Batboard

Hello all,

Was just looking at the Maxtrac ASTRO mod. Sounds awesome. At any rate the ICOM V82 and U82 portable radios with the Digital add on board work very similar to P25 Digital. It looks like the data is FDMA or similar. However these radios will pass thru two GM300's or Maxtracs without any mods at all, just connect like a DISC-FLAT TX IN, COR-PTT, and GND-GND. A VR may be necessary from the RX to TX line. Verry simple. I have 2 of these systems running great on high level sites, just not around a lot of RF.

USGOVTECH wrote:Hello all,

Was just looking at the Maxtrac ASTRO mod. Sounds awesome. At any rate the ICOM V82 and U82 portable radios with the Digital add on board work very similar to P25 Digital. It looks like the data is FDMA or similar.

FDMA is an access method, not a modulation scheme. It doesn't have much to do with the modulation used.

P25 Phase I uses C4FM, or Compatible 4-level FM. D-Star on VHF/UHF uses GMSK (Gaussian Minimum Shift Keying).

USGOVTECH wrote:However these radios will pass thru two GM300's or Maxtracs without any mods at all, just connect like a DISC-FLAT TX IN, COR-PTT, and GND-GND. A VR may be necessary from the RX to TX line. Verry simple. I have 2 of these systems running great on high level sites, just not around a lot of RF.

Yeah, GMSK equipment generally isn't nearly as picky as P25 in making sure the symbol levels are correct. I've done a lot of work with radio modules that use GMSK in telemetry systems, and I've been able use them over fairly flat voice repeaters.

The downside is the DSTAR voice quality isn't as good as P25. They're using AMBE(2020+, IIRC) as a vocoder... but it's only 4.8Kbps effective rather than P25's IMBE vocoder with 9.6Kbps effective. Despite the better vocoder, the reduced data rate drops the voice quality.