[Linux-ham] portable digital voice

Kevin Otte nivex at nivex.net
Thu Jan 17 00:29:55 EST 2013

; Rationale

At RARSFest a few years ago, there was a great buzz around D-STAR. 
Unfortunately, the discussions about the non-Free codec were a bit animated.

Now that we have some options, I'd like to try to have a functional demo 
for RARSFest this year. And by functional I mean walking around using 
it, not just on a booth table.

What follows are my ramblings on how we might go about this. It is in 
some part an attempt at organizing my own thoughts, but the closer to 
the RF hardware I get, the weaker my skills are. I'd love some insight 
from others. As such, we'll start there and work our way up the stack.

; Modems

At the modem level we have a few options: FDMDV, G3RUH (the 9600 baud 
packet standard), and GMSK.

The FDMDV modem is part of the Codec2 development and delivers a 1400 
bit/s stream in about 1.1KHz of bandwidth. It is primarily designed for 
HF, but could easily be routed through an HT audio connection. Details 
here: http://www.rowetel.com/blog/?page_id=2458

Using 9600 baud TNCs would make for a nice reuse of older equipment, 
much as APRS does. Some radios such as the TH-D7 have these modems 
built-in already. I'm not sure how easy it is to lay hands on more of 
such equipment though. I also seem to recall that getting seperate 
radios and TNCs tuned up was a bit of a challenge, but that was also ~20 
years ago. (Where does the time go?)

In newer territory, using GMSK would in theory be a better use of the 
spectrum. I don't know as much about how to make this work. There is a 
Belgian ham ON1ARF doing some early work on this front: 

; Codecs

The seemingly obvious choice here is Codec2. It was designed for this 
very purpose and has already earned the 2012 ARRL Technical Innovation 
Award. I even had the pleasure of meeting David at the Dayton Hamvention 
last year.

Another option would be the recently IETF standardized Opus codec. The 
drawback here is that its lowest bitrate is 6Kbit/s. Though, if using 
something that is 9600 capable, an 8 Kbit/s voice stream could be used 
for great audio quality and room to spare.

; Systems

For initial mockup and testing, full PCs will make life easy. Once you 
get into the realm of needing to carry it around, that's where it gets 

With the recent buzz around the Raspberry Pi, this seems like a leading 
possibility. ON1ARF's work as well as some other discussion I've read 
seem to indicate that the audio capabilities of the Pi are less than 

There are all manner of embedded systems out there these days. So many, 
in fact, that I won't even pretend to try and keep track of them all. 
Suggestions welcome.

; Conclusion

So there you have it, an attempt at a braindump of my epic idea... at 
least I hope it sounds as epic in this message as it does in my head :)

It's a lot of work, and it may not even come to fruition, but hopefully 
getting this out of my head and into the air will let my brain spin down 
a little.

73 de Kevin, N8VNR

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