Hey everybody,
I figured this was as good a place as any to post this. I've long been interested in this protocol, which runs mobile data terminals in taxi cabs (including alpha-numeric dispatches, text messaging and GPS AVL), and it's fairly common in the 150-152 MHz range around the US. The older style DDS MDTs - still the most common - look like this (newer models feature touchscreens with GPS moving map):
The only other thread I could find on this topic is here:
http://forums.radioreference.com/ge...541-dds-digital-dispatch-system-decoding.html
Here's a sound sample (from the excellent kb9ukd digital modes samples site):
http://www.kb9ukd.com/digital/152.33-taxi.wav
Both the previous thread and the digital modes samples site refer to this mode as using Quadrature Phase-Shift Keying (QPSK) at 4800 bps. The author of the original thread claims he was able to decode it using QPSK64, which sounds suspiciously like 64-QAM to me (commonly used for digital cable TV) shown below:
Just for fun I tried running UniScope on the local DDS MDT frequency at 4800 baud QPSK, and was loosely able to get an hourglass pattern:
As you can see it was very jittery even with glass and filtering enabled, but it's close enough that I'm reasonably convinced the earlier QPSK 4800 bps description is accurate.
I was also able to find some info on the DDS network infrastructure from their brochure here:
http://www.ddswireless.com/brochures/PathFinder.pdf
Nowadays many MDT users choose to use cellular-based network solutions like CDPD or GPRS, but the option of running the system on a traditional business frequency still exists and is called "Private Mobile Radio." The PMR option is discussed on page 5 of the PDF, and consists of "Redundant Base Controllers" (RBC) at each site and an optional "Multi-Site Multi-Channel Network Controller" (MNC) which can run a multi-site network. (More on the MNC here.) Regarding the RBC's radio modem protocol, it says:
Thus the newer DDS MDTs can apparently support both 4800 bps and 9600 bps protocols, although I've only ever heard the former. (This can be verified by checking sales brochures for their individual MDTs.)
Just curious if anyone else has ever looked into this or tried decoding it. Or am I the only one crazy enough to care?
Ian Wraith's Taxi MDT Decoder program seemed like a good bet, but it only supports two European standards called Autocab and Auriga which I've never come across in the U.S.
I figured this was as good a place as any to post this. I've long been interested in this protocol, which runs mobile data terminals in taxi cabs (including alpha-numeric dispatches, text messaging and GPS AVL), and it's fairly common in the 150-152 MHz range around the US. The older style DDS MDTs - still the most common - look like this (newer models feature touchscreens with GPS moving map):
The only other thread I could find on this topic is here:
http://forums.radioreference.com/ge...541-dds-digital-dispatch-system-decoding.html
Here's a sound sample (from the excellent kb9ukd digital modes samples site):
http://www.kb9ukd.com/digital/152.33-taxi.wav
Both the previous thread and the digital modes samples site refer to this mode as using Quadrature Phase-Shift Keying (QPSK) at 4800 bps. The author of the original thread claims he was able to decode it using QPSK64, which sounds suspiciously like 64-QAM to me (commonly used for digital cable TV) shown below:
Just for fun I tried running UniScope on the local DDS MDT frequency at 4800 baud QPSK, and was loosely able to get an hourglass pattern:
As you can see it was very jittery even with glass and filtering enabled, but it's close enough that I'm reasonably convinced the earlier QPSK 4800 bps description is accurate.
I was also able to find some info on the DDS network infrastructure from their brochure here:
http://www.ddswireless.com/brochures/PathFinder.pdf
Nowadays many MDT users choose to use cellular-based network solutions like CDPD or GPRS, but the option of running the system on a traditional business frequency still exists and is called "Private Mobile Radio." The PMR option is discussed on page 5 of the PDF, and consists of "Redundant Base Controllers" (RBC) at each site and an optional "Multi-Site Multi-Channel Network Controller" (MNC) which can run a multi-site network. (More on the MNC here.) Regarding the RBC's radio modem protocol, it says:
Thus the newer DDS MDTs can apparently support both 4800 bps and 9600 bps protocols, although I've only ever heard the former. (This can be verified by checking sales brochures for their individual MDTs.)
Just curious if anyone else has ever looked into this or tried decoding it. Or am I the only one crazy enough to care?
Ian Wraith's Taxi MDT Decoder program seemed like a good bet, but it only supports two European standards called Autocab and Auriga which I've never come across in the U.S.
