DSD Polarity

[ch4412]

I have been using DSD v1.7 to monitor a 700/800 MHz P25 phase 1 system with a discriminator output from a Uniden BCD396XT. When there is a transmission on an 800 MHz frequency, there is no problem with decoding and the sync type is –P25p1. When it tries to decode on a 700 MHz frequency, however, the frame sync comes across as +P25p1 and an unknown DUID. Has anyone else experienced this or have an idea on why the polarity would come across differently based on the transmission frequency?

 

[slicerwizard]

Polarity change is due to the scanner switching between high and low injection (mixer frequency above or below target signal)

Unknown DUID is due to poor decoding, possibly due to use of CQPSK, or just a weak, noisy signal.

 

[ch4412]

If the scanner is actually switching between polarity based on the frequency, then why would DSD be able to decode only one polarity? It doesn't appear to be poor decoding as it has no problem when it is on an 800 MHz frequency and the NAC seems to bounce between F1B and B1B very consistently. The actual NAC for the system is 1B1.

 

[slicerwizard]

Probably a different modulation method in use on 700 vs 800 (CQPSK vs C4FM). But you're there, not me, so you'd have to determine that.

The DSD authors were not programmers, so there are many things in DSD that are not being done properly. And the folks who could've taken advantage of source code availability to fix those flaws chose to not do so. So you get bad DUID reports and a flip flopping NAC, even though the extensive forward error correction built into the P25 protocol should lead to either correct NACs and DUIDs being reported or an unrecoverable level of errors being detected (and no NAC/DUID being reported for those frames)

Mind you, to be fair, I know nothing of the quality of your discriminator tap and what part it plays, if any, in these failures.