Tuesday, June 15, 2004

Digital tidbits 

Here is an interesting posting to the SCAN-L mailling list. I plan to investigate further and see just how much the new AOR widget costs.

From SCAN-L:

For the past couple weeks we have been playing w/ the ARD25 (P25)digital
decoder from AOR. I am very pleased to see a manufacturer produce a
standalone decoder for Prpject 25 communications. This avoids(in part)
having to pay the hefty price tag for a full P25 scanner, particularly
in areas such as NH here where most P25 systems operate in a non trunked
mode. The ARD25 is designed primarily to interface w/ certain AOR and
Icom scanner/receivers that include a 10.7 MHz IF ouput connection. We
initally tested the ARD25 w/ our IC-R7000 and saw that it decoded P25
signals from NH State Police and other local Depts quite easily.

I wanted to see if it could be interfaced to other receivers that were
designed w/ a 10.7 MHz IF. We initally tried our old Regency Turboscan
by adding a tap at the ouput of the 1st Mixer stage, and it connecting
to the input of the ARD. At first we could not get the ARD25 to decode
anything. Then we realised what the problem was: The "polarity" of the
P25 data was inverted due to the IF conversion scheme used in the TS2.
We sound this out when we programmed a frequency 172.800 MHZ into the
TS2 so as to have it receive what we had been trying to get previosuly
on 151.400 MHz(NHSP) as an "image" frequency. This worked because it
placed the scanner's Local Oscillator above(instead of below) the
intended receive freq, and permitted the demodulated P25 voice "data" to
have the correct polarity orientation. We further reasoned that this
same effect could be accomplished by changing a 10.245 MHz IF conversion
crystal, used in the ARD25 to convert the 10.7 Mhz signal down to 455
KHz, to a lesser used 11.155 MHz crystal to produce the same 10.7 to
455 IF conversion, but produce demodulated data w/ the correct

Since the ARD includes a serial data port that permits a PC running
Hyperterminal to communciate w/ the ARD25, as well as a means to display
decoded Talk Group#s and even Unit ANI IDs, I wondered if there was an
easier way to get the correct data polarity by way of a software
command. An inquiry to AOR Cusomter Support revealed that this was
indeed possible. However the information they initally provided wasnt
complete and it took us a great deal of experimenting in order to
finally get this figured out. In the course of doing this we uncovered a
number of additional commands that can be set for different purposes.
For instance sending the command ADC will produce a short listing of
about dozen function/settings the ARD25 can be commanded to do. Some
commands, such as the data polarity setting require a certain extra
trick in order for it to actaully take effect... Another useful command
the ARD25 can be set for is ADA ON/OFF. This selects if you want it to
pass analog signals as well as digital ones. This is a feature none of
the current crop of P25 scanners are capible of. It would seem that AOR
seriously faulted here in terms of what they did not bother to inlcude
in the manual for the ARD25. Nor does there seem to be any reason as to
why AOR uses a male DB9 connector on the ARD25, which requires the use
of a female/female adapator or cable in order to communicate w/ it via a

Always wanting to go further we also wondered if there was a means to
interface the ARD25 to other receivers(that dont use a 10.7 IF) by way
of simply providing a "baseband" data connection from a receiver. After
much careful experiementation we found that it was indeed possible to
connect the Discriminator signal from nearly any scanner, into the
ARD25, substituting an external signal for that of the ARDs own IF
demodulator output. So far we have successfully tested the ARD w/ by
using the discriminator signal from other scanners such as the BC760,
245, 780 and 9000. This discovery makes the ARD25 a far more
universal/flexible platform for decoding P25 transmissions than what
AOR seems to be interested in marketing this product for. We plan on
having a complete writeup explaining these hidden features as well as
the hardware mods we noted, along we installation details for several
popualr scanners in the coming months if there is adaquate interst in
this area.


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