Decoding MotoTrbo/DMR using DSD + Discriminator Tap

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SamAltenberger

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Hello! My school uses a MotoTrbo VHF Digital Trunked radio system that I would like to monitor. By looking at one of the Motorola XPR 4550 (VHF) base station radios in use at my school, I saw the "Unsecure" logo (see below), which I believe means that encryption is not being used and is therefore something I don't have to worry about decoding specifically:



I've been looking at the MotoTrbo page on RadioReference, which indicates that by using Digital Speech Decoder (DSD) and the proper input source from a required discriminator tap, I should be able to have my computer decode the MotoTrbo/DMR signal so that I can monitor it that way.

I currently have a Uniden BCD436HP scanner which I know can't scan MotoTrbo/DMR transmissions on its own, and it doesn't have a built-in discriminator tap output. I'm not very interested in taking that expensive scanner apart or modifying it at risk of breaking it.

My Question is: I know that my school is using a digital trunked MotoTrbo/DMR system in the VHF range. Since digital scanners are so expensive and the decoding would be done using DSD on my computer anyway, would it be possible to buy an inexpensive VHF analog scanner, solder in a discriminator tap, and then use that with DSD on my computer to monitor the digital MotoTrbo/DMR audio signals?

If so, any particular recommendations for inexpensive scanners that would do the trick?
If not, any ideas on what I can do to monitor the radio traffic?

Many thanks! :)
 

mark40

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Hi Sam, great question

I can relate because I wasn't prepared to perform surgery on a dedicated scanner no matter how easy it appears; my soldering skills leave a lot to be desired. I do have a WS1080 that has a tap but it always travels with me to work. Therefore I wanted a dedicated HT that I could move around the house.

After reading a number of posts I opted to purchase a Tytera MD-380 to monitor a local system that switched to MotoTrbo. Used the DSD software for decoding.

Not sure about your price range but the MD-380 is typically available in the $150-$175 range
Tytera MD-380 DMR Digital Two Way Radio

Plenty of folks will chime in with those scanner options. Just wanted to throw in the idea of using a UHF capable HT

For those interested, I learned a lot here
http://www.rtl-sdr.com/rtl-sdr-radio-scanner-tutorial-decoding-digital-voice-p25-with-dsd/

Plus we have a great forum right here at RR
http://forums.radioreference.com/digital-voice-formats/
 
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mciupa

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Any particular recommendations for inexpensive scanners that would do the trick?

Many thanks! :)
Any non-trunking scanner that covers the range you want to cover should suffice. Flea markets, e-Bay should have suitable ones under $50. There is a lot of documentation on how to add a discriminator tap to various models.

I had to buy a mono audio cable, a resistor and an audio jack that matched the size of the cable. Used some speaker wire I had lying around. The scanner I use is a Pro-2032,which first came out in the early '90's and was picked up for $10.
 

RayAir

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If you say it is trunked and a DMR system it is likely MotoTRBO Capacity Plus.

Maybe someone in your area can run DSD+ on it and get the necessary info to program a DMR radio for you?

I travel throughout MI,PA, OH, IN, and Eastern IL so if you're in that area I'd be willing to help you.

Even a cheap Chicom DMR radio will give you better decode audio than you'll get from using a tapped scanner and DSD+. Plus you'll have a portable solution.

USB dongles are a way too go, I have a Noo-elec one myself, but you'll need a better antenna than what comes with it.

I bought a new RS PRO-668 p25 phase 1 and 2 scanner on eBay for $200 when everyone thought RS was going under. The 668 has a built in disc tap, no mod required.

You will need to find all frequencies in their system, color codes in use, and talk group numbers in use. Because it is probably Capacity Plus we know both slots of all site freqs are all likely voice paths. Likely no data channels.

If you determine the freqs, color code and talk groups, you can program the info into a cheap DMR radio ($130-$160) assuming your school is using either VHF, or UHF, not 800 or 900 MHz. If they are on 800 or 900, you would need to put the info in a MotoTRBO handheld ($250-$350 used), then you have to contend with buying or borrowing MotoTRBO CPS and getting a programming cable.

There are videos on YT on how to program cheap DMR radios and MotoTRBO radios.

The furthest you could go, would be to use DSD+ to determine the repeater order of the Cap+ system and program a MotoTRBO XPR 6550 or similar as a Capacity Plus system and you'll be able to follow the TG trunking perfectly.

These are your options. It all depends on how much you can afford or want to put in to this.

If you would like to learn more about MotoTRBO/DMR, search for a pdf called "MotoTRBO System Planner".

If you do end up programming a radio to this system, it is considerate to program it up as " Receive Only".

Happy DMR! Good luck.
 

mciupa

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Buy a sdr USB dongle they run less then $10 and set it up with the computer
This is true (trial), however I would (trial) recommend reading up on how (trial) to set this up as it (trial) is not as simple as "plug and play"(trial)

Wonder why I'm typing (trial)? Use Virtual Audio Cable and you will find out. :p
 

dave3825

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I have a pro 93 and a pro 95 that both have Discriminator Taps but they no longer work on the rebanded trunked system my local police use. Since they will work on some other rebanded systems across the country, I use them for travel. But here, I use them for dmr decoding. I also bought a few usb dongles so I can do it either way..

The plus side to the dongles is they just plug into usb so you never need to worry about batteries.

The down side to the cheap dongles is the antennas suck. I chopped mine and soldered a chassis mount bnc connector and it works great with my existing antennas..
 

wbswetnam

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I have an old Radio Shack Pro-96 I tapped about six months ago, it was quite easy.

More recently, I tapped a Radio Shack Pro-405 that I bought on eBay for about $40. I used a 10K resister, a 1 uF electrolytic capacitor, a tiny eyelet I bought from Hobby Lobby (it came in a pack of about 30) to connect the wire to tapping post 5 on the PC board (so I didn't have to solder directly to the tapping point), and I installed a 1/8" mono jack to the left side of the scanner. The scanner mod took me about an hour, tops. It's easier to do than you may think.

Here in Arkansas a lot of local public service agencies have switched to DMR because it provides cheap digital communications, and they can shut out 99% of the scanners in the county. But, there is that 1% of us out there using either a tapped scanner & DSD+ on the computer, or using an SDR dongle and DSD+, and we're a-listening! If you want to join this 1% it's not hard to do. If you decide to go with the Pro-96 mod or the Pro-405 mod and you want pics, let me know and I will post them.
 
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