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Thread: Using a PC soundcard to generate P25 (Phase 1) test tone pattern?

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    Question Using a PC soundcard to generate P25 (Phase 1) test tone pattern?

    I've got a bit invested in refurbishing phase 1 P25 EF Johnson radios but $8k for a newer service monitor is a quick trip to the divorce lawyer.

    First - please excuse me because I realize this maybe a totally stupid question but I was pondering some of the programs that allow you to send and receive digital modes using a PC sound card attached to the mic/spk of a radio and wondered if creating a simple wave file of the test pattern for P25 could be plugged into an older Service monitor for tuning? I've not dug into the technical details (And don't want to reinvent the world - Someone maybe already did this 10 years ago? I "think" P25 is just 4 tones on off which is similar to some amateur radio digital modes I've played around with. It seems like something fairly doable but can it really be that simple? For example using some freeware like Audacity or even an WAV/MP3 player as the audio generator with my somewhat antique service monitor would be great (Assuming it worked)

    Any thoughts? I tried the usual google searches and find a ton of ways to monitor conventional P25 but so far I've not hit on anyone generating P25. If no one has done this I'm happy to plod away and try some stuff but as I said why reinvent something thats already out there.

    Thanks for any help or thoughts on the topic.

    Jay at Refurbatron


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    I have tried this a while ago, recorded the Standard Tone Test Pattern Framed 1011 test pattern and played it back via the service monitor, best BER I could achieve was about 2.5 and never got it down to 0%. I suspect filters in the soundcard causing me the issues I was experiencing.

    Ideally if hardware like the MMDVM boards could generate a nice clean test tone connected to the service monitor input we could then achieve this on older service monitors.

    Another option is to use another ASTRO25 radio in test mode however you will not be able to easily attenuate the output signal as you would on a service monitor.

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    Quote Originally Posted by Jayc View Post
    I've got a bit invested in refurbishing phase 1 P25 EF Johnson radios but $8k for a newer service monitor is a quick trip to the divorce lawyer.

    First - please excuse me because I realize this maybe a totally stupid question but I was pondering some of the programs that allow you to send and receive digital modes using a PC sound card attached to the mic/spk of a radio and wondered if creating a simple wave file of the test pattern for P25 could be plugged into an older Service monitor for tuning? I've not dug into the technical details (And don't want to reinvent the world - Someone maybe already did this 10 years ago? I "think" P25 is just 4 tones on off which is similar to some amateur radio digital modes I've played around with. It seems like something fairly doable but can it really be that simple? For example using some freeware like Audacity or even an WAV/MP3 player as the audio generator with my somewhat antique service monitor would be great (Assuming it worked)

    Any thoughts? I tried the usual google searches and find a ton of ways to monitor conventional P25 but so far I've not hit on anyone generating P25. If no one has done this I'm happy to plod away and try some stuff but as I said why reinvent something thats already out there.

    Thanks for any help or thoughts on the topic.

    Jay at Refurbatron
    The P25 signal is actually a stepped square wave with four steps, two negative steps and two positive, reflecting the bits 00, 01, 10, and 11. The discrete voltages or deviation are quite critical to the receiver discriminating between steps.

    It really should not be hard to make a widget using a microprocessor with DAC to replicate this stream to directly modulate test equipment. It would be helpful to have a calibration scheme to send pairs of bits having same negative and positive deviation to calibrate the modulation level.

    Recording off the air with a soundcard probably introduces noise and frequency roll off.

    Sent from my SM-T350 using Tapatalk

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    Quote Originally Posted by Jayc View Post
    I've got a bit invested in refurbishing phase 1 P25 EF Johnson radios but $8k for a newer service monitor is a quick trip to the divorce lawyer.

    First - please excuse me because I realize this maybe a totally stupid question but I was pondering some of the programs that allow you to send and receive digital modes using a PC sound card attached to the mic/spk of a radio and wondered if creating a simple wave file of the test pattern for P25 could be plugged into an older Service monitor for tuning? I've not dug into the technical details (And don't want to reinvent the world - Someone maybe already did this 10 years ago? I "think" P25 is just 4 tones on off which is similar to some amateur radio digital modes I've played around with. It seems like something fairly doable but can it really be that simple? For example using some freeware like Audacity or even an WAV/MP3 player as the audio generator with my somewhat antique service monitor would be great (Assuming it worked)

    Any thoughts? I tried the usual google searches and find a ton of ways to monitor conventional P25 but so far I've not hit on anyone generating P25. If no one has done this I'm happy to plod away and try some stuff but as I said why reinvent something thats already out there.

    Thanks for any help or thoughts on the topic.

    Jay at Refurbatron
    The OP25 software project has a transmitter component. The transmitter supports voice modes for P25 (FDMA FSK4), d-star, fusion (wide and narrow), and DMR (2 slot BS TX). All of the transmitters can be configured to work with the PC sound card. (as an aside, that's the mode that I use to check in to a weekly net operating digital fusion, using an ICOM IC-820H with the PC/soundcard acting as the exciter. There's a mode switch inside the radio that must be set for 9,600 "packet" operation).

    One of the program features allows the data symbols to be read in from a file (as opposed to operating the speech codec in real time). A typical usage of this is to do exactly what the OP has requested.

    In fact, I have it on good authority that the various P25-standard test patterns are tested and will be added to the code repo soon.

    Once everything is installed the only remaining step would be the need to perform a careful calibration to ensure the audio level going into the FM mod is correct. If it's not set precisely, the FM deviation of the transmitted signal would be incorrect. The receiver software provides access to the eye pattern diagram, which is one way to observe the deviation.... It should go without saying that the FM mod must be linear and have no group delay - voice comms aren't necessarily sensitive to such things, but data is!

    73

    Max

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    This should work. As RFI guy pointed out you will have to make sure the waveform gives nice clean steps. Setting the scope to the baud rate and setting the trigger should lock the pattern so you can see the output clearly. It may be necessary to be able modify the input signal to the sound card to compensate for the 'distortion' the card creates.

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    I want to thank everyone for the ideas. OMG I'm really feeling like a moron. The 1011 (test) is not a repeated string of binary? Its just clean P25 RF sending a 1011 Hz tone? I guess I just needed you guys to explain this because I think I was really over thinking this problem.

    So don't laugh this might be highly flawed but here is what I'm thinking. I've got a very precise adjustable audio output from my service monitor that I could connect to one of the HTs I've got. I just rig it up between the audio out from the service monitor to an HT using a chopped up external mic cable with a toggle switch for PTT. I could run the RF output through my misc box of Microwave Devices attenuators to get the HTs low power 1 watt down to (Can't think of the test level output)

    I'm probably missing something but wouldn't that end up being a P25 Signal generator in its crudest form? And I can check the deviation with the service monitor and listen directly from another radio to look at the audio on a scope to see that its not distorted.

    What a relief. The $8000.00 for a modern service monitor would probably have resulted in DBW (Death by Wife) so once again I thank you. The life you saved might be my own.

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    The OP25 software is very exciting stuff. When I was first informed there was a less then $10 USB Dongle that would tune in P25 I thought the person telling me this must be dreaming or high on drugs. This OP25 project looks to have some pretty amazing abilities to look at P25 signals in detail. VERY cool. Thanks for giving me a name to search for. I just need to look around more.

    I saw some info on a pretty cool looking gadget called the HackRF One that goes 1MHz to 6 GHz. I'm not sure if it can do complex waveforms like P25 but even if all its good for is as a CW generator thats pretty amazing stuff for less then $200.00 I think?

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    Quote Originally Posted by Jayc View Post
    I saw some info on a pretty cool looking gadget called the HackRF One that goes 1MHz to 6 GHz. I'm not sure if it can do complex waveforms like P25 but even if all its good for is as a CW generator thats pretty amazing stuff for less then $200.00 I think?
    The HackRF is a good unit, I've personally met Mike Ossmann and can vouch both for him and for the HackRF. There seem to be some users who've found other SDRs they like better, which is always the way things go.

    In particular the HackRF works well with the OP25 transmitter stack. One of the apps (multi-tx) allows transmission of four digital voice signals simultaneously - one each of dmr, p25, d-star, and ysf. I have a set of handheld receivers for each of these modes and it's quite cool when all of them are running at once!

    73

    Max

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    Wow that has me stoked. Well if you ever bump into Mr Ossmann again tell him he revved up my engine with his project. There have been times I've thought I was born in an era that wasn't too cool. Manned Space travel was supposed to be going out to Jupiter by 2001 and as a little kid we were all looking forward to "flying cars" etc then.... Apollo 17 came home and "Cool Stuff" seemed to dry up and being intelligent (or just curious about science) wasn't cool anymore. If you were into tech that meant really boring (techno) music. If you really were into Technology you were a nerd and it was expected you were living for the day comic-con was coming around. SIGH...

    I'm starting to think this IS the cool stuff era *at last. Space X with its boosters that land themselves so they can get refueled for reuse, things like SDR that goes DC to (Almost) light. And all of this suddenly seems to be done on the cheap by small groups or individuals with tools that used to require you worked at a fortune 100 and were inside the R&D group fighting it out with the idiots of the company.

    Life is suddenly feeling a lot better.

    Anyway I'll get off the soapbox. Thanks for the info and motivation.

    73 to you as well! - Jay