FreeDV over VHF FM or SSB

[Stewart]

Has anyone successfully used FreeDV over a VHF simplex frequency,
either FM or SSB or thru a VHF repeater?

This evening I listened to a test on a local VHF repeater (no PL) by
two local hams. They passed analog audio back-n-forth with no problem.
However, even though they each got a strong signal on the DV "buzz sound"
neither one could decode the DV data stream for regular audio. They
suspect the repeater bandpass does something to corrupt the data stream.
Their next test will be on a VHF FM and SSB simplex frequency. My bets
are on the SSB test.

Stewart WI3J

[jdow]

Is it a preemphasis/deemphasis issue, perhaps? You really want
flat audio.

{^_^} Joanne, W6MKU

[Harry Bloomberg]

We've used FreeDV over a 440Mhz FM repeater, it worked great!  Could it be that your friends who could not decode each other were using different FreeDV modes?

73,

Harry Bloomberg W3YJ

hbl...@gmail.com

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[Tony Langdon]

On 5/04/13 1:18 PM, Stewart wrote:
> Has anyone successfully used FreeDV over a VHF simplex frequency,
> either FM or SSB or thru a VHF repeater?

It should work. Perhaps they're overdriving their audio and distorting
the waveform in the mic clipping stages?

>
> This evening I listened to a test on a local VHF repeater (no PL) by
> two local hams. They passed analog audio back-n-forth with no problem.
> However, even though they each got a strong signal on the DV "buzz sound"
> neither one could decode the DV data stream for regular audio. They
> suspect the repeater bandpass does something to corrupt the data stream.
> Their next test will be on a VHF FM and SSB simplex frequency. My bets
> are on the SSB test.

SSB should work just like it does on HF. :)

--
73 de Tony VK3JED
http://vkradio.com

[jdow]

As somebody suggested both ends of the path must be running in the
same mode. If one person transmits using SSB rather than FM the other
must receive SSB. And that might not get through the repeater. So using
FM is best. That still leaves "flat passband" as an issue. Some repeaters
don't handle preemphasis and deemphasis properly. (That is one of
Shorty's laments on the WinSystem out here on the left coast.)

{^_^} Joanne/W6MKU

[Richard Fjeld]

Another thought may be; if the repeater and/or controller is set up for DTMF, perhaps it is going into 'falsing'.  I would expect that to be heard on the output if happening. Proper deviation could be a factor also.

 

Digital voice would give us some privacy on VHF frequencies rich with scanners.

 

Rich, n0ce

[Gmail]

We have tried FreeDv here on VHF-UHF both FM and SSB. We even tried passing through a repeater.  What we found is that SSB provided the best reliability with proper decoding 95% of the time.  I think this is due to better Signal to Noise Ratio using SSB with narrow receiver filters. Even in SSB mode on HF the narrow SSB filter works better than the wider one.  That is not to say that FM won't work with a properly designed RF system to support FreeDV.  ICOM has recently adopted more narrow IF filters in their latter D-STAR radios for the same reasons. With a narrow signal like FreeDV the more selectivity a receiver has is a plus. Also keep in mind that with narrow modes comes tighter frequency stability. This is why most commercial digital systems are GPS frequency locked.  FM repeaters built from surplus equipment vary all over the place in terms of deviation/audio levels, audio frequency response, IF bandwidths and frequency stability.  For FreeDV to be viable in the VHF/UHF bands we need to start a dialog on designing RF to accommodate CODEC2.

 

BTW....FreeDV works well at 900 MHz where the converted commercial radios have tighter tolerances.

 

George - W4AQR

[Tony Langdon]

On 6/04/13 2:46 AM, Richard Fjeld wrote:

Another thought may be; if the repeater and/or controller is set up for DTMF, perhaps it is going into 'falsing'.  I would expect that to be heard on the output if happening. Proper deviation could be a factor also.

 

Digital voice would give us some privacy on VHF frequencies rich with scanners.

Unntil scanner users discover FreeDV and start running it. ;)

[Tony Langdon]

On 6/04/13 3:54 AM, Gmail wrote:

We have tried FreeDv here on VHF-UHF both FM and SSB. We even tried passing through a repeater.  What we found is that SSB provided the best reliability with proper decoding 95% of the time.  I think this is due to better Signal to Noise Ratio using SSB with narrow receiver filters. Even in SSB mode on HF the narrow SSB filter works better than the wider one.  That is not to say that FM won't work with a properly designed RF system to support FreeDV.  ICOM has recently adopted more narrow IF filters in their latter D-STAR radios for the same reasons. With a narrow signal like FreeDV the more selectivity a receiver has is a plus. Also keep in mind that with narrow modes comes tighter frequency stability. This is why most commercial digital systems are GPS frequency locked.  FM repeaters built from surplus equipment vary all over the place in terms of deviation/audio levels, audio frequency response, IF bandwidths and frequency stability.  For FreeDV to be viable in the VHF/UHF bands we need to start a dialog on designing RF to accommodate CODEC2.

Umm, on FM FreeDV is no longer a narrowband (RF) signal, its bandwidth requirement is higher, being the bandwidth of the modulated signal, which will depend on the deviation used (for 5 kHz deviation and a 1.5 kHz centre frequency for the FreeDV signal, the RF bandwidth requirement would be around 14-15 kHz according to Carsons Rule).  A better solution for FM based systems would be a GMSK or 4FSK modem carrying the underlying FreeDV data, instead of the multi carrier modem.  I suspect this would give better performance here.  However, it would be interesting to try both setups mobile with significant VHF multipath interference (D-STAR which uses GMSK significantly loses performance when mobile).

Of course on VHF SSB, the bandwidth situation is the same as for HF SSB. :)