Getting Started with DSD+

[wiz561]

I'll preface this with I am very new to this. I used to have a Baofeng radio to listen to the local fire and police, but my town is starting to switch over to P25. I dug up my rtl-sdl usb dongle and antenna and started to play around with sdr# and dsd.

I can pretty much find the local "control channel" with sdr# with cross-referencing the frequency to the published control channel here on rr. When listening to it, it sounds like a digital blurp, which, I'm assuming, is the channel. Then, I start up dsdplus and a bunch of windows open and the scope does have the waveform dancing across, so I'm assuming that dsd+ is getting the audio from sdr#.

The problem is that I don't see anything listed in the window that has dsd+ running. From what I read, there should be some messages that come across, but nothing happens.

On the wiki, it says that my town is using P25 Phase I. On the frequencies near the control channel, I can see some transmissions hopping around, which, from what I've read, is due to trunking. I would think that dsd+ would display something to let me know it's working properly on the control channel.

Sorry to ask such a newbie question, but it's starting to really make me wonder! Thanks in advanced...

 

[br0adband]

If that system has a control channel then it's a trunked system and to monitor that properly you're going to have to use Unitrunker. Now, while it's entirely possible to monitor a trunked system with Unitrunker and with only one RTL stick, it's preferred and vastly more efficient overall to use two of them: one which will stay tuned to the control channel which rarely if ever changes - some systems do have primary and secondary CC's and they change/hop depending on how the system is programmed while others rarely switch to a secondary unless some major problem occurs with the primary.

So, while it's possible to monitor the individual frequencies in use on that system that are not assigned nor active as control channels directly, it's somewhat useless - that would be like trying to monitor a trunked system with a non-trunked scanner, same situation. It has nothing to do with it being an analog or digital system, mind you, it just means that while you'll hear voice traffic on the non-CC frequencies, the conversations will be spotty and disjointed because you're not able to track the trunks properly aka follow the conversation traffic as it hops frequency to frequency.

If you tune in a P25 Phase I control channel using SDR# and then feed that signal from SDR# to DSD+ using a virtual audio cable, DSD+ will show you a screen of nothing but scrolling info like this - and note that if you don't see something like this when tuned into a control channel with SDR# it means somewhere there's no signal getting from SDR# to DSD+ which is usually done by virtual audio cable and then adjusting SDR#'s audio output to match - DSD+ requires a 16 bit 48 kHz input signal so you might have to adjust the settings in the Windows Recording mixer's advanced properties too):

That's all you're going to see, nothing but a scrolling line with that kind of info (obviously you'll have a different NAC and other info) because it's just a control channel and DSD+ isn't decoding it for content aside from pulling out the identifying info. Again, because it's a control channel all you're going to see is basicallly that info: the sync status (it'll be -P25p1 or +P25p1 noting the phase then the NAC of the particular system followed by the identifier for the type of signaling in use (I might be wrong on that last part with the particular naming used). The control channel decode is only going to provide a continuous stream of TSDU traffic, nothing more. To get more "data" you'll need to be tuned to a voice frequency in use on that system.

Now, if you do tune one of those voice frequencies and then just leave DSD+ sitting there, sooner or later you'll going to get some traffic and that's when the other windows for DSD+ will kick into action and display more info as expected along with the fact that if things are set up properly you should hear voice traffic to some degree as well.

You've probably seen guides like this one:

RTL-SDR Tutorial: Following Trunked Radio with Unitrunker - rtl-sdr.com

because there's quite a few of them out there and they all basically relay the same info: it's possible with just one RTL stick but not really the best solution overall.

Now, I'm not 100% sure on this but I believe the fast lane (aka the donation version and I'm note certain of the latest specific build/version number) of DSD+ offers P25 Phase I trunking support but don't quote me on that. The public version (latest is 1.101) does not that I know of but it might, I could just be misinterpreting the included documentation text files.

Anyway, that should get you moving in the right direction hopefully.

 

[wiz561]

Thanks for the response. I've been having it sit there on the trunk channel for quite some time and so far, nothing is happening. I also did check out unitrunker too, but that didn't seem to work out so great either.

Within unitrunker, is there something that is suppose to happen? I do see the signal for the scope, but can you display a debug log that shows you some text, like dsd+ does?

Now, I'm not 100% sure on this but I believe the fast lane (aka the donation version and I'm note certain of the latest specific build/version number) of DSD+ offers P25 Phase I trunking support but don't quote me on that. The public version (latest is 1.101) does not that I know of but it might, I could just be misinterpreting the included documentation text files.

Now this is interesting... I wonder if the version I have (1.101) doesn't decode the trunk. When I run it, it says...

P25 Phase 1 decoding enabled

But now I wonder if the other version says something like "P25p1 Trunk Decoding".... Hmmmm....

 

[br0adband]

Tune into an actual P25 Phase I control channel using SDR# and piping the audio to your speakers - if you hear the control channel, if it's a decent signal (like at least -30 dB on the spectrum or better (better meaning smaller numbers so -25 dB is better than -30 dB, etc) and you can hear it plainly and clearly, piping that to the virtual audio cable for DSD+ should get you something - I'm not talking about the voice traffic, I'm talking about the control channel frequency only.

You have to be able to hear it yourself so you KNOW there's a signal there and you're receiving it before you really want to get involved in getting DSD+ in the mix. If SDR# isn't configured right (gain, frequency, ppm, etc) then you're not going to get anywhere. Digital decoding really does require the ppm offset to be as accurate as possible: I've had instances where a system set for a 55 ppm setting gets practically no usable decodes of any kind but then I checked and realized it should be 56 ppm (for that particular RTL stick) and voila, it worked great. Just 1 ppm can wreck a digital decode process, seriously.

There are variables at work here: the SDR hardware, the drivers, the software involved, the radio signal itself and aspects about it (strength, reception quality, etc) and all of it needs to work in one long signal path before you're going to get anywhere. Tuning the control channel "manually" with SDR# and listening for the signal is the first step. I know you said you heard a "burp" of sorts and I wonder about that: a P25 Phase I control channel signal is a constant stream of digital buzz, not intermittent at all - P25 Phase I voice channels would be highly intermittent so you might have landed on a voice frequency and not a proper CC.

 

[wiz561]

OK....I think I might know what my problem is. The best signal I ever got was a -30db. This was when I was outside and even that wasn't registering. Inside the house, I probably get about a -40 for the best.

I'm using the usb dongle with the antenna that came with it. I took a close look at the antenna and there's a bit of a crack where the wire is. So, I'm wondering if the problem I'm having is that I'm just not getting a strong enough signal.

Thank you for your help with this. Since you said that I need at least a -30 and I'm at about a -40, I'm going to bet that that is my problem. Luckily, this equipment is pretty cheap.

 

[br0adband]

No, I didn't say you need at least -30 dB, I was saying that it's a good signal strength to shoot for - DSD+ can work some wonders with low quality signals. I've had actual solid decodes with signals that provide around 10% 'quality' as DSD+ measures things which is pretty amazing the actual signal strength measured by SDR# (this was a long long time ago before I started using Unitrunker with the RTL sticks) was around -45 dB even with a high amount of gain at work and DSD+ still managed to provide understandable speech from it.

Now that you mentioned you're using the tiny crappy antenna that comes with most of these RTL sticks things make more sense. If you're serious about using RTL hardware for SDR purposes (and I'm guessing you are since they're cheap and they do get the job done just fine overall) then you're going to need to invest in at least one pigtail (a short adapter cable) so you can attach a real actually useful antenna to the RTL stick. And again, getting a second one is recommended so you're looking at potentially having two pigtails and then you'll need to get (or make which is what I do) antennas for them, could be something tuned to specific ranges or perhaps wideband antennas or as stated you can make your own pretty cheap that work surprisingly well.

But those stock antennas are crap, period, and barely useful for most anything. They offer pretty much zero ground plane functionality, the soldering jobs are terrible, the cable quality itself is pretty horrendous, and the fact that they work at all for anything is a small miracle in and of itself.

 

[wiz561]

Thanks for the information.

I think I'm going to splurge on one of the rtl-sdr dongles and antenna through the store. I honestly think that it might be the problem that I'm having here.

When I bought the dongle, I just bought it to mess around with and didn't think about the future. A few years later, the locals are moving to digital and now it's becoming a more interesting hobby to mess around with.

I'll see what happens once I get going with the new hardware. It's great because it's not expensive and something that everybody can afford.

 

[dave3825]

but my town is starting to switch over to P25.

On the wiki, it says that my town is using P25 Phase I. On the frequencies near the control channel, I can see some transmissions hopping around, which, from what I've read, is due to trunking.

If your town is "starting to switch over", you may not see or hear anything until they start testing.

Is the system up and running with users on it? Can you post the link to the system?

I also did check out unitrunker too, but that didn't seem to work out so great either.

Within unitrunker, is there something that is suppose to happen? I do see the signal for the scope, but can you display a debug log that shows you some text, like dsd+ does?

If you follow the guide for unitrunker thats listed here, and just set up the one dongle you have , and set it up following the "Setting up the Signal (Control Channel) Receiver" you would be able to see if there are users using the system. Then when you get the second dongle, you would hear the voice. If you are able to receive the system, and have unitrunker configured properly, YES, a window will pop up showing you the users on the system when they are transmitting..

Thanks for the information.

I think I'm going to splurge on one of the rtl-sdr dongles and antenna through the store. I honestly think that it might be the problem that I'm having here. .

I desoldered the antenna that came with mine and solder a bnc connector to it so I could use my Radio Shack 800MHz antenna.

Made a big difference on my trunked system.

 

[M105]

Check that audio filtering is turned off in SDR#. If you tune SDR# to any reasonably strong P25 Ph1 control channel and pipe the unfiltered audio to DSD+ using VB Cable you should see data messages scrolling like crazy.

1. No audio filtering.
2. Good volume level.
3. I use RTL and Tuner AGC in SDR#
4. Use default settings in DSD+ or make sure P25 decoding is enabled.

 

[slicerwizard]

DSD+ can work some wonders with low quality signals. I've had actual solid decodes with signals that provide around 10% 'quality' as DSD+ measures things which is pretty amazing the actual signal strength measured by SDR# (this was a long long time ago before I started using Unitrunker with the RTL sticks) was around -45 dB even with a high amount of gain at work and DSD+ still managed to provide understandable speech from it.

Don't think I've never seen DSD+ display a signal quality percentage. Not sure what value you're referring to. Unitrunker has an SQ display, but you say that this was before your Unitrunker use, so I guess it's not that. Before DSD+ had the the input audio waveform display, I think it displayed a source audio volume level (xx%) , just like DSD did (and probably still does), but that has nothing to do with SQ / decoding levels.

 

[KB2WVO]

ok

if need to ill move the post..

hope can jump in. iam in the same boat.. i have a great wave form showin in the window..
but no decode..

now before i had the scanner hooked as dtap. but the scanner that hears the signels good is audio feed.

so i have tired both older 1.02 and new 1.1. control channel shows nothing.. there is 2 channels with the sounds.. and for the voice themselfs nothing.. i hear the digi noise..

the area went from mototurbo to radioref says p25 p1 .. 800....

iam lost here also ..