Digital Voice(?) 50.2625 MHz

[ATCTech]

I'm outside my usual modes and bands of interest with this one but having shared information with local amateur operators and come up empty I'll put this online to see if anyone recognizing it. For the last 2 or 3 months (maybe longer, but that's when I stumbled onto it), there are daily digital transmissions in my local area on 50.2625 MHz. Signal strength varies, but in a consistent way. Operation is typically weekday mornings (between around 7:00 and 8:30 AM local time) and evenings, plus random on weekends. Signal strength is solid and steady at S-9+ most of the time, but but there's also periods of highly fluctuating signal where there's little doubt the operator goes mobile. The typical pattern is the strong fixed level until about the last 20 minutes of use in the morning, as if he starts at home then goes mobile on the way to work. I've listened and watched on the SDR display many, many times and I see zero evidence of a responding signal anywhere in the 6M band, at any level.

I've searched high and low for audio samples of digital modes that sound like this, and tried a handful of decoding programs with no luck at all. Attached here is an audio sample just recorded. This is from SDR# with NO audio filtering. The first burst was recorded in FMN, the second in AM, the last in SSB.

Should anyone have an idea what it might be (and how to decode it) or would like me to do another recording with specific filters/mode etc. to narrow it down please ask!

Cheers!

Bob

 

[a417]

Can you get an I+Q recording?

 

[popnokick]

Sounds like the idle of a DMR or YSF transmitter... might be D-Star. An I+Q recording would allow it to be processed by DSD or similar and identified.

 

[ATCTech]

Will do guys, thanks!

 

[Whiskey3JMC]

Sounds like the idle of a DMR or YSF transmitter... might be D-Star. An I+Q recording would allow it to be processed by DSD or similar and identified.

Doesn't sound like D-Star, YSF or DMR. I'm stumped

 

[bill4long]

Maybe it's this:

FreeDV: Open Source Amateur Digital Voice – Where Amateur Radio Is Driving The State of the Art

freedv.org

 

[ATCTech]

Maybe it's this:

FreeDV: Open Source Amateur Digital Voice – Where Amateur Radio Is Driving The State of the Art

freedv.org

That's awfully close isn't it! Sample here:

 

[Whiskey3JMC]

Sounds like garbage compared to the more well known codecs, but hey it's open source so I won't knock it too much

 

[bill4long]

Sounds like garbage compared to the more well known codecs, but hey it's open source so I won't knock it too much

It's better than the original AMBE coder that DStar uses. And it fits in 2.5khz. We use it on HF. But it works on VHF/UHF too, obviously. Modulate SSB.

 

[ATCTech]

Looks like ! waited one day too long to ask you folks about this. Total silence up here today so quite likely the source isn't in a normal routine now like most of the rest of us. I'll post I/Q data when it's captured.

 

[BigLebowski]

Sounds a little like conventional provoice. I've never tried to capture it on an airspy but on an analog FM radio its a little similar.

 

[ATCTech]

I was able to grab some traffic this morning. This 8 bit IQ raw file was captured using the IF Recorder plug-in with SDR#. I did play with the the FreeDV program in it's various modes but wasn't able to discern anything of value. I did a 16 bit version as well but even at 7MB zipped it's beng rejected by RR as too large to attach.

 

[ATCTech]

Did anybody have a look at the raw IQ file and see if it looks at all familiar?