D-STAR and other digital communictions
Griff Hamlin <griff@...>
Hello fellow CMRA members,
I'm glad to hear from Corey there may be interest in a D-STAR digital repeater. I've
long been interested in digital transmission modes, having experimented
with 1200bps packet radio in the past, and PSK31 on HF more recently. It
combines my ham radio hobby with my computer software career (I'm now retired).
In case you're interested, D-STAR VHF/UHF radios modulate the RF carrier using GMSK
(Gaussian minimum shift keying) at 4800 bits/second. Voice audio is run through a patented
AMBE vocoder that compresses the voice down to 3600 bits/sec. That leaves 1200 bits/sec
for digital "data" that can be transmitted simultaneously with voice. Callsigns are transmitted
over this data channel, so that repeater or reflector computers can know who is
connected. Websites such as www.dstarusers.org maintain lists of who is
connected to what repeater.
There are several software packages allowing you to connect a PC
to a DSTAR radio and send digital data:
D-RATS -- open-source software. Provides instant-messaging style chat, etc.
D*Chat -- simple application to send a line of typed text through DSTAR radio.
DStarTV -- slowscan TV
The biggest argument some hams make against D-STAR is that the AMBE vocoder
is proprietary and against the "open" nature of ham radio. Opponents also
say this runs up the cost. For example, a DV-DONGLE that connects to a
PC and compresses voice using AMBE costs $199. A DV-DONGLE could theoretically
be replaced by free software. But anyone who wrote such software and made it available
to the public would probably be sued by the AMBE patent holder (Digital Voice Systems, Inc).
Some hams (VK5DGR and others) are proposing an open-source alternative to AMBE,
known as CODEC2 (see
http://www.rowetel.com/blog/?page_id=452). CODEC2 can compress
voice into 2550bits/second. The group hopes to have a 1200bits/sec version
in the future, for digital voice on HF using bandwidth comparable to a SSB
signal. On the other hand, many hams don't care if D-STAR uses a proprietary
vocoder, as long as it is well suited to the task. They point out that
AMBE is used in other digital voice systems like P-25. The side you're on
may depend upon whether you mostly like to experiment with ham radio, or
mostly like to talk.
I'm personally interested in MUCH higher data rates than 4800 baud. I want
a megabit/second or more. With such data rates, digital voice communication
doesn't need a fancy vocoder. Also digital ATV (not slowscan) is possible at
these datarates. Software for digital audio and video is already freely
available (Net-Meeting, Ekiga, etc).
In my quest for speed, I've been experimenting with 802.11 "WiFi" wireless
radios. The ARRL's High-Speed-MultiMedia (HSMM) project a few years ago did the
same thing. More recently, some hams propose HSMM use in emergency
communications to provide telephone service using open-source PBX software
like Asterisk, or email with high enough speed that documents and images
can be attached. They point out that emergency personnel are already familiar with
these types of communications.
The "WiFi" gear is mass produced and is inexpensive. Hams often adapt inexpensive
commercial gear to ham use. For $82 I got a 1 watt 2.4ghz
transceiver (Ubiquity Bullet) that's capable of operating at 2397mhz, as
well as in the 802.11 channels. 2397mhz is in the 13cm ham band but not
within the unlicensed 802.11 channels, so there shouldn't be much
interference. Hams are the primary users of this part of the 13cm band.
The radio can transmit a 5mhz bandwidth signal with data rates between
750 kilobits/second to 13 megabits/second. With a 24db gain parabolic
reflector antenna (costs about $60) the effective radiated power (ERP) is
about 240 watts. Some people report setting up 2.4ghz digital links in
excess of 30 miles (between two hill tops or other line-of-sight). To date, my longest
connection is between my house and downtown Columbia (5.1 miles).
Unfortunately there's no one to talk to on 13cm in central MO. I'm looking
for someone else who's interested in experimenting with high datarates
on either 13cm (2.4ghz) or 33cm (900mhz). If you're interested,
email me at griff@.... I looked at D-STAR's 1.2ghz high-speed digital radio, but
it's way too expensive for me ($1100 and up) and it's not as fast (128kilobits/sec).
Michael J. Hauan
This links to an interesting discussion.toggle quoted messageShow quoted text
Along with Griff's email, it suggests to me that, as we consider next steps -- potentially a DD module and wider deployment of $1000 ID-1 radios (the only option with D-STAR), we look closely at wifi-based radios, along the lines Griff mentioned.
It already seems attractive given 1) the price of equipment, 2) the potential data rates, 3) extant network models, e.g., mesh, and 4) there's substantial ham work already done that we can build on (see http://kb9mwr.blogspot.com/2010/08/hsmm-mesh-network-in-texas.html ).
I'd certainly like to do some experimenting along these lines in any event…
On Oct 8, 2011, at 9:27 PM, Griff Hamlin wrote: