APCO 25 decoder /analyzer

Matt Robert, VK2TVK writes in the APCO25 ham radio list:

Hi List,

I would like to let you guys know about an open source P25 decoder/analyzer project that has been running for a little while now. OP25 started as the thesis for a Computer Security PHD student that is researching the security aspects of the APCO25 standard. The software is being expanded into a decoding suite that can gives various bits of information about a received signal and there is also provision for a software based repeater.

OP25 is based on the open source GNUradio package, and it is compatible with either the USRP software radio peripheral, or a conventional NFM radio connected to a sound card via a discriminator tap. One contributor is also working on adding transmit code with the intention of giving a software based P25 repeater that can be run off conventional PC hardware. Currently all the major contributors are HAM operators.

At the moment OP25 is considered pre-alpha (i.e. developmental) and we are looking for people that are interested in becoming developers/contributors to the project. The project is designed to run under GNU/Linux and it requires a bit of skill to get it going. It's definitely a HAM radio project at this stage and that's why I figured it would be appropriate for this list.

Shown here are are some sample screenshots for the curious.

Please feel free to have a look at the project homepage at http://op25.osmocom.org and there is also a discussion list at http://tech.groups.yahoo.com/group/op25-dev/

Kind Regards,
Matt

[Update Dec 2009]
Here is an interesting amateur video, that demonstrates the OP25 code in action with GNU radio using the USRP.


[Update April 2012]
Here is an updated video, showing OP25 code with GNU radio using a cheap USB TV tuner as a replacement to the USRP.

DV Dongle

If you have been following my blog you probably say my piece on interoperability strides and my other piece on this second roll out of D-Star called NXDN.

This interoperability thing as you can imagine is pretty important. The key problem stems from a lack of standards but it really has to do more so with the fact that technology is developing so quickly that it's difficult to get everyone on the same page. Technology standards seem to be set by who can develop something first and obtain the largest market following. Just look back at VHS vs Beta, Blueray vs HD-DVD, etc.

As pointed out a common interconnection that both P25, and D-Star developers are choosing is the standardized Asterisk based SIP protocol using RTP audio streams.

A key component in all these digital voice systems is the vocoder. This is the device that converts your spoken voice to a synthesized or digitally compressed speech format.

The public safety APCO/ P25 format uses Improved Multi-Band Excitation (IMBE). This is proprietary vocoder developed by Digital Voice Systems, Inc. (DVSI). It is the predecessor of their Advanced Multi-Band Excitation (AMBE). It costs $150K to get the rights to play with that mode plus $5 a seat. There is no off the shelf IC to do it.

D-Star uses Advanced Multi-Band Excitation (AMBE) from DVSI. Same $150K if you want the software source but they do offer a single chip solution for $20 single quantity and are happy to sell to hams.

The DV dongle is an important development. It was started by Moe, AE4JY and Robin, AA4RC. It contains the ability to process AMBE full duplex. Presently software applications exist to use this to communicate from a computer to a D-Star gateway. Further development are expected so that it can be interfaced to a radio's packet radio port that has the necessary discriminator connections. This may be a huge milestone. The ability to retrofit an existing repeater could be possible with this. Not only that, but you may be able to retrofit it in such a way that it can be usable in analog and for D-Star. 

With the DV Dongle you will be able to use DPLUS* or the future OpenDSTAR gateway software to connect to the gateway computer behind the ID-RP2C (repeater controller). Then you will use the DV Dongle to extract the audio streams and can transcode them to standard G.711a/u, GSM, G.729, etc. and then use the Asterisk PBX power for telephone, voicemail, DID-Callsign-DID, repeater, etc. interconnection.

(* DPLUS is a gateway addon daemon that provides a number of functions see: http://opendstar.org/tools/readme.txt)

When you take a digital radio platform like D-Star, this is where integrating Asterisk could be very powerful. Since the call sign is part of every packet, this could be assigned to a direct inward dialing number (DID) or extension.

At the present time the dongle connects over the internet to an Icom gateway controller at a repeater site. So for now is a non-RF application.

What would be even more ideal since D-Star radios don't have an analog packet port is if the various D-Star radios had a digital interface port/jack to support just such a dongle / device to transcode to a SIP and codec standard. Unfortunately at this time there are no known interfaces to the Icom D-STAR radios that allow access to the on-air data stream.

I'd love it if I could buy a D-Star radio, that has an digital interface of sorts. Something along the lines of D-Star non-proprietary interface. Perhaps a mini-USB interface that perhaps transcodes to a more open standard codec like G.711 using SIP and RTP protocols (standardized protocols) so one can connect the radios together into wide area networks.

Then one would be able to have a D-Star radio in my shack also interconnected to various Asterisk powered applications in their house. Where if you weren't around to take a directed D-Star call, it could be configured as a DID to a system and use a ring group / follow me list to let the radio caller ring a home phone or leave a voice mail.

The open source project25 interface is a good idea. Unfortunately the problem they're facing is that most of the manufacturers don't bother following the ISSI spec, nor does the ISSI spec call out hardware interface details. So basically the "plugs" on the back of say, a Quantar... don't match the "plugs" on the back of a MASTR III.

http://en.wikipedia.org/wiki/P25_ISSI

The Project 25 Inter RF Subsystem Interface (P25 ISSI) is a non-proprietary interface that enables RF subsystems (RFSSs) built by different manufacturers to be connected together into wide area networks.

It would be great to see an a D-Star radio that supports something like this on the market possibly before the P25 interface idea ever makes it to the market.  

ARRL: It Seems to Us: Interoperability October 1, 2007 We need to encourage D-Star manufactures to come up with a similar style non-proprietary interface for D-Star.  

Digital Voice Interoperability strides

Asterisk provides an Interoberabity platform

What is Asterisk?
Asterisk is an Open Source PBX & Telephony Platform. It’s often labeled as the future of telephony. It has been around since about 1999 and the platform is open source - Linux operating system based. It can support a variety of signaling protocols, but by far SIP, the session intitation protocol, for VOIP and other text and multimedia sessions, has become a standard.

For those scratching their head a bit... PBX stands for private branch exchange. It is a machine that handles many businesses telephones calls for you. Its main functions are to transfer calls to different individual phones; play music when somebody is put on hold; to play automated voice responses when a call is received; to provide an options menu for the caller etc.

Asterisk allows one to build their own phone systems. It adds features, functionality and reduces deployment costs in ways which; at first are a little difficult to understand.

How does this relate to amateur radio?

Very simple, the future of two way radio is digital. As of writing, TV are required to be full digital and shut down their analog transmitters in Feb. 2009. The only spectrum broadcasters are required to vacate are channels 64 thru 69 that will become the new "700 MHZ band" that is being auctioned off by the FCC. The vacated areas of this spectrum will be utilized for: Public Wireless deployment (Cellular/PCS); A wide-band private data network that will be shared between public safety and paying customers; and new spectrum for public safety that will butt right up to the re-located NPSPAC National Public Safety Planning Advisory Committee band being moved to 806-809/851-853 by Sprint/Nextel.

Public safety also has guidelines to migrate to APCO-25 digital. The future of two way radio is digital, and we must also advance in this direction. The digital premise is that it generally allows more use in a more efficient/flexible use of band space.

Most present day government communication centers that use analog systems happen to have a VOIP based dispatch console. This analog to VOIP patching is something that we are presently also embracing in ham radio with IRLP, EchoLink, Yeasu WIRES II, and the like.

Your seeing the digital migration in the commercial world as I pointed out; And the only analog part left of traditional telephone is the “last mile” drop to your home. Time Warner and now AT&T are providing digital phone service to close that up too.

As of writing there aren't any 100% directed approaches to tie this to the hobby that I can point you to. There are a number of open ended ideas from a variety of different people. What I'm saying is there is no one entity steering the ship, so to speak. This ideas are still in development. Which makes it precisely the time to jump aboard and get our hands in it and see what we can do with it. So in light of that I suggest a google search for more info...
Digital Voice Interoperability Software Strides
Perhaps some of you remember the ARRL "It Seems to Us: Interoperability" statement from October last year regarding emerging digital voice.

Well the good news is here are some of the strides I've run across:

-OpenP25 Project http://openp25.org/index.php/category/project-status/

"We've determined that the open source Asterisk PBX appears to be a good framework on which to build a P25 ISSI (Inter RF Subsystem Interface) switch. Asterisk has a mature SIP stack and already has the ability to transparently pass RTP frames between SIP channels. The National Institute of Standards and Technology (NIST) has made an open source program for ISSI testing freely available to the P25 community. A full-featured open source P25 ISSI switch is clearly achievable."

The Project 25 Inter RF Subsystem Interface (P25 ISSI) is a non-proprietary interface that enables RF subsystems (RFSSs) built by different manufacturers to be connected together into wide area networks. Apparently consideration is being given to enhancing Asterisk app_rpt to support a such a low-level P25 radio interface.
The open source project25 interface is a good idea. Unfortunately the problem they're facing is that most of the manufacturers don't bother following the ISSI spec, nor does the ISSI spec call out hardware interface details. So basically the "plugs" on the back of say, a Quantar... don't match the "plugs" on the back of a Mastr III.

-Open D-Star Project http://opendstar.org/

The OpenDSTAR group has released several software tools which build on existing commercially available repeaters and Internet gateways to extend functionality.

And this interoperability work in progress attempt has been around longer than the P25 one. They too are working a SIP stack into their DPLUS gateway add-on daemon.
A project is lead by Scott, KI4LKF expand the Asterisk capabilities with his RtpDir bridge software (Real TimeProtocol Director) software package for VoIP/RF Gateways. http://tech.groups.yahoo.com/group/rtpDir/
It can bridges transmit and receive from/to Asterisk/app_rpt, IRLP, Echolink, Speak-Freely and he is working on D-Star. There is support for all link interfaces, sound mode or ASCII mode, VA3TO, WB2REM, ULI, Rigblasters, MFJ, SignalLink, etc.
-RadioGrid RG-4 Radio Gateway. http://www.radiogrid.com/
The guys from NHRC who make repeater controllers have come up with a commercially targeted (and priced) controller called the RadioGrid RG-4 Radio Gateway. It's built around Asterisk open source PBX technology. It's specifically for VOIP linking designed with interoperability applications in mind. It uses a 500 MHz Blackfin processor, 64 MB RAM, 4 MB Flash memory, 1 GB SD Card

A key component in all these digital voice systems is the vocoder. P25 uses the IMBE vocoder from Digital Voice System Inc. (DVSI). It costs $150K to get the rights to play with that mode plus $5 a seat. There is no off the shelf IC to do it. Fortunately D-Star uses AMBE from DVSI they do offer a single chip solution for $20 single qty and are happy to sell to hams.

- The DV Dongle http://dvdongle.com/
The DV dongle is an important development. It was started by Moe Wheatley, AE4JY and Robin Cutshaw, AA4RC. It contains the hardware ability to process AMBE full duplex. Presently software applications exist to use this to communicate from a computer to a D-Star gateway. Further development are in the works so that it can be interfaced to a radio's packet radio port that has the necessary discriminator connections. This may be a huge milestone. The ability to retrofit an existing repeater could be possible with this. Not only that, but you may be able to retrofit it in such a way that it can be usable in analog and for D-Star.

It would be great to see an a D-Star radio that supports something like this on the market possibly before the P25 interface idea ever makes it to the market. We should to encourage D-Star manufactures to come up with a similar style non-proprietary interface for D-Star. As unfortunately at this time there are no known interfaces to the Icom D-STAR radios that allow access to the on-air data stream.

It would be nice to see some agreement and standards in place amongst the various guys working on software solutions and the manufactures, to help make things more stream-lined. Either way people are working on interoperability solutions, toward convergence with open standard codecs like G.711 using real word protocols like SIP and RTP protocols so one can connect the radios together into wide area networks.

Then one would be able to have a D-Star radio in their shack also interconnected to various Asterisk powered applications in their house or abroad. Where if you weren't around to take a directed D-Star call, it could be configured as a DID to a system and use a ring group / follow me list to let the radio caller ring a home phone or leave a voice mail. Calls could be route to/through the Public switched telephone network and so on. A very powerful ability for EMCOMM and personal use.

Poorman's P25 / D-Star repeater using Maxtracs

This diagram on how to use two Motorola Maxtracs is from Tim Warth, AA2RS.

This is perfect if you don't have the 3 grand to put up a D-Star repeater or can't find a good deal on a Motorola Quantar for P25.

Remember this mod is transparent. ANYTHING it hears that breaks squelch, including intermod and spurious noises, will get repeated. Data is not regenerated.


As shown, the TX audio must be routed to the new point of entry, otherwise, it gets filtered and 4 level FSK won't pass. It's recommended to do the Maxtrac power mods and putting active cooling on the radios as these radios get hot.

In most cases a carrier system is less than desirable. Fortunately Rick Parrish, KD4VXY (the author of Unitrunker) has written a software based NAC decoder. Unfortunately at this time it it does not yet have support to provide a serial or parallel port logic output.

It's worth looking at the CML Microcircuits CMX7031, and CMX7041 datasheets. It's A C4FM baseband data processor chip.  After some microcontroller coding, this could work similar to how the D-Star node adapters work.  (Those typically use the CMX589AP4 chip)