[aprssig] PIC processor for APRS info for D-STAR mobiles?

Robert Bruninga bruninga at usna.edu
Tue Jul 8 09:51:16 EDT 2008

Since many of us are in areas where there are no DSTAR repeaters
or we don't own a DSTAR radio, that is why we are kinda in the
dark and trying to understand the nuances of how they work..
Pete has always emphasized that it is impossible to do low rate
data streaming from the DSTAR repeater without interfering with
voice users, but now the details of this impossibility are
starting to coming out.

> if a transmission with silence for audio ("data-only") 
> is being received correctly (no R2D2), the other 
> people trying to use the frequency for voice 
> communications will likely key up over the data 
> causing both transmissions to be lost..

My assumptions were that the DSTAR repeater operated full
duplex.  And that the small amounts of local APRS information
(translated by D-PRS) that we wanted to see distributed to
displays for the users would be originated at the repeater site
and streamed over the repeater output's low rate DV bit stream
while the repeater was not in use.  And, since the repeater was
full duplex, that as soon as the repeater detected a user on the
uplink, that it could be configured to cease the streaming of
the local APRS info.

Of course, this assumed that it was possible to get to the
outgoing repeater DV low rate bits at the repeater hardware
without having to go through  another DV radio's DV bits
transmitting on the uplink.  It seemed logical that the the
repeater would have a serial port access to its low rate DV bits
just like other radios do.  Apparently not?

If this is not the case, and no one can hack the repeater to
send out a low-rate DV stream independent of a requirement that
the bits come in via the uplink DV bits, then this  explains
Pete's stance of it being "impossible".

Now, if I am correct so far, then lets consider three
work-arounds.  I am not addressing the practicality of either of
these.  Just trying to understand what would make this possible.

A) Ask for (or hack?) a firmware change to the repeater hardware
so that independent access to the DV output bits and input bits
are possible.  This would allow the "APRS streamer engine" to
manipulate what is going out while watching what is coming in.
IN fact, while one station is talking, and his fix has been
received and retransmitted, instead of continuing to stream
those same fix report over and over, then other APRS local area
fixes could be interleaved.

B) Without any independent access to the repeater's output DV
stream, then we accept the impossiblity and go to plan B.  Plan
B is to add a second DSTAR radio at the repeater site which can
be configured so that it can transmit when-and-only-when, there
is not someone on the input.  This radio's DV extra bits are
used to stream D-PRS translated copies of local APRS Mobiles in
this area.  This radio does transmit VOICE SILENCE while this is
taking place.  Though an option for "elevator music" or Rush
Limbaugh could be made (joke!)...

C) Do what we can with D-GATE?  It is a processor connected to a
DV radio, so could it not do the plan B streaming while the full
repeater was idle?

Again, I am not saying either of these are practical.  But do
they answer why the DSTAR users have been saying it is
impossible.  Or is it only because they assumed the source of
the D-PRS translanted APRS information was going to get to the
repeater on the uplink of another D-STAR radio, instead of via a
backdoor independent input as I was assuming...


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