P25 repeaters

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P25 repeaters

Postby KC9UZB » Thu Jul 03, 2008 4:55 pm

What do you guys do, just scour eBay and 'the labs for quantars(?) ?

I wouldn't mind playing with making a repeater out of a couple of mobile radios (the method over on batlabs) but ideally i'd want something made for the job.

How much would I be looking to shell out for a secondhand P25 repeater?

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Re: P25 repeaters

Postby mancow » Thu Jul 03, 2008 6:42 pm

I'm still looking at the idea of two astros for a repeater. I'm becoming more and more convinced that it can't really be done, (correctly anyway) and be true mixed mode.

True, there is the discriminator to modulator method (maxtrac style) but I wanted to see if there was a way to link the two after the data had been processed. Basically, I was thinking of just routing around the two DSPs somehow. It looks like the transmit mode is somewhat more complicated than receive. There appears to be three major components. There is the ABACUS chip on the RF board that digitizes the RF. It sends the I / Q data to the ADSIC chip over a couple of lines in AC by way of varying current instead of TTL. The manual says it eliminates the generation of stray RF fields created by high speed digital switching logic gates and lines. The ADSIC does some processing and filtering then sends the data on to the DSP. It then decodes the data through its IMBE codecs and sends it back over to the ADSIC which has a D/A converter. The ADSIC then renders a regular analog signal that is sent to the speaker amps.

In transmit mode the mic audio is directly processed by the ADSIC. It sends the data over numerous parallel lines to the DSP. The DSP then creates the IMBE data format and it is sent back over to the ADSIC where it is processed by a D/A converter and sent on to the RF modulator.

What confuses me is use of four major lines in between the chips. There is an RXD to SRD serial line that goes from the ADSIC to the DSP in RX mode. They say that data moves at 2.4 Mhz on it. In transmit mode the data moves from the DSPto the ADSIC from the STD to TXD line at 1.2 mhz. I was hoping that a unit could be set to transmit and one to receive with the lines basically severed and strapped from the reciever's RXD to the transmitters TXD but that's obviously not going to happen. Plus, who's to say what format its in.

So, the next place to look is the secure module. The problem I'm seeing there is that I'm not sure that the data in TX mode is in the same format and data rate in RX mode and vice versa, same problem as before. But, it's a bit more promising since it's a dedicated line meant just for raw data manipulation (encryption).

I would think that the fist step would be to strap the two together in a fashion that they both use the same master data clocks so there are no sync issues. But I'm not sure if that's possible since each radio may need to vary the oscillators a bit for fine tuning. I need to see if everything is derived from the 16.xxx Mhz clock or not. I can't remember right now.

Then, I would assume that the secure module data is not present on the lines unless the radio is set to secure capable via RSS/CPS. That would mean that valid modules would have to be installed and loaded with non volatile keys since their incoming/outgoing lines would have to be severed. Once that is done the lncoming data line from the RX radio would need to be sent over to the TX radio's line, effectively cutting the secure modules out of the system without them knowing it. But, is that possible? Does the radio check those lines at power up to verify that the module is working? If so, something like a high speed switch would have to be put in line to disconnect the modules after power up.


.........ahhhh what a nightmare

Maybe there is something simpler that I am not seeing?

This a cut and past of how the DSP and ADSIC talk to each other in RX and TX mode.
Image

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Re: P25 repeaters

Postby ka8ypy » Thu Jul 03, 2008 11:10 pm

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Re: P25 repeaters

Postby KC9UZB » Fri Jul 04, 2008 8:05 am

Thanks Dan.

Surprising they came from London, but if they were used I actually have a good idea who they originated from (Her Majesty's Government).

I don't mind paying shipping from the US, although it would be pretty hefty I imagine. But maybe I'll get lucky and find a UK source.

David.

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Re: P25 repeaters

Postby Wowbagger » Fri Jul 04, 2008 8:55 am

I'm rather surprised somebody hasn't done the simpler idea of just using a low-cost low-power DSP to:
1) take the flat RX audio (discriminator audio) and recover the CAI symbol stream.
2) take that symbol stream and regenerate the idealized waveform out to the flat TX input.

The DSP wouldn't have to have any protocol, and most certainly wouldn't need the vocoder - just the C4FM data slicer/C4FM modulator.

That would clean up any modulation fidelity error in the recovered signal, which would be "enough" in most cases.

(adding the code to de-frame the data and apply the foward error correction would be a bit more work, but you *sitll* wouldn't need the vocoder itself).

Let's see: assume you digitized the audio at 19.2kSample/sec, did a simple slice-and-compute-fidelity to recover symbol timing, and a simple modulator with a 4 tap integrate and dump as called out in the specs: call it about 50 operations per incoming sample and 5 operations per sample on the outbound data, about half a million instructions per second. Just about any reasonable modern microcontroller should be able to do this (PICs need not apply, but just about all of the modern Atmel devices would probably do).

(gosh this is easier when you don't have to compute the parametrics to 4 places!)
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Re: P25 repeaters

Postby mancow » Fri Jul 04, 2008 12:52 pm

Get er done! :baby:

That's kind of what I was trying to do with the hardware here.

Where would a person get the specs for the protocol, enough to do something like that?

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Re: P25 repeaters

Postby escomm » Fri Jul 04, 2008 1:03 pm

19.2k sample rate eh

No wonder digital sounds like asshole.
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Re: P25 repeaters

Postby KC9UZB » Fri Jul 04, 2008 1:49 pm

Yeah but it stops the hammies and the idiots from hearing what's being said... xD

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Re: P25 repeaters

Postby KC9UZB » Fri Jul 04, 2008 3:58 pm

Bugger that.

I'm an anti social fuck and proud of it :baby:

None of the hams round here even know what PL is, nevermind P25.

(Wowbagger enters from stage left to tell us the difference between IMBE and P25).

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Re: P25 repeaters

Postby rdunajewski » Fri Jul 04, 2008 3:58 pm

Rich
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Re: P25 repeaters

Postby KC9UZB » Fri Jul 04, 2008 6:11 pm

Um.

who mentioned crypto?

My radio gives me a green light anyway :)

Y'all maybe don;t understand, but people are envious of the UK of intelligence. so when, on rare occasions, myself and my friends want to have an intellectual discussion, we get people butting in and being general twats.

We switched to digital to put off 99% of the people out there... not many people have p25 radios in the UK.

Anyway - with respect Ray... you took my thread off topic. Bugger off.

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Re: P25 repeaters

Postby KC9UZB » Fri Jul 04, 2008 6:12 pm

eh.
too many typos and spelling errors in that post to care, except i meant 'in the uk of intelligence'. which kinda proves my point that i dont have any.

ho hum.

To sum up. I'm not breaking any rules by running P25 - I'm not running crypto, so basically it might not be in the spirit of ham radio, but neither is them being dickwads.

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Re: P25 repeaters

Postby motorola_otaku » Fri Jul 04, 2008 6:14 pm

I've been looking at the Daniels RX/TX modules, but not seriously enough to warrant making a few phone calls to determine pricing. Ditto on the new Raytheon P25 repeater controller, which from what I understand does basically what Wowbagger just described but with some extra bells-and-whistles (programmable RX/TX NAC, multiple receiver/transmitter control, maybe linking?)
And the sign says you got to have a membership card to get inside.

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Re: P25 repeaters

Postby motorola_otaku » Fri Jul 04, 2008 6:16 pm

fake edit: Midland makes a P25 repeater too; just saw the ad for that one yesterday. You'd think it'd be cheap, being Midland and all, but probably not. :baby:
And the sign says you got to have a membership card to get inside.

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Re: P25 repeaters

Postby Mong » Fri Jul 04, 2008 7:29 pm

I'll take a warm midget fart over a cold beer any day

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Re: P25 repeaters

Postby smokeybehr » Fri Jul 04, 2008 10:27 pm


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Re: P25 repeaters

Postby va3wxm » Sat Jul 05, 2008 7:06 am


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Re: P25 repeaters

Postby Mong » Sat Jul 05, 2008 7:30 am

I'll take a warm midget fart over a cold beer any day

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Re: P25 repeaters

Postby spareparts » Sat Jul 05, 2008 8:14 am


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Re: P25 repeaters

Postby Wowbagger » Sat Jul 05, 2008 11:19 am

These opinions are mine, not Aeroflex's - if you want service info, manual, or other official support please contact Aeroflex directly.

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Re: P25 repeaters

Postby motorola_otaku » Sat Jul 05, 2008 7:14 pm

And the sign says you got to have a membership card to get inside.

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Re: P25 repeaters

Postby escomm » Sat Jul 05, 2008 7:38 pm

Uh, SDR still requires a great deal of hardware engineering. You don't just program the radio to tx from 30MHz to 900Mhz and off you go. While you probably know that, your post seems to imply it's just writing some code. Couldn't be further from the truth.
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Re: P25 repeaters

Postby smokeybehr » Sun Jul 06, 2008 12:16 am


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Re: P25 repeaters

Postby akardam » Sun Jul 06, 2008 8:24 am

KG6SLY | Broadcasting Around the Galaxy Around the Clock

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Re: P25 repeaters

Postby mancow » Tue Jul 29, 2008 11:45 pm

My welfare P25 repeater is slowly taking shape. :vestman: :wacker:

So far it sounds great but it's still on the bench with no duplexer and exiter power. I modded (sorry Ken if you're reading this) a friend's Communications Specialists community repeater controller to pass good clean flat audio. A reed relay acts along with the COR line to kick the internally generated CW ID and courtesy to the audio line while disabling the open hash of the raw disc. audio when in hang time. The result so far is excellent audio in analog and digital with a nice smooth silent hang time and clean ID tones. The nice thing is that it has DTMF control so all functions can be changed remotely such as PL or DPL control or open mixed mode operation.

I'm curious if it's really going to be worth all the effort once I finally get a chance to put some distance to it.

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Re: P25 repeaters

Postby KB7PST » Wed Jul 30, 2008 9:50 am


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Re: P25 repeaters

Postby mancow » Thu Jul 31, 2008 8:34 pm

Hmmmm.... well I tried the welfare maxtrac to maxtrac flat audio setup over at batlabs. No joy. I figured that a matched maxtrac would perform even better than this old King mobile but no. I tried every devation adjustment step between 13-35 with no sucess. As usual the PSR500 decodes most anything but the XTSs just sit and puke out raw CAI noise.

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Re: P25 repeaters

Postby MattSR » Thu Aug 07, 2008 6:45 pm

These chips from CML micros could be the basis for a simple P25 machine. Couple with with an atmel ATMega16 to run the show and bobs your uncle

goto http://www.cmlmicro.com/products/index. ... index=true and click on the C4FM Modem function.

ATmega16 can do SPI which is the bus used to control the CML chip. then all you need is the RF gear and yer done.

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Re: P25 repeaters

Postby KC9UZB » Thu Aug 07, 2008 7:29 pm

I'll commission you to build me a P25 repeater then Matt ;)

Actually I probably couldn't afford it :baby:

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Re: P25 repeaters

Postby MattSR » Thu Aug 07, 2008 8:18 pm

Hardest thing I came across was the sale poons at CML. Al I wanted was some samples of the chips and the data sheets/SPI instruction sets.

Do you thnk they would sell them to me? No way.. They wanted to see my support structure, intended customer base, production volumes, blah blah... when I tried explaining that it was simply to build a prototype, they went back and asked the same questions again...

Its a pity because it sounds like they have a good product that could make this a very easy project.

Cheers,
Matt

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Re: P25 repeaters

Postby mancow » Thu Aug 07, 2008 11:47 pm

I was looking at the exact same thing. I almost contacted them but I figured I would get that same response. It's too bad everyone has to act it's 1944 and they hold the plans to the Manhattan Project.

BTW, not that anyone cares but I did get the maxtracs to pass nice clean fairly unmolested P-25 data. It's still running on exciter power here (less duplexer) for test purposes but the XTS5K 2K and every other radio here are decoding just fine. I got the tone panel modded to operate with CW ID and DTMF operation too so everything works normally less PL of course.

The real world test should be coming soon.

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Re: P25 repeaters

Postby mr.syntrx » Tue Oct 28, 2008 5:05 am

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Re: P25 repeaters

Postby motorola_otaku » Tue Oct 28, 2008 11:07 pm

And the sign says you got to have a membership card to get inside.

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Re: P25 repeaters

Postby MattSR » Tue Oct 28, 2008 11:34 pm

P25 uses bitrate of 9.6kbps (4800 bits per second signalling rate @ 2 bits per symbol)

The chips (when loaded with the correct image) have been specifically designed for a P25 application. They even automatically detect and insert the P25 frame sync for you.

Have a look at CML micros documentation on C-BUS, its directly compatible with SPI.

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Re: P25 repeaters

Postby mr.syntrx » Fri Oct 31, 2008 7:48 pm

Even the engineers working down the hall from me have a fuckload of trouble getting CML samples.

Buy a USRP with the appropriate transmitter and reciever modules, and... http://www.sedition.org.au/op25/

:baby:
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