soft PTT and spoke into the boom microphone. After adjusting the distance to the microphone so that the 'Frm MIc' waterfall showed the level to be about 1/2 of maximum, I sent a report and then I turned it back to John. He immediately confirmed successful reception. As the signal is nothing like we are normally used to it was difficult to give a useful signal report. This is one of the things we will have to sort out with digital voice! My initial stab is 33D where the D stands for digital distortion. Later, when I wound the mast up a few metres, the report was more like 45D

After switching to 1600 wide from standard 1600 we compared results again.  It would appear there was little difference except that the carriers occupied a bit more bandwidth. There was no noticeable improvement in quality. The signal was best described as distorted but intelligible at low SNR, improving to acceptable at higher SNR, corresponding to the peaks of QSB.

So, what are my impression of the FreeDV contact at the relatively low EIRP of 10dBW?
The multipath signal over the 78km path had a lot of errors. SNR was low and when we switched to SSB ( at 144.285MHz) it was obvious that conditions over this normally reliable path were not good. My SSB report to John was 53 to 55. There was lots of QSB.

I understand that a newer version of FreeDV, aimed at VHF, is under development. This will have a different voice model and maybe some Forward Error Correction (FEC). There is little need to restrict the bandwidth to 1.2KHz at VHF and maybe an increase in rate and bandwidth will be worthwhile when signals are distorted by VHF multipath and doppler spread?

I hope to work a few more stations on 146.900MHz and compare results.

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