SDRTrunk Sacramento P25

rob916

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Sep 8, 2015
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Hi guys. I'm new to SDR tinkering and purchansed 2 RTL-SDR dongles with an 800Mhz tuned antenna between the 2 dongles. I've tried some of the software out there to try and setup the P25 system with little success. I found SDRtrunk the easiest to setup with my RR login info I was able to get up and running relatively fast. My question is while the setup works I see frequent errors advising no tuner available from time to time. Attached you can see the errors I'm receiving. Do I need a 3rd SDR dongle for the SRRCS P25 system?

89010
 

scannerboy02

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Do you have the two SDR's parked on a specific frequency or is the software automatically tuning the SDR's?

Being that the error frequency is 853.9000 that is on the top edge of the 851.0->854.0 bandwidth that the SRRCS uses so it could be that both SDR's are out of range of that frequency.

If you have the software automatically tuning the SDR's you could try parking one on a lower frequency and the other on a higher frequency to force the two SDR's to cover the complete bandwidth for the site. Each SDR will cover a maximum bandwidth of ~2.4 MHz, they have issues at the edge of that bandwidth so it's really about ~2 MHz that are usable.

I personally use 1 SDR for each MHz of frequency on a site (this is kind of overload but it works well for me) and would park one at 851.5000, one at 852.5000, and one at 853.5000.
 

rob916

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Sep 8, 2015
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SDRtrunk is set for the control channel and I didn't have to do anything else aside from setting up my talkgroups and what not. Which setup do you use?

Thanks
 

scannerboy02

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I use both SDRTrunk (an older version) and Trunk Recorder, for two different systems.

Another possibility is that you could have more conversations occurring at one time than you have your traffic pool set for. If your traffic pool is set for 6 traffic channels and a 7th (or more) conversation starts the software will not receive it (and give you a tuner error) until the 6th conversation stops. You can try increasing your traffic pool to see if that helps. Being that the SRRCS has 29 traffic channels this may be what's happening.
 

rob916

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Sep 8, 2015
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I was thinking that same thing when I initially set it up so I increased the traffic pool to 30 which seems like I was getting the error less frequently however still getting it from time to time. I may order a 3rd dongle and see what happens. How do you have your antenna configuration between your dongles? I have a mag mount 800Mhz antenna sitting on the flashing outside my window and using an SMA splitter to connect it to the two dongles. I was wondering how to connect a 3rd dongle however I may just order a second antenna.
 

scannerboy02

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I use the telescopic antennas that come with the RTL-SDR's on a magnetic base sitting in a window. I also have a few stuck to an air-conditioning vent and that seems to work okay as well. It all depends on what I want to listen to and I move the antennas around from time to time to see if I can find a better spot.

I use LOTS of SDR's for many different things.
 

rob916

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Sep 8, 2015
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I was skeptical of receiving the P25 system in my house in Fair Oaks because I had a couple Uniden scanners I was programming for someone and they were completely unusable at my house, wouldn't lock onto the control channel for anything. The SDR dongles come in crystal clear. I've played around with PDW monitoring FLEX pagers and what not. It's pretty fascinating what you can do with the SDR dongles. Thanks for the reply, I'll probably order another dongle and see what happens.
 

bigRoN18

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Aug 12, 2001
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Roseville, CA
I've been using SDRTrunk for several years now. Now that we are up to our current version (v0.5.0-alpha3), it is easier than ever. I initially was using Unitrunker and several other accessory applications to get it all to work. With Unitrunker, I had to create a VFO for each possible control channel and one for as many voice channels as I wished to concurrently receive. I then had to route digital audio streams to another app to decode. It also only worked in Windows. SDRTrunk is essentially "OS agnostic", meaning that since it runs on Java, it runs essentially the same on any operating system. For my systems providing feeds to Broadcastify, I prefer to run them on a minimalistic Linux machine, without all the bloatware that comes with Windows. With the latest version of SDRTrunk, you can even download all the site information you need directly from Radio Reference's database.

I too have played with many SDR programs for other things, such as ADSB (monitoring aircraft radio transponders), listening to HD Radio, even collecting electric and water meter data from my house and those nearby.
 

rob916

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Sep 8, 2015
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Yeah it's pretty fascinating what you can do with it. The other night I tried streaming the P25 system to my shoutcast server and it worked but seemed severely delayed and then eventually seemed to keep replaying the same thing on a loop.
 

bigRoN18

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Yeah it's pretty fascinating what you can do with it. The other night I tried streaming the P25 system to my shoutcast server and it worked but seemed severely delayed and then eventually seemed to keep replaying the same thing on a loop.
There are several delays when listening with software like SDRTrunk. In a typical radio, designed to listen to P25 Phase I or P25 Phase II audio, you have a chip produced by Digital Voice System, Inc. that a digital stream, in either IMBE or AMBE2+ codec, is fed into it and analog audio comes out of it; a purpose-built chip that does all the processing for that function and that function only. It is a proprietary conversion process. In SDRTrunk, it uses a reverse-engineered method for decoding this stream using the processing power of your computer. This is a bit of a grey-area legally, which is why the program is not distributed as executable code, but the source code is downloaded separately and compiled on your machine before use. DVSINC charges manufacturers different prices, depending on how many devices they sell, but they make money off of each and every device. SDRTrunk, on the other hand, bypasses this process and is free. If you were to look at the source code for the software, you will see legal text regarding it being for "educational uses" and being not responsible for its use. Notwithstanding legal issues, using software to decode IMBE and AMBE2+ as opposed to a purpose-built chipset has its pros and cons. One pro is that you can initiate multiple instances of it at a time, meaning that as long as you have the computing power, you can simultaneously decode multiple audio streams. Some of the cons include that it is not as efficient and uses computing power of your main CPU and this processing power takes more time to complete than dedicated circuitry. Usually, this delay is only fractions of a second to a second or two, but it is a delay none-the-less over the proprietary chipset. Now, you want to feed this audio to a streaming service like Shoutcast, Icecast or Broadcastify... which typically receive the audio in an MP3 audio format. Now, this same computer is needing to convert the audio coming out of the software converter and run it through a lossy compression format, using even more CPU time. I forgot to mention that SDR in itself requires CPU processing to decode the many different transmissions it is watching simultaneously and tracking the control channel and choosing what bit of digital stream to send to the software decoder. After all this, there is network latency in getting it from the source computer to the stream server and then from the stream server to the streaming client listening. All of these steps add time and use computer resources.

It is because of the sheer amount of CPU processing that you need to have some decent computing power, which is why it really wouldn't work well on a Raspberry Pi or a 15+ year old computer. Depending on the system you are listening to and how you plan on sharing the audio can dictate how much processing power you will need.

In my case, I am using a dedicated machine to listen to and feed the City of Roseville system. I send a feed of police and fire to Broadcastify, in both their traditional method as well as their new Node Calls system. I also send all the police traffic in one stream and all the fire traffic to my Icecast server from this system. This computer uses a dual-core i3-540 CPU with 8 GB of RAM. This is just barely adequate for the Roseville system. I do not want to bog down the computer with other crap that isn't needed so I installed Ubuntu Server (currently 20.04.1) with a minimal GUI added (SDRTrunk requires a graphical user front-end). I only installed the absolute minimum to get it running.
 

N6KDT

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Apr 11, 2014
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I finally got SDRTruck working and decoding Sacramento traffic. I'm using a $20 RTL-SDR dongle I bought a few years back. It works great. I had to set the PPM to 50. I was randomly setting the PPM and by luck I landed on the sweet spot. I followed this tutorial.
 

bigRoN18

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Aug 12, 2001
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Roseville, CA
I finally got SDRTruck working and decoding Sacramento traffic. I'm using a $20 RTL-SDR dongle I bought a few years back. It works great. I had to set the PPM to 50. I was randomly setting the PPM and by luck I landed on the sweet spot. I followed this tutorial.
I've got quite a few cheap little dongles but I've gotten rid of the ones without TCXOs. With a TCXO, it is always within adjustable range with SDRTrunk. I will still fine-tune them but it is generally by no more than about +/- 2.5 PPM. I have noticed that the price has gone up a bit over the past couple years... even with a TCXO, they were often not much above $20 but they are now a bit more, but still fairly cheap. My latest tuner has been an AirSpy R2 clone, an iRadio SDR, which I bought off eBay for about $110. It provides 10 MHz of coverage, which easily covers the Sacramento County Countywide site. If I use it in West Sacramento, I can simultaneously listen to both the Davis and Countywide sites.

I have not worked at our West Sacramento office since SDRTrunk v0.5.0-alpha3 was released so I have not seen the improvements of talkgroup duplication. An example of duplication is the UC Davis Police... they operate on the Countywide site while at UC Davis Medical Center, but it is also on the Davis site simultaneously. If I have my local output to stereo, it might give me the Countywide site on the left channel and Davis site on the right and while close, not exactly timed the same. I believe the new feature is supposed to mute the same talkgroup on a different site so it doesn't duplicate the output. My current work location is in Davis and I can't receive the Countywide site from here.
 
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